application of nano bio sensors and biochips in health care

8/6/2019 Application of Nano Bio Sensors and Biochips in Health Care

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Application of Nanobiosensors and Biochips

in Health Care : A Review

Submitted by Murugan Veerapandian on Thu, 09/04/2008 - 13:53Average:3.714285Your rating: None Average: 3.7 (7 votes)

This review is an attempt to analyze the different

applications of nanobiosenors and biochips in health care.

The biological and medical fields have seen great advances in biomolecules. This review ismeant to provide an overview of the various types of biosensors and biochips that have beendeveloped for biological and medical applications, along with significant advances over the lastseveral years in these technologies. It also attempts to describe various classification schemesthat can be used for categorizing the different biosensors and provide relevant examples of theseclassification schemes from recent literature.

1. Introduction

The emergence of nanotechnology is opening new horizons for the development of nanosensorsand nanoprobes with submicron-sized dimensions that are suitable for intracellularmeasurements. A biosensor is defined as a device that uses specific biochemical reactionsmediated by isolated enzymes, immunosystems, tissues, organelles or whole cells to detectchemical compounds usually by electrical, thermal or optical signals1. In recent years, manytypes of biosensors have been developed and used in a wide array of biomedical and othersettings. Although it is impossible to survey this entire fast-moving field, this issue presentsarticles about some of the many types of biosensors and biosensor-based applications to give thereader a sense of the enormous importance and potential for these devices. One of the earliestreferences to the concept of a biosensor is from Dr. Leland C Clark who created many of the

early biosensors in the early 1960s2

using an enzyme electrode for measuring glucoseconcentration with the enzyme Glucose Oxidase (GOD). The success of single analyte sensorswas followed by development of integrated multi-analyte sensors capable of morecomprehensive analyses, such as a single instrument for glucose, lactate, and potassiumdetection. Technical developments in manufacturing enabled the development of miniaturizedintegrated biosensors for determination of glucose, lactate, and urea in micro samples ofundiluted whole blood or plasma. Miniaturization also allowed additional analytical tools to beadded to the biosensor, such as chromatography or capillary electrophoresis. The newest


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generation of biosensors includes miniaturized multi-analyte immunosensor devices with high-throughput capabilities and more than 1000 individually addressable electrodes per squarecentimeter. These instruments can detect analytes present in the attomole range3. Modernfabrication techniques such as ink-jet printing, photolithography,

Fig.1 Biosensing principle

microcontact printing, and self-assembly continue to contribute to more advanced biosensors,and the next type of devices to emerge may include miniature biosensors with high-densityligands, selfcontained lab-on-a-chip capabilities, and nanoscale biosensors. Biosensor thatincludes transducers based on integrated circuit microchips are often referred to as biochips.(Fig.1) illustrates the conceptual principle of biosensing process. Biosensors and biochips can beclassified either by their bioreceptor or their transducer type ( Fig. 2).

Fig.2 Classification Schemes of Biosensors/Biochip

2. Nanobiosensors


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Nanomaterials are exquisitely sensitive chemical and biological sensors. Nanosensors withimmobilized bioreceptor probes that are selective for target analyte molecules are callednanobiosensors. They can be integrated into other technologies such as lab-on-a-chip to facilitatemolecular diagnostics. Their applications include detection of microorganisms in varioussamples, monitoring of metabolites in body fluids and detection of tissue pathology such as

cancer. Their portability makes them ideal for pathogenesis of cancer (POC) applications butthey can be used in the laboratory setting as well.

2.1. Nanowire biosensors

Since their surface properties are easily modified, nanowires can be decorated with virtually anypotential chemical or biological molecular recognition unit, making the wires themselves analyteindependent. The nanomaterials transduce the chemical binding event on their surface into achange in conductance of the nanowire in an extremely sensitive, real time and quantitativefashion. Boron-doped silicon nanowires (SiNWs) have been used to create highly sensitive, real-time electrically based sensors for biological and chemical species4. Biotin-modified SiNWs

were used to detect streptavidin down to at least a picomolar concentration range. The small sizeand capability of these semiconductor nanowires for sensitive, label-free, real-time detection of awide range of chemical and biological species could be exploited in array-based screening and invivo diagnostics.

