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Stripping voltammetric measurement of trace metalions using screen-printed carbon and modified carbonpaste electrodes on river water from the Eerste-KuilsRiver SystemVernon S. Somerset a , Lucas H. Hernandez b & Emmanuel I. Iwuoha ca NRE, Council for Scientific and Industrial Research (CSIR), Stellenbosch, South Africab Facultad de Ciencias, Universidad Autónoma de Madrid, Madrid, Spainc SensorLab, Chemistry Department, University of the Western Cape, Bellville, South AfricaPublished online: 19 Nov 2010.

To cite this article: Vernon S. Somerset , Lucas H. Hernandez & Emmanuel I. Iwuoha (2011) Stripping voltammetricmeasurement of trace metal ions using screen-printed carbon and modified carbon paste electrodes on river water fromthe Eerste-Kuils River System, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances andEnvironmental Engineering, 46:1, 17-32, DOI: 10.1080/10934529.2011.526075

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Journal of Environmental Science and Health Part A (2011) 46, 17–32Copyright C© CSIRISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2011.526075

Stripping voltammetric measurement of trace metal ionsusing screen-printed carbon and modified carbon pasteelectrodes on river water from the Eerste-Kuils River System

VERNON S. SOMERSET1, LUCAS H. HERNANDEZ2 and EMMANUEL I. IWUOHA3

1NRE, Council for Scientific and Industrial Research (CSIR), Stellenbosch, South Africa.2Facultad de Ciencias, Universidad Autonoma de Madrid, Madrid, Spain.3SensorLab, Chemistry Department, University of the Western Cape, Bellville, South Africa.

Screen-printed carbon electrodes (SPCEs) and carbon paste electrodes (CPEs) were prepared as “mercury-free” electrochemicalsensors for the determination of trace metal ions in aqueous solutions. SPCEs were coated with conducting polymer layers ofeither polyaniline (PANI), or polyaniline-poly(2,2′-dithiodianiline) (PANI-PDTDA). Furthermore, CPEs containing electroactivecompounds with reactivity towards metal ions were employed to obtain enhanced selectivity. Optimised experimental conditionsfor Hg2+, Pb2+, Ni2+ and Cd2+ determination included the supporting electrolyte concentration, deposition potential (Ed) andaccumulation time (tacc). For the modified carbon paste sensors (MCPEs) it was found that −400 mV is an adequate depositionpotential and an accumulation time of 120 s was adequate for the determination using the different constructed electrodes. Initialresults showed linearity in the examined concentration range between 1 × 10−9 M and 1 × 10−6 M using the SPCE/PANI-PDTDAsensor on laboratory prepared standard solutions, while good selectivity for the different metal ions were obtained. Furthermore, thelimit of detection (LOD) was determined for each of the sensors and for the SPCE/PANI-PDTDA sensor it was found to be 2.2 ×10−13 M, while for the SPCE/PANI sensor the LOD was determined to be 8.4 × 10−11 M. The MCPE sensors also showed goodlinearity between the concentration range of 1 × 10−3 to 1 × 10−9 M. The LOD values for the various MCPE sensors, were found tobe Hg(II) - 1.3 × 10−7 M; Cd(II) – 2.9 × 10−7 M; Ni(II) – 3.2 × 10−7 M; and Pb(II) – 1.7 × 10−7 M for the CPE/PANI-PDTDAsensor. For the CPE/PANI sensor the LOD values were Hg(II) – 1.5 × 10−5 M; Cd(II) – 8.6 × 10−7 M; Ni(II) – 9.5 × 10−7 M; andPb(II) – 1.3 × 10−6 M. For the CPE/MBT sensor the LOD values were Hg(II) – 3.8 × 10−5 M; Cd(II) – 1.4 × 10−6 M; Ni(II) – 1 ×10−6 M; and Pb(II) – 6.3 × 10−5 M. Very low detection was obtained for the SPCE/PANI-PDTDA sensor in Hg2+ determination,while the MCPE sensors delivered sensitive simultaneous detection for Hg2+, Pb2+, Ni2+ and Cd2+ metal ions.

Keywords: Anodic stripping voltammetry, modified carbon paste electrodes, mercury, lead, nickel, cadmium

Introduction

A high emphasis has been placed on the determinationof heavy metals in environmental monitoring, since theseelements such as mercury, lead, cadmium, arsenic andchromium pose serious human health concerns due to theirtoxicity, even at low concentrations (<2 ppb). The analysisof heavy metal ions in environmental samples remains achallenging task since these metals ions are present at verylow levels in the samples with some sample matrices beingvery complex in nature. The search is therefore ongoing

Address correspondence to Vernon S. Somerset, CSIR, NRE, POBox 320, Stellenbosch, 7599, South Africa; E-mail: vsomerset@csir.co.zaReceived June 5, 2010.

to find new sensing materials with suitable recognition ele-ments that can respond selectively and reversibly to specificmetal ions.[1−3]

Several methods have been established for the simultane-ous analysis and determination of metal ions, which includeinductively coupled plasma-mass spectrometry (ICP-MS),X-ray fluorescence spectrometry (XRF), atomic absorptionspectrometry (AAS), and atomic fluorescence spectrome-try (AFS). There is no doubt that these methods are highlyefficient, but they require tedious sample pre-treatments,highly qualified technicians and sophisticated instruments.Furthermore, these methods are also known to be timeconsuming and not suitable for field analysis of multiplesamples.[2,4−7]

In the last few decades, electrochemistry has played anincreasing role in the determination of heavy metals, espe-cially since the invention of polarography as a technique.

