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Sensors and Actuators B 191 (2014) 192– 203

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locate /snb

hiol surfactant assembled on gold nanoparticles ion exchanger forcreen-printed electrode fabrication. Potentiometric determination ofe(III) in environmental polluted samples

amer Awad Alia,∗, Gehad G. Mohamedb, E.M.S. Azzama, Ali A. Abd-elaala

Egyptian Petroleum Research Institute (EPRI), 1 Ahmed El-Zomor St., 11727 Cairo, EgyptChemistry Department, Faculty of Science, Cairo University, 12613 Giza, Egypt

r t i c l e i n f o

rticle history:eceived 19 May 2013eceived in revised form3 September 2013ccepted 27 September 2013vailable online xxx

eywords:

a b s t r a c t

A new modified screen-printed electrode (SPE) based on a recently synthesized ligand 1,4-bis-(8-mercaptooctyloxy)-benzene (I), self-assembled to gold nanoparticles (GNP) as suitable carrier for Ce(III)ion determination with potentiometric method is described. The proposed potentiometric method wasbased on the fabrication of modified gold nanoparticles-screen-printed (GNPs-SPE) and modified screen-printed (MSPE) sensors. These potentiometric sensors respond to Ce(III) ions in the wide linear rangeof 3.25 × 10−10 to 1.0 × 10−1 and 1.0 × 10−7 to 1.0 × 10−2 mol L−1 with Nernstian slopes of 19.95 ± 0.97and 17.04 ± 1.02 mV decade−1 for GNPs-SPE and MSPE, respectively. The detection limit of 3.25 × 10−10

−8 −1

old nanoparticles screen-printed sensorscreen-printed sensorserium(III) ionotentiometric determination

and 9.5 × 10 mol L was obtained at pH range 2.8–8.5 and 3.5–7.5 for GNPs-SPE and MSPE, respec-tively. It has a fast response with response time of about 4 and 7 s, and can be used for at least 7 and5 months without any considerable divergences in the potentials for GNPs-SPE and MSPE, respectively.Such abilities promote new opportunities for determining Ce(III) ions in a wide range of real samples. Theresults obtained compared well with those obtained using inductively coupled plasma atomic absorptionspectrometry (ICP-AES).

. Introduction

Cerium is a member of lanthanide group of elements and theost abundant of them. It is found in monazite, ceric bastnaesite

nd silicate rock [1–3]. It has many industrial applications in thereas of lighting and television, metallurgy, glass and ceramics ands one of the active components of catalytic converters in vehicles.ue to its usage in many fields of human endeavor, there is growingeed to study the environmental, medical and biological effects oferium [4–6]. Hence, the availability of rapid, sensitive and selectiveethods for cerium determination is of importance [7,8].Analytical techniques such as ICP-AES [9], electrothermal

tomic absorption, spectrofluorometry [10,11], ICP-AES/HPLC8,12] and stripping voltammetry [13,14] have been used to deter-

ine cerium (III) cations; however, these methods are expensivend may be unavailable in some areas. Potentiometric electrodes

ossess several advantages, including the direct, simple, rapid,

nexpensive and selective detection of ionic activity. The selectiv-ty of these sensors stems from the interactions between the highly

∗ Corresponding author. Tel.: +20 10 06890640.E-mail address: dr tamerawad@yahoo.com (T.A. Ali).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.09.110

© 2013 Elsevier B.V. All rights reserved.

selective complexing agent incorporated in the membrane materialand the target species [15,16].

In spite of the significant progress in the design of highly selec-tive ionophores for various metal ions, reports on the developmentof selective ionophores for lanthanum and other lanthanide ionswith relatively good selectivity and sensitivity have been presented[17,18]. Nevertheless, in the majority of these studies, disadvan-tages such as high detection limits, narrow dynamic range andserious interferences were observed [15].

Electrodes modified by gold nanoparticles show attractive prop-erties with respect to improving the analytical sensitivity andselectivity during last few years [19,20]. Ionophores have also beenemployed as electrode materials in their reduced form with pro-nounced current rectifying character [21]. In continuation to recentstudies on the use of nano material modified electrodes [22–24],these new sensors were utilized for Ce(III) determination in pureand spiked samples.

One of the alternatives for potentiometry development is thesearch for novel materials that can be applied as receptor layersor can be used to improve existing sensors. Recently this trend

has manifested itself through interest in micro- or nanomaterials[25–34] in potentiometric sensors construction. Different materi-als have been studied, ranging from conducting polymer micro-and nanostructures [31–33] to gold nanoparticles [30] applied as

T.A. Ali et al. / Sensors and Actuators B 191 (2014) 192– 203 193

1,4-bis(8-bromoo ctyloxy)benzene

O O

1,4-dihydroxy benze ne

OHHO

1,8 dibromoctane

1-sodium ethoxied2-refulx 3 hr/N2

2

reflux 6 hr/N2

OS O S

isothiour nium salt intermediate

O O

CCNH2

NH2

H2N

H2N 2Br

Hydro lysis by aq NaOH , thenAcidification by H2SO4

BrBr

BrBr

HSSH

H2NC

S

NH2

2+

-

bis(8-

tmi[bilmpba(whito[lnems

a[Ftnaaofi

Scheme 1. Synthesis of 1,4-

ransducers (solid contact) under the plastic, solvent polymericembranes. Alternatively, polymeric microstructures [34], plat-

num nanoparticles [29] or ionophore modified gold nanoparticles23], were introduced to the ion-selective membrane phase. Car-on nanotubes were also successfully applied for constructional

mprovements serving as transducer layers [25–28,35] of excel-ent properties also for biosensors [36]. Also other carbon based

aterials like nanostructured three-dimensionally ordered macro-orous carbon [37], fullerene [38], or recently graphene [39] haveeen used as transducers in potentiometric sensors. Nanomateri-ls are also applied as alternative (to polymeric, poly(vinylchloride)PVC) or polyacrylate based) receptor phases. Carbon nanotubesere used directly as receptor layers for detection of aromaticydrocarbons [40]. Modification of nanostructures resulting in

on potentiometric sensitivity/selectivity is also of interest. Novelype potentiometric sensors of high selectivity for lead ions werebtained due to covalent linking of ionophore to carbon nanotubes41]. Graphene layers noncovalently modified with complexingigand were recently proposed for Zn(II) sensors. Spontaneous orga-ization of the thiophene derivative of crown ether ligand, priorlectropolymerization together with thiophene leading to poly-er nanoparticles, was shown to result in a layer of potentiometric

ensitivity toward potassium ions [42].Recently in the case of modified gold surface for potentiometric

pplications, nanoparticles (GNPs) seem to be especially attractive20], among others due to a high surface area compared to volume.or potentiometric applications, contrary to most typical applica-ions of GNP, formation of a layer instead of application of singleanoparticles is preferred. Conveniently, GNPs can be drop-cast

pplied on the surface of a substrate electrode. Moreover, GNPsre known to spontaneously form organized structures, clustersr layers [43,44], which have intriguing properties. Formation oflms from GNPs, especially if the latter are modified with ligands,

mercaptooctyloxy)benzene.

can be an attractive alternative to application of gold nanochannels[24] for potentiometric sensing. It is known that in gold nanopar-ticle monolayer films, the interparticle edge-to-edge distances aregenerally equal to the chain length of the capping agent [44], i.e.,typically in the range of a few nanometers. Thus, it seems ratio-nal to expect that in a multilayer structure, the distances would bebigger, however, at the most in the range of less than 10 nm, thusmuch smaller compared to the diameter of nanopores [24] but stillbig enough to enable solution cations to penetrate the layer.

