Sensors and biosensors based on magnetic nanoparticlesTeresa AP Rocha-Santos Department of Chemistry amp CESAM University of Aveiro Campus de Santiago Aveiro 3810-193 PortugalISEITViseu Instituto Piaget Estrada do Alto do Gaio Galifonge Lordosa Viseu 3515-776 Portugal

A R T I C L E I N F O

KeywordsAnalytical figure of meritBiosensorElectrochemicalLabelMagnetic fieldMagnetic nanoparticleOpticalPiezoelectricSensorTransducer

A B S T R A C T

Magnetic nanoparticles (MNPs) have attracted a growing interest in the development and fabrication ofsensors and biosensors for several applications MNPs can be integrated into the transducer materialsandor be dispersed in the sample followed by their attraction by an external magnetic field onto theactive detection surface of the (bio)sensor This review describes and discusses the recent applicationsof MNPs in sensors and biosensors taking into consideration their analytical figures of merit This workalso addresses the future trends and perspectives of sensors and biosensors based on MNPs

copy 2014 Elsevier BV All rights reserved

Contents

1 Introduction 282 Synthesis properties and characterization of magnetic nanoparticles 293 Sensors and biosensors based on magnetic nanoparticles 29

31 Electrochemical 2932 Optical 3233 Piezoelectric 3234 Magnetic field 34

4 Conclusions and future trends 35Acknowledgements 35References 35

1 Introduction

Nanotechnology has been one of the most important researchtrends in material sciences Nanomaterials (nanoparticle (NP) sizerange 1ndash100 nm) compared with non-NP materials show remark-able differences in physical and chemical properties such as uniqueoptical electrical catalytic thermal and magnetic characteristicsdue to their small size [1] In recent years considerable efforts weretherefore made to develop magnetic NPs (MNPs) due to their ownadvantages such as their size physicochemical properties and lowcost of production [23] MNPs exhibit their best performance at sizesof 10ndash20 nm due to supermagnetism which makes them especial-ly suitable when looking for a fast response due to applied magnetic

fields [4] MNPs also have large surface area and high mass trans-ference Since the properties of MNPs depend strongly on theirdimensions their synthesis and their preparation have to be de-signed in order to obtain particles with adequate size-dependentphysicochemical properties MNPs possessing adequatephysicochemistry and tailored surface properties have been syn-thesized under precise conditions for a plethora of applications suchas sample preparation [5ndash7] wastewater treatment [8] water pu-rification [9] disease therapy [310] disease diagnosis (magneticresonance imaging) [31112] cell labelling and imaging [311] tissueengineering [3] and sensors biosensors and other detection systems[13ndash17] Furthermore MNPs have been used to enhance the sen-sitivity and the stability of sensors and biosensors for the detectionof several analytes in clinical food and environmental applica-tions Taking into consideration the broad application of MNPs insensing and biosensing systems this review describes and dis-cusses the current state of recent applications of MNPs in sensorsand biosensors

Tel +351 232 910 100 Fax +351 232 910 183E-mail address teralexuapt teralexsgmailcom (TAP Rocha-Santos)

httpdxdoiorg101016jtrac2014060160165-9936copy 2014 Elsevier BV All rights reserved

Trends in Analytical Chemistry 62 (2014) 28ndash36

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2 Synthesis properties and characterization ofmagnetic nanoparticles

In the past few years many types of MNP were synthesized in-cluding iron oxides (Fe2O3 and Fe3O4) ferrites of manganese cobaltnickel and magnesium FePt cobalt iron nickel CoPt and FeCo par-ticles and multifunctional composite MNPs such as Fe3O4-Ag Fe3O4-Au FePt-Ag and CdS-FePt heterodimers of NPs MNPs can be synthetizedby physical methods (eg gas-phase deposition and electron-beam li-thography) wet chemical methods (eg coprecipitation high-temperature thermal decomposition andor reduction sol-gel synthesisflow-injection synthesis oxidation method electrochemical methodaerosolvapor-phase method supercritical fluid method and synthe-sis using nanoreactors) and microbial methods [2314]

According to Reddy et al [3] the physical methods are limitedby their inability to control particle size down to the nanometer scalewhile the microbial approach ensures high yield good reproduc-ibility and stability associated with low cost A detailed discussionof MNP synthesis beyond the scope of this review can be foundelsewhere [3111819]

MNPs need to be stabilized in order to prevent irreversible ag-glomeration and to enable dissociation Such stabilization can beperformed by surface coating using appropriate polymerssurfactants[eg dextran and poly(ethylene glycol)] generating polymeric shellsthat avoid cluster growth after nucleation and hold the particledomains against attractive forces (eg nanosphere and nanocapsule)and formation of lipid-like coatings around the magnetic core (egliposomes) [3]

Materials are classified by their response to a magnetic fieldapplied externally and there are the five basic types of magnetism(ie diamagnetism paramagnetism ferromagnetism antiferro-magnetism and ferrimagnetism) [2] Materials whose atomicmagnetic moments are uncoupled display paramagnetism [2] Dueto their small volume MNPs are generally superparamagnetic whichmeans that they have no net magnetic dipole Thus thermal fluc-tuations cause random orientation of the spins (ie thermal energymay be enough to cause the spontaneous change in the magneti-zation of each MNP) Therefore in the absence of an electromagneticfield the net magnetic moment of an MNP will be zero at highenough temperatures but when a magnetic field is applied to theNP a magnetic dipole is induced and there will be a net alignmentof magnetic moments After the external magnetic field is removedthe MNPs randomly orient and return to their native non-magneticstate The shape and the size of NPs will also contribute to deter-mine their magnetic behavior The superparamagnetism in NPs isdetermined by the crystallinity of the structures the type of ma-terial and the number of spins and there is no general rule thatpredicts the magnetic properties of an MNP Magnetism is usuallyevaluated using a magnetometer that monitors magnetization asa function of applied magnetic field [5]

The common analytical techniques used to measure the con-centration and the composition of metallic NPs were recentlydescribed by Silva et al [20] including

bull scanning electron microscopy (SEM) near field scanning opticalmicroscopy (NSOM) transmission electron microscopy (TEM)scanning transmission electron microscopy (STEM) atomic forcemicroscopy (AFM) and environmental scanning electron mi-croscopy (ESEM) to assess the size and the shape of NPs and

bull energy-dispersive X-ray transmission - electron microscopy (EDX-EM) electron-energy-loss spectrometry (EELS) X-raydiffractometry (XRD) and X-ray fluorescence (XRF) to measurethe elemental compositions of single NPs

Those methods were also the most commonly used for charac-terization of MNPs applied in sensing and biosensing systems

[572122] so detailed discussion on such methods is beyond thescope of this review

3 Sensors and biosensors based on magnetic nanoparticles

Sensing strategies based on MNPs offer advantages in terms ofanalytical figures of merit such as enhanced sensitivity low limitof detection (LOD) high signal-to-noise ratio and shorter time ofanalysis than non-MNP-based strategies [2324] In sensing appli-cations MNPs are used through direct application of tagged supportsto the sensor being integrated into the transducer materials andor dispersion of the MNPs in the sample followed by their attractionby an external magnetic field onto the active detection surface ofthe (bio)sensor

Table 1 shows examples of MNP-based sensors and biosensorsfor the detection of several analytes in different samples [2225ndash59]taking into consideration their analytical figures of merit such asLOD and linear range Table 1 shows that these sensors andbiosensors are based on different transduction principles (electro-chemical optical piezoelectric and magnetic field) which we presentand discuss in the following sub-sections according to their clas-sification

31 Electrochemical

Electrochemical (EC) devices measure EC signals (current voltageand impedance) induced by the interaction of analytes and elec-trodes that can be coated with chemicals biochemical materials orbiological elements to improve their surface activity [6061] ECdevices possess advantages of rapidity high sensitivity low cost andeasy miniaturization and operation so being attractive in applica-tions such as clinical environmental biological and pharmaceutical[1360] EC devices can be classified as amperometric potentio-metric voltammetric chemiresistive and capacitive according totheir working principles [60] The EC immunosensors and enzymetissue and DNA biosensors are designed through immobilizingbiological-recognition elements of antibodies enzyme tissue andDNA respectively on the working electrode surface To improve thesensitivity of EC devices signal amplification has been attemptedusing MNPs MNPs can be used in EC devices through their contactwith the electrode surface transport of a redox-active species tothe electrode surface and formation of a thin film on the elec-trode surface For MNP-based EC biosensors [2225ndash2732ndash39]Table 1 shows different detection modes such as voltammetry[25ndash31] amperometry [3233] potentiometry [3435]electrochemiluminescence (ECL) [3637] and EC impedance [3839]which were used for analyte detection and quantification Amongthe sensors the detection mode most used was voltammetry[28ndash31]

Due to its superparamagnetic property biocompatibility with an-tibodies and enzymes and ease of preparation Fe3O4 is mostcommonly used in developing biosensors However Fe3O4 magnet-ic dipolar attraction and its large ratio of surface area to volume maylead to aggregation in clusters when exposed to biological solu-tions Functionalization can overcome this problem and also enhancebiocompatibility

A broad variety of functionalized MNPs have been used such ascore-shell Au-Fe3O4 [25] core-shell Au-Fe3O4SiO2 [32] core-shellFe3O4SiO2 [28] Au-Fe3O4 composite NPs [22] Fe3O4SiO2MWCNTs[33] Fe3O4 anchored on reduced graphene oxide [29] and Fe3O4Au-MWCNT-chitosan [30]

Core-shell Fe3O4SiO2 is one of the most used in biosensors sinceit contributes to stabilization of MNPs in solution and enhances thebinding of ligands at the surface of MNPs Core-shell Fe3O4SiO2 isalso much used in modifying electrode surfaces since its charac-teristics such as good electrical conductivity large surface area and

29TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

Table 1Selected examples of sensors and biosensors based on magnetic nanoparticles

Transductionprinciple

Sensor type Modes of magnetic nanoparticles Detection limit Detection range Analyte Ref

Electrochemical Voltammetric immunosensor Core-shell Au-Fe3O4 001 ng mLminus1 0005ndash50 ng mLminus1 Carcinoembryonic antigen (NA) [25]Voltammetric immunosensor Fe3O4 Au nanoparticles 022 ng mLminus1 05ndash2000 ng mLminus1 Clenbuterol (pork) [26]Voltammetric enzyme based biosensor Au-Fe3O4 composite nanoparticles 56 times 10minus4 ng mLminus1 10 times 10minus3ndash10 ng mLminus1 Organochloride pesticides (cabbage) [22]Voltammetric enzyme based biosensor Fe3O4 Au nanoparticles 20 times 10minus5 M 20 times 10minus5ndash25 times 10minus3 M H2O2 (contact lens care solution) [27]Voltammetric sensor Core-shell Fe3O4SiO2 18 times 10minus8 M 50 times 10minus8ndash10 times 10minus6 M Metronidazole (milk honey) [28]Voltammetric sensor Fe3O4 anchored on reduced graphene oxide ND 02ndash06 nM Cr(III) (NA) [29]Voltammetric sensor Fe3O4Au-MWCNT-chitosan 15 times 10minus9 mol Lminus1 10 times 10minus6-10 times 10minus3 mol Lminus1 Streptomycin (NA) [30]Voltammetric sensor Core-shell Fe3O4SiO2MWCNT 013 μM 060ndash1000 μM Uric acid (blood serum urine) [31]Amperometric enzyme based biosensor Core-shell Au-Fe3O4SiO2 001 mM 005ndash10 mM 10 mMndash80 mM Glucose (human serum) [32]Amperometric enzyme based biosensor Fe3O4SiO2MWCNT 800 nM 1 μMndash30 mM Glucose (glucose solution) [33]Potentiometric immunosensor Magnetic beads Dynabeads Protein G 0007 μg mLminus1 ND Zearalenone (maize certified

reference material baby food cerealwheat rice maize barley oats sorghumrye soya flour)

[34]

Potentiometric enzyme based biosensor Core-shell Fe3O4 05 μM 05 μMndash34 mM Glucose (human serum) [35]Electrochemoluminescent immunosensor Core-shell Fe3O4 Au nanoparticles 02 pg mLminus1 00005ndash50 ng mLminus1 α-fetoprotein (human serum) [36]Electrochemoluminescent immunosensor Core-shell Fe3O4Au 025 ng mLminus1 0ndash6 ng mLminus1 Cry1Ac (NA) [37]Electrochemical impedance immunosensor Iron oxide carboxyl-modified magnetic

nanoparticles001 ng mLminus1 001ndash5 ng mLminus1 Ochratoxin A (wine) [38]

Electrochemical impedance biosensor FeAu nanoparticles-2-aminoethanethiolfunctionalized graphene nanoparticles

20 times 10minus15 M 10 times 10minus4ndash10 times 10minus8 M DNA (NA) [39]

Optical SPR immunosensor Magnetic nanoparticles (fluidMAG-ARA)with iron oxide core

045 pM ND β-human chronic gonadotropin (NA) [40]

SPR immunosensor Fe3O4Au magnetic nanoparticles 065 ng mLminus1 10ndash2000 ng mLminus1 α-fetoprotein (NA) [41]SPR immunosensor Fe3O4 magnetic nanoparticles 0017 nM 027ndash27 nM Thrombin (NA) [42]SPR immunosensor Fe3O4AgAu magnetic nanocomposites ND 015ndash4000 μg mLminus1 Dog IgG (NA) [43]SPR immunosensor Fe3O4-Au nanorod ND 015ndash4000 μg mLminus1 Goat IgM (NA) [44]SPR immunosensor Coreshell Fe3O4SiO2 ND 125ndash2000 μg mLminus1 Rabbit IgG (NA) [45]SPR immunosensor Coreshell Fe3O4AgSiO2 ND 030ndash2000 μg mLminus1 Rabbit IgG (NA) [45]SPR immunosensor Iron oxide carboxyl-modified magnetic

nanoparticles094 ng mLminus1 1ndash50 ng mLminus1 Ochratoxin A (wine) [38]

Fluorescence immunosensor Fe3O4 ND 103ndash108 cfu mLminus1 Escherichia coli (NA) [46]Piezoelectric QCM immunosensor Iron oxide magnetic nanobeads 00128 HA unit 0128ndash128 HA unit Avian influenza virus H5N1 (chicken

tracheal swab)[47]

QCM biosensor Iron oxide magnetic nanoparticles ND 18 times 104ndash18 times 107 cfu mLminus1 D desulfotomaculum (NA) [48]QCM immunosensor Fe3O4SiO2 03 pg mLminus1 0001ndash100 ng mLminus1 C-reactive protein (human serum) [49]Electrochemical QCM immunosensor Core-shell Fe3O4Au-MWCNTcomposites 03 pg mLminus1 0001ndash5 ng mLminus1 Myoglobin (human serum) [50]QCM immunosensor Iron oxide magnetic nanoparticles 53 cfu mLminus1 ND Escherichia coli O157H7 (Milk) [51]

Magnetic field Giant magnetoresistive immunosensor Cubic FeCo nanoparticles 83 fM ND Endoglin (human urine) [52]Giant magnetoresistive immunosensor Cubic FeCo nanoparticles ND 125 fMndash415 pM Interleukin-6 (human serum) [53]Giant magnetoresistive sensor Iron oxide with polyethylene glycol coating 8 Oe shift ND NA [54]Magneto-optical fiber sensor Fe3O4 nanoparticles 5928 pm Oeminus1 ND NA [55]Magneto-optical fiber sensor Fe3O4 in magnetic fluid 16206 pm mTminus1 ND NA [56]Superconducting quantuminterference device sensor

Carboxyl functionalized iron oxide nanoparticles 13 times 106 cells ND MCF7Her2-18 breast cancer cells (mice cells) [57]

Hall sensor Manganese-doped ferrite (MnFe2O4) ND 101ndash105 cells Rare cells MDA-MB-468 cancer cells (whole blood) [58]Hall sensor Manganese-doped ferrite (MnFe2O4) ND 101ndash106 counts Staphylococcus aureus Enterococcus faecalis and

Micrococcus luteus (spiking cultured bacteriain liquid media)

[59]

Shift due to deposition of 7 MNPs Sensitivity

MWCNT Multiwalled carbon nanotube NA not applied ND not determined QCM Quartz-crystal microbalance SPR Surface-plasmon resonance

30TA

PRocha-SantosTrendsin

AnalyticalChem

istry62

(2014)28ndash36

more electroactive interaction sites can provide enhanced masstransport and easier accessibility to the active sites thus increas-ing the analytical signal and the sensitivity

Carbon materials such as carbon nanotubes (CNTs) are alsowidely used to functionalize MNPs due to their physical proper-ties such as large surface area chemical and thermal stabilitycontrolled nanoscale structure and electronic and optical proper-ties [30] Recently a nanocomposite of multi-walled CNTs (MWCNTs)decorated with magnetic core-shell Fe3O4SiO2 was synthetized andused to fabricate a modified carbon-paste electrode (CPE) for thedetermination of uric acid (Fe3O4SiO2MWCNT-CPE) [31] The EC-sensing characteristics were studied by cyclic voltammetry for anMNP-modified CPE (Fe3O4SiO2MWCNT-CPE) an unmodified CPEand an MWCNT-CPE The anodic peak current of MNP-modified CPEwas found to be 27 times higher than that of the MWCNT-CPE and46 times higher than that of the unmodified CPE The increased sen-sitivity can be attributed to the core-shell Fe3O4SiO2MWCNT thathas fast electron-transfer kinetics and a larger electroactive surfacearea compared to the other two electrodes (MWCNT-CPE and un-modified CPE)

Au-Fe3O4-composite NPs [22] are also used due to their easeof preparation large specific surface area good biocompatibilitystrong adsorption ability and good conductivity enhanced by usingAuNPs As an example Gan et al [22] modified a screen-printedcarbon electrode using a composite of MNPs Fig 1 shows the bio-sensor apparatus and the biosensor-detection principle oforganophosphorous pesticides In this device acetylcholinester-ase (AChE)-coated Fe3O4Au MNPs were synthetized and thenabsorbed on the surface of a CNTnano-ZrO2Prussian blueNafion-modified screen-printed carbon electrode The biosensor was appliedto determine dimethoate in cabbage and showed performance com-parable to gas chromatography coupled to flame photometricdetector (GC-FPD) The biosensor showed advantages such as a fastresponse adequate linear range (Table 1) and adequate sensitivityfor the detection of organophosphorous pesticides due to the con-ductive Fe3O4Au MNPs that were used to provide a large electrodesurface area to amplify the current response signal of thiocholine(TCh) and to enhance sensitivity Furthermore the biosensor surfacecan easily be renewed on removing Fe3O4AuAChE from the bio-sensor by applying an external magnetic field due to itssuperparamagnetism Nevertheless the easy immobilization ofenzymeMNPs (Fe3O4AuAChE) on the screen-printed carbon elec-trode reduces the manufacturing costs since it has the advantages

of integration of the electrodes simple manipulation low con-sumption of sample reduced use of expensive reagents and simpleexperimental design

As another example Zamfir et al [38] developed an EC-impedance immunosensor for the detection of ochratoxin-A basedon anti-ochratoxin-A monoclonal-antibody-iron-oxide carboxyl-modified MNPs at the surface of an Au working electrode The useof iron-oxide carboxyl-modified MNPs for anti-ochratoxin-Amonoclonal-antibody immobilization allows easy regeneration ofthe electrode and also reduces the impedance of the system thusincreasing its sensitivity

In both these examples the MNPs were concentrated onelectrode-surface materials and have advantages such as in-creased sensitivity and stability besides ease of renewing theelectrode by releasing the MNPs and replacing them with new MNPs

