The Application of Nafion Metal Catalyst Free Carbon NanotubeModified Gold Electrode: Voltammetric Zinc Detection in Serum

Wei Yue,a Adam Bange,b Bill L. Riehl,c Jay M. Johnson,c Ian Papautsky,d William R. Heineman*a

a Department of Chemistry, University of Cincinnati, Cincinnati, OH, 45221-0172, USAb Department of Chemistry, Xavier University, Cincinnati, OH, 45207, USAc SCNTE LTD P.O. Box 38, Alpha, OH 45301, USAd BiomicroSystems Labs, School of Electronic and Computing Systems, University of Cincinnati, Cincinnati, OH, 45221-0030, USA*e-mail: heinemwr@uc.edu

Received: April 5, 2013Accepted: June 24, 2013Published online: September 5, 2013

AbstractMetal catalyst free carbon nanotube (MCFCNT) whiskers were first used as an electrode modification material ona gold electrode surface for zinc voltammetric measurements. A composite film of Nafion and MCFCNT whiskerswas applied to a gold electrode surface to form a mechanically stable sensor. The sensor was then used for zinc de-tection in both acetate buffer solution and extracted bovine serum solution. A limit of detection of 53 nM was ach-ieved for a 120 s deposition time. The zinc in bovine serum was extracted via a double extraction procedure usingdithizone in chloroform as a zinc chelating ligand. The modified electrode was found to be both reliable and sensi-tive for zinc measurements in both matrices.

Keywords: Metal catalyst free carbon nanotubes, Nafion, Bovine serum, Dithizone, Anodic stripping voltammetry,Zinc

DOI: 10.1002/elan.201300158

1 Introduction

Zinc is an essential trace element that is required in thehuman body for catalyzing enzyme activity, transcriptionand protein functions [1]. Maintaining controlled levels ofzinc is critical for multiple physiological functions, includ-ing the immune system, insulin action, and anti-oxidantsystems [2,3]. Lack of zinc can cause immune dysfunc-tion, growth and reproduction problems; also, inflamma-tion and infection are associated with low zinc level inblood [2]. It has been widely accepted that normal zinclevel is necessary to maintain a healthy immune system[4, 5]. As there are health concerns associated with zincdeficiencies, it is often necessary to analyze such complexbiological matrices as blood, serum, tissue, and urine. Avariety of methods has been reported in the literature, in-cluding inductively coupled plasma – mass spectrometry(ICP-MS) and other spectroscopic and electrochemicalmethods [6–8]. Of these, electrochemical methods haveadvantages associated with high sensitivity, low detectionvolumes, compact instrumentation, and low cost [9, 10].

Carbon nanotubes (CNTs) are an increasingly impor-tant group of nanomaterials possessing chemical, physical,and mechanical properties that make them well suited forelectrochemical sensing applications [11,12]. Nanotubeshave high electrical conductivity, are relatively inertchemically, can be used over a wide potential range, andhave a very high microscopic surface area due to their

surface structure [13, 14]. CNTs can be prepared in a vari-ety of ways, including arc discharge, laser ablation andchemical vapor deposition [15,16]. A hindrance sharedby these methods is the general use of metal catalysts intheir preparation that can contaminate the CNTs andreduce their effectiveness for stripping voltammetry ap-plications [17,18]. Previously, metal catalyst free carbonnanotubes (MCFCNTs) fabricated via a solid-phasegrowth mechanism have been reported as a good elec-trode material with fast electron transfer rate and clearcyclic voltammetry background as well as for trace metaldetection with stripping voltammetry with low limit of de-tection and high sensitivity [19,20]. The MCFCNT whisk-er used in this paper is a powder material with a whiskershape, which is different from the solid CNT electrodethat we used previously [19,20]. In this work, we continueto explore the potential of MCFCNTs for anodic strip-ping voltammetry (ASV) by using this whisker materialas an electrode modifier for zinc detection in biologicalsamples.

A potential limitation of stripping analyses in complexmatrices such as blood is the presence of organic and in-organic substances that may display redox activity nearthe potential of the analyte of interest, interfering withthe detection signal. In addition, organic biological sub-stances that can adsorb on the electrode surface can de-crease its effective surface area and passivate (foul) the

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electrode surface. Also, a large fraction of the zinc inblood or serum is coordinated with binding sites, makingit unavailable for detection by ASV [21,22]. Thus,a robust analytical procedure must overcome these chal-lenges in order to reliably determine the analyte concen-tration. A variety of techniques can be used to overcomethe matrix interferences, such as UV irradiation decom-position, ultrasound-assisted extraction and microwave-digestion [7, 23, 24]. Of these techniques, analyte extrac-tion which physically separates the targeted analyte fromthe complicated matrix is much simpler since it does notrequire an external energy supply. A double extractionmethod using dithizone as a chelating reagent has beenreported as an effective method for extracting metal ionsfrom different matrices [7,25–27].