2.2 Ion Channel Switch biosensor technologies

The Ion Channel Switch (ICS), a novel biosensor technology of Ambri Ltd (Chatswood,Australia), is based upon a synthetic self-assembling membrane, which acts like a biologicalswitch that detecting the signaling the presence of specific molecules by triggering an electricalcurrent5. This is the basis of the companys SensiDx System, a nanobiosensor device that has

been designed for POC testing in critical care environments in hospitals. By delivering preciseand quantitative test results in an immediate timeframe, the SensiDx System reduces the time ofemergency diagnoses from hours down to minutes.

2.3. Electronic nanobiosensors

The Biodetect system of Integrated Nanotechnologies ( Henrietta , NY ) works by electronicallydetecting the binding of a target DNA molecule to sensors on a microchip. The target moleculesform a bridge between two electrically separated wires. In order to create a strong clear signal,the bound target molecules are chemically developed to form conductive DNA wires, which aremetalized and can be seen by electron microscopy. The bridges, which can be observed byfluorescent imaging techniques, are readily detected by measuring the resistance or otherelectrical properties of the sensor. These DNA wires turn on a sensor much like an on/offswitch12. Each chip contains multiple sensors, which can be independently addressed withcapture probes for different target DNA molecules from the same or different organisms. Aproprietary DNA Lithography process is used to attach capture probes to each of the sensors onthe chip. These chips now have billions of capture probes per sensor, which greatly improvessensitivity.


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2.4 Viral nanosensor

Virus particles are essentially biological nanoparticles. Herpes simplex virus (HSV) andadenovirus have been used to trigger the assembly of magnetic nanobeads as a nanosensor forclinically relevant viruses6 .The nanobeads have a supramagnetic iron oxide core coated with

dextran. Protein G is attached as a binding partner for antivirus antibodies. Anti-HSV antibodiesare conjugated directly to nanobeads using a bifunctional linker to avoid nonspecific interactionsbetween medium components and protein G. By using a magnetic field, as few as five viralparticles can be detected in a 10 l serum sample. This system is more sensitive than ELISA-based methods and is an improvement over PCR-based detection because it is cheaper, faster andhas fewer artifacts.

2.5 PEBBLE nanosensors

Probes Encapsulated by Biologically Localized Embedding (PEBBLE) nanosensors consist ofsensor molecules entrapped in a chemically inert matrix by a microemulsion polymerization

process that produces spherical sensors in the size range of 20 to 200 nm

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. These sensors arecapable of real-time inter- and intra-cellular imaging of ions and molecules and are insensitive tointerference from proteins. PEBBLE can also be used for early detection of cancer. PEBBLEnanosensors also show very good reversibility and stability to leaching and photobleaching, aswell as very short response times and no perturbation by proteins. In human plasma theydemonstrate a robust oxygen sensing capability, little affected by light scattering andautofluorescence 8.

2.6 Optical biosensors

Many biosensors that are currently marketed rely on the optical properties of lasers to monitor

and quantify interactions of biomolecules that occur on specially derived surfaces or biochips.Surface plasmon resonance technology is the best-known example of this technology.

2.6.1 Surface plasmon resonance technology

Surface plasmon resonance (SPR) is an opticalelectrical phenomenon involving the interactionof light with the electrons of a metal. The opticalelectronic basis of SPR is the transfer of theenergy carried by photons of light to a group of electrons (a plasmon) at the surface of a metal.The next generation microarray-based SPR systems are designed to help researchers profile andcharacterize biomolecular interactions in a parallel format. Miniature optical sensors thatspecifically identify low concentrations of environmental and biological substances are in highdemand. Currently, there is no optical sensor that provides identification of the aforementionedspecies without amplification techniques at naturally occurring concentrations. Triangular silvernanoparticles have remarkable optical properties and their enhanced sensitivity to their nanoenvironment has been used to develop a new class of optical sensors using localized SPRspectroscopy9.