ECL immunosensors currently use MNPs as labeling agent or im-mobilization support The ECL signal is based on a sequence of stagessuch as EC (single electron redox processes of substance) chemi-cal (biradical combinations) and optical (emission of the ECL quanta)[62] The ECL assays can have three main formats (ie direct inter-action competition assay and sandwich-type assay) [62] Quantumdots such as CdS CdSe or coreshell type ZnSCdSe have been ofgreatest interest in ECL applications due to the quantum confine-ment effect having optical and electronic properties that make themexcellent labels for improving the sensitivity of transducer sur-faces coated with MNPs and magnetic capture probes

An ECL immunosensor was developed for detecting α-fetoprotein(AFP) based on a sandwich immunoreaction strategy using mag-netic particles as capture probes and quantum dots as signal tags[36] Fig 2 shows the process used for preparing magnetic captureprobes Fe3O4-Auprimary AFP antibody (Ab1) and signal tag of CdS-Au secondary AFP antibody (Ab2) The Ab1 was first anchored inthe surface of Fe3O4-Au nanospheres by the Au-S bond The prod-ucts with an Ab1 immobilized on the surface of Fe3O4-Au capturedAFP (antigen) from a solution Finally the protein-labeled CdS-AuNPs were introduced to the immunoreaction with the exposedpart of AFP The Fe3O4-AuAb1AFPAb2CdS-Au was used to con-struct the ECL immunosensor It was observed that the Fe3O4 MNP-modified electrode in the solution had almost no ECL signal whilethe Fe3O4-Au MNP-modified electrode had a slightly enhanced ECLsignal The signal of the immunosensor was therefore further en-hanced by adding CdS-Au as a label compared to the non-labeledsystem (Fe3O4-AuAb1AFP) It was also observed that when the

Fig 1 Example of an electrochemical (voltammetric enzyme-type) biosensor view of the apparatus from (a) plane and (b) vertical directions (c) detection principle forthe detection of organophosphorous pesticides (OPs) CV Cyclic voltammetry DPV differential pulse voltammetry SPCEs screen printed carbon electrodes TCh thiocholineAChE Acetylcholinesterase ATCh Acetylthiocholine GMP Fe3O4Au (GMP) magnetic nanoparticles GMP-AChE Acetylcholinesterase-coated Fe3O4Au magnetic nanoparticlesPB Prussian blue CHI 660B Electrochemical workstation Reprinted from Open Access [22] copy2010 MDPI

31TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

CdS-Au composite film was used instead of CdS NPs the ECL signalincreased 25 times This increase can be attributed to the cata-lytic activity of AuNPs that enhanced electrical conductivity andsensitivity The immunosensor showed performance comparable toELISA in detecting AFP in human serum and therefore potential forclinical application

32 Optical

Optical devices have been applied to the detection of severalanalytes in clinical samples [2463] environmental samples [64ndash66]and food samples [67] due to their main characteristics such as lowsignal-to-noise ratio reduced interferences and reduced costs ofmanufacture Optical devices can be classified by their principlesof detection (ie fluorescence spectroscopy interferometry reflec-tance chemiluminescence (CL) light scattering and refractive index)CL-detection systems have to be enhanced in emission intensity andimproved in selectivity for use in quantitative analysis of complexmatrices such as biological and environmental samples In orderto overcome such limitations MNPs can play a useful part in theCL reactions as catalyst biomolecule carrier and separation tool [16]Iranifam [16] recently reviewed and discussed the analytical ap-plications of CL-detection systems assisted by MNPs so a detailedpresentation and discussion on such methods is beyond the scopeof this review

Table 1 shows that among the MNP-based optical devices thedetection modes used were surface plasmon resonance (SPR)[3840ndash45] and fluorescence spectroscopy [46] Fig 3 shows animmunosensor that combines SPR technology with MNP assays fordetection and manipulation of β human chorionic gonadotropin (β-hCG) [40] The approach is based on a grating-coupled SPR sensorchip that is functionalized by antibodies recognizing the targetanalyte (β-hCG) The MNPs were conjugated with antibodies andwere used both as labels for enhancing refractive-index changes due

to the capture of analyte and also as carriers for fast delivery of theanalyte at the sensor surface thus enhancing the SPR-sensor re-sponse A magnetic field was used to capture the MNPs-antibody-analyte on the sensor surface The use of MNPs together with itscollection on the sensor surface by applying a magnetic field im-proved the sensitivity by four orders of magnitude with respect toregular SPR using direct detection This enhancement was attrib-uted to the larger mass and higher refractive index of MNPs An LODof 045 pM was achieved for the detection of β-hCG This workingprinciple should be further investigated for the analysis of analytessuch as viruses or bacterial pathogens since it can overcome theproblems of the low sensitivity of SPR-biosensor technology due tomass transfer to the sensor surface being strongly hindered by dif-fusion for these analytes

The analytical signal associated with fluorescence intensity canalso be enhanced using MNPs such as Fe3O4 A microfluidicimmunosensor chip was developed having circular microchannels[46] for detection of Escherichia coli The methodology used in-volves in a first step the conjugation of Fe3O4 MNPs with antibodyand in a second step the in-flow capture of antigens in themicrochannels The captured MNPs create a heap-like structure atthe detection site under the influence of a reversed magnetic flowthat increases the retention time of antigens at the site of captureand the capture efficiency of antigens so enhancing the intensityof the fluorescence signal

33 Piezoelectric

Piezoelectric devices can be quartz-crystal microbalance(QCM) and surface acoustic wave (SAW) Table 1 shows that theMNP-based piezoelectric sensors and biosensors are based onQCM transduction [47ndash51] The QCM is a quartz-crystal diskwith metal electrodes in each side of the disk [68ndash70] that vi-brates under the influence of an electric field The frequency of

Fig 2 Example of the preparation procedure of an electrochemiluminescent (ECL) immunosensor BSA Bovine serum albumin AFP α-fetoprotein Ab1 Primary antibodyof AFP Ab2 CdS-Au labeled secondary antibody Reprinted [36] copy 2012 with permission from Elsevier

32 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

this oscillation depends on the cut and the thickness of the diskThis resonant frequency changes as compound(s) adsorb or desorbfrom the surface of the crystal A reduction in frequency is propor-tional to the mass of adsorbed compound QCMs are small androbust inexpensive and capable of giving a rapid response downto a mass change of 1 ng The major drawback of these devices isthe increase in noise with the decrease in dimensions due to in-stability as the surface area-to-volume ratio increases Moredisadvantages of QCM are the interference from atmospheric hu-midity and the difficulty in using them for the determination ofanalytes in solution [71]

MNPs with piezoelectric properties can easily eliminate theseproblems since they offer an attractive transduction mechanism andrecognition event with advantages such as solid-state construc-tion and cost effectiveness The frequency enhancement in thepresence of MNPs can be due to

(1) the MNPs possessing some inherent piezoelectricity(2) the MNPs binding and helping to concentrate the analyte mol-

ecules at the QCM surface and(3) the MNPs acting as matrix carriers to load labels

A QCM immunosensor for detection of C-reactive protein (CRP)in serum was developed In a first step a sandwich-typeimmunoreaction was made between the capture probe (silicondioxide-coated magnetic Fe3O4 NPs) labeled with primary CRP an-tibody (MNs-CRPAb1) CRP and signal tag [horseradish peroxidase(HRP) coupled with HRP-linked secondary CRP antibody co-immobilized on AuNPs (AuNPs-HRPHRP-CRP Ab2)] [49] In a secondstep the immunocomplex was exposed to 3-amino-9-ethylcarbazole(AEC) and hydrogen peroxide Fig 4 shows the preparation proce-dures and the detection principle The capture probe containing theMNPs (MNs-CRPAb1) enhanced the analytical signal due to bothmagnetic separation and immobilization at the electrode surfaceFurther the advantages of the magnetic beads (Fe3O4SiO2) for la-beling CRPAb1 include the mono-disperse size distribution and easypreparation of the labeled conjugates The performance of the QCMmethodology was comparable with the ELISA methodology whendetecting CRP in human serum Moreover the QCM-sensor surfacecan be regenerated easily and used repeatedly due to the use of theMNPs

More research is needed on the development of magneticnanostructures characterization of their piezoelectric behavior andtheir application in piezoelectric sensors and biosensors since theypromise to overcome the sensitivity and stability issues character-istic of these kind of devices

Fig 3 Example of a surface-plasmon resonance (SPR) immunosensor (A) Opticalsensor set-up and (B) a sensor chip of the magnetic nanoparticle (NP)-enhancedgrating coupled SPR sensor (C) The analytical signal before and after immobiliza-tion of the capture antibody Reprinted with permission from [40] copy2011 AmericanChemical Society

Fig 4 Example of a quartz-crystal-microbalance (QCM) immunosensor (Left) Procedures of the preparation of Fe3O4SiO2-Ab1 and AuNPs-HRPHRP-Ab2 conjugations(Right) Detection principle TEOS Tetraethyl orthosilicate EDC 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide NHS Amine-reactive N-hydroxysuccinimide CRP C-reactiveprotein Ab1 Primary CRP antibody Ab2 Secondary CRP antibody AuNP Gold nanoparticle HRP Horseradish peroxidase AEC 3-amino-9-ethylcarbazole MNP Fe3O4SiO2 nanoparticle Reprinted from [49] copy2013 with the permission from Elsevier

33TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

34 Magnetic field

Table 1 shows that the magnetic field devices using MNPs [52ndash59]include giant magnetoresistive (GMR) Hall Effect magneto-optical and superconducting quantum interference sensors

Magnetoresistive sensors are based on the intrinsic magnetore-sistance of a ferromagnetic material or on ferromagneticnon-magnetic heterostructures [72] Depending on the nanostructureof the nanomaterial layer these devices can show the GMR effector the tunneling magnetoresistance effect In these devices the an-alytical signal (change in electrical resistance) is measured followingthe analyte binding in the presence of a magnetic field The ana-lytical signal can therefore be obtained by small changes in themagnetic field and depends on the magnetic field along the sensorarea [73] When using a GMR device and MNPs for interleukin-6(analyte) detection two methodologies have been attempted (Fig 5)[53] In the first possible methodology the GMR sensor isfunctionalized with capture antibodies and the analyte binds tothe capture antibody The detection antibodies labeled with MNPsbind to the analyte captured The second detection methodologyinvolves functionalization of the GMR sensor with capture anti-bodies and then the direct capture of the MNP-labeled analyte onthe GMR biosensor In both cases the GMR biosensor detects thedipole field generated by the MNPs captured on the sensor surfacewhich is sensitive to distance The quality of the MNPs is very im-portant for successful magnetoresistive detection so ideal probesshould be superparamagnetic having high magnetic moment and

large susceptibility in order to enable their magnetization in a smallmagnetic field The MNPs also need to have uniform size and shapesince the magnetic signal depends on it and to be stable in phys-iological solutions so that their coupling with biomolecules canbe controlled [73] Moreover the choice of MNPs with highmagnetic moment leads to increased signal and therefore high sen-sitivity Taking this into consideration for sensitive magnetoresistivedetection the ideal candidates have been metallic Fe Co or theiralloy MNPs [73] According to Li et al [53] considering thesame NP volume and an applied field of 10 Oe the net magneticmoment of one FeCo NP is 7ndash11 times higher than that of oneFe3O4 NP

MNPs can also be used in microfluidic devices which due to theirpermanent magnetic moment can be controlled via external in-homogeneous magnetic fields and also detected by magnetoresistivesensors There are also two types of microfabricated magnetic fielddevices which are the magnetoresistive and the Hall Effect A micro-Hall sensor was developed for the enumeration of rare cells ex vivo[58] The microfluidic chip-based micro-Hall sensor measures themagnetic moments of cells in flow that have been labeled withMNPs The micro-Hall sensor integrates several technological ad-vances for accurate measurements of biomarkers on individual cellssuch as

(1) linear response which enables operation at such high mag-netic fields (gt01 T) that MNPs can be completely magnetizedto generate maximal signal strength

Fig 5 Example of the use of magnetic nanoparticles (MNPs) and giant magneto-resistive (GMR) sensors in two different methodologies (A) Sandwich-type approach wherethe GMR sensor is functionalized with capture antibodies for subsequent analyte binding The detection antibodies labeled with MNPs are then applied and bind to thecaptured analyte (B) Two-layer approach where the GMR sensor is functionalized with capture antibodies for the direct application and capture of the MNP-modified analyte(C) GMR biosensor working principle Reprinted with permission from [53] copy2010 American Chemical Society

34 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

(2) the Hall element is similar size to the cells that pass over itthus increasing the sensitivity of the device

(3) an array of eight sensors constituting the micro-Hall sensorallows less-stringent fluidic control than if the cells had tobe focused over a single sensor and

(4) an array that integrates the overall magnetic flux from eachcell enables measurement of the total magnetic moment ofa single cell The micro-Hall sensor is capable of high-throughput screening and has demonstrated clinical utilityby detecting circulating tumor cells in whole blood of 20ovarian cancer patients at higher sensitivity than currentlypossible with clinical standards

A magnetic field sensor was developed combining a magneticfluid (Fe3O4 NPs) and an optical fiber Loyt-Sagnac interferometer[55] The sensor takes advantage of the magnification of the bire-fringence effect of the magnetic fluid by the properly designed opticalfiber Loyt-Sagnac interferometer structure The sensor demon-strated a sensitivity enhanced by 1ndash3 orders of magnitude comparedto existing magnetic fluid sensors

Magnetic field sensors are not easily extended to the detectionof multi-analytes since the analytical signal arises from the mag-netic moment m which is a single physical parameter By usingsuperparamagnetic NPs with different sizes or different materialsthe analytical signals can be distinguished by their unique non-magnetization curves thus enabling multi-analyte detection bymagnetic field devices [58]

4 Conclusions and future trends

In the past decade MNPs have gained much attention and wereused in several analytical applications such as sensors andbiosensors In (bio)sensing devices MNPs can be applied in thesensor surface or as labels Magnetic labeling of biomolecules is anattractive proposition due to the absence of magnetic back-ground in almost every biological sample However implementationof magnetic labels requires biocompatibility monodispersion andadequate functionalization to reduce non-specific binding Thefunctionalized MNPs with proper functional groups and the surfaceimmobilization technique can therefore play a vital role in signif-icant improvement in the sensitivity of (bio)sensing devices In thiscontext research focused on synthesis and characterization of MNPcomposites and their behavior in (bio)sensing devices is still neededWe therefore recommend further work investigating more suit-able functionalized magnetic nanomaterials that will be fit for multi-analyte detection systems in the future

The majority of the developed devices using MNPs as labels orintroduced into the transducer material are based on EC transduc-tion EC devices were successfully applied to sensitively quantifyingdifferent multi-analytes in environmental clinical and food samplesThese devices can be disposable labeled or label-free integratedinto microfluidic structures and inexpensive

Optical devices have been developed almost always based on CLdetection and a few used detection by SPR and fluorescence spec-troscopy so more research is needed on the development of newoptical sensors and biosensors using MNPs

Concerning piezoelectric devices more research is needed on thedevelopment of new sensors and biosensors since the magneticnanostructures have the potential to overcome sensitivity and sta-bility problems

Magnetic field sensors have been used as detectors of MNP labelsIn MNP-based magnetic field sensors the next step is to take thetechnology to the micrometer and nanometer scale and extend theirapplication to a broad range of environmental food and clinicalsamples since MNPs can enhance the analytical signal Sensing mul-tiple analytes into a single magnetic field device also needs to be

further developed by the use of superparamagnetic NPs with dif-ferent characteristics such as size and type of material

We recommend integration of MNP-based devices andmicrofluidic structures onto single chips since it will enable the com-bination of several steps such as sample preparation molecularlabeling detection and analysis into a single device for multi-analyte detection

Acknowledgements

This work was supported by European Funds through COMPETEand by National Funds through the Portuguese Science Founda-tion (FCT) within project PEst-CMARLA00172013 This work wasalso funded by FEDER under the ldquoPrograma de Cooperaccedilatildeo Territo-rial Europeia INTERREG IV B SUDOErdquo within the framework of theresearch project ORQUE SUDOE SOE3P2F591

References

[1] M Farreacute J Sanchiacutes D Barceloacute Anaysis and assessement of the occurrence thefate and the behavior of nanomaterials in the environment Trend Anal Chem30 (2011) 515ndash527

[2] A Akbarzadeh M Samiei S Daravan Magnetic nanoparticles preparationphysical properties and applications in biomedicine Nanoscale Res Lett 7(2012) 1ndash13

[3] LH Reddy JL Arias J Nicolas P Couvreur Magnetic nanoparticles design andcharacterization toxicity and biocompatibility pharmaceutical and biomedicalapplications Chem Rev 112 (2012) 5818ndash5878

[4] CGCM Netto HE Toma LH Andrade Superparamagnetic nanoparticles asversatile carriers and supporting materials for enzymes J Mol Catal B Enzym85ndash86 (2013) 71ndash92

[5] X-S Li G-T Zhu Y-B Luo B-F Yuan Y-Q Feng Synthesis and applicationsof functionalized magnetic materials in sample preparation Trend Anal Chem45 (2013) 233ndash247

[6] Y Moliner-Martinez A Ribera E Coronado P Campiacutens-Falcoacute Preconcentrationof emerging contaminants in environmental water samples by using silicasupported Fe3O4 magnetic nanoparticles for improving mass detection incapillary liquid chromatography J Chromatogr A 1218 (2011) 2276ndash2283

[7] L Chen T Wang J Tong Application of derivatized magnetic materials to theseparation and the preconcentration of pollutants in water samples Trend AnalChem 30 (2011) 1095ndash1108

[8] SCN Tang IMC Lo Magnetic nanoparticles essential factors for sustainableenvironmental applications Water Res 47 (2013) 2613ndash2632

[9] RD Ambashta M Sillanpaa Water purification using magnetic assistance areview J Hazardo Mater 180 (2010) 38ndash49

[10] JK Oh JM Park Iron oxide-based superparamagnetic polymeric nanomaterialsdesign preparation and biomedical application Progr Polym Sci 36 (2011)168ndash189

[11] M Colombo S Carregal-Romero MF Casula L Gutieacuterrez MP Morales IBBohm et al Biological applications of magnetic nanoparticles Chem Soc Rev12 (2012) 4306ndash4334

[12] S-H Huang R-S Juang Biochemical and biomedical applications ofmultifunctional magnetic nanoparticles a review J Nanopart Res 13 (2011)4411ndash4430

[13] K Aguilar-Arteaga JA Rodriguez E Barrado Magnetic solids in analyticalchemistry a review Anal Chim Acta 674 (2010) 157ndash165

[14] JS Beveridge JR Stephens ME Williams The use of magnetic nanoparticlesin analytical chemistry Annu Rev Anal Chem 4 (2011) 251ndash273

[15] S Carregal-Romero E Caballero-Diacuteaz L Beqa AM Abdelmonem M Ochs DHuhn et al Muliplexed sensing and imaging with colloidal nano- andmicroparticles Annu Rev Anal Chem 6 (2013) 53ndash81

[16] M Iranifam Analytical applications of chemiluminescence-detection systemsassisted by magnetic microparticles and nanoparticles Trend Anal Chem 51(2013) 51ndash70

[17] Y Xu E Wang Electrochemical biosensors based on magnetic micronanoparticles Electrochim Acta 84 (2012) 62ndash73

[18] L-Y Lu L-N Yu X-G Xu Y Jiang Monodisperse magnetic metallicnanoparticles sunthesis performance enhancement and advanced applicationsRare Met 32 (2013) 323ndash331