Based on these investigations, in the present work we introducea simple and selective potentiometric method for determinationof cerium (III) ions in aqueous samples. Here, for the first timea ligand with two –SH groups at the heads was bonded to thegold nanoparticles (GNPs) and used as ionophore in a modifiedscreen-printed electrode. In this process a macrocycle was formedwhich interacted with cerium ions selectively. In this manner, 1,4-bis-(8-mercaptooctyloxy)-benzene (I), (Scheme 1) was synthesizedand self-assembled to the gold nanoparticles in the screen-printedelectrode. The modified paste was used as a selective electrode indetermination of Ce(III) ions potentiometrically. Finally, this elec-trode was compared with other similar Ce(III) selective electrodesand in many aspects such as detection limit, concentration range,and selectivity show superior behaviours.

2. Experimental

2.1. Reagents

All the reagents were of the analytical grade and deion-

ized water was used throughout the experiments. 1,4-bis(8-bromopentyloxy)benzene and 1,8-dibromo-pentane were sup-plied from Merck. Hydrogen tetracholoroaurate (HAuCl4·3H2O),trisodium citrate and Ce(III) nitrate were obtained from Aldrich.

194 T.A. Ali et al. / Sensors and Actuators B 191 (2014) 192– 203

tyloxy

CoCts

2

sarfB

2

JrfdawwtmEws1IomI

2

obhoh

1d4i

Scheme 2. Synthesized 1,4-bis(8-mercaptooc

arbon graphite powder (synthetic 1–2 �m) (Aldrich), paraffinil (Merck) was used for the fabrication of different electrodes.hloride salts of zinc, magnesium, cadmium, chromium, stron-ium, nickel, calcium, potassium, manganese, lead, barium, cobalt,odium, ferrous and aluminum are used as interfering materials.

.1.1. SamplesWater samples included formation water (Sample 1 and 2 were

upplied from Meleiha, Western Desert, Agiba Petroleum Companynd Karama, Al-Wahhat-Al-Bahhriyah, Qarun Petroleum Company,espectively), tap water (sample 3), river water (sample 4 suppliedrom Shoubra, Aga, Dakahliya), sea water (sample 5 supplied fromaltim area, Kafr El-Sheikh) and red sea (sample 6).

.2. Apparatus

Laboratory potential measurements were performed usingenway 3505 pH-meter. Silver–silver chloride double-junctioneference electrode (Metrohm 6.0726.100) in conjugation with dif-erent ion selective electrodes was used. pH measurements wereone using Thermo Orion, model Orion 3 stars, USA. Prior tonalysis, all glassware used were washed carefully with distilledater and dried in the oven before use. UV–vis absorption spectraere recorded using the double-beam in-time spectrophotome-

er (Philips-PU8750) and the temperature of the cell holder wasaintained at 25 ± 0.1 ◦C. The measurements were carried out in

gyptian Petroleum Research Institute. All spectra were recordedith 2 cm−1 resolution at an angle of incidence 80◦ relative to the

urface normal using the Fourier Transform Infrared, (Nicolet IS-0). The FTIR were carried out in Egyptian Petroleum Researchnstitute. Transmission Electron Microscope (TEM) (JEOL JEM-2100perating at 200 kV attached to a CCD camera) was used. Theeasurements were carried out in Egyptian Petroleum Research

nstitute.

.3. Synthesis of 1,4-bis(8-mercaptooctyloxy)benzene

Synthesis of 1,4-bis(8-mercaptoalkyloxy)benzene was carriedut in two steps as shown in Scheme 1 [45]. 1,4-bis(8-romooctyloxy)benzene was prepared by the reaction of 1 mol ofydroquinone with 2 mol of 1,8-dibromopentane in the presencef sodium ethoxide under nitrogen flow to prevent oxidation ofydroquinone [45].

A mixture of 1,4-bis(8-bromooctyloxy)benzene (3.8 g, 7.7 mmol,

equiv.) and thiourea (3 g, 30.9 mmol, 4 equiv.) in 50 mL ofimethylformamide was heated to 100 ◦C under nitrogen flow for

h to produce the isothiouronium salt. The basic hydrolysis ofsothiouronium salt was performed, under a nitrogen atmosphere,

)benzene with the gold nanoparticles (GNPs).

by heating the mixture of isothiouronium salt and solution of NaOH(1.6 g, 32.6 mmol, 3 equiv.) in 10 mL of water at 110 ◦C for 2 h. Then,the mixture was acidified with dilute H2SO4 to obtain 1,4-bis(8-mercaptooctyloxy)benzene (I) [46] (Scheme 1).

2.4. Preparation of the gold nanoparticles (AuNPs)

In this study, gold nanoparticles were synthesized as men-tioned in the literature [47]. Briefly, gold chloroauric acid salt(HAuCl4) was used as gold salt in the experiments. Trisodiumcitrate (Na3C6H5O7·2H2O) was used as reducing agent. Gold saltwas mixed and boiled to the boiling point (97.5 ◦C) at prepared con-centration to start the synthesis reaction. Gold salt was yellow atthat time. After adding the prepared sodium citrate to the solution,sodium citrate turned to citric acid. At that stage yellow colouredsolution suddenly became transparent and colourless. It changed toblack and after than slowly to wine red [48]. The solution was kepton mixing at all the colour changing stages and it was kept on hotsurface. When the colour change was ended, gold particle solutionwas kept on mixing and kept in the refrigerator at 4 ◦C. Gold saltsynthesis was concluded at this point.