2.6.2 Laser nanosensors


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In a laser nanosensor, laser light is launched into the fiber, and the resulting evanescent field atthe tip of the fiber is used to excite target molecules bound to the antibody molecules. Aphotometric detection system is used to detect the optical signal (e.g., fluorescence) originatingfrom the analyte molecules or from the analyte-bioreceptor reaction10. Laser nanosensors can beused for invivo analysis of proteins and biomarkers in individual living cells.

2.7 Nanoshell biosensors

Gold nanoshells have been used in a rapid immunoassay capable of detecting analyte withincomplex biological media without any sample preparation11. Aggregation of antibody/nanoshellconjugates with extinction spectra in the near infrared is monitored spectroscopically in thepresence of analyte. Successful detection was achieved in this system constitutes a simpleimmunoassay capable of detecting sub-nanogram-per-milliliter quantities of immunoglobulins insaline, serum, and whole blood. Nanoshells are already being developed for applicationsincluding cancer diagnosis, cancer therapy, and testing for proteins associated with Alzheimersdisease. Nanoshells can enhance chemical sensing by as much as 10 billion times. That makes

them about 10,000 times more effective at Raman scattering than traditional methods. Whenmolecules and materials scatter light, a small fraction of the light interacts in such a way that itallows scientists to determine their detailed chemical makeup. This property, known as Ramanscattering, is used by medical researchers, drug designers, chemists and other scientists todetermine the nature of various materials. An enormous limitation in the use of Raman scatteringhas been its extremely weak sensitivity. Nanoshells can provide large, clean, reproducibleenhancements of this effect, opening the door for new, and all-optical sensing applications.Scientists at the Laboratory of Nanophotonics of Rice University (

Houston, TX ) have found that nanoshells are extremely effective at magnifying the Ramansignature of molecules, each individual nanoshell acting as an independent Raman enhancer.

That creates an opportunity to design alloptical nanoscale sensorsessentially new molecularlevel diagnostic instrumentsthat could detect as little as a few molecules of a target substance,which could be anything from a drug molecule or a key disease protein to a deadly chemicalagent. The metal cover of the nanoshell captures passing light and focuses it, a property thatdirectly leads to the enormous Raman enhancements observed. Furthermore, nanoshells can betuned to interact with specific wavelengths of light by varying the thickness of their shells. Thistunability allows for the Raman enhancements to be optimized for specific wavelengths of light.The finding that individual nanoshells can vastly enhance the Raman effect opens the door forbiosensor designs that use a single nanoshell, something that could prove useful for engineerswho are trying to probe the chemical processes within small structures such as individual cells,or for the detection of very small amounts of a material, like a few molecules of a deadlybiological or chemical agent12.

3. Clinical applications of biosensors

There has been a great demand for rapid and reliable methods which can be used in biochemicallaboratories for determination of substances in biological fluids such as blood, serum and urine,etc. There is also a demand to move clinical analysis from centralized laboratories to a doctorsclinic and patients self-testing at home. Most of the methods available in the market for rapid


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detection are based on enzyme electrodes. They provide for a negligible enzyme consumption of


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fluid outside the probe, defuse in as soon as the substances are carried out of the body by theperfusion liquid, the concentration can be determined by coupling it with a biosensor. Otherapproaches, which have been proposed for in vivo monitoring include enzyme basedelectrochemical, enzyme based field effect transistor (ENFET), enzyme based thermoelectric,electrochemical and optical approach.

4. Biosensors for health care

4.1. Glucose biosensor

Detection of glucose has been the most studied analyte in diabetic patients. The level of theglucose can be monitored either in vivo or in vitro. The first approach for in vitro study waspioneered by Shichiri et al.15Mascini et al.16 reported an artificial pancreas for continuousmeasurement of glucose. A number of glucose biosensors have been reported which are based onconducting polymers 17, 18, 19 . Ramanathan et al.19 covalently attached glucose oxidase on poly(o-amino benzoic acid) and fabricated the screen printed electrodes made of this material. These

electrodes have been shown to be useful for glucose estimation from 1 to 40 mM and stability ofabout 6 days. The i-STAT portable clinical analyser which is a significant commerciallyavailable biosensor, can measure a range of parameters: sodium, chloride, potassium, glucose,blood urea nitrogen (BUN) and haematocrit. The NPL Glucosense developed at the NationalPhysical Laboratory, India is based on the screen printed graphite electrode having a mediatorincorporated in the working electrode. The product is available with the Indian markets for theconsumers. The sensors are fabricated using thin film microfabrication technology on adisposable cartridge20. Recently, Singhal et al.17 reported that poly (3-dodecylthiophene) /stearicacid /glucose oxidase (P3DT/SA/GOX) LangmuirBlodgett films based glucose biosensor canbe used for at least 35 measurements and was found to be stable upto 40 days.