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The hanging drop mercury electrode (HDME) and mer-cury film electrode (MFE) were traditionally used as work-ing electrodes for the simultaneous voltammetric determi-nation of heavy metals. Recently, alternative working elec-trodes has been the quest of many researchers since the tox-icity of mercury has been highlighted and the use of it beenconsidered undesirable. The use of various mercury-freeelectrodes including bismuth film electrodes, gold-coatedelectrodes, silver electrodes, glassy carbon electrodes, car-bon paste electrodes, carbon nanotube electrodes or screen-printed carbon electrodes has been applied for sensitivemetal determinations.[2,4,8,9]

With the focus on modified carbon paste electrodes, themost sensitive detection limits achieved using various ap-proaches (not containing Hg) and employing differentialpulse anodic stripping voltammetry (DPASV) as electro-analytical procedure include: (i) Hg2+, 5 × 10−10 M; (ii)Pb2+, 4 × 10−9 M; (iii) Ni2+, 4 × 10−8 M; (iv) Cd2+, 5× 10−10 M. These detection limits were obtained for theanalysis of water samples and or model solutions.[10] Itshould also be noted that in these experiments, single ortwo metal ions were determined simultaneously in the re-spective electro-analytical procedures employed.

The use of disposable sensors for the analyses of toxicmetal ions has become very popular in the last two decadesdue to their simplicity, affordability, and ease-of-use. Themicro fabrication of thick film electrodes is now well-established through the use of screen-printing techniques.This process involves the sequential deposition of layersof different conductive or non-conductive inks on a va-riety of inert substrates. Therefore, a growing preferencefor screen-printed carbon electrodes (SPCEs) can be foundwith stripping voltammetric techniques applied for metalion determination, since they allow a high degree of sen-sitivity and selectivity. The production of screen-printedelectrodes (SPCEs) has enabled the mass production of dis-posable electrodes and presents the most promising routefor inexpensive and yet highly reproducible chemical sens-ing devices.[8,11−13]

Carbon paste electrodes (CPEs) under the field of chem-ically modified electrodes (CMEs), have received consider-able attention for several years, due to its ability to accumu-late metal ions on the basis of the interaction of these ionswith a functional group on the electrode surface. CPEs canbe easily prepared, regenerated and modified by mixingwith various ligands depending on the application. Theyfurther offer a renewable and modified surface, are cheapand offer very low background current interferences.[14,15]

The voltammetric determination of various trace metalsafter their pre-concentration at CMEs receives high inter-est as a research area in electroanalysis. These electrodesurfaces are not only solid-sate and mercury-free; they mayexploit a chemical reaction in open circuit for the accumula-tion of a selected species prior to its voltammetric quantifi-cation. Several modifiers have been used in the constructionof CMEs and the selection of modifier is determined by the

application of the CMEs. Modifiers commonly used are or-ganic polymers, ligands, as well as inorganic ion exchangerssuch as clays depending on the analyte of interest.[13,16−20]

The aim of this work was to demonstrate the applica-tion of SPCEs coated with conducting polymer films inthe stripping voltammetric determination of selected metalions in electroanalysis. In a previous paper that forms partof this study, it was reported that SPCE coated with PANI-PDTDA can be applied to the determination of Hg2+ions in aqueous solutions. In this work, the use of PANI-PDTDA and other compounds in modified carbon pasteelectrodes are described. The use of CPEs modified with theconducting polymers of polyaniline (PANI) or polyaniline-poly(2,2′-dithiodianiline) (PANI-PDTDA), as well as theuse of mercaptobenzothiazole (MBT) and the zeolite calledfaujasite (FAUY), have shown that these electrode surfacescan be used for the selective pre-concentration and quan-titation of Hg2+, Pb2+, Ni2+ and Cd2+ metal ions by dif-ferential pulse anodic stripping voltammetry (DPASV) inriver water samples.

Materials and methods

Reagents and materials

The reagents aniline (99%), N,N-dimethylformamide(98%), graphite powder (<20 micron) and mineral oil wereobtained from Aldrich, Germany. Potassium chloride, sul-phuric acid (95%) and hydrochloric acid (32%) were pur-chased from Merck, South Africa. Mercury(II) chloride(99.5%, ACS), diethyl ether (99.8%), mercaptobenzothia-zole (MBT) and ammonium persulfate (APS) were pur-chased from Fluka (Germany) and used as received. Allother chemicals were of analytical grade, or better, andwere used as received. All solutions were always preparedusing Milli-Q (Millipore) water.