2.4.1. Preparation of the cross linked gold nanoparticle networkGold nanoparticle solution (20 mL) was mixed with

5 mL saturated solution of the synthesized 1,4-bis(8-mercaptooctyloxy)benzene (I) in DMF and stirred continuouslyfor 24 h till the colour change. The resulting solution was usedfor ultra violet experiments and TEM image then the resultingsolution was centrifuged at 12,000 rpm to obtain the precipitate ofthiol-coated gold nanoparticles in Scheme 2. The precipitate waswashed with cold water to remove any unreacted materials. Thewashed precipitate was used for Infrared experiments and XRDanalysis.

2.5. Preparation of the 1,4-bis(8-mercaptooctyloxy)benzene(I)-modified screen-printed electrodes

Modified SPEs were printed in arrays of six couples consist-ing of the working electrodes (each 5 mm × 35 mm) following theprocedures previously described [49–55].

A polyvinyl chloride flexible sheet (0.2 mm) was used as a sub-strate which was not affected by the curing temperature or theink solvent and easily cutted by scissors. The working electrodeswere prepared depending on the method of fabrication. The work-

ing electrode was printed using homemade carbon ink (preparedby mixing 2.5–10 mg ionophore, 400 �L of paraffin oil, 1.25 g ofpolyvinyl chloride 8% and 0.75 g carbon powder). A modified pastewas prepared in a similar fashion, except that the graphite powder

T.A. Ali et al. / Sensors and Actuators B 191 (2014) 192– 203 195

Table 1Optimization of the screen-printed ingredients.

Composition Electrode characteristics

No. GP (mg) PO (mg) I (mg) PVC (mg) GNP (�L) Slope (mV decade−1) LR (mol L−1) R

I 66 34 – 100 – 10.66 ± 2.34 1.0 × 10−5 to 1.0 × 10−2 0.902II 64 32 4 100 – 12.33 ± 1.68 5.0 × 10−6 to 1.0 × 10−2 0.923III 67 28.5 4.5 100 200 18.98 ± 0.74 1.0 × 10−9 to 1.0 × 10−2 0.970IV 66.5 30.5 3 100 200 15.86 ± 1.21 1.0 × 10−8 to 1.0 × 10−2 0.964V 65 29 6 100 – 14.19 ± 2.32 1.0 × 10−6 to 1.0 × 10−2 0.958VI 63 32 5 100 200 19.22 ± 1.48 6.5 × 10−10 to 2.5 × 10−2 0.990VII 65 28 7 100 – 16.73 ± 1.50 5.5 × 10−7 to 1.0 × 10−2 0.988VIIIa 67 27.5 5.5 100 200 19.95 ± 0.97 3.2 × 10−10 to 3.5 × 10−2 0.999

200

−9 −2

–

wgTfewtd

2

2

ctaw

2

i0ttoeipwt

2

a

A

ttCoaticw

l

IX 64 28 8 100

Xa 63 29.5 7.5 100

a VIII = GNPs-SPE and X = MSPE.

as mixed with a desired weight of (I) ligand and (I) assembled onold nanoparticles to get different composition as given in Table 1.hey were printed using homemade carbon ink and cured at 50 ◦Cor 30 min. A layer of an insulator was then placed onto the printedlectrodes, leaving a defined rectangular shaped (5 mm × 5 mm)orking area and a similar area (for the electrical contact) on

he other side. Fabricated electrodes were stored at 4 ◦C and usedirectly in the potentiometric measurements.

.6. Potentiometric electrodes and measurements

.6.1. Effect of pHA series of pH solution ranging from 2 to 9 were prepared at

onstant ion concentration, i.e. (1 × 10−3 mol L−1). The pH varia-ions were brought about by the addition of dilute acid (HCl) andlkali (NaOH) solution. The value of electrode potential at each pHas recorded and was plotted against pH.

.6.2. The response timeThe method of determining response time in the present work

s being outlined as follows. The electrode was first dipped in a.1 mol L−1 solution of the ion concerned and immediately shiftedo another solution of the same ion (10 fold higher in concentra-ion), and the solutions were continuously stirred. The potentialf the solution was read at zero second, just after dipping of thelectrode in the second solution and subsequently recorded at thentervals of 4 and 7 s for GNPs-SPE and MSPE, respectively. Theotentials were then plotted versus the time. The time duringhich the potentials attain constant value represents the response

ime of the electrode.

.6.3. Electrodes system and emf measurementsAll emf measurements were carried out with the following cell

ssembly:

g/AgCl|Samplesolution|MSPEorGNPs-SPE

All the emf observations were made relative to a double junc-ion Ag/AgCl electrode with a pH/mV meter. The performance ofhe GNPs-SPE electrodes was investigated by measuring the emf ofe(III) nitrate solution which is prepared with a concentration rangf 10−1 to 10−10 mol L−1 by serial dilution. Each solution was stirrednd the potential reading was recorded when it became stable, andhen plotted as a logarithmic function of Ce(III) activity. The activ-ties of metal ions were based on the activity coefficient (�), whichalculated from the modified form of the Debye–Hückel equation,

hich is applicable to any ion:

og � = −0.511Z2

((�)1/2

((1 + 1.5(�)1/2) − 0.2�)

)

22.12 ± 1.64 4.5 × 10 to 1.0 × 10 0.98017.04 ± 1.91 9.5 × 10−8 to 1.0 × 10−2 0.994

where � is the ionic strength and Z is the valency. All measurementswere carried out at 25 ± 0.1 ◦C.

5 mL aliquots of 10−10 to 10−1 mol L−1 Ce(III) solution or watersamples spiked with different concentrations of Ce(III) ion weretransferred into 25 mL beaker at 25 ◦C. The pH of each solution wasadjusted to 4 using NaOH/HCl, followed by immersing the elec-trode GNPs-SPE and MSPE in conjugation with Ag/AgCl referenceelectrode in the solution. The potential change was plotted againstthe logarithm of Ce(III) concentration from which the calibrationcurves were constructed. The method was repeated several timesto check the accuracy and reproducibility of the proposed method.The electrodes were washed several times with de-ionized waterbetween each measurement.

3. Results and discussion

Self-assembled monolayer (SAM) provides a simple method ofelectrode functionalization by strong chemisorption of free anchorgroups such as thiols, disulfides, amines, silanes or acids, and hasfound potential applications such as in sensors, corrosion inhibi-tion, wetting control, and other biomolecular electronic devices.

Due to its simplicity, versatility, and having high level order on amolecular scale, SAM has been widely used in fabrication of highlyorganized and homogenized modified screen-printed electrodeinterface. With this regard, many studies have been devoted tofabricate electrochemical sensors and biosensors by SAM technol-ogy. The material frequently used for the SAM preparation is goldbecause of its good affinity to sulfur and inertness. Gold nanopar-ticle modifications could largely increase the immobilized amountof S-functionalized compounds and enhance the Au–S bond andstability of SAMs layer.