4.2 Lactate biosensor

Lactate measurement is helpful in respiratory insufficiencies, shocks, heart failure, metabolicdisorder and monitoring the physical condition of athletes. Many biosensors have been reportedto date21, 22. Two different technologies have been approached for the development ofminiaturized systems. Thin film electrodes have been developed, which can be used as eitherimplantable catheter type devices or for in vivo monitoring in combination with microdialysissystem23, 24. Secondly, disposable type sensors were developed for the purpose of on-lineanalysis25, 26. Group at the National Physical Laboratory, India has recently developed a screenprinted electrode based lactate biosensor. Li and coworkers28have recently reported the solgelencapsulation of lactate dehydrogenase for optical sensing of L-lactate. Such a disposable lactate

sensor has a linear dynamic range from 0.2 to 1 mM of lactate and stability of about 3weeks. Thesensor was found to have a diminished enzyme activity (about 10%) and leaching of the enzymefrom the matrix.

4.3. Urea and creatinine biosensors

Urea estimation is of utmost importance in monitoring kidney functions and disorders associatedwith it. Most of the urea biosensors available in literature are based on detection of NH4

+ or


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HCO3sensitive electrodes28, 29, 30. Osaka et al.31 constructed a highly sensitive and rapidflow

injection system for urea analysis with a composite film of electropolymerized inactivepolypyrrole and a poly ion complex. Gambhir et al.32 have recently co-immobilized urease andglutamate dehydrogenase on electrochemically prepared polypyrrole/polyvinyl sulphonate forthe fabrication of urea biosensor. Singhal et al.28 have recently immobilized urease on poly (N-

vinyl carbazole/stearic acid).

4.4. Cholesterol biosensor

Determination of cholesterol is clinically very important because abnormal concentrations ofcholesterol are related with hypertension , hyperthyroidism, anemia and coronary artery diseases.

Determination based on the inherent specificity of an enzymatic reaction provides the mostaccurate means for obtaining true blood cholesterol concentration. Reports on the developmentof cholesterol biosensors are available33-39. Recently, Vengatajalabathy and Mizutani40demonstrated an amperometric biosensor for cholesterol determination by a layer-by-layer self-

assembly using ChOx and poly (styrenesulfonate) on a monolayer of microperoxidase covalentlyimmobilised on Au-alkanethiolate electrodes. The sensor was found to be responsive even in thepresence of potential electrical interferents, L-ascorbic acid, pyruvic acid and uric acid. Kumar etal.33 presented a cholesterol biosensor by co immobilization of cholesterol oxidase andperoxidase on solgel films and utilized these films for estimations of cholesterol.

4.5. Uric acid biosensor

Uric acid is one of the major products of purine breakdown in humans and therefore itsdetermination serves as a market for the detection of range of diosorders associated with alteredpurine metabolism, notably gout, hyperuricaemia and LeschNyhan Syndrome. Elevated levels

of uric acid are observed in a wide range of conditions such as leukaemia, pneumonia, kidneyinjury, hypertension, ischemia, etc. Additionally, as a reducing agent uric acid scavenges freeoxygen radicals, preventing their destructive action towards tissue and cells. Various attemptshave been made to develop a biosensor for the estimation of uric acid41-47.