Apparatus

Differential pulse anodic stripping voltammetry (DPASV)was performed with the use of a conventional three-electrode cell using a BASi Epsilon Electrochemical An-alyzer and Workstation. Screen-printed carbon electrodes(SPCEs) and carbon paste electrodes (CPEs) were used asthe working electrodes. an Ag/AgCl electrode (saturatedKCl) and a platinum wire (diam. 1 mm) were used as thereference and auxiliary electrode, respectively. All electro-chemical experiments were carried out in a single compart-ment electrochemical cell and a room temperature of 22± 1◦C.[2] Scanning Electron Microscopy (SEM) measure-ments for morphology studies were performed using a LEO1525 Field Emission Scanning Electron Microscope (FE-SEM) with interchangeable accelerating voltages (maxi-mum of 15.00 kV) for optimal sensitivity. Samples were

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Stripping voltammetric measurement of trace metal ions 19

Fig. 1. Map showing the location of the river water samples collected on the Kuils and Eerste Rivers in the Berg Water ManagementArea (WMA) in the Western Cape Province, South Africa.

mounted on aluminium stubs using conductive glue andwere then coated with a thin layer of carbon.[8]

Site description

River water samples were collected on the Kuils and EersteRivers in the Berg Water Management Area (WMA) inthe Western Cape Province, South Africa. In Figure 1 it isshown that the Kuils River starts near the town of Dur-banville and flows south towards the sea. The Eerste Riverstarts near the town of Stellenbosch and flows also southto meet the Kuils River, after which it is known as theEerste/Kuils River System. The Eerste/Kuils River Systemthen flows through the Eerste River Estuary and drains intoFalse Bay. It should be noted that the Eerste/Kuils RiverSystem is highly impacted by discharges from wastewatertreatment works.

The location of the sampling sites on the different riversand the GPS coordinates are displayed in Table 1.

Electrode preparation

SPCEs were obtained from the Sensors and SeparationsGroup, Department of Chemical Sciences, Dublin CityUniversity, Dublin 9, Ireland. The fabrication of the SPCEis described in the paper of Somerset et al.[8] Four differ-ent kinds of modified carbon paste electrodes (MCPEs)were prepared, consisting of: (i) polyaniline (PANI), ii)

Table 1. Identification of sampling sites, their location on thedifferent rivers and their respective GPS coordinates.

Site Code River Latitude Longitude

S1 Kuils 33◦ 55′ 02.5′′ 18◦ 40′ 31.2′′S2 Kuils 33◦ 57′ 12.9′′ 18◦ 39′ 52.0"S3 Kuils 34◦ 2′ 47.6′′ 18◦ 43′ 30.6′′S4 Eerste 33 ◦ 58′ 08.5′′ 18 ◦ 47′ 22.0′′S5 Eerste/Kuils 34◦ 4′ 12.4′′ 18◦ 45′ 51.9′′

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polyaniline-poly(2,2′-dithiodianiline (PANI-PDTDA), and(iii) mercaptobenzothiazole (MBT) dispersed within car-bon paste. The modified CPEs were prepared by thoroughlyhomogenising PANI powder (or MBT) with graphite pow-der and mineral oil using an agate mortar and pestle. Thepaste mixtures were packed firmly into piston-driven elec-trode holders made of polytetrafluoroethylene cylindricaltube (i.d. 3.5 mm). The electrical contact for the carbonpaste working was established with a copper rod.[21,22]

Preparation of polymer films

Polyaniline was prepared chemically and dried for incor-poration into the modified CPE construction. Chemicalsynthesis of polyaniline consisted of applying equimo-lar (with respect to aniline) amounts of oxidizing agent,which was ammonium persulfate (APS) in this study.[11]

The co-polymer film of polyaniline (PANI) and poly(2,2′-dithiodianiline) was grown electrochemically on the surfaceof a SPCE by repetitive cyclic voltammetric scanning at 50mV/s from − 200 to + 1100 mV, for 10 cycles at 25◦C.Polyaniline was also electropolymerised as a monopolymeron a SPCE surface, using a 10 mL solution of 0.2 M anilineand aqueous 1 M HCl, and cycling repetitively at 100mV/sfrom −200 to + 1100mV for 10 cycles at 25◦C. Each SPCEwas then rinsed with Milli-Q water and immersed in thewater until use.[8]

Analytical procedure

The analysis of Hg2+, Pb2+, Ni2+ and Cd2+ was per-formed using differential pulse anodic stripping voltam-metry (DPASV) that was carried out in 2.0 mL aliquot

samples. DPASV involved the following steps: (a) the pre-concentration step at −0.4 V for 120 s; (b) the differentialpulse anodic stripping voltammograms were recorded whenswept from −0.4 V to 1.5 V after 2 s quiescence. The exper-imental parameters used were: deposition potential, −0.4V; accumulation time, 120 s; differential pulse amplitude(peak to peak), 50 mV; step amplitude, 4 mV; pulse width,50 ms; pulse period, 200 ms; frequency, 100 Hz.[8]

Results and discussion

Voltammetric analysis of co-polymer electropolymerisation

In Figure 2a, the results for the electropolymerisation ofthe PANI-PDTDA co-polymer on the SPCE surface inacidic medium are displayed. These results show the verygood redox activity obtained for the co-polymerisation,when the potential was cycled between − 200 and +1100mV at a scan rate of 100 mVs−1 for 10 cycles. Theresults further show that three redox couples for the PANI-PDTDA co-polymerisation can be seen. The three re-dox couples of (A/A′), (B/B′) and (C/C′) were observedto form at approximately the same potentials as the re-dox couples obtained for the electropolymerisation ofPANI on a SPCE surface, showing that there is a strongPANI backbone in the formation of the PANI-PDTDAco-polymer.[8]

Further analysis of the cyclic voltammogram (CV) forthe PANI-PDTDA co-polymer, show that oxidation peaksA and C represent the transformation of leucoemeral-dine base to emeraldine salt and the emeraldine salt topernigraniline salt forms, while oxidation peak B indicatesthe formation of benzoquinone. Analysis of the reduction

Fig. 2. Cyclic voltammetric (CV) results displaying the electropolymerisation of the co-polymer of PANI-PDTDA on a SPCE surfacein (a). The potential was cycled between − 200 and + 1100mV at a scan rate of 100mVs−1 for 10 cycles. In (b) the 12th cycle obtainedfor the PANI-PDTDA electropolymerisation can be viewed.