[19] O Philippova A Barabanova V Molchanov A Khokhlov Magnetic polymerbeads recent trends and developments in synthetic design and applicationsEur Polym J 47 (2011) 542ndash559

[20] BF Silva S Peacuterez P Gardinalli RK Singhal AA Mozeto D Barceloacute Analyticalchemistry of metallic nanoparticles in natural environments Trend Anal Chem30 (2011) 528ndash540

[21] Y-X Ma Y-F Li G-H Zhao L-Q Yang J-Z Wang X Shan et al Preparationand characterization of graphite nanosheets decorated with Fe3O4 nanoparticlesused in the immobilization of glucoamylase Carbon 50 (2012) 2976ndash2986

35TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

Table 1Selected examples of sensors and biosensors based on magnetic nanoparticles

Transductionprinciple

Sensor type Modes of magnetic nanoparticles Detection limit Detection range Analyte Ref

Electrochemical Voltammetric immunosensor Core-shell Au-Fe3O4 001 ng mLminus1 0005ndash50 ng mLminus1 Carcinoembryonic antigen (NA) [25]Voltammetric immunosensor Fe3O4 Au nanoparticles 022 ng mLminus1 05ndash2000 ng mLminus1 Clenbuterol (pork) [26]Voltammetric enzyme based biosensor Au-Fe3O4 composite nanoparticles 56 times 10minus4 ng mLminus1 10 times 10minus3ndash10 ng mLminus1 Organochloride pesticides (cabbage) [22]Voltammetric enzyme based biosensor Fe3O4 Au nanoparticles 20 times 10minus5 M 20 times 10minus5ndash25 times 10minus3 M H2O2 (contact lens care solution) [27]Voltammetric sensor Core-shell Fe3O4SiO2 18 times 10minus8 M 50 times 10minus8ndash10 times 10minus6 M Metronidazole (milk honey) [28]Voltammetric sensor Fe3O4 anchored on reduced graphene oxide ND 02ndash06 nM Cr(III) (NA) [29]Voltammetric sensor Fe3O4Au-MWCNT-chitosan 15 times 10minus9 mol Lminus1 10 times 10minus6-10 times 10minus3 mol Lminus1 Streptomycin (NA) [30]Voltammetric sensor Core-shell Fe3O4SiO2MWCNT 013 μM 060ndash1000 μM Uric acid (blood serum urine) [31]Amperometric enzyme based biosensor Core-shell Au-Fe3O4SiO2 001 mM 005ndash10 mM 10 mMndash80 mM Glucose (human serum) [32]Amperometric enzyme based biosensor Fe3O4SiO2MWCNT 800 nM 1 μMndash30 mM Glucose (glucose solution) [33]Potentiometric immunosensor Magnetic beads Dynabeads Protein G 0007 μg mLminus1 ND Zearalenone (maize certified

reference material baby food cerealwheat rice maize barley oats sorghumrye soya flour)

[34]

Potentiometric enzyme based biosensor Core-shell Fe3O4 05 μM 05 μMndash34 mM Glucose (human serum) [35]Electrochemoluminescent immunosensor Core-shell Fe3O4 Au nanoparticles 02 pg mLminus1 00005ndash50 ng mLminus1 α-fetoprotein (human serum) [36]Electrochemoluminescent immunosensor Core-shell Fe3O4Au 025 ng mLminus1 0ndash6 ng mLminus1 Cry1Ac (NA) [37]Electrochemical impedance immunosensor Iron oxide carboxyl-modified magnetic

nanoparticles001 ng mLminus1 001ndash5 ng mLminus1 Ochratoxin A (wine) [38]

Electrochemical impedance biosensor FeAu nanoparticles-2-aminoethanethiolfunctionalized graphene nanoparticles

20 times 10minus15 M 10 times 10minus4ndash10 times 10minus8 M DNA (NA) [39]

Optical SPR immunosensor Magnetic nanoparticles (fluidMAG-ARA)with iron oxide core

045 pM ND β-human chronic gonadotropin (NA) [40]

SPR immunosensor Fe3O4Au magnetic nanoparticles 065 ng mLminus1 10ndash2000 ng mLminus1 α-fetoprotein (NA) [41]SPR immunosensor Fe3O4 magnetic nanoparticles 0017 nM 027ndash27 nM Thrombin (NA) [42]SPR immunosensor Fe3O4AgAu magnetic nanocomposites ND 015ndash4000 μg mLminus1 Dog IgG (NA) [43]SPR immunosensor Fe3O4-Au nanorod ND 015ndash4000 μg mLminus1 Goat IgM (NA) [44]SPR immunosensor Coreshell Fe3O4SiO2 ND 125ndash2000 μg mLminus1 Rabbit IgG (NA) [45]SPR immunosensor Coreshell Fe3O4AgSiO2 ND 030ndash2000 μg mLminus1 Rabbit IgG (NA) [45]SPR immunosensor Iron oxide carboxyl-modified magnetic

nanoparticles094 ng mLminus1 1ndash50 ng mLminus1 Ochratoxin A (wine) [38]

Fluorescence immunosensor Fe3O4 ND 103ndash108 cfu mLminus1 Escherichia coli (NA) [46]Piezoelectric QCM immunosensor Iron oxide magnetic nanobeads 00128 HA unit 0128ndash128 HA unit Avian influenza virus H5N1 (chicken

tracheal swab)[47]

QCM biosensor Iron oxide magnetic nanoparticles ND 18 times 104ndash18 times 107 cfu mLminus1 D desulfotomaculum (NA) [48]QCM immunosensor Fe3O4SiO2 03 pg mLminus1 0001ndash100 ng mLminus1 C-reactive protein (human serum) [49]Electrochemical QCM immunosensor Core-shell Fe3O4Au-MWCNTcomposites 03 pg mLminus1 0001ndash5 ng mLminus1 Myoglobin (human serum) [50]QCM immunosensor Iron oxide magnetic nanoparticles 53 cfu mLminus1 ND Escherichia coli O157H7 (Milk) [51]

Magnetic field Giant magnetoresistive immunosensor Cubic FeCo nanoparticles 83 fM ND Endoglin (human urine) [52]Giant magnetoresistive immunosensor Cubic FeCo nanoparticles ND 125 fMndash415 pM Interleukin-6 (human serum) [53]Giant magnetoresistive sensor Iron oxide with polyethylene glycol coating 8 Oe shift ND NA [54]Magneto-optical fiber sensor Fe3O4 nanoparticles 5928 pm Oeminus1 ND NA [55]Magneto-optical fiber sensor Fe3O4 in magnetic fluid 16206 pm mTminus1 ND NA [56]Superconducting quantuminterference device sensor

Carboxyl functionalized iron oxide nanoparticles 13 times 106 cells ND MCF7Her2-18 breast cancer cells (mice cells) [57]

Hall sensor Manganese-doped ferrite (MnFe2O4) ND 101ndash105 cells Rare cells MDA-MB-468 cancer cells (whole blood) [58]Hall sensor Manganese-doped ferrite (MnFe2O4) ND 101ndash106 counts Staphylococcus aureus Enterococcus faecalis and

Micrococcus luteus (spiking cultured bacteriain liquid media)

[59]

Shift due to deposition of 7 MNPs Sensitivity

MWCNT Multiwalled carbon nanotube NA not applied ND not determined QCM Quartz-crystal microbalance SPR Surface-plasmon resonance

30TA

PRocha-SantosTrendsin

AnalyticalChem

istry62

(2014)28ndash36

more electroactive interaction sites can provide enhanced masstransport and easier accessibility to the active sites thus increas-ing the analytical signal and the sensitivity

Carbon materials such as carbon nanotubes (CNTs) are alsowidely used to functionalize MNPs due to their physical proper-ties such as large surface area chemical and thermal stabilitycontrolled nanoscale structure and electronic and optical proper-ties [30] Recently a nanocomposite of multi-walled CNTs (MWCNTs)decorated with magnetic core-shell Fe3O4SiO2 was synthetized andused to fabricate a modified carbon-paste electrode (CPE) for thedetermination of uric acid (Fe3O4SiO2MWCNT-CPE) [31] The EC-sensing characteristics were studied by cyclic voltammetry for anMNP-modified CPE (Fe3O4SiO2MWCNT-CPE) an unmodified CPEand an MWCNT-CPE The anodic peak current of MNP-modified CPEwas found to be 27 times higher than that of the MWCNT-CPE and46 times higher than that of the unmodified CPE The increased sen-sitivity can be attributed to the core-shell Fe3O4SiO2MWCNT thathas fast electron-transfer kinetics and a larger electroactive surfacearea compared to the other two electrodes (MWCNT-CPE and un-modified CPE)

Au-Fe3O4-composite NPs [22] are also used due to their easeof preparation large specific surface area good biocompatibilitystrong adsorption ability and good conductivity enhanced by usingAuNPs As an example Gan et al [22] modified a screen-printedcarbon electrode using a composite of MNPs Fig 1 shows the bio-sensor apparatus and the biosensor-detection principle oforganophosphorous pesticides In this device acetylcholinester-ase (AChE)-coated Fe3O4Au MNPs were synthetized and thenabsorbed on the surface of a CNTnano-ZrO2Prussian blueNafion-modified screen-printed carbon electrode The biosensor was appliedto determine dimethoate in cabbage and showed performance com-parable to gas chromatography coupled to flame photometricdetector (GC-FPD) The biosensor showed advantages such as a fastresponse adequate linear range (Table 1) and adequate sensitivityfor the detection of organophosphorous pesticides due to the con-ductive Fe3O4Au MNPs that were used to provide a large electrodesurface area to amplify the current response signal of thiocholine(TCh) and to enhance sensitivity Furthermore the biosensor surfacecan easily be renewed on removing Fe3O4AuAChE from the bio-sensor by applying an external magnetic field due to itssuperparamagnetism Nevertheless the easy immobilization ofenzymeMNPs (Fe3O4AuAChE) on the screen-printed carbon elec-trode reduces the manufacturing costs since it has the advantages

of integration of the electrodes simple manipulation low con-sumption of sample reduced use of expensive reagents and simpleexperimental design

As another example Zamfir et al [38] developed an EC-impedance immunosensor for the detection of ochratoxin-A basedon anti-ochratoxin-A monoclonal-antibody-iron-oxide carboxyl-modified MNPs at the surface of an Au working electrode The useof iron-oxide carboxyl-modified MNPs for anti-ochratoxin-Amonoclonal-antibody immobilization allows easy regeneration ofthe electrode and also reduces the impedance of the system thusincreasing its sensitivity

In both these examples the MNPs were concentrated onelectrode-surface materials and have advantages such as in-creased sensitivity and stability besides ease of renewing theelectrode by releasing the MNPs and replacing them with new MNPs

ECL immunosensors currently use MNPs as labeling agent or im-mobilization support The ECL signal is based on a sequence of stagessuch as EC (single electron redox processes of substance) chemi-cal (biradical combinations) and optical (emission of the ECL quanta)[62] The ECL assays can have three main formats (ie direct inter-action competition assay and sandwich-type assay) [62] Quantumdots such as CdS CdSe or coreshell type ZnSCdSe have been ofgreatest interest in ECL applications due to the quantum confine-ment effect having optical and electronic properties that make themexcellent labels for improving the sensitivity of transducer sur-faces coated with MNPs and magnetic capture probes

An ECL immunosensor was developed for detecting α-fetoprotein(AFP) based on a sandwich immunoreaction strategy using mag-netic particles as capture probes and quantum dots as signal tags[36] Fig 2 shows the process used for preparing magnetic captureprobes Fe3O4-Auprimary AFP antibody (Ab1) and signal tag of CdS-Au secondary AFP antibody (Ab2) The Ab1 was first anchored inthe surface of Fe3O4-Au nanospheres by the Au-S bond The prod-ucts with an Ab1 immobilized on the surface of Fe3O4-Au capturedAFP (antigen) from a solution Finally the protein-labeled CdS-AuNPs were introduced to the immunoreaction with the exposedpart of AFP The Fe3O4-AuAb1AFPAb2CdS-Au was used to con-struct the ECL immunosensor It was observed that the Fe3O4 MNP-modified electrode in the solution had almost no ECL signal whilethe Fe3O4-Au MNP-modified electrode had a slightly enhanced ECLsignal The signal of the immunosensor was therefore further en-hanced by adding CdS-Au as a label compared to the non-labeledsystem (Fe3O4-AuAb1AFP) It was also observed that when the

Fig 1 Example of an electrochemical (voltammetric enzyme-type) biosensor view of the apparatus from (a) plane and (b) vertical directions (c) detection principle forthe detection of organophosphorous pesticides (OPs) CV Cyclic voltammetry DPV differential pulse voltammetry SPCEs screen printed carbon electrodes TCh thiocholineAChE Acetylcholinesterase ATCh Acetylthiocholine GMP Fe3O4Au (GMP) magnetic nanoparticles GMP-AChE Acetylcholinesterase-coated Fe3O4Au magnetic nanoparticlesPB Prussian blue CHI 660B Electrochemical workstation Reprinted from Open Access [22] copy2010 MDPI

31TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

CdS-Au composite film was used instead of CdS NPs the ECL signalincreased 25 times This increase can be attributed to the cata-lytic activity of AuNPs that enhanced electrical conductivity andsensitivity The immunosensor showed performance comparable toELISA in detecting AFP in human serum and therefore potential forclinical application

32 Optical

Optical devices have been applied to the detection of severalanalytes in clinical samples [2463] environmental samples [64ndash66]and food samples [67] due to their main characteristics such as lowsignal-to-noise ratio reduced interferences and reduced costs ofmanufacture Optical devices can be classified by their principlesof detection (ie fluorescence spectroscopy interferometry reflec-tance chemiluminescence (CL) light scattering and refractive index)CL-detection systems have to be enhanced in emission intensity andimproved in selectivity for use in quantitative analysis of complexmatrices such as biological and environmental samples In orderto overcome such limitations MNPs can play a useful part in theCL reactions as catalyst biomolecule carrier and separation tool [16]Iranifam [16] recently reviewed and discussed the analytical ap-plications of CL-detection systems assisted by MNPs so a detailedpresentation and discussion on such methods is beyond the scopeof this review

Table 1 shows that among the MNP-based optical devices thedetection modes used were surface plasmon resonance (SPR)[3840ndash45] and fluorescence spectroscopy [46] Fig 3 shows animmunosensor that combines SPR technology with MNP assays fordetection and manipulation of β human chorionic gonadotropin (β-hCG) [40] The approach is based on a grating-coupled SPR sensorchip that is functionalized by antibodies recognizing the targetanalyte (β-hCG) The MNPs were conjugated with antibodies andwere used both as labels for enhancing refractive-index changes due

to the capture of analyte and also as carriers for fast delivery of theanalyte at the sensor surface thus enhancing the SPR-sensor re-sponse A magnetic field was used to capture the MNPs-antibody-analyte on the sensor surface The use of MNPs together with itscollection on the sensor surface by applying a magnetic field im-proved the sensitivity by four orders of magnitude with respect toregular SPR using direct detection This enhancement was attrib-uted to the larger mass and higher refractive index of MNPs An LODof 045 pM was achieved for the detection of β-hCG This workingprinciple should be further investigated for the analysis of analytessuch as viruses or bacterial pathogens since it can overcome theproblems of the low sensitivity of SPR-biosensor technology due tomass transfer to the sensor surface being strongly hindered by dif-fusion for these analytes

The analytical signal associated with fluorescence intensity canalso be enhanced using MNPs such as Fe3O4 A microfluidicimmunosensor chip was developed having circular microchannels[46] for detection of Escherichia coli The methodology used in-volves in a first step the conjugation of Fe3O4 MNPs with antibodyand in a second step the in-flow capture of antigens in themicrochannels The captured MNPs create a heap-like structure atthe detection site under the influence of a reversed magnetic flowthat increases the retention time of antigens at the site of captureand the capture efficiency of antigens so enhancing the intensityof the fluorescence signal

33 Piezoelectric

Piezoelectric devices can be quartz-crystal microbalance(QCM) and surface acoustic wave (SAW) Table 1 shows that theMNP-based piezoelectric sensors and biosensors are based onQCM transduction [47ndash51] The QCM is a quartz-crystal diskwith metal electrodes in each side of the disk [68ndash70] that vi-brates under the influence of an electric field The frequency of

Fig 2 Example of the preparation procedure of an electrochemiluminescent (ECL) immunosensor BSA Bovine serum albumin AFP α-fetoprotein Ab1 Primary antibodyof AFP Ab2 CdS-Au labeled secondary antibody Reprinted [36] copy 2012 with permission from Elsevier

32 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

this oscillation depends on the cut and the thickness of the diskThis resonant frequency changes as compound(s) adsorb or desorbfrom the surface of the crystal A reduction in frequency is propor-tional to the mass of adsorbed compound QCMs are small androbust inexpensive and capable of giving a rapid response downto a mass change of 1 ng The major drawback of these devices isthe increase in noise with the decrease in dimensions due to in-stability as the surface area-to-volume ratio increases Moredisadvantages of QCM are the interference from atmospheric hu-midity and the difficulty in using them for the determination ofanalytes in solution [71]

MNPs with piezoelectric properties can easily eliminate theseproblems since they offer an attractive transduction mechanism andrecognition event with advantages such as solid-state construc-tion and cost effectiveness The frequency enhancement in thepresence of MNPs can be due to

(1) the MNPs possessing some inherent piezoelectricity(2) the MNPs binding and helping to concentrate the analyte mol-

ecules at the QCM surface and(3) the MNPs acting as matrix carriers to load labels

A QCM immunosensor for detection of C-reactive protein (CRP)in serum was developed In a first step a sandwich-typeimmunoreaction was made between the capture probe (silicondioxide-coated magnetic Fe3O4 NPs) labeled with primary CRP an-tibody (MNs-CRPAb1) CRP and signal tag [horseradish peroxidase(HRP) coupled with HRP-linked secondary CRP antibody co-immobilized on AuNPs (AuNPs-HRPHRP-CRP Ab2)] [49] In a secondstep the immunocomplex was exposed to 3-amino-9-ethylcarbazole(AEC) and hydrogen peroxide Fig 4 shows the preparation proce-dures and the detection principle The capture probe containing theMNPs (MNs-CRPAb1) enhanced the analytical signal due to bothmagnetic separation and immobilization at the electrode surfaceFurther the advantages of the magnetic beads (Fe3O4SiO2) for la-beling CRPAb1 include the mono-disperse size distribution and easypreparation of the labeled conjugates The performance of the QCMmethodology was comparable with the ELISA methodology whendetecting CRP in human serum Moreover the QCM-sensor surfacecan be regenerated easily and used repeatedly due to the use of theMNPs

More research is needed on the development of magneticnanostructures characterization of their piezoelectric behavior andtheir application in piezoelectric sensors and biosensors since theypromise to overcome the sensitivity and stability issues character-istic of these kind of devices

Fig 3 Example of a surface-plasmon resonance (SPR) immunosensor (A) Opticalsensor set-up and (B) a sensor chip of the magnetic nanoparticle (NP)-enhancedgrating coupled SPR sensor (C) The analytical signal before and after immobiliza-tion of the capture antibody Reprinted with permission from [40] copy2011 AmericanChemical Society

Fig 4 Example of a quartz-crystal-microbalance (QCM) immunosensor (Left) Procedures of the preparation of Fe3O4SiO2-Ab1 and AuNPs-HRPHRP-Ab2 conjugations(Right) Detection principle TEOS Tetraethyl orthosilicate EDC 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide NHS Amine-reactive N-hydroxysuccinimide CRP C-reactiveprotein Ab1 Primary CRP antibody Ab2 Secondary CRP antibody AuNP Gold nanoparticle HRP Horseradish peroxidase AEC 3-amino-9-ethylcarbazole MNP Fe3O4SiO2 nanoparticle Reprinted from [49] copy2013 with the permission from Elsevier

33TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

34 Magnetic field

Table 1 shows that the magnetic field devices using MNPs [52ndash59]include giant magnetoresistive (GMR) Hall Effect magneto-optical and superconducting quantum interference sensors

Magnetoresistive sensors are based on the intrinsic magnetore-sistance of a ferromagnetic material or on ferromagneticnon-magnetic heterostructures [72] Depending on the nanostructureof the nanomaterial layer these devices can show the GMR effector the tunneling magnetoresistance effect In these devices the an-alytical signal (change in electrical resistance) is measured followingthe analyte binding in the presence of a magnetic field The ana-lytical signal can therefore be obtained by small changes in themagnetic field and depends on the magnetic field along the sensorarea [73] When using a GMR device and MNPs for interleukin-6(analyte) detection two methodologies have been attempted (Fig 5)[53] In the first possible methodology the GMR sensor isfunctionalized with capture antibodies and the analyte binds tothe capture antibody The detection antibodies labeled with MNPsbind to the analyte captured The second detection methodologyinvolves functionalization of the GMR sensor with capture anti-bodies and then the direct capture of the MNP-labeled analyte onthe GMR biosensor In both cases the GMR biosensor detects thedipole field generated by the MNPs captured on the sensor surfacewhich is sensitive to distance The quality of the MNPs is very im-portant for successful magnetoresistive detection so ideal probesshould be superparamagnetic having high magnetic moment and

large susceptibility in order to enable their magnetization in a smallmagnetic field The MNPs also need to have uniform size and shapesince the magnetic signal depends on it and to be stable in phys-iological solutions so that their coupling with biomolecules canbe controlled [73] Moreover the choice of MNPs with highmagnetic moment leads to increased signal and therefore high sen-sitivity Taking this into consideration for sensitive magnetoresistivedetection the ideal candidates have been metallic Fe Co or theiralloy MNPs [73] According to Li et al [53] considering thesame NP volume and an applied field of 10 Oe the net magneticmoment of one FeCo NP is 7ndash11 times higher than that of oneFe3O4 NP

MNPs can also be used in microfluidic devices which due to theirpermanent magnetic moment can be controlled via external in-homogeneous magnetic fields and also detected by magnetoresistivesensors There are also two types of microfabricated magnetic fielddevices which are the magnetoresistive and the Hall Effect A micro-Hall sensor was developed for the enumeration of rare cells ex vivo[58] The microfluidic chip-based micro-Hall sensor measures themagnetic moments of cells in flow that have been labeled withMNPs The micro-Hall sensor integrates several technological ad-vances for accurate measurements of biomarkers on individual cellssuch as

(1) linear response which enables operation at such high mag-netic fields (gt01 T) that MNPs can be completely magnetizedto generate maximal signal strength

Fig 5 Example of the use of magnetic nanoparticles (MNPs) and giant magneto-resistive (GMR) sensors in two different methodologies (A) Sandwich-type approach wherethe GMR sensor is functionalized with capture antibodies for subsequent analyte binding The detection antibodies labeled with MNPs are then applied and bind to thecaptured analyte (B) Two-layer approach where the GMR sensor is functionalized with capture antibodies for the direct application and capture of the MNP-modified analyte(C) GMR biosensor working principle Reprinted with permission from [53] copy2010 American Chemical Society

34 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

(2) the Hall element is similar size to the cells that pass over itthus increasing the sensitivity of the device

(3) an array of eight sensors constituting the micro-Hall sensorallows less-stringent fluidic control than if the cells had tobe focused over a single sensor and

(4) an array that integrates the overall magnetic flux from eachcell enables measurement of the total magnetic moment ofa single cell The micro-Hall sensor is capable of high-throughput screening and has demonstrated clinical utilityby detecting circulating tumor cells in whole blood of 20ovarian cancer patients at higher sensitivity than currentlypossible with clinical standards

A magnetic field sensor was developed combining a magneticfluid (Fe3O4 NPs) and an optical fiber Loyt-Sagnac interferometer[55] The sensor takes advantage of the magnification of the bire-fringence effect of the magnetic fluid by the properly designed opticalfiber Loyt-Sagnac interferometer structure The sensor demon-strated a sensitivity enhanced by 1ndash3 orders of magnitude comparedto existing magnetic fluid sensors

Magnetic field sensors are not easily extended to the detectionof multi-analytes since the analytical signal arises from the mag-netic moment m which is a single physical parameter By usingsuperparamagnetic NPs with different sizes or different materialsthe analytical signals can be distinguished by their unique non-magnetization curves thus enabling multi-analyte detection bymagnetic field devices [58]

4 Conclusions and future trends

In the past decade MNPs have gained much attention and wereused in several analytical applications such as sensors andbiosensors In (bio)sensing devices MNPs can be applied in thesensor surface or as labels Magnetic labeling of biomolecules is anattractive proposition due to the absence of magnetic back-ground in almost every biological sample However implementationof magnetic labels requires biocompatibility monodispersion andadequate functionalization to reduce non-specific binding Thefunctionalized MNPs with proper functional groups and the surfaceimmobilization technique can therefore play a vital role in signif-icant improvement in the sensitivity of (bio)sensing devices In thiscontext research focused on synthesis and characterization of MNPcomposites and their behavior in (bio)sensing devices is still neededWe therefore recommend further work investigating more suit-able functionalized magnetic nanomaterials that will be fit for multi-analyte detection systems in the future

The majority of the developed devices using MNPs as labels orintroduced into the transducer material are based on EC transduc-tion EC devices were successfully applied to sensitively quantifyingdifferent multi-analytes in environmental clinical and food samplesThese devices can be disposable labeled or label-free integratedinto microfluidic structures and inexpensive

Optical devices have been developed almost always based on CLdetection and a few used detection by SPR and fluorescence spec-troscopy so more research is needed on the development of newoptical sensors and biosensors using MNPs

Concerning piezoelectric devices more research is needed on thedevelopment of new sensors and biosensors since the magneticnanostructures have the potential to overcome sensitivity and sta-bility problems

Magnetic field sensors have been used as detectors of MNP labelsIn MNP-based magnetic field sensors the next step is to take thetechnology to the micrometer and nanometer scale and extend theirapplication to a broad range of environmental food and clinicalsamples since MNPs can enhance the analytical signal Sensing mul-tiple analytes into a single magnetic field device also needs to be

further developed by the use of superparamagnetic NPs with dif-ferent characteristics such as size and type of material

We recommend integration of MNP-based devices andmicrofluidic structures onto single chips since it will enable the com-bination of several steps such as sample preparation molecularlabeling detection and analysis into a single device for multi-analyte detection

Acknowledgements

This work was supported by European Funds through COMPETEand by National Funds through the Portuguese Science Founda-tion (FCT) within project PEst-CMARLA00172013 This work wasalso funded by FEDER under the ldquoPrograma de Cooperaccedilatildeo Territo-rial Europeia INTERREG IV B SUDOErdquo within the framework of theresearch project ORQUE SUDOE SOE3P2F591

References

[1] M Farreacute J Sanchiacutes D Barceloacute Anaysis and assessement of the occurrence thefate and the behavior of nanomaterials in the environment Trend Anal Chem30 (2011) 515ndash527

[2] A Akbarzadeh M Samiei S Daravan Magnetic nanoparticles preparationphysical properties and applications in biomedicine Nanoscale Res Lett 7(2012) 1ndash13

[3] LH Reddy JL Arias J Nicolas P Couvreur Magnetic nanoparticles design andcharacterization toxicity and biocompatibility pharmaceutical and biomedicalapplications Chem Rev 112 (2012) 5818ndash5878

[4] CGCM Netto HE Toma LH Andrade Superparamagnetic nanoparticles asversatile carriers and supporting materials for enzymes J Mol Catal B Enzym85ndash86 (2013) 71ndash92

[5] X-S Li G-T Zhu Y-B Luo B-F Yuan Y-Q Feng Synthesis and applicationsof functionalized magnetic materials in sample preparation Trend Anal Chem45 (2013) 233ndash247

[6] Y Moliner-Martinez A Ribera E Coronado P Campiacutens-Falcoacute Preconcentrationof emerging contaminants in environmental water samples by using silicasupported Fe3O4 magnetic nanoparticles for improving mass detection incapillary liquid chromatography J Chromatogr A 1218 (2011) 2276ndash2283

[7] L Chen T Wang J Tong Application of derivatized magnetic materials to theseparation and the preconcentration of pollutants in water samples Trend AnalChem 30 (2011) 1095ndash1108

[8] SCN Tang IMC Lo Magnetic nanoparticles essential factors for sustainableenvironmental applications Water Res 47 (2013) 2613ndash2632

[9] RD Ambashta M Sillanpaa Water purification using magnetic assistance areview J Hazardo Mater 180 (2010) 38ndash49

[10] JK Oh JM Park Iron oxide-based superparamagnetic polymeric nanomaterialsdesign preparation and biomedical application Progr Polym Sci 36 (2011)168ndash189

[11] M Colombo S Carregal-Romero MF Casula L Gutieacuterrez MP Morales IBBohm et al Biological applications of magnetic nanoparticles Chem Soc Rev12 (2012) 4306ndash4334

[12] S-H Huang R-S Juang Biochemical and biomedical applications ofmultifunctional magnetic nanoparticles a review J Nanopart Res 13 (2011)4411ndash4430

[13] K Aguilar-Arteaga JA Rodriguez E Barrado Magnetic solids in analyticalchemistry a review Anal Chim Acta 674 (2010) 157ndash165

[14] JS Beveridge JR Stephens ME Williams The use of magnetic nanoparticlesin analytical chemistry Annu Rev Anal Chem 4 (2011) 251ndash273

[15] S Carregal-Romero E Caballero-Diacuteaz L Beqa AM Abdelmonem M Ochs DHuhn et al Muliplexed sensing and imaging with colloidal nano- andmicroparticles Annu Rev Anal Chem 6 (2013) 53ndash81

[16] M Iranifam Analytical applications of chemiluminescence-detection systemsassisted by magnetic microparticles and nanoparticles Trend Anal Chem 51(2013) 51ndash70

[17] Y Xu E Wang Electrochemical biosensors based on magnetic micronanoparticles Electrochim Acta 84 (2012) 62ndash73

[18] L-Y Lu L-N Yu X-G Xu Y Jiang Monodisperse magnetic metallicnanoparticles sunthesis performance enhancement and advanced applicationsRare Met 32 (2013) 323ndash331

[19] O Philippova A Barabanova V Molchanov A Khokhlov Magnetic polymerbeads recent trends and developments in synthetic design and applicationsEur Polym J 47 (2011) 542ndash559

[20] BF Silva S Peacuterez P Gardinalli RK Singhal AA Mozeto D Barceloacute Analyticalchemistry of metallic nanoparticles in natural environments Trend Anal Chem30 (2011) 528ndash540

[21] Y-X Ma Y-F Li G-H Zhao L-Q Yang J-Z Wang X Shan et al Preparationand characterization of graphite nanosheets decorated with Fe3O4 nanoparticlesused in the immobilization of glucoamylase Carbon 50 (2012) 2976ndash2986

35TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

more electroactive interaction sites can provide enhanced masstransport and easier accessibility to the active sites thus increas-ing the analytical signal and the sensitivity

Carbon materials such as carbon nanotubes (CNTs) are alsowidely used to functionalize MNPs due to their physical proper-ties such as large surface area chemical and thermal stabilitycontrolled nanoscale structure and electronic and optical proper-ties [30] Recently a nanocomposite of multi-walled CNTs (MWCNTs)decorated with magnetic core-shell Fe3O4SiO2 was synthetized andused to fabricate a modified carbon-paste electrode (CPE) for thedetermination of uric acid (Fe3O4SiO2MWCNT-CPE) [31] The EC-sensing characteristics were studied by cyclic voltammetry for anMNP-modified CPE (Fe3O4SiO2MWCNT-CPE) an unmodified CPEand an MWCNT-CPE The anodic peak current of MNP-modified CPEwas found to be 27 times higher than that of the MWCNT-CPE and46 times higher than that of the unmodified CPE The increased sen-sitivity can be attributed to the core-shell Fe3O4SiO2MWCNT thathas fast electron-transfer kinetics and a larger electroactive surfacearea compared to the other two electrodes (MWCNT-CPE and un-modified CPE)

Au-Fe3O4-composite NPs [22] are also used due to their easeof preparation large specific surface area good biocompatibilitystrong adsorption ability and good conductivity enhanced by usingAuNPs As an example Gan et al [22] modified a screen-printedcarbon electrode using a composite of MNPs Fig 1 shows the bio-sensor apparatus and the biosensor-detection principle oforganophosphorous pesticides In this device acetylcholinester-ase (AChE)-coated Fe3O4Au MNPs were synthetized and thenabsorbed on the surface of a CNTnano-ZrO2Prussian blueNafion-modified screen-printed carbon electrode The biosensor was appliedto determine dimethoate in cabbage and showed performance com-parable to gas chromatography coupled to flame photometricdetector (GC-FPD) The biosensor showed advantages such as a fastresponse adequate linear range (Table 1) and adequate sensitivityfor the detection of organophosphorous pesticides due to the con-ductive Fe3O4Au MNPs that were used to provide a large electrodesurface area to amplify the current response signal of thiocholine(TCh) and to enhance sensitivity Furthermore the biosensor surfacecan easily be renewed on removing Fe3O4AuAChE from the bio-sensor by applying an external magnetic field due to itssuperparamagnetism Nevertheless the easy immobilization ofenzymeMNPs (Fe3O4AuAChE) on the screen-printed carbon elec-trode reduces the manufacturing costs since it has the advantages

of integration of the electrodes simple manipulation low con-sumption of sample reduced use of expensive reagents and simpleexperimental design

As another example Zamfir et al [38] developed an EC-impedance immunosensor for the detection of ochratoxin-A basedon anti-ochratoxin-A monoclonal-antibody-iron-oxide carboxyl-modified MNPs at the surface of an Au working electrode The useof iron-oxide carboxyl-modified MNPs for anti-ochratoxin-Amonoclonal-antibody immobilization allows easy regeneration ofthe electrode and also reduces the impedance of the system thusincreasing its sensitivity

In both these examples the MNPs were concentrated onelectrode-surface materials and have advantages such as in-creased sensitivity and stability besides ease of renewing theelectrode by releasing the MNPs and replacing them with new MNPs

ECL immunosensors currently use MNPs as labeling agent or im-mobilization support The ECL signal is based on a sequence of stagessuch as EC (single electron redox processes of substance) chemi-cal (biradical combinations) and optical (emission of the ECL quanta)[62] The ECL assays can have three main formats (ie direct inter-action competition assay and sandwich-type assay) [62] Quantumdots such as CdS CdSe or coreshell type ZnSCdSe have been ofgreatest interest in ECL applications due to the quantum confine-ment effect having optical and electronic properties that make themexcellent labels for improving the sensitivity of transducer sur-faces coated with MNPs and magnetic capture probes

An ECL immunosensor was developed for detecting α-fetoprotein(AFP) based on a sandwich immunoreaction strategy using mag-netic particles as capture probes and quantum dots as signal tags[36] Fig 2 shows the process used for preparing magnetic captureprobes Fe3O4-Auprimary AFP antibody (Ab1) and signal tag of CdS-Au secondary AFP antibody (Ab2) The Ab1 was first anchored inthe surface of Fe3O4-Au nanospheres by the Au-S bond The prod-ucts with an Ab1 immobilized on the surface of Fe3O4-Au capturedAFP (antigen) from a solution Finally the protein-labeled CdS-AuNPs were introduced to the immunoreaction with the exposedpart of AFP The Fe3O4-AuAb1AFPAb2CdS-Au was used to con-struct the ECL immunosensor It was observed that the Fe3O4 MNP-modified electrode in the solution had almost no ECL signal whilethe Fe3O4-Au MNP-modified electrode had a slightly enhanced ECLsignal The signal of the immunosensor was therefore further en-hanced by adding CdS-Au as a label compared to the non-labeledsystem (Fe3O4-AuAb1AFP) It was also observed that when the

Fig 1 Example of an electrochemical (voltammetric enzyme-type) biosensor view of the apparatus from (a) plane and (b) vertical directions (c) detection principle forthe detection of organophosphorous pesticides (OPs) CV Cyclic voltammetry DPV differential pulse voltammetry SPCEs screen printed carbon electrodes TCh thiocholineAChE Acetylcholinesterase ATCh Acetylthiocholine GMP Fe3O4Au (GMP) magnetic nanoparticles GMP-AChE Acetylcholinesterase-coated Fe3O4Au magnetic nanoparticlesPB Prussian blue CHI 660B Electrochemical workstation Reprinted from Open Access [22] copy2010 MDPI

31TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

CdS-Au composite film was used instead of CdS NPs the ECL signalincreased 25 times This increase can be attributed to the cata-lytic activity of AuNPs that enhanced electrical conductivity andsensitivity The immunosensor showed performance comparable toELISA in detecting AFP in human serum and therefore potential forclinical application

32 Optical

Optical devices have been applied to the detection of severalanalytes in clinical samples [2463] environmental samples [64ndash66]and food samples [67] due to their main characteristics such as lowsignal-to-noise ratio reduced interferences and reduced costs ofmanufacture Optical devices can be classified by their principlesof detection (ie fluorescence spectroscopy interferometry reflec-tance chemiluminescence (CL) light scattering and refractive index)CL-detection systems have to be enhanced in emission intensity andimproved in selectivity for use in quantitative analysis of complexmatrices such as biological and environmental samples In orderto overcome such limitations MNPs can play a useful part in theCL reactions as catalyst biomolecule carrier and separation tool [16]Iranifam [16] recently reviewed and discussed the analytical ap-plications of CL-detection systems assisted by MNPs so a detailedpresentation and discussion on such methods is beyond the scopeof this review

Table 1 shows that among the MNP-based optical devices thedetection modes used were surface plasmon resonance (SPR)[3840ndash45] and fluorescence spectroscopy [46] Fig 3 shows animmunosensor that combines SPR technology with MNP assays fordetection and manipulation of β human chorionic gonadotropin (β-hCG) [40] The approach is based on a grating-coupled SPR sensorchip that is functionalized by antibodies recognizing the targetanalyte (β-hCG) The MNPs were conjugated with antibodies andwere used both as labels for enhancing refractive-index changes due

to the capture of analyte and also as carriers for fast delivery of theanalyte at the sensor surface thus enhancing the SPR-sensor re-sponse A magnetic field was used to capture the MNPs-antibody-analyte on the sensor surface The use of MNPs together with itscollection on the sensor surface by applying a magnetic field im-proved the sensitivity by four orders of magnitude with respect toregular SPR using direct detection This enhancement was attrib-uted to the larger mass and higher refractive index of MNPs An LODof 045 pM was achieved for the detection of β-hCG This workingprinciple should be further investigated for the analysis of analytessuch as viruses or bacterial pathogens since it can overcome theproblems of the low sensitivity of SPR-biosensor technology due tomass transfer to the sensor surface being strongly hindered by dif-fusion for these analytes

The analytical signal associated with fluorescence intensity canalso be enhanced using MNPs such as Fe3O4 A microfluidicimmunosensor chip was developed having circular microchannels[46] for detection of Escherichia coli The methodology used in-volves in a first step the conjugation of Fe3O4 MNPs with antibodyand in a second step the in-flow capture of antigens in themicrochannels The captured MNPs create a heap-like structure atthe detection site under the influence of a reversed magnetic flowthat increases the retention time of antigens at the site of captureand the capture efficiency of antigens so enhancing the intensityof the fluorescence signal