4.6. DNA biosensor

DNA biosensors have an enormous application in clinical diagnostics for inherited diseases,

Rapid detection of pathogenic infections and screening of cDNA colonies are required inmolecular biology. Conventional methods for the analysis of specific gene sequences are based

on either direct sequencing or DNA hybridization

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. Because of its simplicity, most of thetraditional techniques in molecular biology are based on hybridization. Several immobilizationtechniques such as adsorption49, covalent attachment50, or immobilization involving avidinbiotin complexation51 were adopted for a DNA probe to the surface of an electrochemicaltransducer. The transducer was made from carbon52; gold53-55; or conducting polymer56, 57. In thecase of a common sandwich assay the signal generating species is an enzyme, such ashorseradish peroxidase58. Lund et al.59 linked the tagged DNA to the surface of the microsphereusing a suitable reagent. Another effort is the use of microfabrication system and micro


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mechanical technology to the preparation of DNA samples and their analysis (e.g. DNA chip).Gambhir et al.57 have recently attempted to immobilize DNA on conducting polypyrrole/polyvinyl sulphonate films and demonstrated the adsorption characteristics. They believed thatanion doped polypyrrole undergoes ion exchange with PO4

of DNA to facilitating theadsorption. Presently, DNA probes and biosensors have widely attracted attention for diagnosis

of various disorders

60-63

.

4.7. Immuno-sensors

Immuno-sensors are small, portable instruments for analysis of complex fluids and are designedfor the ease of use by un-trained personnel, rapid assay and sensitivity comparable to that ofELISA. During the past decade, a number of methods for immunoassay by specific interactionsbetween antibodies and antigens to analyze microorganisms, viruses, pesticides and industrialpollutants have been developed64-67. Immuno-sensors are the analytical systems based onimmuno-chemical principles that can automatically carry out estimation of desired parameter.Barnett et al. have detected thaumatin using antibody containing polypyrrole electrodes67. In the

recent past, immunoassays have relied on complex indirect enzyme methods in which theresultant product of the enzyme immuno reaction can be measured. Recently, antibodies havebeen raised against the conducting polymer, carbazole as a hapten, which may react to modulatethe polymer electrochemistry.

It has been observed by cyclic voltammetry that the reaction of the antiserum influences thepolymer matrix electrochemistry by an amperometric response.

5. Biosensor Transducers

The transducer converts the biochemical interactions into a measurable electronic signal.

Electrochemical, electrooptical, acoustical, and mechanical transducers are among the manytypes

found in biosensors. The transducer works either directly or indirectly.

5.1. Direct detection biosensors

Direct detection sensors, in which the biological interaction is directly measured in real time,typically use non-catalytic ligands such as cell receptors or antibodies. The most common directdetection biosensor systems employ evanescent wave, or surface plasmon resonance (SPR),technology which measures resonant oscillation of electrons on the surface of a metal.

5.2 Indirect detection biosensors

The second class of transducers, indirect detection sensors, relies on secondary elements that areoften catalytic elements such as enzymes. Some examples of secondary elements are the enzymealkaline phosphatase and fluorescently tagged antibodies that enhance detection of a sandwichcomplex. Dr. Vincent Gaus and Dr. Omowunmi Sadiks groups describe the use ofelectrochemical transducers to measure the oxidation or reduction of an electroactive compound


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substrates,

Products, gases, pollutants,antibiotics, vitamins, etc.

Optoelectronic/wave guide

and fiber optic device

Optical pH, enzyme substrates and

immunological systems

Ion sensitive electrode (ISE) Potentiometric Ions in biological media,enzyme electrodes, enzyme

immunosensors

Field effect transistor (FET) Potentiometric Ions, gases, enzymesubstrates and

immunological analytes

6. Biochip

Biochips are the basis for miniaturized biochemical assays, and offer many advantages over theconventional analytical methods, the most significant of which are: i) a variety of analytes can beinvestigated simultaneously in the same sample, ii) the required sample quantities are minimal,iii) low consumption of scarce reagents, iv) high miniaturization and v) high sample throughput.These advantages become very evident if we consider the workflow during a typical drugscreening process. At the beginning there is the need for a selection of few eligible compoundsout of a large variety of molecules for a given purpose. Since the most limiting factor is the ratiobetween the number of data points per day and the cost per data point, this first mass-selection is

best done at a molecular level where DNA-chips and protein microarrays find their mostcommon application. After that the investigations will move to the whole cells, where theChannelomics can deliver very clear informations about the effect of drugs on the physiology ofthe cells68.