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Stripping voltammetric measurement of trace metal ions 21

Fig. 3. Scanning electron micrographs showing the surface features of screen-printed carbon electrode surfaces modified with PANIin (a), PANI-PDTDA in (b), PANI-PDTDA in (c). (Continued)

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Fig. 3. Continued.

peaks, shows that peaks A′ and C′ corresponds to the con-version of pernigraniline salt to emeraldine salt and emeral-dine salt to leucoemeraldine base, while peak B′ can beattributed to the formation of hydroquinone.[23,24]

On closer inspection of the PANI-PDTDA co-polymerresults for the 12th cycle shown in Figure 2b, it is seenthat redox couple (B/B′) represents 2 separate redox pro-cesses occurring during electrosynthesis. It indicates thatduring benzoquinone and hydroquinone formation in thePANI backbone, another process of self-doping/undopingof thiolate anions (S−) formed by the reductive cleavage ofS-S bonds in the PDTDA backbone, of the PANI-PDTDAco-polymer is also occurring.[23,24]

Morphology studies of different electrodes

Screen-printed carbon electrode surfaces were coated withPANI and PANI-PDTDA using electropolymerisation ofthe monomer solutions. The morphology of the differentmodified electrodes was characterised using scanning elec-tron microscopy (SEM) analysis. The results obtained canbe viewed in Figure 3.

The results shown in the SEM micrographs in Figure 3illustrate how the morphology of the clean SPCE surface in(a) changes, when it is coated with PANI in (b) and PANI-

PDTDA in (c). These results clearly indicate the change insurface morphology and are indicative of good morphologyfor both the polymers of PANI in (b) and PANI-PDTDAin (c). For both polymers nanostructured materials wereformed with the co-polymer of PANI-PDTDA containingstrands with well-defined structure, while clusters were ob-tained for PANI.[8]

SEM analysis was also done for the CPEs modified withdifferent electroactive compounds, in order to characteriseand compare the morphology of the different MCPEs. Forthe SEM micrographs displayed in Figure 4 it can be seenthat randomly shaped micrometer-sized flakes of graphite(Fig. 4a) can be seen. Similar SEM results were obtainedfor the MCPEs modified with the electroactive compoundsof PANI (Figure 4b) and PANI-PDTDA (Fig. 4c). In Fig-ure 4d it is seen that MBT crystals are dispersed betweenthe flakes of graphite and it seems that more milling wasrequired to obtain a more homogenous electrode composi-tion.

Voltammetric scan rate studies

The voltammetric behaviour of the different MCPEs wasexamined using a 0.1 M HCl solution and the peak currentdata was obtained at different scan rates. All experiments

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Stripping voltammetric measurement of trace metal ions 23

Fig. 4. Scanning electron micrographs showing the surface features of an unmodified CPE in (a) and various modified carbon pasteelectrodes (MCPEs) with the different modifying compounds of PANI in (b), PANI-PDTDA in (c), MBT in (d). (Continued)

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Fig. 4. Continued.

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Stripping voltammetric measurement of trace metal ions 25

were performed at room temperature and each electrodewas used in a single assay. The results obtained for thecyclic voltammetry (CV) evaluation are shown in Figure 5,with the results of the Randles-Sevcik plots conducted forthe peak current data of the first redox couples of each CValso shown.

The results in Figures 5a and b are that of the PANImodified CPE, in Figures 5c and d it is the PANI-PDTDACPE, while in Figures 5e and f the MBT modified CPE areshown. The CV results obtained for the PANI and PANI-PDTDA modified CPEs have shown characteristic redoxcouples of the polymer material, corresponding to the re-dox couples of the polymer materials itself (Fig. 2). This wasvery promising results obtained indicating that sufficientpolymer material was incorporated into the MCPE. Goodvoltammetric results were also obtained for the CPE+MBTsensor and the redox activity of the well-defined redox cou-ples can be seen in Figure 5. The results obtained for theRandles-Sevcik plots were determined at scan rates of 5,10, 20, 30, 40, 60, 80, 100, 150, 200, 300 and 400 mV/s.These plots in Figures 5b, d, and f are linear indicating thatthe peak currents depend linearly on the square root ofthe scan rate for v ≤ 150 mV/s. This information also pro-vides evidence that the charge transport process is diffusioncontrolled at the electrode surfaces.[8,26−28]

Effect of accumulation potential

The optimisation of the accumulation or deposition poten-tial involved the investigation of several potentials rangingbetween −600 and −200 mV (vs. Ag/AgCl). Initial depo-sition potential (Ed) data was collected using a solutioncontaining 0.1 M H2SO4, 0.5 M HCl and 1 × 10−6 M M2+ions. The results obtained for the peak current data usingdifferent Ed values, have shown that Ed = − 400mV hadthe highest current and the error was the smallest (no plot-ted results shown). It was then decided to use an Ed valueof − 300 mV for further studies since the precision at thispotential was much better.[4,8,29−31]