33 Piezoelectric

Piezoelectric devices can be quartz-crystal microbalance(QCM) and surface acoustic wave (SAW) Table 1 shows that theMNP-based piezoelectric sensors and biosensors are based onQCM transduction [47ndash51] The QCM is a quartz-crystal diskwith metal electrodes in each side of the disk [68ndash70] that vi-brates under the influence of an electric field The frequency of

Fig 2 Example of the preparation procedure of an electrochemiluminescent (ECL) immunosensor BSA Bovine serum albumin AFP α-fetoprotein Ab1 Primary antibodyof AFP Ab2 CdS-Au labeled secondary antibody Reprinted [36] copy 2012 with permission from Elsevier

32 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

this oscillation depends on the cut and the thickness of the diskThis resonant frequency changes as compound(s) adsorb or desorbfrom the surface of the crystal A reduction in frequency is propor-tional to the mass of adsorbed compound QCMs are small androbust inexpensive and capable of giving a rapid response downto a mass change of 1 ng The major drawback of these devices isthe increase in noise with the decrease in dimensions due to in-stability as the surface area-to-volume ratio increases Moredisadvantages of QCM are the interference from atmospheric hu-midity and the difficulty in using them for the determination ofanalytes in solution [71]

MNPs with piezoelectric properties can easily eliminate theseproblems since they offer an attractive transduction mechanism andrecognition event with advantages such as solid-state construc-tion and cost effectiveness The frequency enhancement in thepresence of MNPs can be due to

(1) the MNPs possessing some inherent piezoelectricity(2) the MNPs binding and helping to concentrate the analyte mol-

ecules at the QCM surface and(3) the MNPs acting as matrix carriers to load labels

A QCM immunosensor for detection of C-reactive protein (CRP)in serum was developed In a first step a sandwich-typeimmunoreaction was made between the capture probe (silicondioxide-coated magnetic Fe3O4 NPs) labeled with primary CRP an-tibody (MNs-CRPAb1) CRP and signal tag [horseradish peroxidase(HRP) coupled with HRP-linked secondary CRP antibody co-immobilized on AuNPs (AuNPs-HRPHRP-CRP Ab2)] [49] In a secondstep the immunocomplex was exposed to 3-amino-9-ethylcarbazole(AEC) and hydrogen peroxide Fig 4 shows the preparation proce-dures and the detection principle The capture probe containing theMNPs (MNs-CRPAb1) enhanced the analytical signal due to bothmagnetic separation and immobilization at the electrode surfaceFurther the advantages of the magnetic beads (Fe3O4SiO2) for la-beling CRPAb1 include the mono-disperse size distribution and easypreparation of the labeled conjugates The performance of the QCMmethodology was comparable with the ELISA methodology whendetecting CRP in human serum Moreover the QCM-sensor surfacecan be regenerated easily and used repeatedly due to the use of theMNPs

More research is needed on the development of magneticnanostructures characterization of their piezoelectric behavior andtheir application in piezoelectric sensors and biosensors since theypromise to overcome the sensitivity and stability issues character-istic of these kind of devices

Fig 3 Example of a surface-plasmon resonance (SPR) immunosensor (A) Opticalsensor set-up and (B) a sensor chip of the magnetic nanoparticle (NP)-enhancedgrating coupled SPR sensor (C) The analytical signal before and after immobiliza-tion of the capture antibody Reprinted with permission from [40] copy2011 AmericanChemical Society

Fig 4 Example of a quartz-crystal-microbalance (QCM) immunosensor (Left) Procedures of the preparation of Fe3O4SiO2-Ab1 and AuNPs-HRPHRP-Ab2 conjugations(Right) Detection principle TEOS Tetraethyl orthosilicate EDC 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide NHS Amine-reactive N-hydroxysuccinimide CRP C-reactiveprotein Ab1 Primary CRP antibody Ab2 Secondary CRP antibody AuNP Gold nanoparticle HRP Horseradish peroxidase AEC 3-amino-9-ethylcarbazole MNP Fe3O4SiO2 nanoparticle Reprinted from [49] copy2013 with the permission from Elsevier

33TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

34 Magnetic field

Table 1 shows that the magnetic field devices using MNPs [52ndash59]include giant magnetoresistive (GMR) Hall Effect magneto-optical and superconducting quantum interference sensors

Magnetoresistive sensors are based on the intrinsic magnetore-sistance of a ferromagnetic material or on ferromagneticnon-magnetic heterostructures [72] Depending on the nanostructureof the nanomaterial layer these devices can show the GMR effector the tunneling magnetoresistance effect In these devices the an-alytical signal (change in electrical resistance) is measured followingthe analyte binding in the presence of a magnetic field The ana-lytical signal can therefore be obtained by small changes in themagnetic field and depends on the magnetic field along the sensorarea [73] When using a GMR device and MNPs for interleukin-6(analyte) detection two methodologies have been attempted (Fig 5)[53] In the first possible methodology the GMR sensor isfunctionalized with capture antibodies and the analyte binds tothe capture antibody The detection antibodies labeled with MNPsbind to the analyte captured The second detection methodologyinvolves functionalization of the GMR sensor with capture anti-bodies and then the direct capture of the MNP-labeled analyte onthe GMR biosensor In both cases the GMR biosensor detects thedipole field generated by the MNPs captured on the sensor surfacewhich is sensitive to distance The quality of the MNPs is very im-portant for successful magnetoresistive detection so ideal probesshould be superparamagnetic having high magnetic moment and

large susceptibility in order to enable their magnetization in a smallmagnetic field The MNPs also need to have uniform size and shapesince the magnetic signal depends on it and to be stable in phys-iological solutions so that their coupling with biomolecules canbe controlled [73] Moreover the choice of MNPs with highmagnetic moment leads to increased signal and therefore high sen-sitivity Taking this into consideration for sensitive magnetoresistivedetection the ideal candidates have been metallic Fe Co or theiralloy MNPs [73] According to Li et al [53] considering thesame NP volume and an applied field of 10 Oe the net magneticmoment of one FeCo NP is 7ndash11 times higher than that of oneFe3O4 NP

MNPs can also be used in microfluidic devices which due to theirpermanent magnetic moment can be controlled via external in-homogeneous magnetic fields and also detected by magnetoresistivesensors There are also two types of microfabricated magnetic fielddevices which are the magnetoresistive and the Hall Effect A micro-Hall sensor was developed for the enumeration of rare cells ex vivo[58] The microfluidic chip-based micro-Hall sensor measures themagnetic moments of cells in flow that have been labeled withMNPs The micro-Hall sensor integrates several technological ad-vances for accurate measurements of biomarkers on individual cellssuch as

(1) linear response which enables operation at such high mag-netic fields (gt01 T) that MNPs can be completely magnetizedto generate maximal signal strength

Fig 5 Example of the use of magnetic nanoparticles (MNPs) and giant magneto-resistive (GMR) sensors in two different methodologies (A) Sandwich-type approach wherethe GMR sensor is functionalized with capture antibodies for subsequent analyte binding The detection antibodies labeled with MNPs are then applied and bind to thecaptured analyte (B) Two-layer approach where the GMR sensor is functionalized with capture antibodies for the direct application and capture of the MNP-modified analyte(C) GMR biosensor working principle Reprinted with permission from [53] copy2010 American Chemical Society

34 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

(2) the Hall element is similar size to the cells that pass over itthus increasing the sensitivity of the device

(3) an array of eight sensors constituting the micro-Hall sensorallows less-stringent fluidic control than if the cells had tobe focused over a single sensor and

(4) an array that integrates the overall magnetic flux from eachcell enables measurement of the total magnetic moment ofa single cell The micro-Hall sensor is capable of high-throughput screening and has demonstrated clinical utilityby detecting circulating tumor cells in whole blood of 20ovarian cancer patients at higher sensitivity than currentlypossible with clinical standards

A magnetic field sensor was developed combining a magneticfluid (Fe3O4 NPs) and an optical fiber Loyt-Sagnac interferometer[55] The sensor takes advantage of the magnification of the bire-fringence effect of the magnetic fluid by the properly designed opticalfiber Loyt-Sagnac interferometer structure The sensor demon-strated a sensitivity enhanced by 1ndash3 orders of magnitude comparedto existing magnetic fluid sensors

Magnetic field sensors are not easily extended to the detectionof multi-analytes since the analytical signal arises from the mag-netic moment m which is a single physical parameter By usingsuperparamagnetic NPs with different sizes or different materialsthe analytical signals can be distinguished by their unique non-magnetization curves thus enabling multi-analyte detection bymagnetic field devices [58]

4 Conclusions and future trends

In the past decade MNPs have gained much attention and wereused in several analytical applications such as sensors andbiosensors In (bio)sensing devices MNPs can be applied in thesensor surface or as labels Magnetic labeling of biomolecules is anattractive proposition due to the absence of magnetic back-ground in almost every biological sample However implementationof magnetic labels requires biocompatibility monodispersion andadequate functionalization to reduce non-specific binding Thefunctionalized MNPs with proper functional groups and the surfaceimmobilization technique can therefore play a vital role in signif-icant improvement in the sensitivity of (bio)sensing devices In thiscontext research focused on synthesis and characterization of MNPcomposites and their behavior in (bio)sensing devices is still neededWe therefore recommend further work investigating more suit-able functionalized magnetic nanomaterials that will be fit for multi-analyte detection systems in the future

The majority of the developed devices using MNPs as labels orintroduced into the transducer material are based on EC transduc-tion EC devices were successfully applied to sensitively quantifyingdifferent multi-analytes in environmental clinical and food samplesThese devices can be disposable labeled or label-free integratedinto microfluidic structures and inexpensive

Optical devices have been developed almost always based on CLdetection and a few used detection by SPR and fluorescence spec-troscopy so more research is needed on the development of newoptical sensors and biosensors using MNPs

Concerning piezoelectric devices more research is needed on thedevelopment of new sensors and biosensors since the magneticnanostructures have the potential to overcome sensitivity and sta-bility problems

Magnetic field sensors have been used as detectors of MNP labelsIn MNP-based magnetic field sensors the next step is to take thetechnology to the micrometer and nanometer scale and extend theirapplication to a broad range of environmental food and clinicalsamples since MNPs can enhance the analytical signal Sensing mul-tiple analytes into a single magnetic field device also needs to be

further developed by the use of superparamagnetic NPs with dif-ferent characteristics such as size and type of material

We recommend integration of MNP-based devices andmicrofluidic structures onto single chips since it will enable the com-bination of several steps such as sample preparation molecularlabeling detection and analysis into a single device for multi-analyte detection

Acknowledgements

This work was supported by European Funds through COMPETEand by National Funds through the Portuguese Science Founda-tion (FCT) within project PEst-CMARLA00172013 This work wasalso funded by FEDER under the ldquoPrograma de Cooperaccedilatildeo Territo-rial Europeia INTERREG IV B SUDOErdquo within the framework of theresearch project ORQUE SUDOE SOE3P2F591

References

[1] M Farreacute J Sanchiacutes D Barceloacute Anaysis and assessement of the occurrence thefate and the behavior of nanomaterials in the environment Trend Anal Chem30 (2011) 515ndash527

[2] A Akbarzadeh M Samiei S Daravan Magnetic nanoparticles preparationphysical properties and applications in biomedicine Nanoscale Res Lett 7(2012) 1ndash13

[3] LH Reddy JL Arias J Nicolas P Couvreur Magnetic nanoparticles design andcharacterization toxicity and biocompatibility pharmaceutical and biomedicalapplications Chem Rev 112 (2012) 5818ndash5878

[4] CGCM Netto HE Toma LH Andrade Superparamagnetic nanoparticles asversatile carriers and supporting materials for enzymes J Mol Catal B Enzym85ndash86 (2013) 71ndash92

[5] X-S Li G-T Zhu Y-B Luo B-F Yuan Y-Q Feng Synthesis and applicationsof functionalized magnetic materials in sample preparation Trend Anal Chem45 (2013) 233ndash247

[6] Y Moliner-Martinez A Ribera E Coronado P Campiacutens-Falcoacute Preconcentrationof emerging contaminants in environmental water samples by using silicasupported Fe3O4 magnetic nanoparticles for improving mass detection incapillary liquid chromatography J Chromatogr A 1218 (2011) 2276ndash2283

[7] L Chen T Wang J Tong Application of derivatized magnetic materials to theseparation and the preconcentration of pollutants in water samples Trend AnalChem 30 (2011) 1095ndash1108

[8] SCN Tang IMC Lo Magnetic nanoparticles essential factors for sustainableenvironmental applications Water Res 47 (2013) 2613ndash2632

[9] RD Ambashta M Sillanpaa Water purification using magnetic assistance areview J Hazardo Mater 180 (2010) 38ndash49

[10] JK Oh JM Park Iron oxide-based superparamagnetic polymeric nanomaterialsdesign preparation and biomedical application Progr Polym Sci 36 (2011)168ndash189

[11] M Colombo S Carregal-Romero MF Casula L Gutieacuterrez MP Morales IBBohm et al Biological applications of magnetic nanoparticles Chem Soc Rev12 (2012) 4306ndash4334

[12] S-H Huang R-S Juang Biochemical and biomedical applications ofmultifunctional magnetic nanoparticles a review J Nanopart Res 13 (2011)4411ndash4430

[13] K Aguilar-Arteaga JA Rodriguez E Barrado Magnetic solids in analyticalchemistry a review Anal Chim Acta 674 (2010) 157ndash165

[14] JS Beveridge JR Stephens ME Williams The use of magnetic nanoparticlesin analytical chemistry Annu Rev Anal Chem 4 (2011) 251ndash273

[15] S Carregal-Romero E Caballero-Diacuteaz L Beqa AM Abdelmonem M Ochs DHuhn et al Muliplexed sensing and imaging with colloidal nano- andmicroparticles Annu Rev Anal Chem 6 (2013) 53ndash81

[16] M Iranifam Analytical applications of chemiluminescence-detection systemsassisted by magnetic microparticles and nanoparticles Trend Anal Chem 51(2013) 51ndash70

[17] Y Xu E Wang Electrochemical biosensors based on magnetic micronanoparticles Electrochim Acta 84 (2012) 62ndash73

[18] L-Y Lu L-N Yu X-G Xu Y Jiang Monodisperse magnetic metallicnanoparticles sunthesis performance enhancement and advanced applicationsRare Met 32 (2013) 323ndash331

[19] O Philippova A Barabanova V Molchanov A Khokhlov Magnetic polymerbeads recent trends and developments in synthetic design and applicationsEur Polym J 47 (2011) 542ndash559

[20] BF Silva S Peacuterez P Gardinalli RK Singhal AA Mozeto D Barceloacute Analyticalchemistry of metallic nanoparticles in natural environments Trend Anal Chem30 (2011) 528ndash540

[21] Y-X Ma Y-F Li G-H Zhao L-Q Yang J-Z Wang X Shan et al Preparationand characterization of graphite nanosheets decorated with Fe3O4 nanoparticlesused in the immobilization of glucoamylase Carbon 50 (2012) 2976ndash2986

35TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

CdS-Au composite film was used instead of CdS NPs the ECL signalincreased 25 times This increase can be attributed to the cata-lytic activity of AuNPs that enhanced electrical conductivity andsensitivity The immunosensor showed performance comparable toELISA in detecting AFP in human serum and therefore potential forclinical application

32 Optical

Optical devices have been applied to the detection of severalanalytes in clinical samples [2463] environmental samples [64ndash66]and food samples [67] due to their main characteristics such as lowsignal-to-noise ratio reduced interferences and reduced costs ofmanufacture Optical devices can be classified by their principlesof detection (ie fluorescence spectroscopy interferometry reflec-tance chemiluminescence (CL) light scattering and refractive index)CL-detection systems have to be enhanced in emission intensity andimproved in selectivity for use in quantitative analysis of complexmatrices such as biological and environmental samples In orderto overcome such limitations MNPs can play a useful part in theCL reactions as catalyst biomolecule carrier and separation tool [16]Iranifam [16] recently reviewed and discussed the analytical ap-plications of CL-detection systems assisted by MNPs so a detailedpresentation and discussion on such methods is beyond the scopeof this review

Table 1 shows that among the MNP-based optical devices thedetection modes used were surface plasmon resonance (SPR)[3840ndash45] and fluorescence spectroscopy [46] Fig 3 shows animmunosensor that combines SPR technology with MNP assays fordetection and manipulation of β human chorionic gonadotropin (β-hCG) [40] The approach is based on a grating-coupled SPR sensorchip that is functionalized by antibodies recognizing the targetanalyte (β-hCG) The MNPs were conjugated with antibodies andwere used both as labels for enhancing refractive-index changes due

to the capture of analyte and also as carriers for fast delivery of theanalyte at the sensor surface thus enhancing the SPR-sensor re-sponse A magnetic field was used to capture the MNPs-antibody-analyte on the sensor surface The use of MNPs together with itscollection on the sensor surface by applying a magnetic field im-proved the sensitivity by four orders of magnitude with respect toregular SPR using direct detection This enhancement was attrib-uted to the larger mass and higher refractive index of MNPs An LODof 045 pM was achieved for the detection of β-hCG This workingprinciple should be further investigated for the analysis of analytessuch as viruses or bacterial pathogens since it can overcome theproblems of the low sensitivity of SPR-biosensor technology due tomass transfer to the sensor surface being strongly hindered by dif-fusion for these analytes

The analytical signal associated with fluorescence intensity canalso be enhanced using MNPs such as Fe3O4 A microfluidicimmunosensor chip was developed having circular microchannels[46] for detection of Escherichia coli The methodology used in-volves in a first step the conjugation of Fe3O4 MNPs with antibodyand in a second step the in-flow capture of antigens in themicrochannels The captured MNPs create a heap-like structure atthe detection site under the influence of a reversed magnetic flowthat increases the retention time of antigens at the site of captureand the capture efficiency of antigens so enhancing the intensityof the fluorescence signal

33 Piezoelectric

Piezoelectric devices can be quartz-crystal microbalance(QCM) and surface acoustic wave (SAW) Table 1 shows that theMNP-based piezoelectric sensors and biosensors are based onQCM transduction [47ndash51] The QCM is a quartz-crystal diskwith metal electrodes in each side of the disk [68ndash70] that vi-brates under the influence of an electric field The frequency of

Fig 2 Example of the preparation procedure of an electrochemiluminescent (ECL) immunosensor BSA Bovine serum albumin AFP α-fetoprotein Ab1 Primary antibodyof AFP Ab2 CdS-Au labeled secondary antibody Reprinted [36] copy 2012 with permission from Elsevier

32 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

this oscillation depends on the cut and the thickness of the diskThis resonant frequency changes as compound(s) adsorb or desorbfrom the surface of the crystal A reduction in frequency is propor-tional to the mass of adsorbed compound QCMs are small androbust inexpensive and capable of giving a rapid response downto a mass change of 1 ng The major drawback of these devices isthe increase in noise with the decrease in dimensions due to in-stability as the surface area-to-volume ratio increases Moredisadvantages of QCM are the interference from atmospheric hu-midity and the difficulty in using them for the determination ofanalytes in solution [71]

MNPs with piezoelectric properties can easily eliminate theseproblems since they offer an attractive transduction mechanism andrecognition event with advantages such as solid-state construc-tion and cost effectiveness The frequency enhancement in thepresence of MNPs can be due to

(1) the MNPs possessing some inherent piezoelectricity(2) the MNPs binding and helping to concentrate the analyte mol-

ecules at the QCM surface and(3) the MNPs acting as matrix carriers to load labels