Another approach that applies to electroanalyticaltechniques is the use of a chemically ion selective elec-trode coating [28]. Because faradaic electron transferoccurs at the interface of the sample with the electrodesurface, a material that limits which chemical species areable to reach the electrode can also be an effective meansof removing interferences. Nafion is a perfluorinated sul-fonated cation-exchanger with the advantages of thermalstability, chemical inertness, mechanical strength and re-sistance to fouling and has been widely used to modifyelectrodes in electrochemistry [29,30]. Nafion ethanolicsolution can also serve as an excellent CNT dispersionsolvent that facilitates the electrode modification proce-dure significantly [29].

The aim of this preliminary work is to develop a simplemetal based sensor that allows accurate and rapid serumzinc detection using square-wave stripping voltammetry.Previously, our group has developed a lab-on-chip sensorwith bismuth film working electrode on gold substrate forzinc detection by ASV [31]. Here we evaluate MCFCNTwhiskers as an electrode material by immobilizing themin a Nafion film coated on a gold substrate. This approachhas improved the limit of detection of the sensor for zincdetection due to the Nafion/MCFCNT whiskers coatingand, more importantly, the new platform worked verywell for detection of zinc in a bovine serum sample.

2 Experimental

2.1 Reagents

All chemicals were purchased without further purifica-tion: Nafion perfluorinated resin solution 5 wt. %, acetatebuffer solution (pH 4.65) and 1000 ppm Zn standard solu-tion for atomic absorption spectroscopy (AAS) werefrom Sigma Aldrich; bovine serum was purchased fromFisher Scientific; MCFCNT whiskers were supplied bySCNTE LLC (Beavercreek, OH) and used without anypretreatment. All other chemicals used in this work wereACS certified reagent grade and all solutions were pre-pared with deionized water (18.2 MW from Milli-QSystem, Barnstead, MA).

2.2 Apparatus

ASV measurements were carried out in a 20 mL conven-tional three-electrode cell containing 15 mL of sample so-lution and consisting of a MCFCNT modified gold(Nafion/whiskers-Au) electrode as working electrode, Ag/AgCl as reference electrode (filled with 3 M KCl solu-tion) and Pt wire as auxiliary electrode. A BASi 100BElectrochemical Analyzer from BASi (West Lafayette,IN) was used as the potentiostat. Basic set-up parametersfor Osteryoung square wave voltammetry were squarewave amplitude=25 mV, step potential=5 mV and fre-quency=25 Hz. The sonicator used in this work wasa FS20D from Fisher Scientific; the centrifuge was a Mar-athon 6K from Fisher Scientific.

Transmission electron microscopy (TEM) images weretaken on a Tecnai F20 at 200 kV high angle annular darkfield (HAADF) in scanning TEM mode. Scanning elec-tron microscopy (SEM) images were taken on a FEI XL20 ESEM in environmental SEM mode. Energy-disper-sive X-ray spectroscopy (EDX) images were taken on anEDAX detector from EDAX Inc.

AAS measurements were done using a VarianAA240FS atomic absorption spectrometer. The extractsamples of bovine serum were diluted in 10% HNO3 andanalyzed using the parameters specified by the instru-ment. For serum samples, the same procedure was fol-lowed except that we spiked the serum with zinc standardand calculated the recovery.

2.3 Preparation of Nafion/Whiskers-Au Electrode

MCFCNT whiskers powder was dispersed in 1 wt. %Nafion ethanol solution assisted by sonication for 10 minto yield a homogeneous dispersion. A gold electrode wasfirst sonicated for 1 min in distilled water to clean theelectrode surface. Then, 10 mL of the MCFCNT whiskersNafion dispersion was applied to the dry gold surface.The coating was left to let ethanol evaporate for 30 minand then used directly for ASV measurements.