Effect of pre-concentration time

The dependence of the anodic peak current on the pre-concentration (deposition) time were also investigated andoptimised. Different deposition times over the interval 0–300 seconds were investigated for the different chemicalsensors constructed, to investigate the dependence betweenthe anodic peak data and the deposition time. The resultsobtained for this investigation (not shown here) indicatethat the anodic peak current increases for deposition timesof 0–120 s and thereafter remains relatively constant forthe period up to 300 s evaluated (no plotted results shown).With the highest anodic peak currents obtained at approxi-mately 120 s, it was then decided to select a deposition timeof 120 s for further studies.[4,29,32]

Optimisation of pH and supporting electrolyte conditions

A solution of 0.1 M H2SO4 was used to maintain the pH <

2 in the sample matrix for Hg2+ determination using strip-ping voltammetry. In optimising the sensor conditions, itwas found that these conditions were also appropriate forthe determination of the Pb2+, Ni2+ and Cd2+ ions, as thepeak current data has shown. Optimisation of the support-ing electrolyte involved the collection of DPASV results fordifferent HCl concentrations of 0, 0.004, 0.01, 0.1, 0.3, 0.5,0.6 and 0.7 M. An increase in sensitivity for Hg2+ deter-mination was observed when HCl was used as supportingelectrolyte, resulting in smooth and reproducible peaks. TheDPASV data have also shown an increase in peak currentvalues as the HCl concentration was increased, whereasthe peak current decreased for 0.6 and 0.7 M concentra-tions (no plotted results shown). Following this analysis, allexperimental data was collected using 0.5 M HCl concen-tration as the supporting electrolyte.[8,21,29−31]

Modified carbon paste electrode composition

Optimisation of carbon paste composition of the MCPEswas also conducted and the voltammetric responses of theMCPEs were evaluated by DPASV. In Table 2 a summaryis provided of the percentage material that was used in theconstruction of the MCPEs. It can be seen that the graphitepowder content was varied between 45–55%, the mineraloil content was kept constant while the modifying agent

Table 2. Results for the optimisation of the modified carbon pasteelectrode composition.

Graphite Mineral oil Modifier MaximumMCPE content content content currentdescription (%) (%) (%) response

CPE+PANI 35 25 40 X40 25 3545 25 3050 25 2555 25 20

CPE+PANI-PDTDA

35 25 40 X

40 25 3545 25 3050 25 2555 25 20

CPE+MBT 35 25 40 X40 25 3545 25 3050 25 2555 25 20

CPE+FAUY 35 25 40 X40 25 3545 25 3050 25 2555 25 20

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Fig. 5. Cyclic voltammograms (CVs) obtained for the different MCPEs are shown in (a), (c), (e) and (g), performed at a scan rate of80 mVs−1 in an electrolyte solution of 0.1 M HCl. The results for the Randel-SevŁik plots are shown in (b), (d), (f) and (h), conductedfor the peak current data of the first redox couples of each CV.

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Stripping voltammetric measurement of trace metal ions 27

Fig. 6. Differential pulse anodic stripping voltammograms (DPASVs) obtained with the SPCE/PANI-PDTDA sensor in (a) and theSPCE/PANI sensor in (b). A solution containing various Hg2+ concentrations, 0.1 M H2SO4 and 0.5 M HCl were evaluated at Ed

= −400 mV and tacc = 120 s.

content was varied between 20–40%. In the case of thepolymers, it was found that a 25% modified compositiongave the maximum current response, while a 20% modifiedcomposition of MBT gave the best results due to the crystalsbeing bigger compared to the fine graphite powder. In thecase of faujasite, a 30% modified composition gave the bestresults after a pre-concentration time of 10 min. in a 1 ×10−3 M M2+ solution.[16,29]

Voltammetric analysis using SPCEs

The results obtained for the use of differential pulse anodicstripping voltammetry (DPASV) for the evaluation of theSPCE/PANI-PDTDA and SPCE/PANI sensors are shownin Figure 6. Voltammograms for the anodic peak currents(Ipa) for three of the Hg2+ standards studied, including thatof a sample blank, with the SPCE/PANI-PDTDA sensorare shown in Figure 6a. The voltammetric results shown inFigure 6b are for the SPCE/PANI sensor and the anodicpeak currents (Ipa) for two of the Hg2+ standards studied,including that of a sample blank are shown.[8]

The voltammetric results in Figure 6a indicate thata good shift in the peak current data for a small in-crease in the Hg2+ ion concentration was experienced forthe SPCE/PANI-PDTDA sensor. Similar trends were ob-served for the SPCE/PANI sensor, although the peakswere not as well defined (Fig. 6b). Analysis of the dif-ferential pulse anodic stripping voltammograms for theSPCE/PANI-PDTDA sensor shows that one major oxi-dation peak at a potential of 501.8 mV (vs. Ag/AgCl) wasobtained that can be attributed to Hg2+ ions. The resultsfor the SPCE/PANI sensor have shown a defined peak forconcentrations of 1 × 10−6 to 1 × 10−3 M Hg2+ ions, while

lower concentrations of 1 × 10−7 to 1 × 10−8 M have shownless defined peak maxima.[8,32]

The SPCE/PANI-PDTDA sensor results were furtherevaluated to determine the limit of detection for this spe-cific sensor. The limit of detection (LOD) was calculated asthe standard deviation multiplied by three and that valuedivided by the slope of the calibration plot in the linearrange. The LOD value for this sensor was found to be 2.2× 10−13 M (n = 10). In the case of the SPCE/PANI sen-sor, the LOD was found to be 8.4 × 10−11 M (n = 10),which are 2 orders lower compared to the SPCE/PANI-PDTDA sensor, indicating that these sensors were ableto sensitively determine Hg2+ ions in laboratory modelsolutions.