A QCM immunosensor for detection of C-reactive protein (CRP)in serum was developed In a first step a sandwich-typeimmunoreaction was made between the capture probe (silicondioxide-coated magnetic Fe3O4 NPs) labeled with primary CRP an-tibody (MNs-CRPAb1) CRP and signal tag [horseradish peroxidase(HRP) coupled with HRP-linked secondary CRP antibody co-immobilized on AuNPs (AuNPs-HRPHRP-CRP Ab2)] [49] In a secondstep the immunocomplex was exposed to 3-amino-9-ethylcarbazole(AEC) and hydrogen peroxide Fig 4 shows the preparation proce-dures and the detection principle The capture probe containing theMNPs (MNs-CRPAb1) enhanced the analytical signal due to bothmagnetic separation and immobilization at the electrode surfaceFurther the advantages of the magnetic beads (Fe3O4SiO2) for la-beling CRPAb1 include the mono-disperse size distribution and easypreparation of the labeled conjugates The performance of the QCMmethodology was comparable with the ELISA methodology whendetecting CRP in human serum Moreover the QCM-sensor surfacecan be regenerated easily and used repeatedly due to the use of theMNPs

More research is needed on the development of magneticnanostructures characterization of their piezoelectric behavior andtheir application in piezoelectric sensors and biosensors since theypromise to overcome the sensitivity and stability issues character-istic of these kind of devices

Fig 3 Example of a surface-plasmon resonance (SPR) immunosensor (A) Opticalsensor set-up and (B) a sensor chip of the magnetic nanoparticle (NP)-enhancedgrating coupled SPR sensor (C) The analytical signal before and after immobiliza-tion of the capture antibody Reprinted with permission from [40] copy2011 AmericanChemical Society

Fig 4 Example of a quartz-crystal-microbalance (QCM) immunosensor (Left) Procedures of the preparation of Fe3O4SiO2-Ab1 and AuNPs-HRPHRP-Ab2 conjugations(Right) Detection principle TEOS Tetraethyl orthosilicate EDC 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide NHS Amine-reactive N-hydroxysuccinimide CRP C-reactiveprotein Ab1 Primary CRP antibody Ab2 Secondary CRP antibody AuNP Gold nanoparticle HRP Horseradish peroxidase AEC 3-amino-9-ethylcarbazole MNP Fe3O4SiO2 nanoparticle Reprinted from [49] copy2013 with the permission from Elsevier

33TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

34 Magnetic field

Table 1 shows that the magnetic field devices using MNPs [52ndash59]include giant magnetoresistive (GMR) Hall Effect magneto-optical and superconducting quantum interference sensors

Magnetoresistive sensors are based on the intrinsic magnetore-sistance of a ferromagnetic material or on ferromagneticnon-magnetic heterostructures [72] Depending on the nanostructureof the nanomaterial layer these devices can show the GMR effector the tunneling magnetoresistance effect In these devices the an-alytical signal (change in electrical resistance) is measured followingthe analyte binding in the presence of a magnetic field The ana-lytical signal can therefore be obtained by small changes in themagnetic field and depends on the magnetic field along the sensorarea [73] When using a GMR device and MNPs for interleukin-6(analyte) detection two methodologies have been attempted (Fig 5)[53] In the first possible methodology the GMR sensor isfunctionalized with capture antibodies and the analyte binds tothe capture antibody The detection antibodies labeled with MNPsbind to the analyte captured The second detection methodologyinvolves functionalization of the GMR sensor with capture anti-bodies and then the direct capture of the MNP-labeled analyte onthe GMR biosensor In both cases the GMR biosensor detects thedipole field generated by the MNPs captured on the sensor surfacewhich is sensitive to distance The quality of the MNPs is very im-portant for successful magnetoresistive detection so ideal probesshould be superparamagnetic having high magnetic moment and

large susceptibility in order to enable their magnetization in a smallmagnetic field The MNPs also need to have uniform size and shapesince the magnetic signal depends on it and to be stable in phys-iological solutions so that their coupling with biomolecules canbe controlled [73] Moreover the choice of MNPs with highmagnetic moment leads to increased signal and therefore high sen-sitivity Taking this into consideration for sensitive magnetoresistivedetection the ideal candidates have been metallic Fe Co or theiralloy MNPs [73] According to Li et al [53] considering thesame NP volume and an applied field of 10 Oe the net magneticmoment of one FeCo NP is 7ndash11 times higher than that of oneFe3O4 NP

MNPs can also be used in microfluidic devices which due to theirpermanent magnetic moment can be controlled via external in-homogeneous magnetic fields and also detected by magnetoresistivesensors There are also two types of microfabricated magnetic fielddevices which are the magnetoresistive and the Hall Effect A micro-Hall sensor was developed for the enumeration of rare cells ex vivo[58] The microfluidic chip-based micro-Hall sensor measures themagnetic moments of cells in flow that have been labeled withMNPs The micro-Hall sensor integrates several technological ad-vances for accurate measurements of biomarkers on individual cellssuch as

(1) linear response which enables operation at such high mag-netic fields (gt01 T) that MNPs can be completely magnetizedto generate maximal signal strength

Fig 5 Example of the use of magnetic nanoparticles (MNPs) and giant magneto-resistive (GMR) sensors in two different methodologies (A) Sandwich-type approach wherethe GMR sensor is functionalized with capture antibodies for subsequent analyte binding The detection antibodies labeled with MNPs are then applied and bind to thecaptured analyte (B) Two-layer approach where the GMR sensor is functionalized with capture antibodies for the direct application and capture of the MNP-modified analyte(C) GMR biosensor working principle Reprinted with permission from [53] copy2010 American Chemical Society

34 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

(2) the Hall element is similar size to the cells that pass over itthus increasing the sensitivity of the device

(3) an array of eight sensors constituting the micro-Hall sensorallows less-stringent fluidic control than if the cells had tobe focused over a single sensor and

(4) an array that integrates the overall magnetic flux from eachcell enables measurement of the total magnetic moment ofa single cell The micro-Hall sensor is capable of high-throughput screening and has demonstrated clinical utilityby detecting circulating tumor cells in whole blood of 20ovarian cancer patients at higher sensitivity than currentlypossible with clinical standards

A magnetic field sensor was developed combining a magneticfluid (Fe3O4 NPs) and an optical fiber Loyt-Sagnac interferometer[55] The sensor takes advantage of the magnification of the bire-fringence effect of the magnetic fluid by the properly designed opticalfiber Loyt-Sagnac interferometer structure The sensor demon-strated a sensitivity enhanced by 1ndash3 orders of magnitude comparedto existing magnetic fluid sensors

Magnetic field sensors are not easily extended to the detectionof multi-analytes since the analytical signal arises from the mag-netic moment m which is a single physical parameter By usingsuperparamagnetic NPs with different sizes or different materialsthe analytical signals can be distinguished by their unique non-magnetization curves thus enabling multi-analyte detection bymagnetic field devices [58]

4 Conclusions and future trends

In the past decade MNPs have gained much attention and wereused in several analytical applications such as sensors andbiosensors In (bio)sensing devices MNPs can be applied in thesensor surface or as labels Magnetic labeling of biomolecules is anattractive proposition due to the absence of magnetic back-ground in almost every biological sample However implementationof magnetic labels requires biocompatibility monodispersion andadequate functionalization to reduce non-specific binding Thefunctionalized MNPs with proper functional groups and the surfaceimmobilization technique can therefore play a vital role in signif-icant improvement in the sensitivity of (bio)sensing devices In thiscontext research focused on synthesis and characterization of MNPcomposites and their behavior in (bio)sensing devices is still neededWe therefore recommend further work investigating more suit-able functionalized magnetic nanomaterials that will be fit for multi-analyte detection systems in the future

The majority of the developed devices using MNPs as labels orintroduced into the transducer material are based on EC transduc-tion EC devices were successfully applied to sensitively quantifyingdifferent multi-analytes in environmental clinical and food samplesThese devices can be disposable labeled or label-free integratedinto microfluidic structures and inexpensive

Optical devices have been developed almost always based on CLdetection and a few used detection by SPR and fluorescence spec-troscopy so more research is needed on the development of newoptical sensors and biosensors using MNPs

Concerning piezoelectric devices more research is needed on thedevelopment of new sensors and biosensors since the magneticnanostructures have the potential to overcome sensitivity and sta-bility problems

Magnetic field sensors have been used as detectors of MNP labelsIn MNP-based magnetic field sensors the next step is to take thetechnology to the micrometer and nanometer scale and extend theirapplication to a broad range of environmental food and clinicalsamples since MNPs can enhance the analytical signal Sensing mul-tiple analytes into a single magnetic field device also needs to be

further developed by the use of superparamagnetic NPs with dif-ferent characteristics such as size and type of material

We recommend integration of MNP-based devices andmicrofluidic structures onto single chips since it will enable the com-bination of several steps such as sample preparation molecularlabeling detection and analysis into a single device for multi-analyte detection

Acknowledgements

This work was supported by European Funds through COMPETEand by National Funds through the Portuguese Science Founda-tion (FCT) within project PEst-CMARLA00172013 This work wasalso funded by FEDER under the ldquoPrograma de Cooperaccedilatildeo Territo-rial Europeia INTERREG IV B SUDOErdquo within the framework of theresearch project ORQUE SUDOE SOE3P2F591

References

[1] M Farreacute J Sanchiacutes D Barceloacute Anaysis and assessement of the occurrence thefate and the behavior of nanomaterials in the environment Trend Anal Chem30 (2011) 515ndash527

[2] A Akbarzadeh M Samiei S Daravan Magnetic nanoparticles preparationphysical properties and applications in biomedicine Nanoscale Res Lett 7(2012) 1ndash13

[3] LH Reddy JL Arias J Nicolas P Couvreur Magnetic nanoparticles design andcharacterization toxicity and biocompatibility pharmaceutical and biomedicalapplications Chem Rev 112 (2012) 5818ndash5878

[4] CGCM Netto HE Toma LH Andrade Superparamagnetic nanoparticles asversatile carriers and supporting materials for enzymes J Mol Catal B Enzym85ndash86 (2013) 71ndash92

[5] X-S Li G-T Zhu Y-B Luo B-F Yuan Y-Q Feng Synthesis and applicationsof functionalized magnetic materials in sample preparation Trend Anal Chem45 (2013) 233ndash247

[6] Y Moliner-Martinez A Ribera E Coronado P Campiacutens-Falcoacute Preconcentrationof emerging contaminants in environmental water samples by using silicasupported Fe3O4 magnetic nanoparticles for improving mass detection incapillary liquid chromatography J Chromatogr A 1218 (2011) 2276ndash2283

[7] L Chen T Wang J Tong Application of derivatized magnetic materials to theseparation and the preconcentration of pollutants in water samples Trend AnalChem 30 (2011) 1095ndash1108

[8] SCN Tang IMC Lo Magnetic nanoparticles essential factors for sustainableenvironmental applications Water Res 47 (2013) 2613ndash2632

[9] RD Ambashta M Sillanpaa Water purification using magnetic assistance areview J Hazardo Mater 180 (2010) 38ndash49

[10] JK Oh JM Park Iron oxide-based superparamagnetic polymeric nanomaterialsdesign preparation and biomedical application Progr Polym Sci 36 (2011)168ndash189

[11] M Colombo S Carregal-Romero MF Casula L Gutieacuterrez MP Morales IBBohm et al Biological applications of magnetic nanoparticles Chem Soc Rev12 (2012) 4306ndash4334

[12] S-H Huang R-S Juang Biochemical and biomedical applications ofmultifunctional magnetic nanoparticles a review J Nanopart Res 13 (2011)4411ndash4430

[13] K Aguilar-Arteaga JA Rodriguez E Barrado Magnetic solids in analyticalchemistry a review Anal Chim Acta 674 (2010) 157ndash165

[14] JS Beveridge JR Stephens ME Williams The use of magnetic nanoparticlesin analytical chemistry Annu Rev Anal Chem 4 (2011) 251ndash273

[15] S Carregal-Romero E Caballero-Diacuteaz L Beqa AM Abdelmonem M Ochs DHuhn et al Muliplexed sensing and imaging with colloidal nano- andmicroparticles Annu Rev Anal Chem 6 (2013) 53ndash81

[16] M Iranifam Analytical applications of chemiluminescence-detection systemsassisted by magnetic microparticles and nanoparticles Trend Anal Chem 51(2013) 51ndash70

[17] Y Xu E Wang Electrochemical biosensors based on magnetic micronanoparticles Electrochim Acta 84 (2012) 62ndash73

[18] L-Y Lu L-N Yu X-G Xu Y Jiang Monodisperse magnetic metallicnanoparticles sunthesis performance enhancement and advanced applicationsRare Met 32 (2013) 323ndash331

[19] O Philippova A Barabanova V Molchanov A Khokhlov Magnetic polymerbeads recent trends and developments in synthetic design and applicationsEur Polym J 47 (2011) 542ndash559

[20] BF Silva S Peacuterez P Gardinalli RK Singhal AA Mozeto D Barceloacute Analyticalchemistry of metallic nanoparticles in natural environments Trend Anal Chem30 (2011) 528ndash540

[21] Y-X Ma Y-F Li G-H Zhao L-Q Yang J-Z Wang X Shan et al Preparationand characterization of graphite nanosheets decorated with Fe3O4 nanoparticlesused in the immobilization of glucoamylase Carbon 50 (2012) 2976ndash2986

35TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

this oscillation depends on the cut and the thickness of the diskThis resonant frequency changes as compound(s) adsorb or desorbfrom the surface of the crystal A reduction in frequency is propor-tional to the mass of adsorbed compound QCMs are small androbust inexpensive and capable of giving a rapid response downto a mass change of 1 ng The major drawback of these devices isthe increase in noise with the decrease in dimensions due to in-stability as the surface area-to-volume ratio increases Moredisadvantages of QCM are the interference from atmospheric hu-midity and the difficulty in using them for the determination ofanalytes in solution [71]

MNPs with piezoelectric properties can easily eliminate theseproblems since they offer an attractive transduction mechanism andrecognition event with advantages such as solid-state construc-tion and cost effectiveness The frequency enhancement in thepresence of MNPs can be due to

(1) the MNPs possessing some inherent piezoelectricity(2) the MNPs binding and helping to concentrate the analyte mol-

ecules at the QCM surface and(3) the MNPs acting as matrix carriers to load labels

A QCM immunosensor for detection of C-reactive protein (CRP)in serum was developed In a first step a sandwich-typeimmunoreaction was made between the capture probe (silicondioxide-coated magnetic Fe3O4 NPs) labeled with primary CRP an-tibody (MNs-CRPAb1) CRP and signal tag [horseradish peroxidase(HRP) coupled with HRP-linked secondary CRP antibody co-immobilized on AuNPs (AuNPs-HRPHRP-CRP Ab2)] [49] In a secondstep the immunocomplex was exposed to 3-amino-9-ethylcarbazole(AEC) and hydrogen peroxide Fig 4 shows the preparation proce-dures and the detection principle The capture probe containing theMNPs (MNs-CRPAb1) enhanced the analytical signal due to bothmagnetic separation and immobilization at the electrode surfaceFurther the advantages of the magnetic beads (Fe3O4SiO2) for la-beling CRPAb1 include the mono-disperse size distribution and easypreparation of the labeled conjugates The performance of the QCMmethodology was comparable with the ELISA methodology whendetecting CRP in human serum Moreover the QCM-sensor surfacecan be regenerated easily and used repeatedly due to the use of theMNPs

More research is needed on the development of magneticnanostructures characterization of their piezoelectric behavior andtheir application in piezoelectric sensors and biosensors since theypromise to overcome the sensitivity and stability issues character-istic of these kind of devices

Fig 3 Example of a surface-plasmon resonance (SPR) immunosensor (A) Opticalsensor set-up and (B) a sensor chip of the magnetic nanoparticle (NP)-enhancedgrating coupled SPR sensor (C) The analytical signal before and after immobiliza-tion of the capture antibody Reprinted with permission from [40] copy2011 AmericanChemical Society

Fig 4 Example of a quartz-crystal-microbalance (QCM) immunosensor (Left) Procedures of the preparation of Fe3O4SiO2-Ab1 and AuNPs-HRPHRP-Ab2 conjugations(Right) Detection principle TEOS Tetraethyl orthosilicate EDC 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide NHS Amine-reactive N-hydroxysuccinimide CRP C-reactiveprotein Ab1 Primary CRP antibody Ab2 Secondary CRP antibody AuNP Gold nanoparticle HRP Horseradish peroxidase AEC 3-amino-9-ethylcarbazole MNP Fe3O4SiO2 nanoparticle Reprinted from [49] copy2013 with the permission from Elsevier

33TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

34 Magnetic field

Table 1 shows that the magnetic field devices using MNPs [52ndash59]include giant magnetoresistive (GMR) Hall Effect magneto-optical and superconducting quantum interference sensors

Magnetoresistive sensors are based on the intrinsic magnetore-sistance of a ferromagnetic material or on ferromagneticnon-magnetic heterostructures [72] Depending on the nanostructureof the nanomaterial layer these devices can show the GMR effector the tunneling magnetoresistance effect In these devices the an-alytical signal (change in electrical resistance) is measured followingthe analyte binding in the presence of a magnetic field The ana-lytical signal can therefore be obtained by small changes in themagnetic field and depends on the magnetic field along the sensorarea [73] When using a GMR device and MNPs for interleukin-6(analyte) detection two methodologies have been attempted (Fig 5)[53] In the first possible methodology the GMR sensor isfunctionalized with capture antibodies and the analyte binds tothe capture antibody The detection antibodies labeled with MNPsbind to the analyte captured The second detection methodologyinvolves functionalization of the GMR sensor with capture anti-bodies and then the direct capture of the MNP-labeled analyte onthe GMR biosensor In both cases the GMR biosensor detects thedipole field generated by the MNPs captured on the sensor surfacewhich is sensitive to distance The quality of the MNPs is very im-portant for successful magnetoresistive detection so ideal probesshould be superparamagnetic having high magnetic moment and

large susceptibility in order to enable their magnetization in a smallmagnetic field The MNPs also need to have uniform size and shapesince the magnetic signal depends on it and to be stable in phys-iological solutions so that their coupling with biomolecules canbe controlled [73] Moreover the choice of MNPs with highmagnetic moment leads to increased signal and therefore high sen-sitivity Taking this into consideration for sensitive magnetoresistivedetection the ideal candidates have been metallic Fe Co or theiralloy MNPs [73] According to Li et al [53] considering thesame NP volume and an applied field of 10 Oe the net magneticmoment of one FeCo NP is 7ndash11 times higher than that of oneFe3O4 NP

MNPs can also be used in microfluidic devices which due to theirpermanent magnetic moment can be controlled via external in-homogeneous magnetic fields and also detected by magnetoresistivesensors There are also two types of microfabricated magnetic fielddevices which are the magnetoresistive and the Hall Effect A micro-Hall sensor was developed for the enumeration of rare cells ex vivo[58] The microfluidic chip-based micro-Hall sensor measures themagnetic moments of cells in flow that have been labeled withMNPs The micro-Hall sensor integrates several technological ad-vances for accurate measurements of biomarkers on individual cellssuch as

(1) linear response which enables operation at such high mag-netic fields (gt01 T) that MNPs can be completely magnetizedto generate maximal signal strength

Fig 5 Example of the use of magnetic nanoparticles (MNPs) and giant magneto-resistive (GMR) sensors in two different methodologies (A) Sandwich-type approach wherethe GMR sensor is functionalized with capture antibodies for subsequent analyte binding The detection antibodies labeled with MNPs are then applied and bind to thecaptured analyte (B) Two-layer approach where the GMR sensor is functionalized with capture antibodies for the direct application and capture of the MNP-modified analyte(C) GMR biosensor working principle Reprinted with permission from [53] copy2010 American Chemical Society