On the other hand, gold nanoparticles have received muchattention when compared to other metal nanoparticles mainly dueto its ease in preparation, high stability and their shape and sizedependent catalytic activity. It could not only increase the assemblyamount of organothiol but also influence the structure and stabilityof SAM.

For the first time, application of a membrane composed of goldnanoparticles decorated with complexing ligand for potentiomet-ric sensing is described by Woznica et al. [20]. Gold nanoparticlesdrop cast from a solution form a porous structure on a substrateelectrode surface. Sample cations can penetrate the gold nanopar-ticles layer and interact with ligand acting as a charged ionophore,resulting in Nernstian potentiometric responses. Anchoring ofcomplexing ligand on the gold surface abolishes the necessity ofionophore application. Moreover, it opens the possibility of prepa-

ration of potentiometric sensors using chelators of significantlydifferent selectivity patterns further enhanced by the absence ofpolymeric membrane matrix. This was clearly seen, for example, forgold nanoparticles stabilizing the applied ligand–dithizone–thiol

196 T.A. Ali et al. / Sensors and Actuators B 191 (2014) 192– 203

merca

cct

ipoctltaipmp

3n

taa

Fig. 1. FTIR-spectrum of (a) 1,4-bis(8-

onformation leading to high potentiometric selectivity towardopper ions, much higher than that of ionophores typically usedo induce selectivity for polymeric ion-selective membranes.

In a similar manner to the previously reported article [20], its expected that the thiol form of surfactant molecule would bereferably bound to the gold surface, resulting in stabilizationf this thiol form. Thus, for gold modified with thiol surfactant,omplexation of Ce(III) ion able to react with the thiol form ofhis compound is expected, increasing the selectivity of the thioligand due to conformational stabilization of the complexing reac-ive center. This system is potentially interesting for analyticalpplications, including potentiometric sensing. Based on thesenvestigations, we used gold nanoparticles and mercapto com-ound self-assembled on gold nanoparticles for construction ofodified carbon paste electrode as ion selective electrodes for

otentiometric determination of Ce(III).

.1. Self assembly of 1,4-bis(8-mercaptooctyloxy)benzene on goldanoparticles (GNPs)

A unique property of the gold nanoparticles encapsulated withhiol surfactants is that they can be handled and characterized as

simple chemical compound. Therefore, NMR, UV–vis, and FT-IRre useful tools to characterize them. The self-assembling of the

ptooctyloxy)benzene (I) and GNPs-(I).

synthesized 1,4-bis(8-mercaptooctyloxy)-benzene on GNPs wasstudied using FTIR, UV, TEM and XRD techniques.

3.1.1. Fourier transform infrared spectroscopy (FTIR)The FTIR spectra of the free thiol (1,4-bis(8-mercapto-

octyloxy)benzene) and it’s nanostructure with gold nanoparticlesare given in Fig. 1. Upon comparison, it is clear that some peaks areaffected and the others remain unchanged upon adding nanopar-ticles. The similarity indicated that thiol compound is a part of thecomposite and the gold nanoparticles are effectively stabilized withthe synthesized thiol surfactants (Fig. 1) [48].

To understand how the thiol molecule passivates metalnanoparticles is therefore an interesting issue. An IR transmissionspectrum of the 1,4-bis(8-mercaptooctyloxy)benzene-passivatedgold nanoparticles dispersed in a KBr pellet is shown in Fig. 1. Forsimplicity, only the spectra in the region from 4000 to 400 cm−1 aregiven, which are mainly associated with ring-stretching and hydro-gen bending modes. For comparison, a transmission spectrum ofthe 1,4-bis(8-mercaptooctyloxy)benzene molecule in KBr is alsogiven. Significant differences between these two classes of spectracan be seen, indicating that the molecules experience a structural

change on adsorption.

The ring stretching combined with the hydrogen in-planewagging modes is predicted to give rise to a medium-intensitypeak around 1657 cm−1, a strong peak at 1510 cm−1 and a

ctuators B 191 (2014) 192– 203 197

raTa1Harbawntaiar

hnn

3

GtiU(tpaagabfign

3

wga

T.A. Ali et al. / Sensors and A

elatively weak peak near 1466 cm−1 compared with 1621, 1511nd 1466 cm−1, respectively, for the thiol and its nanostructure.wo strong peaks are found near 1110 and 826 cm−1, which aressociated with the hydrogen in-plane rocking, and are seen at109 and 824 cm−1 for the thiol and its nanostructure, respectively.ydrogen out-of-plane bending is found around 769 cm−1 but isctually observed at 766 cm−1 for the thiol and its nanostructure,espectively. FTIR spectrum of 1,4-bis(8-mercaptooctyloxy)-enzene showed the characteristic bands for alkyl part at ∼2935nd ∼2854 cm−1 for asymmetric and symmetric stretching (CH)hich are found at 2927 and 2587 cm−1 for thiol-passivatedanoparticles, respectively. The bands at ∼728 and 723 cm−1 forhe thiol and thiol-passivated nanoparticles, respectively, can bessigned for –(CH2)n– rocking mode. The SH and C–O stretch-ng vibration bands were found at 2062 and ∼1236 and 2060nd 1235 cm−1 for the thiol and thiol-passivated nanoparticles,espectively.

From Fig. 1, it is also evident that the thiol spectrum shows aigh degree of similarity with the spectrum of the thiol-passivatedanoparticles. This implies that the thiol species adsorbed to theanoparticles are in a configuration similar to that of the thiol form.

.1.2. Ultraviolet absorption measurements (UV)To have a clear picture about interactions between Ce(III) and

NPs with mercapto compound, UV–vis technique was used. Sincehiol surfactant part in ionophore is common and plays the mostmportant role in interaction with Ce(III) and GNPs, we just presentV–vis studies related to 1,4-bis(8-mercaptopentyloxy)benzene

I) as a sample. The UV–vis absorption spectra showed that inhe absence of nanoparticle, the –SH groups in mercapto com-ound bonded to Ce(III) ions and a charge transfer peaks appeart ∼520 nm, and this peak disappeared when the nanoparticle wasdded to the thiol solution (Fig. 2). This suggests that the –SHroups are bonded to gold nanoparticles [45]. The band observedt 250 nm may be attributed to the �–�* transition within theenzene ring of thiol surfactant. The appearance of this peak con-rms the self assembling of the synthesized thiol compound on theold nanoparticles and the formation of nanoshells with the goldanoparticles.