Voltammetric analysis using MCPEs

The differential pulse anodic stripping voltammetric(DPASV) determination of a series of standard (or model)

Table 3. Summary of optimum conditions for trace metal iondetermination in aqueous samples using the MCPEs constructedin this study.

Determinant Results

Reduction step pH 1–2Reduction potential −400 mVReduction time 120 sSupporting electrolyte 0.5 M HCl

Measurement Supporting electrolyte 0.5 M HClstep Measurement Differential pulse

techniquePotential window −400 to +1500 mV

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Fig. 7. Differential pulse anodic stripping voltammograms (DPASVs) obtained for the analysis of Cd2+, Ni2+, Pb2+ and Hg2+ ionsusing different MCPE sensors. In Fig. 7a the DPASV results for the CPE/PANI-PDTDA sensor is shown, while Fig. 7b shows theCPE/PANI sensor results and Fig. 7c the CPE/MBT results. Deposition potential, - 0.4 V; accumulation time, 120 s; differentialpulse amplitude (peak to peak), 50 mV; step amplitude, 4 mV; pulse width, 50 ms; pulse period, 200ms; frequency, 100 Hz.

solutions of Cd2+, Ni2+, Pb2+ and Hg2+ was performedwith the MCPEs under the optimised working conditionsdescribed in Table 3. The optimum conditions for tracemetal ion determination with the MCPEs were identifiedand all subsequent DPASV measurements were performedwith the outlined experimental steps.

In Figure 7 below, the results obtained for the DPASVanalysis of a mixed solution containing Cd2+, Ni2+, Pb2+and Hg2+ ions is shown. For the CPE/PANI-PDTDA (Fig.7a), CPE/PANI (Fig. 7b) and CPE/MBT (Fig. 7c) sensorsit was found that −400 mV is an adequate deposition poten-tial and an accumulation time of 120 s was adequate for the

determination using the different constructed electrodes.The peak current maxima results obtained have shown thatthe peaks for the different metal ions can be identified asCd2+ in a, Ni2+ in b, Pb2+ in c, and Hg2+ in d. Initial resultsshowed linearity in the examined concentration range be-tween 1 × 10−9 M and 1 × 10−6 M for laboratory preparedsolutions, while good selectivity for the different metal ionswere obtained. Further investigations will be conducted tounderstand current peak data for the various trace metalions.[4]

The MCPE/PANI-PDTDA sensor results were furtherevaluated to determine the limit of detection for this specific

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Stripping voltammetric measurement of trace metal ions 29

Table 4. Results for the comparison of present work with other modified SPCEs and CPEs for the determination of Hg2+, Pb2+,Ni2+ and Cd2+ in model standard solutions and environmental samples are listed.

Limit ofElectrode Modifier Method Linear range detection (LOD) Ref.

SPCEa Gold (Au) DPASVc Cd(II), Pb(II) Cd(II) – 1.4 µg/L; [33]0–50 µg/L; Pb(II) – 0.5 µg/L;Hg(II) Hg(II) – 0.9 µg/L0–100 µg/L

SPCEa Bismuth (Bi) nanoparticles SWASVd Zn(II), Cd(II), Pb(II) Zn (II) – 4.9 ng/mL; [34]8–12 ng/mL Cd(II) – 1.7 ng/mL;

Pb(II) – 1.3 ng/mLSPCEa Bismuth oxide (Bi2O3) SCPe Pb(II), Cd(II) Pb(II) – 8 µg/L; [35]

20–30 µg/L Cd(II) – 16 µg/LSPCEa PANI–PDTDA DPASVc 1 × 10−3–1 × 10−9 M Hg(II) – 2.2 × 10−13 M This workSPCEa PANI DPASVc 1 × 10−3–1 × 10−7 M Hg(II) – 8.4 × 10−11 M This workCPEb Diphenylthio–carbazone DPASVc Pb(II) Pb(II) – 0.08 µM [4]

0.1–2.5 µMCPEb Diacetyldioxime DPASVc 0.04–15 µM Pb(II) – 1 × 10−8 M; [4]

Cd(II) – 4 × 10−8 MCPEb 2–Aminothiazole (AMT) SWASVd NAf Hg(II) – 5 × 10−10 M [10]CPEb 1-Furoylthioureas (FTHD) DPASVc NAf Cd(II) – 5 × 10−10 M [10]CPEb Cyclodextrins DPASVc NAf Pb(II) – 6 × 10−7 M; [10]

Cd(II) – 2 × 10−6 MCPEb Aza-crown compound DPASVc NAf Ni(II) – 4 × 10−8 M; [10]