34 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

(2) the Hall element is similar size to the cells that pass over itthus increasing the sensitivity of the device

(3) an array of eight sensors constituting the micro-Hall sensorallows less-stringent fluidic control than if the cells had tobe focused over a single sensor and

(4) an array that integrates the overall magnetic flux from eachcell enables measurement of the total magnetic moment ofa single cell The micro-Hall sensor is capable of high-throughput screening and has demonstrated clinical utilityby detecting circulating tumor cells in whole blood of 20ovarian cancer patients at higher sensitivity than currentlypossible with clinical standards

A magnetic field sensor was developed combining a magneticfluid (Fe3O4 NPs) and an optical fiber Loyt-Sagnac interferometer[55] The sensor takes advantage of the magnification of the bire-fringence effect of the magnetic fluid by the properly designed opticalfiber Loyt-Sagnac interferometer structure The sensor demon-strated a sensitivity enhanced by 1ndash3 orders of magnitude comparedto existing magnetic fluid sensors

Magnetic field sensors are not easily extended to the detectionof multi-analytes since the analytical signal arises from the mag-netic moment m which is a single physical parameter By usingsuperparamagnetic NPs with different sizes or different materialsthe analytical signals can be distinguished by their unique non-magnetization curves thus enabling multi-analyte detection bymagnetic field devices [58]

4 Conclusions and future trends

In the past decade MNPs have gained much attention and wereused in several analytical applications such as sensors andbiosensors In (bio)sensing devices MNPs can be applied in thesensor surface or as labels Magnetic labeling of biomolecules is anattractive proposition due to the absence of magnetic back-ground in almost every biological sample However implementationof magnetic labels requires biocompatibility monodispersion andadequate functionalization to reduce non-specific binding Thefunctionalized MNPs with proper functional groups and the surfaceimmobilization technique can therefore play a vital role in signif-icant improvement in the sensitivity of (bio)sensing devices In thiscontext research focused on synthesis and characterization of MNPcomposites and their behavior in (bio)sensing devices is still neededWe therefore recommend further work investigating more suit-able functionalized magnetic nanomaterials that will be fit for multi-analyte detection systems in the future

The majority of the developed devices using MNPs as labels orintroduced into the transducer material are based on EC transduc-tion EC devices were successfully applied to sensitively quantifyingdifferent multi-analytes in environmental clinical and food samplesThese devices can be disposable labeled or label-free integratedinto microfluidic structures and inexpensive

Optical devices have been developed almost always based on CLdetection and a few used detection by SPR and fluorescence spec-troscopy so more research is needed on the development of newoptical sensors and biosensors using MNPs

Concerning piezoelectric devices more research is needed on thedevelopment of new sensors and biosensors since the magneticnanostructures have the potential to overcome sensitivity and sta-bility problems

Magnetic field sensors have been used as detectors of MNP labelsIn MNP-based magnetic field sensors the next step is to take thetechnology to the micrometer and nanometer scale and extend theirapplication to a broad range of environmental food and clinicalsamples since MNPs can enhance the analytical signal Sensing mul-tiple analytes into a single magnetic field device also needs to be

further developed by the use of superparamagnetic NPs with dif-ferent characteristics such as size and type of material

We recommend integration of MNP-based devices andmicrofluidic structures onto single chips since it will enable the com-bination of several steps such as sample preparation molecularlabeling detection and analysis into a single device for multi-analyte detection

Acknowledgements

This work was supported by European Funds through COMPETEand by National Funds through the Portuguese Science Founda-tion (FCT) within project PEst-CMARLA00172013 This work wasalso funded by FEDER under the ldquoPrograma de Cooperaccedilatildeo Territo-rial Europeia INTERREG IV B SUDOErdquo within the framework of theresearch project ORQUE SUDOE SOE3P2F591

References

[1] M Farreacute J Sanchiacutes D Barceloacute Anaysis and assessement of the occurrence thefate and the behavior of nanomaterials in the environment Trend Anal Chem30 (2011) 515ndash527

[2] A Akbarzadeh M Samiei S Daravan Magnetic nanoparticles preparationphysical properties and applications in biomedicine Nanoscale Res Lett 7(2012) 1ndash13

[3] LH Reddy JL Arias J Nicolas P Couvreur Magnetic nanoparticles design andcharacterization toxicity and biocompatibility pharmaceutical and biomedicalapplications Chem Rev 112 (2012) 5818ndash5878

[4] CGCM Netto HE Toma LH Andrade Superparamagnetic nanoparticles asversatile carriers and supporting materials for enzymes J Mol Catal B Enzym85ndash86 (2013) 71ndash92

[5] X-S Li G-T Zhu Y-B Luo B-F Yuan Y-Q Feng Synthesis and applicationsof functionalized magnetic materials in sample preparation Trend Anal Chem45 (2013) 233ndash247

[6] Y Moliner-Martinez A Ribera E Coronado P Campiacutens-Falcoacute Preconcentrationof emerging contaminants in environmental water samples by using silicasupported Fe3O4 magnetic nanoparticles for improving mass detection incapillary liquid chromatography J Chromatogr A 1218 (2011) 2276ndash2283

[7] L Chen T Wang J Tong Application of derivatized magnetic materials to theseparation and the preconcentration of pollutants in water samples Trend AnalChem 30 (2011) 1095ndash1108

[8] SCN Tang IMC Lo Magnetic nanoparticles essential factors for sustainableenvironmental applications Water Res 47 (2013) 2613ndash2632

[9] RD Ambashta M Sillanpaa Water purification using magnetic assistance areview J Hazardo Mater 180 (2010) 38ndash49

[10] JK Oh JM Park Iron oxide-based superparamagnetic polymeric nanomaterialsdesign preparation and biomedical application Progr Polym Sci 36 (2011)168ndash189

[11] M Colombo S Carregal-Romero MF Casula L Gutieacuterrez MP Morales IBBohm et al Biological applications of magnetic nanoparticles Chem Soc Rev12 (2012) 4306ndash4334

[12] S-H Huang R-S Juang Biochemical and biomedical applications ofmultifunctional magnetic nanoparticles a review J Nanopart Res 13 (2011)4411ndash4430

[13] K Aguilar-Arteaga JA Rodriguez E Barrado Magnetic solids in analyticalchemistry a review Anal Chim Acta 674 (2010) 157ndash165

[14] JS Beveridge JR Stephens ME Williams The use of magnetic nanoparticlesin analytical chemistry Annu Rev Anal Chem 4 (2011) 251ndash273

[15] S Carregal-Romero E Caballero-Diacuteaz L Beqa AM Abdelmonem M Ochs DHuhn et al Muliplexed sensing and imaging with colloidal nano- andmicroparticles Annu Rev Anal Chem 6 (2013) 53ndash81

[16] M Iranifam Analytical applications of chemiluminescence-detection systemsassisted by magnetic microparticles and nanoparticles Trend Anal Chem 51(2013) 51ndash70

[17] Y Xu E Wang Electrochemical biosensors based on magnetic micronanoparticles Electrochim Acta 84 (2012) 62ndash73

[18] L-Y Lu L-N Yu X-G Xu Y Jiang Monodisperse magnetic metallicnanoparticles sunthesis performance enhancement and advanced applicationsRare Met 32 (2013) 323ndash331

[19] O Philippova A Barabanova V Molchanov A Khokhlov Magnetic polymerbeads recent trends and developments in synthetic design and applicationsEur Polym J 47 (2011) 542ndash559

[20] BF Silva S Peacuterez P Gardinalli RK Singhal AA Mozeto D Barceloacute Analyticalchemistry of metallic nanoparticles in natural environments Trend Anal Chem30 (2011) 528ndash540

[21] Y-X Ma Y-F Li G-H Zhao L-Q Yang J-Z Wang X Shan et al Preparationand characterization of graphite nanosheets decorated with Fe3O4 nanoparticlesused in the immobilization of glucoamylase Carbon 50 (2012) 2976ndash2986

35TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

34 Magnetic field

Table 1 shows that the magnetic field devices using MNPs [52ndash59]include giant magnetoresistive (GMR) Hall Effect magneto-optical and superconducting quantum interference sensors

Magnetoresistive sensors are based on the intrinsic magnetore-sistance of a ferromagnetic material or on ferromagneticnon-magnetic heterostructures [72] Depending on the nanostructureof the nanomaterial layer these devices can show the GMR effector the tunneling magnetoresistance effect In these devices the an-alytical signal (change in electrical resistance) is measured followingthe analyte binding in the presence of a magnetic field The ana-lytical signal can therefore be obtained by small changes in themagnetic field and depends on the magnetic field along the sensorarea [73] When using a GMR device and MNPs for interleukin-6(analyte) detection two methodologies have been attempted (Fig 5)[53] In the first possible methodology the GMR sensor isfunctionalized with capture antibodies and the analyte binds tothe capture antibody The detection antibodies labeled with MNPsbind to the analyte captured The second detection methodologyinvolves functionalization of the GMR sensor with capture anti-bodies and then the direct capture of the MNP-labeled analyte onthe GMR biosensor In both cases the GMR biosensor detects thedipole field generated by the MNPs captured on the sensor surfacewhich is sensitive to distance The quality of the MNPs is very im-portant for successful magnetoresistive detection so ideal probesshould be superparamagnetic having high magnetic moment and

large susceptibility in order to enable their magnetization in a smallmagnetic field The MNPs also need to have uniform size and shapesince the magnetic signal depends on it and to be stable in phys-iological solutions so that their coupling with biomolecules canbe controlled [73] Moreover the choice of MNPs with highmagnetic moment leads to increased signal and therefore high sen-sitivity Taking this into consideration for sensitive magnetoresistivedetection the ideal candidates have been metallic Fe Co or theiralloy MNPs [73] According to Li et al [53] considering thesame NP volume and an applied field of 10 Oe the net magneticmoment of one FeCo NP is 7ndash11 times higher than that of oneFe3O4 NP

MNPs can also be used in microfluidic devices which due to theirpermanent magnetic moment can be controlled via external in-homogeneous magnetic fields and also detected by magnetoresistivesensors There are also two types of microfabricated magnetic fielddevices which are the magnetoresistive and the Hall Effect A micro-Hall sensor was developed for the enumeration of rare cells ex vivo[58] The microfluidic chip-based micro-Hall sensor measures themagnetic moments of cells in flow that have been labeled withMNPs The micro-Hall sensor integrates several technological ad-vances for accurate measurements of biomarkers on individual cellssuch as

(1) linear response which enables operation at such high mag-netic fields (gt01 T) that MNPs can be completely magnetizedto generate maximal signal strength

Fig 5 Example of the use of magnetic nanoparticles (MNPs) and giant magneto-resistive (GMR) sensors in two different methodologies (A) Sandwich-type approach wherethe GMR sensor is functionalized with capture antibodies for subsequent analyte binding The detection antibodies labeled with MNPs are then applied and bind to thecaptured analyte (B) Two-layer approach where the GMR sensor is functionalized with capture antibodies for the direct application and capture of the MNP-modified analyte(C) GMR biosensor working principle Reprinted with permission from [53] copy2010 American Chemical Society

34 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

(2) the Hall element is similar size to the cells that pass over itthus increasing the sensitivity of the device

(3) an array of eight sensors constituting the micro-Hall sensorallows less-stringent fluidic control than if the cells had tobe focused over a single sensor and

(4) an array that integrates the overall magnetic flux from eachcell enables measurement of the total magnetic moment ofa single cell The micro-Hall sensor is capable of high-throughput screening and has demonstrated clinical utilityby detecting circulating tumor cells in whole blood of 20ovarian cancer patients at higher sensitivity than currentlypossible with clinical standards

A magnetic field sensor was developed combining a magneticfluid (Fe3O4 NPs) and an optical fiber Loyt-Sagnac interferometer[55] The sensor takes advantage of the magnification of the bire-fringence effect of the magnetic fluid by the properly designed opticalfiber Loyt-Sagnac interferometer structure The sensor demon-strated a sensitivity enhanced by 1ndash3 orders of magnitude comparedto existing magnetic fluid sensors

Magnetic field sensors are not easily extended to the detectionof multi-analytes since the analytical signal arises from the mag-netic moment m which is a single physical parameter By usingsuperparamagnetic NPs with different sizes or different materialsthe analytical signals can be distinguished by their unique non-magnetization curves thus enabling multi-analyte detection bymagnetic field devices [58]

4 Conclusions and future trends

In the past decade MNPs have gained much attention and wereused in several analytical applications such as sensors andbiosensors In (bio)sensing devices MNPs can be applied in thesensor surface or as labels Magnetic labeling of biomolecules is anattractive proposition due to the absence of magnetic back-ground in almost every biological sample However implementationof magnetic labels requires biocompatibility monodispersion andadequate functionalization to reduce non-specific binding Thefunctionalized MNPs with proper functional groups and the surfaceimmobilization technique can therefore play a vital role in signif-icant improvement in the sensitivity of (bio)sensing devices In thiscontext research focused on synthesis and characterization of MNPcomposites and their behavior in (bio)sensing devices is still neededWe therefore recommend further work investigating more suit-able functionalized magnetic nanomaterials that will be fit for multi-analyte detection systems in the future

The majority of the developed devices using MNPs as labels orintroduced into the transducer material are based on EC transduc-tion EC devices were successfully applied to sensitively quantifyingdifferent multi-analytes in environmental clinical and food samplesThese devices can be disposable labeled or label-free integratedinto microfluidic structures and inexpensive

Optical devices have been developed almost always based on CLdetection and a few used detection by SPR and fluorescence spec-troscopy so more research is needed on the development of newoptical sensors and biosensors using MNPs

Concerning piezoelectric devices more research is needed on thedevelopment of new sensors and biosensors since the magneticnanostructures have the potential to overcome sensitivity and sta-bility problems

Magnetic field sensors have been used as detectors of MNP labelsIn MNP-based magnetic field sensors the next step is to take thetechnology to the micrometer and nanometer scale and extend theirapplication to a broad range of environmental food and clinicalsamples since MNPs can enhance the analytical signal Sensing mul-tiple analytes into a single magnetic field device also needs to be

further developed by the use of superparamagnetic NPs with dif-ferent characteristics such as size and type of material

We recommend integration of MNP-based devices andmicrofluidic structures onto single chips since it will enable the com-bination of several steps such as sample preparation molecularlabeling detection and analysis into a single device for multi-analyte detection

Acknowledgements

This work was supported by European Funds through COMPETEand by National Funds through the Portuguese Science Founda-tion (FCT) within project PEst-CMARLA00172013 This work wasalso funded by FEDER under the ldquoPrograma de Cooperaccedilatildeo Territo-rial Europeia INTERREG IV B SUDOErdquo within the framework of theresearch project ORQUE SUDOE SOE3P2F591

References

[1] M Farreacute J Sanchiacutes D Barceloacute Anaysis and assessement of the occurrence thefate and the behavior of nanomaterials in the environment Trend Anal Chem30 (2011) 515ndash527

[2] A Akbarzadeh M Samiei S Daravan Magnetic nanoparticles preparationphysical properties and applications in biomedicine Nanoscale Res Lett 7(2012) 1ndash13

[3] LH Reddy JL Arias J Nicolas P Couvreur Magnetic nanoparticles design andcharacterization toxicity and biocompatibility pharmaceutical and biomedicalapplications Chem Rev 112 (2012) 5818ndash5878

[4] CGCM Netto HE Toma LH Andrade Superparamagnetic nanoparticles asversatile carriers and supporting materials for enzymes J Mol Catal B Enzym85ndash86 (2013) 71ndash92

[5] X-S Li G-T Zhu Y-B Luo B-F Yuan Y-Q Feng Synthesis and applicationsof functionalized magnetic materials in sample preparation Trend Anal Chem45 (2013) 233ndash247

[6] Y Moliner-Martinez A Ribera E Coronado P Campiacutens-Falcoacute Preconcentrationof emerging contaminants in environmental water samples by using silicasupported Fe3O4 magnetic nanoparticles for improving mass detection incapillary liquid chromatography J Chromatogr A 1218 (2011) 2276ndash2283

[7] L Chen T Wang J Tong Application of derivatized magnetic materials to theseparation and the preconcentration of pollutants in water samples Trend AnalChem 30 (2011) 1095ndash1108

[8] SCN Tang IMC Lo Magnetic nanoparticles essential factors for sustainableenvironmental applications Water Res 47 (2013) 2613ndash2632

[9] RD Ambashta M Sillanpaa Water purification using magnetic assistance areview J Hazardo Mater 180 (2010) 38ndash49

[10] JK Oh JM Park Iron oxide-based superparamagnetic polymeric nanomaterialsdesign preparation and biomedical application Progr Polym Sci 36 (2011)168ndash189

[11] M Colombo S Carregal-Romero MF Casula L Gutieacuterrez MP Morales IBBohm et al Biological applications of magnetic nanoparticles Chem Soc Rev12 (2012) 4306ndash4334

[12] S-H Huang R-S Juang Biochemical and biomedical applications ofmultifunctional magnetic nanoparticles a review J Nanopart Res 13 (2011)4411ndash4430

[13] K Aguilar-Arteaga JA Rodriguez E Barrado Magnetic solids in analyticalchemistry a review Anal Chim Acta 674 (2010) 157ndash165

[14] JS Beveridge JR Stephens ME Williams The use of magnetic nanoparticlesin analytical chemistry Annu Rev Anal Chem 4 (2011) 251ndash273

[15] S Carregal-Romero E Caballero-Diacuteaz L Beqa AM Abdelmonem M Ochs DHuhn et al Muliplexed sensing and imaging with colloidal nano- andmicroparticles Annu Rev Anal Chem 6 (2013) 53ndash81

[16] M Iranifam Analytical applications of chemiluminescence-detection systemsassisted by magnetic microparticles and nanoparticles Trend Anal Chem 51(2013) 51ndash70

[17] Y Xu E Wang Electrochemical biosensors based on magnetic micronanoparticles Electrochim Acta 84 (2012) 62ndash73

[18] L-Y Lu L-N Yu X-G Xu Y Jiang Monodisperse magnetic metallicnanoparticles sunthesis performance enhancement and advanced applicationsRare Met 32 (2013) 323ndash331

[19] O Philippova A Barabanova V Molchanov A Khokhlov Magnetic polymerbeads recent trends and developments in synthetic design and applicationsEur Polym J 47 (2011) 542ndash559

[20] BF Silva S Peacuterez P Gardinalli RK Singhal AA Mozeto D Barceloacute Analyticalchemistry of metallic nanoparticles in natural environments Trend Anal Chem30 (2011) 528ndash540

[21] Y-X Ma Y-F Li G-H Zhao L-Q Yang J-Z Wang X Shan et al Preparationand characterization of graphite nanosheets decorated with Fe3O4 nanoparticlesused in the immobilization of glucoamylase Carbon 50 (2012) 2976ndash2986

35TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

(2) the Hall element is similar size to the cells that pass over itthus increasing the sensitivity of the device

(3) an array of eight sensors constituting the micro-Hall sensorallows less-stringent fluidic control than if the cells had tobe focused over a single sensor and

(4) an array that integrates the overall magnetic flux from eachcell enables measurement of the total magnetic moment ofa single cell The micro-Hall sensor is capable of high-throughput screening and has demonstrated clinical utilityby detecting circulating tumor cells in whole blood of 20ovarian cancer patients at higher sensitivity than currentlypossible with clinical standards