6.1 DNA Biochip

A DNA-chip consists in most cases in a glass or quartz slide acting as a carrier, which isfunctionalized with an array of probes (features). A single probe contains identical molecules, forthis reason it is also called feature. The basic principle of operation of a DNA-chip consists ofthree main steps: 1) Functionalization, i.e. the immobilization of different DNA sequences onto

different positions (probes), 2) Hybridization, bring the analytes in contact with the probes, byflooding the chip with the sample solution and let the hybridization take place, 3) Readout, afterwashing the chip to remove all non-bound molecules, the array is scanned to detect on whichprobes a hybridization took place. Depending on the different fields of application, e.g. geneexpression for drugs screening, or medical diagnosis of cancer or genetic diseases, therequirements for the DNA-chip may include high sensitivity, wide dynamic range and highspecificity. The first two parameters reflect the ability to deliver a correct response at different


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intensities of hybridization, while the latter shows the efficiency in rejecting analytes with anyminimal mismatch68.

6.2 Integrated DNA biochip

The development of a truly integrated biochip having a phototransistor integrated circuit (IC)microchip has been reported by Vo-Dinh and coworkers69, 70. This work involves the integrationof a 4 X 4 and 10 X 10 optical biosensor array onto an integrated circuit (Fig. 3). Most opticalbiochip technologies are very large when the excitation source and detector are considered,making them impractical for anything but laboratory usage. In this biochip the sensors,amplifiers, discriminators and logic circuitry are all built onto the chip. In one biochip system,each of the sensing elements is composed of 220 individual phototransistor cells connected inparallel to improve the sensitivity of the instrument. The ability to integrate light emitting diodes(LEDs) as the excitation sources into the system is also discussed. An important element in thedevelopment of the multifunctional biochip (MFB) involves the design and development of anIC electro-optic system for the microchip detection elements using the complementary metal

oxide silicon (CMOS) technology. With this technology,

Fig.3 Schematic diagram of an integrated DNA biochip system69

highly integrated biochips are made possible partly through the capability of fabricating multipleoptical sensing elements and microelectronics on a single system. Applications of the biochip are

illustrated by measurements of the HIV1 sequence-specific probes using the DNA biochip devicefor the detection of a gene segment of the AIDS virus70. Recently, a MFB which allowssimultaneous detection of several disease end-points using different bioreceptors, such as DNA,antibodies, enzymes, and cellular probes, on a single biochip system was developed71. The MFBdevice was a self-contained system based on an integrated circuit including photodiode sensorarrays, electronics, amplifiers, discriminators and logic circuitry. The multi-functional capabilityof the MFB biochip device is illustrated by measurements of different types of bioreceptors using


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DNA probes specific to gene fragments of theMycobacterium Tuberculosis (TB) system, andantibody probes targeted to the cancer related tumor suppressor gene p53.

6.3 Micro fluidics-based biochip system

The analysis of complex liquid samples for specific components necessitates some means ofselective detection. In some cases, discrimination is afforded through spectral characteristics ofthe components in a complex sample, particularly with infrared, Raman and mass spectroscopes.Otherwise, hyphenated techniques which couple spectroscopic techniques with separationsystems are routinely used for the analysis of complex samples (e.g. gas chromatography-massspectrometry (GC-MS) 72-74, liquid chromatography-mass spectrometry (LC-MS) 75, massSpectrometry - mass

Fig. 4 Schematic diagram of micro fluidics based biochip system71

spectrometry (MS- MS)76, liquid Chromatography - infrared absorption spectrometry (LC-IR)77,78, ion trap MS79.80, gas chromatography-infrared absorption spectrometry (GC-IR)73,74,77,capillary electrophoresis (CE), etc.81-83.However, with the exception of some innovative MSdevice-based systems e.g. ion traps79, 80, such instruments tend to be bulky, expensive, labor-

intensive, and require skilled operators. Furthermore, individual chromatographic-basedseparation systems generally have limited applicability in terms of ranges of molecular size and/or functionality. Even with hyphenated techniques, analysis of a complex sample may requiremultiple pretreatment steps as well as multiple chromatographic systems. Schematic diagram ofmicro fluidics based biochip system71 (Fig. 4)