Co(II) – 1 × 10−7 MCPEb PANI–PDTDA DPASVc 1 × 10−3–1 × 10−9 M Hg(II) – 1.3 × 10−7 M; This work

Cd(II) – 2.9 × 10−7 M;Ni(II) – 3.2 × 10−7 M;Pb(II) – 1.7 × 10−7 M

CPEb PANI DPASVc 1 × 10−3–1 × 10−8 M Hg(II) – 1.5 × 10−5 M; This workCd(II) – 8.6 × 10−7 M;Ni(II) – 9.5 × 10−7 M;Pb(II) – 1.3 × 10−6 M

CPEb MBT DPASVc 1 × 10−3–1 × 10−8 M Hg(II) – 3.8 × 10−5 M; This workCd(II) – 1.4 × 10−6 M;Ni(II) – 1 × 10−6 M;Pb(II) – 6.3 × 10−5 M

a Screen–printed carbon electrode; b Carbon paste electrode; c Differential pulse anodic stripping voltammetry; d Square wave anodic strippingvoltammetry; e Stripping chronopotentiometry; f NA.

sensor. The limit of detection (LOD) was calculated as thestandard deviation multiplied by three and that value di-vided by the slope of the calibration plot in the linear range.The calculated LOD values for the Hg2+ ion determina-tion using the different MCPE sensors were found to be:(i) CPE/PANI-PDTDA = 1.3 × 10−7 M (n = 10); (ii)CPE/PANI = 1.5 × 10−5 M (n = 10); and (iii) CPE/MBT= 3.8 × 10−5 M (n= 10).

Further analysis has shown that for the Cd2+ determi-nation, the LOD values were found to be: (i) CPE/PANI-PDTDA = 2.9 × 10−7 M (n = 10); (ii) CPE/PANI = 8.6× 10−7 M (n = 10); and (iii) CPE/MBT = 1.4 × 10−6 M(n = 10).

In the case of Ni2+ determination, the LOD values were:(i) CPE/PANI-PDTDA = 3.2 × 10−7 M (n = 10); (ii)

CPE/PANI = 9.5 × 10−7 M (n = 10); and (iii) CPE/MBT= 1 × 10−6 M (n = 10).

Lastly, for the Pb2+ determination, the LOD values were:(i) CPE/PANI-PDTDA = 1.7 × 10−7 M (n = 10); (ii)CPE/PANI = 1.3 × 10−6 M (n = 10); and (iii) CPE/MBT= 6.3 × 10−5 M (n = 10).

Analysis of the preceding results shows that the variousMCPE sensors investigated in this work had very low detec-tion limits for the simultaneous determination of specificheavy metal ions. In Table 4, the results obtained in thisstudy is compared to that obtained in other investigations.From the results listed in Table 4, it can be seen that boththe SPCE/PANI-PDTDA and SPCE/PANI sensors hadvery low detection limits, comparing favourably and bet-ter than the other SPCE sensors reported. In the case of

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Table 5. Determination of heavy metal ions in river water samples using the SPCE and MCPE sensors.

Sensor description Sample Reference (ppb) Detected (ppb) Recovery (%)

SPCE/PANI–PDTDA S1 80 (Added); [Hg2+] 73.6 ± 1.3; [Hg2+] 92 [Hg2+]SPCE/PANI–PDTDA S4 120 (Added); [Hg2+] 114.1 ± 1.2; [Hg2+] 95 [Hg2+]CPE/PANI– S1 80 (Added); [Hg2+; 64.1 ± 1.2 80 [Hg2+]PDTDA Pb2+; Ni2+; Cd2+] 60.3 ± 2.1 75 [Pb2+]

56.1 ± 1.4 70 [Ni2+]48.1 ± 2.3 60 [Cd2+]

CPE/PANI– S2 80 (Added); [Hg2+; 66.4 ± 1.5 83 [Hg2+]PDTDA Pb2+; Ni2+; Cd2+] 56.8 ± 0.8 71 [Pb2+]

54.4 ± 1.2 68 [Ni2+]52.3 ± 0.9 65 [Cd2+]

CPE/PANI– S3 80 (Added); [Hg2+; 68.8 ± 1.1 86 [Hg2+]PDTDA Pb2+; Ni2+; Cd2+] 62.4 ± 0.7 78 [Pb2+]

54.4 ± 1.8 68 [Ni2+]46.4 ± 0.5 58 [Cd2+]

CPE/PANI– S4 80 (Added); [Hg2+; 65.6 ± 2.1 82 [Hg2+]PDTDA Pb2+; Ni2+; Cd2+] 55.2 ± 1.9 69 [Pb2+]

60.1 ± 0.6 75 [Ni2+]46.4 ± 0.9 58 [Cd2+]

CPE/PANI– S5 80 (Added); [Hg2+; 69.6 ± 0.9 87 [Hg2+]PDTDA Pb2+; Ni2+; Cd2+] 56.1 ± 0.3 70 [Pb2+]

60.2 ± 0.8 75 [Ni2+]54.1 ± 1.4 68 [Cd2+]

the MCPE sensors, the results obtained in this study com-pares also favourably to previous studies and relatively lowdetection limits were obtained for the simultaneous detec-tion of 4 heavy metals ions with the CPE/PANI-PDTDA,CPE/PANI and CPE/MBT sensors.