A magnetic field sensor was developed combining a magneticfluid (Fe3O4 NPs) and an optical fiber Loyt-Sagnac interferometer[55] The sensor takes advantage of the magnification of the bire-fringence effect of the magnetic fluid by the properly designed opticalfiber Loyt-Sagnac interferometer structure The sensor demon-strated a sensitivity enhanced by 1ndash3 orders of magnitude comparedto existing magnetic fluid sensors

Magnetic field sensors are not easily extended to the detectionof multi-analytes since the analytical signal arises from the mag-netic moment m which is a single physical parameter By usingsuperparamagnetic NPs with different sizes or different materialsthe analytical signals can be distinguished by their unique non-magnetization curves thus enabling multi-analyte detection bymagnetic field devices [58]

4 Conclusions and future trends

In the past decade MNPs have gained much attention and wereused in several analytical applications such as sensors andbiosensors In (bio)sensing devices MNPs can be applied in thesensor surface or as labels Magnetic labeling of biomolecules is anattractive proposition due to the absence of magnetic back-ground in almost every biological sample However implementationof magnetic labels requires biocompatibility monodispersion andadequate functionalization to reduce non-specific binding Thefunctionalized MNPs with proper functional groups and the surfaceimmobilization technique can therefore play a vital role in signif-icant improvement in the sensitivity of (bio)sensing devices In thiscontext research focused on synthesis and characterization of MNPcomposites and their behavior in (bio)sensing devices is still neededWe therefore recommend further work investigating more suit-able functionalized magnetic nanomaterials that will be fit for multi-analyte detection systems in the future

The majority of the developed devices using MNPs as labels orintroduced into the transducer material are based on EC transduc-tion EC devices were successfully applied to sensitively quantifyingdifferent multi-analytes in environmental clinical and food samplesThese devices can be disposable labeled or label-free integratedinto microfluidic structures and inexpensive

Optical devices have been developed almost always based on CLdetection and a few used detection by SPR and fluorescence spec-troscopy so more research is needed on the development of newoptical sensors and biosensors using MNPs

Concerning piezoelectric devices more research is needed on thedevelopment of new sensors and biosensors since the magneticnanostructures have the potential to overcome sensitivity and sta-bility problems

Magnetic field sensors have been used as detectors of MNP labelsIn MNP-based magnetic field sensors the next step is to take thetechnology to the micrometer and nanometer scale and extend theirapplication to a broad range of environmental food and clinicalsamples since MNPs can enhance the analytical signal Sensing mul-tiple analytes into a single magnetic field device also needs to be

further developed by the use of superparamagnetic NPs with dif-ferent characteristics such as size and type of material

We recommend integration of MNP-based devices andmicrofluidic structures onto single chips since it will enable the com-bination of several steps such as sample preparation molecularlabeling detection and analysis into a single device for multi-analyte detection

Acknowledgements

This work was supported by European Funds through COMPETEand by National Funds through the Portuguese Science Founda-tion (FCT) within project PEst-CMARLA00172013 This work wasalso funded by FEDER under the ldquoPrograma de Cooperaccedilatildeo Territo-rial Europeia INTERREG IV B SUDOErdquo within the framework of theresearch project ORQUE SUDOE SOE3P2F591

References

[1] M Farreacute J Sanchiacutes D Barceloacute Anaysis and assessement of the occurrence thefate and the behavior of nanomaterials in the environment Trend Anal Chem30 (2011) 515ndash527

[2] A Akbarzadeh M Samiei S Daravan Magnetic nanoparticles preparationphysical properties and applications in biomedicine Nanoscale Res Lett 7(2012) 1ndash13

[3] LH Reddy JL Arias J Nicolas P Couvreur Magnetic nanoparticles design andcharacterization toxicity and biocompatibility pharmaceutical and biomedicalapplications Chem Rev 112 (2012) 5818ndash5878

[4] CGCM Netto HE Toma LH Andrade Superparamagnetic nanoparticles asversatile carriers and supporting materials for enzymes J Mol Catal B Enzym85ndash86 (2013) 71ndash92

[5] X-S Li G-T Zhu Y-B Luo B-F Yuan Y-Q Feng Synthesis and applicationsof functionalized magnetic materials in sample preparation Trend Anal Chem45 (2013) 233ndash247

[6] Y Moliner-Martinez A Ribera E Coronado P Campiacutens-Falcoacute Preconcentrationof emerging contaminants in environmental water samples by using silicasupported Fe3O4 magnetic nanoparticles for improving mass detection incapillary liquid chromatography J Chromatogr A 1218 (2011) 2276ndash2283

[7] L Chen T Wang J Tong Application of derivatized magnetic materials to theseparation and the preconcentration of pollutants in water samples Trend AnalChem 30 (2011) 1095ndash1108

[8] SCN Tang IMC Lo Magnetic nanoparticles essential factors for sustainableenvironmental applications Water Res 47 (2013) 2613ndash2632

[9] RD Ambashta M Sillanpaa Water purification using magnetic assistance areview J Hazardo Mater 180 (2010) 38ndash49

[10] JK Oh JM Park Iron oxide-based superparamagnetic polymeric nanomaterialsdesign preparation and biomedical application Progr Polym Sci 36 (2011)168ndash189

[11] M Colombo S Carregal-Romero MF Casula L Gutieacuterrez MP Morales IBBohm et al Biological applications of magnetic nanoparticles Chem Soc Rev12 (2012) 4306ndash4334

[12] S-H Huang R-S Juang Biochemical and biomedical applications ofmultifunctional magnetic nanoparticles a review J Nanopart Res 13 (2011)4411ndash4430

[13] K Aguilar-Arteaga JA Rodriguez E Barrado Magnetic solids in analyticalchemistry a review Anal Chim Acta 674 (2010) 157ndash165

[14] JS Beveridge JR Stephens ME Williams The use of magnetic nanoparticlesin analytical chemistry Annu Rev Anal Chem 4 (2011) 251ndash273

[15] S Carregal-Romero E Caballero-Diacuteaz L Beqa AM Abdelmonem M Ochs DHuhn et al Muliplexed sensing and imaging with colloidal nano- andmicroparticles Annu Rev Anal Chem 6 (2013) 53ndash81

[16] M Iranifam Analytical applications of chemiluminescence-detection systemsassisted by magnetic microparticles and nanoparticles Trend Anal Chem 51(2013) 51ndash70

[17] Y Xu E Wang Electrochemical biosensors based on magnetic micronanoparticles Electrochim Acta 84 (2012) 62ndash73

[18] L-Y Lu L-N Yu X-G Xu Y Jiang Monodisperse magnetic metallicnanoparticles sunthesis performance enhancement and advanced applicationsRare Met 32 (2013) 323ndash331

[19] O Philippova A Barabanova V Molchanov A Khokhlov Magnetic polymerbeads recent trends and developments in synthetic design and applicationsEur Polym J 47 (2011) 542ndash559

[20] BF Silva S Peacuterez P Gardinalli RK Singhal AA Mozeto D Barceloacute Analyticalchemistry of metallic nanoparticles in natural environments Trend Anal Chem30 (2011) 528ndash540

[21] Y-X Ma Y-F Li G-H Zhao L-Q Yang J-Z Wang X Shan et al Preparationand characterization of graphite nanosheets decorated with Fe3O4 nanoparticlesused in the immobilization of glucoamylase Carbon 50 (2012) 2976ndash2986

35TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36

[22] N Gan X Yang D Xie Y Wu W Wen A disposable organophosphoruspesticides enzyme biosensor based on magnetic composite nano-particlesmodified screen printed carbon electrode Sensors 10 (2010) 625ndash638

[23] CIL Justino TAP Rocha-Santos S Cardoso AC Duarte Strategies for enhancingthe analytical performance of nanomaterial-based sensors Trends Anal Chem47 (2013) 27ndash36

[24] CIL Justino TAP Rocha-Santos AC Duarte Review of analytical figures ofmerit of sensors and biosensors in clinical applications Trends Anal Chem 29(2010) 1172ndash1183

[25] J Li H Gao Z Chen X Wei CF Yang An Electrochemical immunosensor forcarcinoembryonic antigen enhanced by self assembled nanogold coatings onmagnetic particles Anal Chim Acta 665 (2010) 98ndash104

[26] X Yang F Wu D-Z Chen H-W Lin An electrochemical immunosensor forrapid determination of clenbuterol by using magnetic nanocomposites to modifyscreen printed carbon electrode based on competitive immunoassay modeSensor Actuat B-Chem 192 (2014) 529ndash535

[27] Y Xin X Fu-bing L Hong-wei W Feng C Di-zhao W Zhao-yang A novel H2O2biosensor based on Fe3O4-Au magnetic nanoparticles coated horseradishperoxidase and grapheme sheets-Nafion film modified screen-printed carbonelectrode Electrochim Acta 109 (2013) 750ndash755

[28] D Chen J Deng J Liang J Xie C Hue K Huang A core-shell molecularlyimprinted polymer grafted onto a magnetic glassy carbon electrode as aselective sensor for the determination of metronidazole Sensor Actuat B-Chem183 (2013) 594ndash600

[29] A Prakash S Chandra D Bahadur Structural magnetic and textural propertiesof iron oxide-reduced graphene oxide hybrids and their use for theelectrochemical detection of chromium Carbon 50 (2012) 4209ndash4212

[30] Y Hu Z Zang H Zhang L Luo S Yao Selective and sensitive molecularlyimprinted sol-gel film-based electrochemical sensor combining mecaptoaceticacid modified PbS nanoparticles with Fe3O4Au-multi-walled carbonnanotubes-chitosan J Solid State Electrochem 16 (2012) 857ndash867

[31] M Arvand M Hassannezhad Magnetic core-shell Fe3O4SiO2MWCNTnanocomposite modified carbon paste electrode for amplified electrochemicalsensing of uric acid Mater Sci Eng C 36 (2014) 160ndash167

[32] X Chen J Zhu Z Chen C Xu Y Wang C Yao A novel bienzyme glucosebiosensor based on three layer Au-Fe3O4SiO2 magnetic nanocomposite SensorActuat B-Chem 159 (2011) 220ndash228

[33] TT Baby S Ramaprabhu SiO2 coated Fe3O4 magnetic nanoparticle dispersedmultiwalled carbon nanotubes based amperometric glucose biosensor Talanta80 (2010) 2016ndash2022

[34] M Hervaacutes MA Loacutepez A Escarpa Simplified calibration and analysis onscreen-printed disposable platforms for electrochemical magnetic bead-basedinmunosensing of zearalenone in baby food samples Biosens Bioelectron 25(2010) 1755ndash1760

[35] Z Yang C Zhang J Zhang W Bai Potentiometric glucose biosensor basedcore-shell Fe3O4-enzyme-polypyrrole nanoparticles Biosens Bioelectron 51(2014) 268ndash273

[36] H Zhou N Gan T Li Y Cao S Zeng L Zheng et al The sandwich-typeelectroluminescence immunosensor for a-fetoprotein based on enrichment byFe3O4-Au magnetic nano probes and signal amplification by CdS-Au compositenanoparticles labeled anti-AFP Anal Chim Acta 746 (2012) 107ndash113

[37] J Li Q Xu X Wei Z Hao Electrogenerated chemiluminescence immunosensorfor Bacillus thuringiensis Cry1Ac based on Fe3O4Au nanoparticles J Agric FoodChem 61 (2013) 1435ndash1440

[38] L-G Zamfir I Geana S Bourigua L Rotariu C Bala A Errachid et al Highlysensitive label-free immunosensor for ochratoxin A based on functionalizedmagnetic nanoparticles and EISSPR detection Sensor Actuat B-Chem 159(2011) 178ndash184

[39] ML Yola T Eren N Atar A novel and sensitive electrochemical DNA biosensorbased on FeAu nanoparticles decorated grapheme oxide Electrochim Acta125 (2014) 38ndash47

[40] Y Wang J Dostalek W Knoll Magnetic nanoparticle-enhanced biosensor basedon grating-coupled surface plasmon resonance Anal Chem 83 (2011) 6202ndash6207

[41] R-P Liang G-H Yao L-X Fan J-D Qiu Magnetic Fe3O4Au composite-enhanced surface plasmon resonance for ultrasensitive detection of magneticnanoparticle-enriched α-fetoprotein Anal Chim Acta 737 (2012) 22ndash28

[42] J Wang Z Zhu A Munir HS Zhou Fe3O4 nanoparticles-enhanced SPR sensingfor ultrasensitive sandwich bio-assay Talanta 84 (2011) 783ndash788

[43] J Wang D Song H Zhang J Zhang Y Jin H Zhang et al Studies of Fe3O4AgAucomposites for immunoassay based on surface plasmon resonance biosensorColloids Surf B 102 (2013) 165ndash170

[44] H Zhang Y Sun J Wang J Zhang H Zhang H Zhou et al Preparation andapplication of novel nanocomposites of magnetic-Auu nanorod in SPR biosensorBiosens Bioelectron 34 (2012) 137ndash143

[45] L Wang Y Sun J Wang J Wang A Yu H Zhang et al Preparation of surfaceplasmon resonance biosensor based on magnetic coreshell Fe3O4SiO2 andFe3O4AgSiO2 nanoparticles Colloids Surf B 84 (2011) 484ndash490

[46] S Agrawal K Paknikar D Bodas Development of immunosensor usingmagnetic nanoparticles and circular microchannels in PDMS MicroelectronEng 115 (2014) 66ndash69

[47] D Li J Wang R Wang Y Li D Abi-Ghanem L Berghman et al A nanobeadsamplified QCM immunosensor for the detection of avian influenza virus H5N1Biosens Bioelectron 26 (2011) 4146ndash4154

[48] Y Wan D Zhang B Hou Determination of sulphate-reducing bacteria basedon vancomycin-functionalised magnetic nanoparticles using modification-freequartz crystal microbalance Biosens Bioelectron 25 (2010) 1847ndash1850

[49] J Zhou N Gan T Li H Zhou X Li Y Cao et al Ultratrace detection of C-reactiveprotein by a piezoelectric immunosensor based on Fe3O4SiO2 magnetic capturenanoprobes and HRP-antibody co-immobilized nano gold as signal tags SensorActuat B-Chem 178 (2013) 494ndash500

[50] N Gan L Wang T Li W Sang F Hu Y Cao A novel signal-amplifiedimmunoassay for Myoglobin using magnetic core-shell Fe3O4Au multi walledcarbon nanotubes composites as labels based on one piezoelectric sensor IntegrFerroelectr 144 (2013) 29ndash40

[51] Z-Q Shen J-F Wang Z-G Qiu M Jun X-W Wang Z-L Chen et al QCMimmunosensor detection of Escherichia coli O157H7 based beaconimmunomagnetic nanoparticles and catalytic growth of colloidal gold BiosensBioelectron 26 (2011) 3376ndash3381

[52] B Srinivasan Y Li Y Jing C Xing J Slaton J-P Wang A three-layercompetition-based giant magnetoresistive assay for direct quantification ofendoglin from human urine Anal Chem 83 (2011) 2996ndash3002

[53] Y Li B Srinivasan Y Jing X Yao MA Hugger J-P Wang et al Nanomagneticcompetition assay for low-abundance protein biomarker quantification inunprocessed human sera J Am Chem Soc 132 (2010) 4388ndash4392

[54] T Klein J Lee W Wang T Rahman RI Vogel J-P Wang Interaction of domainwalls and magnetic nanoparticles in giant magnetoresistive nanostrips forbiological applications IEEE T Magn 49 (2013) 3414ndash3417

[55] P Zu CC Chan GW Koh WS Lew Y Jin HF Liew et al Enhancement ofthe sensitivity of magneto-optical fiber sensor by magnifying the birefringenceof magnetic fluid film with Loyt-Sagnac interferometer Sensor Actuat B-Chem191 (2014) 19ndash23

[56] M Deng D Liu D Li Magnetic field sensor based on asymmetric optical fibertaper and magnetic fluid Sensor Actuat A- Phys (2014) httpdxdoiorg101016jsna201402014

[57] HJ Hattaway KS Butler NL Adolphi DM Lovato R Belfon D Fegan et alDetection of breast cancer cells using targeted magnetic nanoparticles andultra-sensitive magnetic field sensors Breast Cancer Res 13 (2011) 1ndash13

[58] D Issadore J Chung H Shao M Liong AA Ghazani CM Castro et alUltrasensitive clinical enumeration of rare cells ex vivo using a μ-Hall detectorSci Transl Med 141 (2012) 1ndash22

[59] D Issadore HJ Chung J Chung G Budin R Weissleder H Lee μ-hall chipfor sensitive detection of bacteria Adv Healthcare Mater 2 (2013) 1224ndash1228

[60] K Duarte CIL Justino AC Freitas TAP Rocha-Santos AC Duarte Directreading methods for analysis of volatile organic compounds and nanoparticlesa review Trends Anal Chem 53 (2014) 21ndash32

[61] Y Xu E Wang Electrochemical biosensors based on magnetic micronanoparticles Electrochim Acta 84 (2012) 62ndash73

[62] K Muzyka Current trends in the development of the electrochemioluminescentimmunosensors Biosens Bioelectron 54 (2014) 393ndash407

[63] LIB Silva FDP Ferreira AC Freitas TAP Rocha-Santos AC Duarte Opticalfiber biosensor coupled to chromatographic separation for screening ofdopamine norepinephrine and epinephrine in human urine and plasma Talanta80 (2009) 853ndash857

[64] C Elosua I Vidondo FJA Arregui C Bariain A Luquin M Laguna et al Lossymode resonance optical fiber sensor to detect organic vapors Sensor ActuatB-Chem 187 (2013) 65ndash71

[65] LIB Silva TAP Rocha-Santos AC Duarte Development of a fluorosiloxanepolymer coated optical fibre sensor for detection of organic volatile compoundsSensor Actuat B-Chem 132 (2008) 280ndash289

[66] LIB Silva TAP Rocha-Santos AC Duarte Comparison of a gaschromatography-optical fibre (GC-OF) detector with a gas chromatography-flame ionization detector (GC-FID) for determination of alcoholic compoundsin industrial atmospheres Talanta 76 (2008) 395ndash399

[67] LIB Silva FDP Ferreira AC Freitas TAP Rocha-Santos AC Duarte Opticalfiber-based micro-analyzer for indirect measurements of volatile amines levelsin fish Food Chem 123 (2010) 806ndash813

[68] MT Gomes TA Rocha-Santos AC Duarte JAPB Oliveira Determination ofsulfur dioxide in wine using a quartz crystal microbalance Anal Chem 68(1996) 1561

[69] X Wang B Ding J Yu M Wang F Pan A highly sensitive humidity sensor basedon a nanofibrous membrane coated quartz crystal microbalanceNanotechnology 21 (2010) 55502

[70] MT Gomes TA Rocha-Santos AC Duarte JAPB Oliveira The performanceof a tetramethylammonium fluoride tetrahydrated coated piezoelectric crystalfor carbon dioxide detection Anal Chim Acta 335 (1996) 235

[71] K Catterjee S Sarkar KJ Rao S Paria Coreshell nanoparticles in biomedicalapplications Adv Colloid Interface Sci (2014) httpdxdoiorg101016jcis201312008

[72] PP Freitas R Ferreira S Cardoso F Cardoso Magnetoresistive sensors J PhysCondens Matter 19 (2007) 165221ndash165242

[73] X Sun D Ho L-M Lacroix JQ Xiao S Sun Magnetic nanoparticlesfor magnetoresistance-based biodetection IEEE Trans Nanobiosci 11 (2012)46ndash53

36 TAP Rocha-SantosTrends in Analytical Chemistry 62 (2014) 28ndash36