2.4 Extraction Procedures

First, 10 mL (5 mM) dithizone in chloroform was depro-tonated by mixing with 10 mL (pH 9) 1 M ammonia/0.5 M ammonium buffer solution. Then the deprotonatedform of dithizone was mixed with the solution containingZn (II) and 0.5 mL of 0.05 M potassium thiocyanate inethanol and sonicated for 5 min. After sonication, the so-lution was transferred to a 50 mL plastic tube and centri-fuged for 10 min at 4000 rpm to separate the two phases.The organic phase was collected and sonicated with10 mL 1 M sulfuric acid for another 5 min. Then the clearaqueous phase was collected and mixed with 0.1 M ace-tate buffer and the pH was adjusted to 6 with 5 M NaOHsolution. The resulting solution was used for ASV meas-urements with Osteryoung square wave mode.

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3 Results and Discussion

3.1 MCFCNT Whiskers

Traditionally, transition metals are used as catalyst to syn-thesize CNTs [32]. The residual metal oxide cannot befully removed even by harsh acid wash, which can resultin an electrochemical interference such as reduction ofthe metal oxide in the CNTs [33,34]. In addition, batchto batch variation of CNTs in terms of metallic impuritycan introduce variation for the application of CNTs inelectrochemistry [13, 35]. The MCFCNT whiskers usedhere were synthesized via the carbo thermal carbide con-version method, which grows carbon nanotubes on a sili-con carbide matrix [35, 36]. Since no metal catalyst isused in the synthesis process, batch to batch variability iseliminated in terms of impurities and no post treatment isrequired. High purity material obtained with this processhas been reported as a good material for electrodes [19].The TEM images of the MCFCNT whiskers in Figure 1clearly show the whisker shape of the MCFCNT.

3.2 Comparison of Bare Gold, Nafion Coated Gold(Nafion-Au) and Nafion/Whiskers-Au Electrodes

In order to separate the effect of the Nafion coating fromthe MCFCNT whiskers, control experiments for zinc de-tection were conducted on a bare gold electrode anda Nafion-Au electrode without MCFCNT whiskers.Figure 2 shows anodic stripping voltammograms of 4 mMzinc on these different electrodes. A stripping peak forZn on bare gold electrode occurs at 480 mV. When com-paring the bare gold electrode to the Nafion-Au elec-trode, it can be seen that the Nafion coating causesa small negative shift in the peak potential to 520 mV. Al-

though the coating does not increase the zinc strippingpeak current, it improves the electrochemical response interms of smoother background and better defined peakshape. On both bare gold electrode and Nafion-Au elec-trode, the zinc stripping peak is at ca. �500 mV, which isshifted to a more positive potential compared to theNernst potential of 0.98 V (vs. Ag/AgCl) due to the un-derpotential deposition (UPD) of depositing zinc on goldsubstrate [37]. For the Nafion/whiskers-Au electrode,a double peak pattern is observed: a major stripping peakat ca. �1050 mV, which is zinc stripping from MCFCNTwhiskers, followed by a minor stripping peak at ca.�500 mV, which is zinc stripping from gold. We attributethis to incomplete coverage of the gold surface byMCFCNT whiskers. The height of the “Zn stripping fromgold” peak is diminished compared to this peak on baregold and the Nafion-Au electrodes because most of thegold is now covered by MCFCNT whiskers. The “Znstripping from MCFCNT whiskers” peak is substantiallylarger than the “Zn stripping from gold” peak, indicatingthat it is now the primary surface area for deposition ofZn during the deposition step.

The surface coverage of gold by MCFCNT whiskerscan be seen in the scanning electron micrographs inFigure 3. Figures 3A and 3B were obtained at the samesurface with different detectors. Figure 3A was taken witha gaseous secondary electron detector which shows thesurface topology of the Nafion/whiskers-Au electrodewhere the MCFCNT whiskers coating can be identifiedby the rough layers. Figure 3B, which was taken witha backscattered electron detector, shows the surface com-position of the Nafion/whiskers-Au electrode. The darkercolored part in Figure 3B shows the film of Nafion/whisk-ers coating which correlates to the ridged part in Fig-ure 3A. Figure 4 shows the EDX results for the bright

Fig. 1. TEM images of MCFCNT whiskers: A) 4000� magnification; B) 200 000� magnification

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and dark areas in Figure 3B as indicated with arrows inFigure 3B. Clearly, Figure 4A shows a strong Au signalwhich indicates the exposed gold surface on the Nafion/whiskers-Au electrode (horizontal arrow in Figure 3B).On the other hand, the signature gold peak has disap-peared in Figure 4B, indicating that the gold surface wasfully covered by the Nafion/whiskers coating for the darkarea (vertical arrow in Figure 3B).