7. Future issues in the development of Nanobiosensor and Biochip


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New biosensors and biosensor arrays are being developed using new materials, nanomaterialsand microfabricated materials including new methods of patterning. Biosensor components willuse nanofabrication technologies. Use of nanotubes, Buckminster fullerenes (buckyballs), silicaand its derivatives can produce nanosized devices. Some of the challenges will be: Developmentof real-time non-invasive technologies that can be applied to detection and quantitation of

biological fluids without the need for multiple calibrations using clinical samples. Developmentof biosensors utilizing new technologies that offers improved sensitivity for detection with highspecificity at the molecular level. Development of biosensor arrays that can successfully detect,quantify and quickly identify individual components of mixed gases and liquid in an industrialenvironment. It would be desirable to develop multiple integrated biosensor systems that utilizedoped oxides, polymers, enzymes or other components to give the system the requiredspecificity. A system with all the sensor components, software, plumbing, reagents and sampleprocessing are an example of an integrated sensor. There is also a need for reliable fluid handlingsystems for dirty fluids and for relatively small quantities of fluids (nanoliter to attoliterquantities). These should be low cost, disposable, reliable and easy to use as part of an integratedsensor system. Sensing in picoliter to attoliter volumes might create new problems in

development of micro reactors for sensing and novel phenomenon in very small channels

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.

The size of device to move (levitate) tiny fluid droplet has been reduced by using micron scalediamagnets to create a magnetic micromanipulation chip, which operates with femtodropletslevitated in air84. The droplets used are 1 billion times smaller in volume than has beendemonstrated by conventional methods. The levitated particles can be manipulated andpositioned with accuracy within a range up to 300 nm. Use of this technology on a lab-on-a-chipwould refine the examination of fluid droplets containing trace chemicals and viruses. Eventhough micro array/biochip methods employing the detection of specific biomolecularinteractions are now an indispensable tool for molecular diagnostics, there are some limitations.DNA micro arrays and enzyme-linked immunosorbent assay (ELISA) rely on the labeling ofsamples with a fluorescent taga procedure that is time consuming and expensive.Nanotechnologies can provide label-free detection and are being applied to overcome some ofthe limitations of biochip technology.

8. Summary

As biosensor technology advances, the range of applications broadens. Biosensors are now beingdeveloped for detection of microbial pathogens and their toxins (see articles by Drs. Ligler, Gau,and Sadik and their colleagues), monitoring of glucose and other metabolites, blood analysis (seearticles by Drs. Battrell and Gau and colleagues), and other physiological monitoring (seearticles by Drs. Yingfu Li and Homola and colleagues), cancer detection (see articles by Drs.

Gau, Thundat, Soper, and Battrell and their colleagues), and monitoring. In addition, biosensortechnology is being applied to allergen detection, food and biomaterial quality testing, and basicresearch on molecular interactions. Biosensors offer several advantages over other analyticalmethods including rapid and even real-time measurements, high sensitivity, selectivity, andspecificity even when a complex or turbid sample matrix is used. As the technology advances,producing lab-on-a-chip devices (as described by Drs. Soper and Battrell and their colleagues),these self-contained portable instruments will allow measurements outside the laboratory, in thefield or at the bedside. Biochip technologies could offer a unique combination of performance


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capabilities and analytical features of merit not available in any other bioanalytical systemcurrently available. With its multichannel capability, biochip technology allows simultaneousdetection of multiple biotargets. Biochip systems have great promise to offer several advantagesin size, performance, fabrication, analysis and production cost due to their integrated opticalsensing microchip. The small sizes of the probes (micro liter to nanoliter) minimize sample

requirement and reduce reagent and waste requirement. Highly integrated systems lead to areduction in noise and an increase in signal due to the improved efficiency of sample collectionand the reduction of interfaces. The capability of large-scale production using low-costintegrated circuit (IC) technology is an important advantage. The assembly process of variouscomponents is made simple by integration of several elements on a single chip. For medicalapplications, this cost advantage will allow the development of extremely low cost, disposablebiochips that can be used for in-home medical diagnostics of diseases without the need ofsending samples to a laboratory for analysis.

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