.1.3. Transmission electron microscope (TEM)

The size distribution of the nanoparticles was characterized

ith electron microscopy. Representative TEM micrographs of theold nanoparticles before and after the addition of thiol surfactantre shown in Fig. 3. It is obvious from this figure that the prepared

Fig. 3. TEM image of the individual GNPs and of nanostruct

Fig. 2. UV–vis absorption spectra of GNPs and GNPs-1,4-bis(8-mercaptooctyoxy)benzene (I).

gold nanoparticles almost take spherical forms and the size ofmost of the gold particles is about 12–20 nm. After addition ofthiol surfactant (1,4-bis(8-mercaptooctyloxy)benzene) as shownin Fig. 3, the gold nanoparticles also exhibited similar particlediameter but exists more dispersed than gold nanoparticles alonedue to presence of alkyl chain which decrease the aggregation ofgold nanoparticles in addition to the increase of the alkyl chainlead to decrease the aggregation of particles and increased thedispersion in solutions.

It is clear that the synthesized surfactant shows the best stabi-lization of the AuNPs as shown in Fig. 3. This is related to the factthat the adsorption of surfactant molecules lowers the free energyof the surface and, therefore, the reactivity of the particles. The ratioof surfactant to metal precursor can control the size distribution ofthe nanoparticles. The steric bulk of the surfactants provide a phys-ical barrier that prevents the metal surfaces from contacting eachother directly. They can also change the surface charge of a clusterand thus change its stability toward aggregation. The combinationof the energetic stabilization of the metal surface by the surfac-tant, the consequences of charge–charge interactions, and the stericrepulsion between particles prevents the system from forming

aggregates. According to the above TEM results the stabilizationof the gold nanoparticles size is clear after the self assemblingof the surfactant molecules under investigation as shown inFig. 3.

ure of GNPs-1,4-bis(8-mercaptooctyloxy)benzene (I).

198 T.A. Ali et al. / Sensors and Actuators B 191 (2014) 192– 203

esized

3

fa2Atrsi3ttm

matbbttstcnosootG

3

wcmmsdiagrTtw

the Ce(III) ion concentration in solution, over a concentration rangefrom 1.0 × 10−2 to 1.0 × 10−9 mol L−1. The actual potential versustime traces is shown in Fig. 6. As can be seen, in whole concentra-tion range the electrode reaches its equilibrium response in a very

Table 2Response characteristics of GNPs-SPE and MSPE potentiometric sensors.

Parameter Response characteristics

GNPs-SPE MSPE

Slope (mV decade−1) 19.95 ± 0.97 17.04 ± 1.91Correlation coefficient, r 0.999 0.994Lower detection limit (mol L−1) 3.2 × 10−10 9.5 × 10−8

Response time (s) 4 7Working pH range 2.8–8.5 3.5–7.5Usable range (mol L−1) 3.2 × 10−10 to 1 × 10−1 1 × 10−7 to 1.0 × 10−2

SD of slope (mV decade−1) 0.105 0.572

Fig. 4. XRD spectra GNPs and nanostructure of synth

.1.4. X-Ray diffraction (XRD)In the current study, the modification to nanoparticle sur-

aces is probed by measuring the XRD spectra of the unmodifiednd modified GNPs (Fig. 4). The data show diffraction peaks at� = 38.2◦, 44.4◦, 64.6◦ and 77.5◦, which can be indexed to Au(1 1 1),u(2 0 0), Au(2 2 0) and Au(3 1 1), planes of pure gold. It confirmed

hat the main composition of the nanoparticles was gold. Fig. 4epresents the powder XRD pattern of the nanostructure for theynthesized 1,4-bis(8-mercaptooctyloxy)benzene with the GNPs. Its also found that there are some peaks at 2� = 18.5◦, 21◦, 24.5◦and4.5◦ appeared in addition to the decrease in the intensity ofhe peaks of GNPs which confirm the formation of nanoshell ashe result of the self-assembling of the synthesized 1,4-bis(8-

ercaptooctyloxy)benzene on the GNPs.Gold nanoparticles can be functionalized with a wide range of

aterials. Polymers such as polyvinylpyrrolidone (PVP) and tanniccid are capping agents typically used to stabilize gold nanopar-icles. Binding molecules to a gold surface can be accomplishedy physical adsorption or by taking advantage of extremely sta-le thiol-gold bonds. Particles can be functionalized with moleculeshat ‘flip’ the surface charge of the negatively charged gold nanopar-icles to a positively charged surface. The gold nanoparticles haveurface area/volume ratios that are extremely high. Thus, many ofhe physical properties of the gold nanoparticles such as solubilityhange the optical properties and stability are dominated by theature of the nanoparticle surface. The intensity of the XRD spectraf the modified gold nanoparticles is relatively lowered in compari-on with the unmodified one, reflecting the successful modificationf the surface. Moreover, the noisy background appeared in the casef the modified GNPs (due to the interference of the substrate andhe modifier as well) decreased the sharpness of the XRD peaks ofNPs.

.2. Composition and characteristics of the electrodes

Preliminary experiments showed that screen-printed electrodehich do not contain a modifier have low response towards

erium ion. For this purpose, a new thiol surfactant of 1,4-bis-(8-ercaptooctyloxy)benzene (I) was synthesized and investigated asodifier for the present electrode. It is well known that the sen-

itivity and selectivity obtained for a given ion selective electrodeepends not only on the nature of ionophore used, but also signif-

cantly on the paste composition. The influence of modifier ligandmounts in the screen-printed electrodes response was investi-ated. However, unmodified screen-printed electrodes do not have

esponse for cerium ions. On the other hand, as can be seen fromable 1, increasing levels of thiol surfactant in the paste composi-ion resulted in increasing the screen printed electrodes responseshere they display larger slopes and lower detection limits. In

1,4-bis(8-mercaptooctyloxy)benzene (I) with GNPs.

addition, the electrodes without gold nanoparticles gave a limitedworking concentration range, relative high detection limit and lowsensitivity (MSPE) if it is compared with the electrodes having thiolsurfactant assembled on gold nanoparticles. It is probably due to thepower of gold to self-assembled the ionophore from –SH group ofthiol surfactant compound used as modifier (GNPs-SPE). Besides,the gold nanoparticles may lead to the expanding of the surface ofpaste by fabrication of three dimensional nanostructures; as wellas diminish of the Ohmic resistance of the paste.