3.3 MCFCNT Whiskers Amount Optimization

The effect of amount of MCFCNT whiskers in the Nafionfilm coated on the gold electrode was investigated. Asshown in Figure 5, the peak current for zinc stripping at�1050 mV from the MCFCNT whiskers increases almostlinearly with increasing amount of the MFCNT whiskers

dispersion up to about 3 mg/mL above which it levels off.Also, as more CNT coated on the gold surface, the Zincpeak at �500 mV gets smaller as more of the gold is cov-ered. As shown by the error bars, reproducibility wasmuch poorer at concentrations above 3 mg/mL. Thethicker whiskers films are less mechanically stable andcan be disrupted by hydrogen bubbles formed by electrol-ysis of water during the Zn deposition step, which causeslarger variation [38]. Overall, 3 mg/mL gives a good com-promise for zinc ASV detection with respect to both peakheight and reproducibility. Therefore, the optimized filmcomposition of 10 mL of 3 mg/mL MCFCNT whiskers in1 wt. % Nafion ethanol solution was used for all subse-quent experiments.

Between measurements, the electrode was cleaned byapplying a constant potential of 500 mV for 1 min to

Fig. 2. ASV voltammograms of 4 mM Zn2+ in acetate buffer solution (pH 6) on different electrodes (bare gold electrode, Nafion-Augold electrode and Nafion/whiskers-Au electrode). Deposition time 120 s; deposition potential �1400 mV.

Fig. 3. SEM images of Nafion/whiskers-Au electrode: A) surface topology of Nafion/whiskers-Au electrode with a gas secondaryelectron detector; B) surface composition of Nafion/whiskers-Au electrode with a backscattered electron detector.

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remove any zinc residue from the electrode surface. Asingle electrode can give reproducible results for 50–80measurements. The relative standard deviation of zinc de-tection (4 mM in acetate buffer) with different Nafion/whiskers-Au electrodes (N=5) was found to be 5.1%.

3.4 pH, Deposition Potential and Deposition TimeOptimization

Electrolysis of water is an important factor to considerfor ASV detection of zinc since hydrogen evolution caninterfere with zinc deposition on the electrode surface be-

cause of the very negative potential required. Hydrogenevolution is quite sensitive to solution pH, deposition po-tential and electrode material. Figure 6A shows cyclic vol-tammograms of the Nafion/whiskers-Au electrode in ace-tate buffer with different pH values. Electrolysis of waterstarts at ca. �1000 mV at pH values lower than 6.0, whichwill interfere with zinc deposition for ASV measurementsat ca. �1300 mV. Figure 6B shows how buffer pH affectszinc ASV detection at the Nafion/whiskers-Au electrode.The ASV peak currents are lower and with larger varia-tions at lower pH values compared with pH 6.0. Overall,ASV at pH 6 buffer has the most moderate background

Fig. 4. EDX images of Nafion/whiskers-Au electrode: A) The bright area (horizontal arrow) in Figure 3B; B) The dark area (verticalarrow) in Figure 3B.

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and the most reproducible peak current since significanthydrogen evolution occurs at lower pH. Also, ASV hasthe sharpest and highest stripping peak at a depositionpotential of �1400 mV. The peak current drops at morenegative deposition potentials due to hydrogen gas evolu-tion at the electrode surface, which interferes with themass transport of Zn2+ analyte to the electrode surfaceand damages the Nafion/whiskers film. Figure 7 shows

the deposition potential optimization of ASV zinc mea-surement at the optimized pH value. �1400 mV waschosen as optimized deposition potential due to the high-est peak height and acceptable reproducibility for the Znstripping peak at �1050 mV. Thus, pH 6 and a depositionpotential of �1400 mV were used for subsequent experi-ments. The optimization of deposition time was also ex-plored with those optimized experimental parameters. Itwas found that the peak current increases linearly withdeposition time up to 120 s after which the curve slopestarts to decrease as coverage of the CNTs on the elec-trode surface increases with longer deposition time. So,120 s was selected as optimized deposition time.

3.5 Calibration Data

ASV determination of Zn2 + in a series of standard solu-tions was carried out in 0.1 M pH 6 acetate buffer solu-tion using optimized experimental parameters. Figure 9shows ASV voltammograms in the linear range of zincdetection with increasing concentrations from 0.5 to7 mM. The double peak pattern is also seen in the voltam-mogram and the major zinc peak at �1050 mV was usedfor zinc stripping peak measurements. Each voltammo-gram represents three replicates of zinc measurements forthe same concentration. A plot of peak current versus

Fig. 5. Effect of CNT concentration on peak current for ASVZn2+ detection. Coating: 10 mL of MCFCNTs whisker dispersionin 1 wt. % Nafion ethanolic solution with different CNT concen-trations. Deposition time 120 s, deposition potential �1400 mV;zinc concentration 5 mM.