It can be seen from Table 2 that the GNPs-SPE-(I) (GNPs-SPE)and modified screen-printed electrodes (MSPE) with the graphitepowder/paraffin oil/(I) weight percent ratio of 67/27.5/5.5 with200 �L GNPs and 63/29.5/7.5 were selected as the one with theoptimal composition of the paste for GNP-SPE and MSPE, respec-tively. Both electrodes exhibit a Nernstian slope of 19.95 ± 0.97 and17.04 ± 1.91 mV decade−1 in a wide cerium ion concentration rangefrom 3.25 × 10−10 to 1.0 × 10−1 and 1.0 × 10−7 to 1.0 × 10−2 mol L−1

for GNPs-SPE and MSPE, respectively (Fig. 5). The LOD for the elec-trodes was found to be 3.25 × 10−10 and 9.5 × 10−8 mol L−1 forGNPs-SPE and MSPE, respectively. This LOD was calculated whenthe linear regions of the calibration graphs were extrapolated tothe baseline potentials.

3.3. Response time

The average time required for the Ce(III) ion-selective electrodeto reach a potential within ±1 mV of the final equilibrium valueafter successive immersion of a series of cerium ion solutions, eachhaving a 10-fold difference in concentrations, was measured. Inthis study, the practical response time was recorded by changing

Intercept (mV) 115.17 ± 0.22 170.62 ± 1.10Life time (months) 7 5Accuracy (%) 99.96 98.99Precision (%) 0.67 0.91

T.A. Ali et al. / Sensors and Actuators B 191 (2014) 192– 203 199

F(

srdaouc

3

GmMcFn

3

wi0stt

ig. 5. Calibration curve for (a) Ce(III)-screen-printed based on GNP-(I) modifierGNPs-SPE) and (b) Ce(III)-screen-printed based on (I) modifier (MSPE).

hort time which is found to be 4 and 7 s for GNPs-SPE and MSPE,espectively. The results, thus, obtained are indicative of a rapidiffusion achievement of equilibrium between the aqueous layernd the paste sensor, and rapid complex formation and exchangef ions in the paste. The sensing behaviour of the GNP-SPE remainednchanged when the potentials recorded either from low to highoncentrations or vice versa.

.4. Life time

The proposed electrodes show fairly a high lifetime over otherNP-SPE sensors [49–55] and could be used for more than 7 and 5onths without any measurable divergence for the GNPs-SPE andSPE, respectively. The effect of paste duration on the response

haracteristics of Ce(III) selective electrode is shown in Table 2 andig. 7. As can be seen, the response of the SPE electrode remainedearly unchanged over very long time.

.5. Effect of pH

The effect of pH of test solution on the response of electrodesere investigated using 1.0 × 10−3 and 1.0 × 10−5 mol L−1 cerium

on solutions. The pH adjustments in solutions were made with

.01 mol L−1 hydrochloric acid or sodium hydroxide solutions. Ashown in Fig. 8 the electrodes responses are independent of pH inhe range of 2.8–8.5 and 3.5–7.5 for GNPs-SPE and MSPE, respec-ively. However, the observed decrease in the potential at higher

Fig. 6. Dynamic response of (a) GNPs-SPE and (b) MSPE obtained by successiveincrease of Ce(III) ion concentration.

pH values could assigned to the formation of some hydroxy com-plexes of Ce(III) ions in solution. On the other hand, at pH valueslower than 2.8, the electrode potential rises. This is probably dueto simultaneous response of the electrodes to H3O+ and Ce(III) ionsor could be due to the protonation of the ion carrier [49–55].

3.6. Effect of temperature

Calibration graphs were constructed as previously described attest solution temperatures of 10, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65and 70 ◦C. The slope, usable concentration range and response timeof the electrodes corresponding to each temperature are reportedin Table 3. It is obvious that the electrodes gave a good Nerns-tian response in the temperature range from 10 to 70 ◦C. FromTable 3, the standard electrode potentials (E◦) were determined,as the intercepts of the calibration graphs at p[Ce] = 0, and usedto obtain the isothermal temperature coefficient (dE◦/dT) of theelectrode by aid of the following equation:

◦ ◦(

dE◦ )

E = E (25) +

dT(T − 25)

where Eo(25) is the standard electrode potential at 25 ◦C, the slope

of the straight-line obtained represents the isothermal coefficient

200 T.A. Ali et al. / Sensors and Actuators B 191 (2014) 192– 203

FS

ofittN

3

asa[

TP

ig. 7. The lifetime of (a) Ce(III)-Screen-printed based on GNP-(I) modifier (GNPs-PE) and (b) Ce(III)-Screen-printed based on (I) modifier (MSPE).

f the electrodes which are found to be 0.000196 and 0.00031 V/◦Cor GNPs-SPE and MSPE, respectively. The values of the obtainedsothermal coefficients of the electrodes indicated that the elec-rodes had fairly high thermal stability within the investigatedemperature range and showed no deviation from the theoreticalernstian behaviour.

.7. Potentiometric selectivity

Potentiometric selectivity coefficient shows the accuracy of

pplication of each electrode for evaluation of analytes content andtrongly depends to analyte content and method of evaluation. Sep-rate solution method (SSM) and fixed interference method (FIM)49–55] has been used for evaluation of selectivity coefficient of

able 3erformance characteristics of Ce(III) screen-printed electrode at different temperatures.

Temperature (◦C) Slope (mV decade−1) Slope (mV decade−1) Usable conce(mol L−1)

GNPs-SPE MSPE GNPs-SPE

10 17.97 14.50 9.5 × 10−9 to

20 18.52 15.99 3.2 × 10−10 to30 18.98 17.04 3.2 × 10−10 to40 19.21 16.26 3.2 × 10−10 to50 19.75 16.88 3.2 × 10−10 to60 20.01 17.00 3.2 × 10−10 to70 20.55 12.75 9.5 × 10−9 to

Fig. 8. Effect of pH of test solutions on the response of (a) GNPs-SPE and (b) MSPE.

proposed electrodes. In SSM, the Nicolskii coefficient determinedby comparing potential of two solutions, containing a salt of theprimary and interfering ion only, while the selectivity coefficient isdetermined using following Eq. (1):

ln KSSMIJ =

(zIF[E2 − E1]

RT

)− ln a

(1 −

(zI

zJ

))(1)

In Eq. (1) it is considered that aI = aJ. E1 and E2 are the responseof the electrode to main and interfering ion, respectively. In the

fixed interference method (FIM), selectivity coefficients of the pro-posed Ce(III) ion selective electrodes were evaluated graphicallyfor solution containing a fixed concentration of the interferingions (1.0 × 10−3 mol L−1) and different amounts of the cerium ion

ntration range Usable concentration range(mol L−1)

E◦elec. (mV) E◦

elec. (mV)

MSPE GNPs-SPE MSPE

1 × 10−2 1 × 10−6 to 1 × 10−2 −238 −341 3.5 × 10−2 1 × 10−7 to 1 × 10−2 −229 −330 3.5 × 10−2 9.5 × 10−8 to 1.0 × 10−2 −216 −300 3.5 × 10−2 9.5 × 10−8 to 1.0 × 10−2 −202 −280 3.5 × 10−2 9.5 × 10−8 to 1.0 × 10−2 −195 −260 3.5 × 10−2 9.5 × 10−8 to 1.0 × 10−2 −180 −2221 × 10−2 1 × 10−6 to 1 × 10−2 −170 −195

T.A. Ali et al. / Sensors and Actuato

Table 4Selectivity coefficients of various ions using GNPs-SPE and MSPE sensors.

Interfering ions KMPBA,B KMPB

A,B KFIMA,B KFIM

A,B

GNPs-SPEa MSPEa GNPs-SPEb MSPEb

Ca2+ 2.5 × 10−7 5.6 × 10−6 4.0 × 10−7 6.1 × 10−6

Cd2+ 8.2 × 10−7 7.5 × 10−6 3.3 × 10−7 5.3 × 10−5

Cu2+ 7.9 × 10−8 5.2 × 10−7 4.7 × 10−7 3.4 × 10−6

Mg2+ 7.0 × 10−8 3.1 × 10−7 9.2 × 10−7 5.4 × 10−6

Ni2+ 6.6 × 10−7 7.4 × 10−6 3.6 × 10−5 7.1 × 10−4

Co2+ 4.1 × 10−8 6.3 × 10−7 3.9 × 10−7 3.2 × 10−6

Hg2+ 8.5 × 10−7 3.1 × 10−6 5.7 × 10−6 2.6 × 10−5

Cr3+ 4.7 × 10−6 3.5 × 10−4 3.6 × 10−5 5.5 × 10−3

Al3+ 9.4 × 10−7 3.7 × 10−5 4.8 × 10−5 8.8 × 10−4

Fe3+ 4.0 × 10−5 2.1 × 10−3 6.12 × 10−1 4.65 × 10−2

Na+ 3.21 × 10−6 6.12 × 10−4 6.22 × 10−5 9.01 × 10−3

K+ 6.33 × 10−5 8.21 × 10−3 5.23 × 10−4 2.29 × 10−4

NO3− 6.89 × 10−6 5.51 × 10−5 3.28 × 10−4 4.24 × 10−3

SO4−2 9.85 × 10−7 7.37 × 10−5 6.43 × 10−4 2.43 × 10−5

Br− 8.03 × 10−6 5.21 × 10−5 7.32 × 10−4 3.24 × 10−3

I− 4.24 × 10−2 1.90 × 10−1 0.87 × 10−2 8.3 × 10−1

Cl− 4.41 × 10−5 4.22 × 10−3 6.0 × 10−4 0.9 × 10−2

ac

K

acrmst

lsmti

tnrrraodneoctmrltbe

irw

was 99.90 ± 0.53 and 98.90 ± 0.89 for GNPs-SPE and MSPE, respec-tively. These results were closely in agreement with those obtained

a Selectivity coefficients found by matched potential method.b Selectivity coefficients found by fixed interference method.

nd the results obtained are shown in Table 4. The selectivityoefficients KFIM

IJ is determined by applying the following equation:

FIMIJ = ai(DL)

ai(BG)Zi/Zj

From Table 4, it can be seen that the proposed electrodesre highly selective towards Ce(III) ion with respect to variety ofations. There is a satisfactory agreement between the two sets ofesults obtained by the SSM and FIM methods. The properties ofetal nanoparticles strongly depend on their size and shape, so a

ynthetic procedure controlling the growth and the morphology ofhe nanoparticles must be followed.

Nanomaterials show higher activity, capability and requireower quantities due to their high specific surface areas and nano-ized effects. Although metal is a poor catalyst in bulk form butetal nanoparticles can exhibit excellent catalytic activity due to

heir relative high surface area-to-volume ratio and the differencen their properties with the bulk material.

The comparison of slope and linear range of the proposed elec-rodes according to Duncan’s separation as reference method showo significant difference in slope, while linear range of electrodeesponse greatly enhanced due to incorporation of nanomate-ial into the screen-printed matrices. These achievements can beelated to the increase in the number of surface conductive atomnd higher surface of graphite matrices that permit high numberf transferred molecules or ions to the surface of electrode throughiffusion or convection pathways [49–55]. Our probable mecha-ism for improvement in the response properties of the proposedlectrode (GNPs-SPE) in the presence of thiol surfactant assembledn gold nanoparticles may be obtained through its affect on theharge density of ionophore that lead to increase in its interac-ion with Ce(III) ions. After all, as it is evident from Table 4 that

ost of the interfering ions do not have significant effect on theesponse of the modified electrode (MSPE), and most of them showow values of selectivity coefficients, indicating no interference inhe performance of the modified screen-printed electrodes assem-ly. Generally, the values in Table 4 indicate that the proposedlectrodes can act as selective toward cerium ions satisfactorily.

An electrode containing sodium tetraphenylborate is preparedn order to check the interference of many anions such as chlo-ide, bromide, iodide, sulphate and nitrate anions. The electrodeas found to have selectivity coefficient values of 4.60 × 10−5,

rs B 191 (2014) 192– 203 201

6.3 × 10−4, 2.25 × 10−3, 5.24 × 10−4 and 7.66 × 10−5 for chloride,bromide, iodide, sulphate and nitrate anions, respectively.

The effect of redox sensitivity on the electrode performance wasstudied where its selectivity towards Ce(IV) ion is investigated. Theelectrode was found to have selectivity coefficient of 0.92 whichindicates that Ce(IV) can interfere seriously with the determina-tion of Ce(III) ion. Also the effect of Fe(III)/Fe(II) and Ce(IV)/Ce(III)redox systems were studied. Solutions with different ratios of 1:1,1:3, 1:5 and 1:10 of Fe(III):Fe(II) and Ce(III):Ce(IV), respectively,were prepared and their interference effect is studied. It is foundthat the selectivity coefficient of the GNP-SPE electrode towardsFe(III)/Fe(II) redox couple is found to be ranged from 3.56 × 10−2 to5.12 × 10−4, which indicate that this redox couple has no effect onthe selectivity of the electrode towards Ce(III) ions. In addition, theelectrode is found to have selectivity coefficient values of 0.22–0.79which indicates that Ce(IV) is seriously interfere.

3.8. Analysis of cerium in water sample

To assess the applicability of the electrodes to real sample analy-ses, they were applied to the determination of cerium ions in spikedwater samples.