Fig. 6. A) Cyclic voltammograms of Nafion/whiskers-Au elec-trode in acetate buffer with different pH values; scan rate:10 mV/s; B) pH optimization of Zn2+ ASV detection on Nafion/whiskers-Au electrode in acetate buffer with optimized coatingparameters from pH 4.5 to 6. Zn2+ concentration 5 mM; deposi-tion potential �1400 mV; deposition time 120 s.

Fig. 7. Deposition potential optimization of Zn2+ detectionwith optimized coating parameters: deposition time 120 s; zincconcentration 5 mM; pH 6.0

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concentration is shown in Figure 8. The correlation equa-tion for the linear range is I (mA)= (3.77�0.03) [Zn(mM)]�(1.05�0.06) (mA) (R2 =0.99 for 9 concentrationsin the range of 0.5–7 mM and the standard deviations areshown for the slope and intercept). The limit of detection

was calculated to be 53 nM (based on 3s/slope) which isclose to the 50 nM that we reported previously fora MCFCNT electrode for zinc detection [19]. Measure-ments of higher concentrations fit into another linearrange which is attributed to stripping of zinc that was de-posited on zinc rather than the zinc deposited directly onMCFCNT whiskers.

The bare gold electrode has a dissatisfactory behaviorfor zinc detection in terms of poor reproducibility, non-linear calibration and distorted background. After coatingwith 10 mL 1 wt.% Nafion solution, the Nafion-Au elec-trode showed excellent reproducibility for zinc detectionand a linear range of 0.5–4 mM was obtained as shown inFigure 9. For concentrations higher than 4 mM, the depo-sition follows a zinc-on-zinc deposition mechanism whichleads to a smaller slope value. The limit of detection(based on 3s) calculated for the Nafion-Au electrodewithin the first linear range was 80 nM. Notably, theNafion-Au electrode has a smoother background anda better defined zinc peak than bare gold electrode at thesame current scale. The Nafion/whiskers-Au electrode ex-hibits 2.4 times increase in peak current compared to theNafion-Au electrode which is attributed to the increasedelectrode surface area provided by the MCFCNT whisk-ers and the fast electron transfer rate at MCFCNT whisk-ers. Compared with the Nafion-Au electrode, the Nafion/whiskers-Au electrode has a much wider linear range,larger calibration curve slope and bigger peak current(shown in Figure 9). Also, the limit of detection of zincfor the Nafion/whiskers-Au electrode is lower than forthe Nafion-Au electrode. In this comparison, all of the ex-perimental parameters were the same optimized valuesdescribed above.

Comparison of zinc detection with different CNT-basedelectrodes is shown in Table 1. The combination of CNT,Nafion film and bismuth film has the lowest limit of de-tection and largest linear range. The Hg film CNT elec-trode has a similar linear range to the MCFCNT elec-trode with a higher limit of detection. One obvious ad-vantage of this work is that there is no extra film neededto be co-deposited (e.g., Hg and Bi film [39, 40]) on theelectrode which allows an easier fabrication process.

3.6 Sample Preparation and Standard Addition ofExtraction from Bovine Serum

Dithizone (H2Dz) has been used as a highly efficient che-lating ligand extraction of metals such as silver, zinc andlead from different matrices [7, 26,27]. Also, it has beenreported that the extraction process can be expedited by

Table 1. Comparison of different CNT based electrodes for ASV Zn2+ detection.

Sensor Deposition time (s) Limit of detection (mM) Linear range (mM)

MWCNTs/Hg film [40] 180 0.43 0.89–9.9MWCNTs/NA/Bi/SPE [39] 120 0.0046 0.0077–1.5This work 120 0.053 0.5–7

Fig. 8. ASV voltammograms of Zn2+ with Nafion/whiskers-Auelectrode in 0.1 M pH 6 acetate buffer in the concentration rangeof 0.5 mM to 7.0 mM. Deposition time 120 s; deposition potential�1400 mV.

Fig. 9. Dynamic ranges of Zn2+ ASV measurements in therange of 0.5 mM–11 mM on Nafion-Au electrode and Nafion/whiskers-Au electrode. Deposition time 120 s; deposition poten-tial �1400 mV.