The analyses were performed by using the direct potentiometriccalibration method. The results given in Table 5 revealed that theyare consistent with those obtained applying inductively coupledplasma atomic absorption spectrometry (ICP-AES). This validatesthe applicability of the GNP-SPE and MSPE for the selective deter-mination of cerium ions in the analyzed environmental samples.

3.9. Method validation

The method was validated with respect to linearity, accuracy,precision, repeatability, robustness and ruggedness accordance toICH guidelines [50].

Under the optimal experimental conditions, linear relationshipsexist between the electrode potential (mV) and the logarithm ofthe corresponding concentration of Ce(III) (Fig. 5). The regressiondata, correlation coefficients (r), and other statistical parameter arepreviously listed in Table 2.

The accuracy of the proposed GNP-SPE and MSPE sensors wasinvestigated by the analysis of Ce(III) ion in its spiked watersamples using standard addition method. The results obtained inTable 2 showed mean percentage recoveries of 99.96 ± 0.42 and98.99 ± 1.01 for GNPs-SPE and MSPE, respectively, revealing goodaccuracy for the determination of cerium in its different samples.

The precision of the proposed potentiometric method was mea-sured as percentage relative standard deviation (RSD%). It wastested by applying the proposed method for determination offive replicates of the investigated Ce(III) ion in its spiked watersamples. The RSD% values for the repeated determinations werefound to be 0.67% and 0.91% for GNPs-SPE and MSPE, respectively,(Table 2). The RSD values are less than 1% indicating good preci-sion.

The robustness of the proposed method was tested by investi-gating the effect of using acetate buffer with pH 4 to introduce smallchanges in pH during the analysis of the tested Ce(III). The proposedGNP-SPE and MSPE sensors remained unaffected by this small vari-ation in method parameters. The calculated percentage recovery

from standard cerium solutions. Also, the reproducibility and theruggedness of the proposed method were evaluated upon usinganother model of pH-meter (Jenway 3505). The obtained result was99.66 ± 0.33 and 98.87 ± 0.64 for GNPs-SPE and MSPE, respectively.

202 T.A. Ali et al. / Sensors and Actuators B 191 (2014) 192– 203

Table 5Determination of cerium ions in spiked water samples using GNPs-SPE and MSPE.

Samples [Ce(III)] (�g L−1)

GNPs-SPE MSPE ICP-AES

Added Found R.S.D. (%) Recovery (%) Found R.S.D. (%) Recovery (%) Found R.S.D. (%) Recovery (%)

1 5 4.92 0.634 98.4 4.89 0.875 97.8 4.81 1.076 96.27.5 7.47 0.345 99.6 7.42 1.110 98.93 7.39 1.092 98.53

10 9.99 0.135 99.9 9.95 1.004 99.5 9.90 0.929 99.00

2 5 4.97 0.453 99.4 4.94 0.648 98.8 4.89 0.894 97.87.5 7.51 0.785 100.13 7.48 0.982 99.73 7.52 1.034 100.26

10 9.98 0.941 99.8 9.96 1.007 99.6 9.91 1.011 99.1

3 10 10.02 0.078 100.2 9.98 0.563 99.8 9.92 0.982 99.212.5 12.49 0.092 99.92 12.47 0.487 99.76 12.44 0.896 99.5215 14.99 0.081 99.93 15.01 0.945 100.06 14.93 1.213 99.53

4 10 9.98 0.156 99.8 9.96 1.006 99.6 10.02 1.096 100.212.5 12.49 0.310 99.92 12.48 0.678 99.84 12.45 1.056 99.615 15.05 0.078 100.33 15.00 1.011 100.00 14.48 1.005 96.53

5 3 3.04 0.235 101.33 2.95 0.678 98.33 2.89 1.004 96.336 5.98 0.176 99.66 5.95 0.734 99.16 5.94 1.023 99.009 9.00 0.098 100.00 8.97 0.973 99.66 8.93 0.986 99.22

6 3 2.99 1.008 99.66 2.94 0.587 98.00 2.82 1.015 94.0082

4

aemca9frTmptoiu

R

[

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6 6.01 0.264 100.16 5.99 8.97 0.476 99.66 9.0

. Conclusions

The new thiol surfactant compound was used for self-ssembled-gold nanoparticles modified screen-printedlectrodes GNP-SPE and applied for potentiometric deter-ination of cerium ion. The proposed sensors have good

haracteristics such as low detection limit of 3.25 × 10−10

nd 9.5 × 10−8 mol L−1, wide concentration range from.30 × 10−10 to 1.0 × 10−1 and 1 × 10−7 to 1.0 × 10−2 mol L−1,ast response time of ∼4 and 7 s for GNPs-SPE and MSPE,espectively, and good selectivity coefficient for many cation.hese values are better than those for other cerium(III) potentio-etric sensors previously published in the literature [23–28]. The

otentiometric responses of these electrodes are independent ofhe pH of the test solution in the pH range 2.8–8.5. The resultsbtained using the proposed sensors for determination of Ce(III)ons in spiked water samples are consistent with those obtainedsing the recommended ICP-AES method.

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ctuato

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from Chemistry Department, Faculty of Science, Zagazig University. He worked as

T.A. Ali et al. / Sensors and A

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Biographies

Tamer Awad Ali received his PhD degree in 2011 in Analytical Chemistry from Fac-ulty of Science, Ain Shams University, Egypt. He is a researcher in Egyptian PetroleumResearch Institute in Department of Petroleum Applications, nanotechnology Lab.In addition he is working in Supervisor Patent office and Member in ChemicalsDevelopment Services Center (CDSC). His research is focused on Ion Selective Elec-trode, Gold Nanoparticles-Carbon paste sensor, Gold nanoparticles-Screen-printedElectrodes, Carbon Nanotubes.

Gehad G. Mohamed, Professor of Inorganic and Analytical Chemistry, Faculty ofScience, Cairo University. His research work is focused on ion selective electrodes(screen printed, carbon paste, PVC electrodes) together with spectrophotometryand electro for drug analysis. Also his research work is interested in preparationand characterization of metal complexes with organic compounds or drugs.

E.M.S. Azzam had obtained his PhD from Cairo University, Beniswef branch in 1999.He is professor at applied surfactants laboratory, petrochemicals department, Egyp-tian petroleum research institute since 2009. His research interested in synthesis,evaluation and applications of surfactants and colloidal nanoparticles. He has manyof international papers published in this filed.

A.A. Abd El-Aal He obtained BSc in science (2004) MSc organic chemistry (2010)

Researcher Ass. in period 2007–2010 and work as Ass. Researcher from 2010 tillnow in Petrochemicals Department, Egyptian Petroleum Research Institute. His cur-rent research focuses on synthesis, evaluation and applications of surfactants andcolloidal nanoparticles.