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using potassium thiocyanate to form a complex with zincand applying sonication during the extraction [25,27].Herein, we use H2Dz as the extraction reagent to extractand quantify zinc in bovine serum with the ASV methodon the Nafion/whiskers-Au electrode. The mechanism isdescribed by the following equations:

H2Dz ðorgÞ þOH� ðaqÞ ! H2OþHDz� ðaqÞ ð1Þ

2HDz� ðaqÞ þ ZnðSCNÞ2 ðaqÞ !ZnðHDzÞ2 ðorgÞ þ 2SCN� ðaqÞ

ð2Þ

ZnðHDzÞ2 ðorgÞ þ 2Hþ ðaqÞ ! Zn2þ ðaqÞ þH2Dz ðorgÞð3Þ

The extraction efficiency was first examined by extract-ing zinc from aqueous zinc AAS standard solution.Table 2 shows the extraction efficiency of various ligandto metal ratios of 10 : 1, 50 : 1 and 100 :1. A ligand tometal ratio of 100 :1 is needed for high extraction yields.Bovine serum samples were treated with the proceduredescribed in the Experimental Section. The resulting solu-tion was adjusted to pH 6 and quantified by ASV witha Nafion/whiskers-Au electrode using the standard addi-tion method. The low concentration of zinc caused us touse a longer deposition time of 5 min to obtain a betterdefined ASV peak for zinc detection in the serum ex-tracts. Figure 10A shows the stripping voltammograms fora bovine serum extract and standard additions from 1–4 mM. The zinc stripping peak at ca. �500 mV is notchanged linearly by the addition of zinc due to the limitedexposed Au surface area. Also, shoulder peaks are pres-ent in Figure 10A at ca. �750 mV which were not presentin Figure 8. These shoulder peaks are possibly due to thepresence of some residual organic species which can alterthe ASV behavior of heavy metals [41]. Figure 10Bshows the plot [I (mA)= (3.24�0.15) [Zn (mM)]+ (2.54�0.33) (mA)] (standard deviations are shown for the slopeand intercept) of standard addition. The plot slope ishigher than the 2.38 mA/mM that we reported before forzinc detection with a MCFCNT electrode [19]. The sensi-tivity increase is due to the Nafion film with MCFCNTwhiskers coating. The zinc concentration in bovine serumwas tested to be 45�0.6 mM which is in close agreementwith an independent AAS result of 48 mM.

4 Conclusions

For the first time, MCFCNT whiskers are used as an elec-trode coating material confined in Nafion on a gold elec-trode. This novel material has advantages over the tradi-tional carbon nanotube material such as low cost, highpurity and good consistency from batch to batch.MCFCNT whiskers are homogeneously dissolved in1 wt. % Nafion ethanolic solution and coated onto thegold electrode surface. The Nafion/whiskers-Au electrodeshows better reproducibility and lower limit of detectionfor zinc detection in acetate buffer than both Au andNafion-Au electrodes. A double peak pattern was ob-served for zinc detection with Nafion/whiskers-Au elec-trode due to UPD on exposed gold. Previously, multiwal-led carbon nanotubes (MWCNTs) have been used as anelectrode modifier for trace level voltammetric zinc de-tection. The double extraction method used in this work

Table 2. Efficiency of extraction from zinc stock solution withdifferent [L] : [Zn2+] ratios.

[L] : [Zn2+] ratio Extraction efficiency (%)

10 :1 68�150 :1 83�2

100 :1 97�1

Fig. 10. A) Voltammograms of standard addition of bovineserum extract for ASV zinc detection with Nafion/whiskers-Auelectrode. B) Standard addition plot of bovine serum extract forASV Zn2+ detection with Nafion/whiskers-Au electrode. Deposi-tion time 300 s; deposition potential �1400 mV; pH 6.

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to extract zinc from bovine serum was optimized to givea high yield of zinc extraction. The Nafion/whiskers-Auelectrode works very well for zinc detection in bovineserum extracts, and the results are in good agreementwith independent AAS measurements. This work showsthe good potential of using MCFCNT materials togetherwith a gold substrate for a sensing system that measureszinc in blood.

Acknowledgement

The authors gratefully acknowledge NIEHSR21ES019255 for financial support, SCNTE LTD for pro-viding the MCFCNT whiskers, and Dr. Necati Kaval fortechnical support on the SEM and EDX.

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