Analysis of river water samples

Clean handling techniques were employed throughout theriver water sample collection to minimise the occurrence oferroneous results. Polyethylene bottles containing approxi-mately 20 mL of 10% hydrochloric acid (HCl) solution wererinsed at the site 3 times with river water before the sam-pling bottle was kept below the water surface to collect thesample. The sampling bottle was filled to the top to removeany air in the bottle, acidified with HCl solution and placedinside double ziplock bags, followed by transportation onice to the laboratory.

The determination of inorganic mercury with theSPCE/PANI-PDTDA sensor was performed on two sam-ples, S1 and S4, to test the analytical procedure outlined inTable 3. The river water sampled at site S1 was spiked with80 ppb (µg/L) of a Hg2+ standard, while sample S4 wasspiked with 120 ppb (µg/L) of the same Hg2+ standard.The results obtained for Hg2+ analysis in samples S1 and S4with the use of the SPCE/PANI-PDTDA sensor is shownin Table 5. These results indicate that the proposed methodwas successfully applied to samples S1 and S4, with goodrecoveries obtained for the amounts of Hg2+ determined.It further showed that the a higher percentage recovery was

obtained with the 120 ppb of Hg2+ added compared to 80ppb used.

More results were collected with the CPE/PANI-PDTDA sensor due to the ease of operation of this sensorand the ability to renew the sensor surface very easily for thenext sample to be analysed. To test the application of theCPE/PANI-PDTDA sensor, a mixed standard containing80 ppb of Hg2+; Pb2+; Ni2+; and Cd2+ ions were preparedand applied. The results obtained (Table 5) for the appli-cation of CPE/PANI-PDTDA sensor to the river samplescollected at sites S1 to S5, showed that the percentage re-coveries for the 4 metal ions ranged between 58% and 87%at the different sites. The results further showed that theaffinity of the sensor for Hg2+ determination was the high-est, since the highest recoveries were obtained for this metalin the five samples analysed. The best percentage recoveryresults were obtained for samples S1, S2 and S5 with noindividual recovery for a single metal ion below 60%. Fur-thermore, with the percentage recoveries lower at sites S4and S5, it may be indicative that a sample pre-cleaning stepis required for this two matrices to increase the percentagerecovery and improve the stripping results for these sites.

Conclusion

A simple and effective electrode system for the singledetermination of Hg2+ ions and simultaneous determi-nation of Hg2+, Pb2+, Ni2+, and Cd2+ ions was devel-oped. Measurements of these analytes in aqueous solutions

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Stripping voltammetric measurement of trace metal ions 31

were carried out at the constructed modified electrode sur-faces using DPASV as technique. The optimised workingconditions for the electroanalysis were identified and de-scribed. The investigation of the redox characteristics ofHg2+ ions at a SPCE sensor surface coated with a PANI-PDTDA polymer film, have shown that well-defined oxida-tion peaks can be obtained in a solution containing 0.1 MH2SO4concentration; 0.5 M HCl, electrolyte concentrationand varying Hg2+ ion concentrations. SEM analyses of theSPCE electrode surfaces coated with different conductingpolymers have shown that nanostructured materials wereobtained. For the CPE/PANI-PDTDA, CPE/PANI andCPE/MBT sensors it was found that −400 mV is an ad-equate deposition potential and an accumulation time of120 s was adequate for the determination using the differ-ent constructed electrodes. Initial results showed linearityin the examined concentration range between 1 × 10−9

M and 1 × 10−6 M for laboratory prepared solutions,while good selectivity for the different metal ions wereobtained.

Furthermore, the limit of detection was determined forselected sensors and for the SPCE/PANI-PDTDA sensorthe LOD value was found to be 2.2 × 10−13 M, while forthe SPCE/PANI sensor the LOD was found to be 8.4 ×10−11 M. The SPCE/PANI-PDTDA and SPCE/PANI sen-sors had very low detection limits, comparing favourablyand better than the other SPCE sensors reported. Verygood results were also obtained for the MCPE sensors inits application to the determination of 4 heavy metals inlaboratory standard solutions, with the individual sensorresults as CPE/PANI-PDTDA sensor: Hg(II) – 1.3 × 10−7

M; Cd(II) – 2.9 × 10−7 M; Ni(II) – 3.2 × 10−7 M; Pb(II) –1.7 × 10−7 M. For the CPE/PANI sensor, the LOD valueswere: Hg(II) – 1.5 × 10−5 M; Cd(II) – 8.6 × 10−7 M; Ni(II)– 9.5 × 10−7 M; Pb(II) – 1.3 × 10−6 M. The CPE/MBT sen-sor delivered LOD values of: Hg(II) – 3.8 × 10−5 M; Cd(II)– 1.4 × 10−6 M; Ni(II) – 1 × 10−6 M; Pb(II) – 6.3 × 10−5 M.The results obtained for the different MCPE sensors, againcompared favourably to previous studies and relatively lowdetection limits were obtained for the simultaneous detec-tion of 4 heavy metals ions with the CPE/PANI-PDTDA,CPE/PANI and CPE/MBT sensors. The application ofthe sensors to the analysis of river water samples haveshown that with the use of the SPCE/PANI-PDTDA sen-sor higher percentage recoveries (92% and 95%) were ob-tained in the determination of Hg2+ in the samples. This isnot surprising as the sensor was applied for single metal ionanalysis. In the multi-analyte analysis with the CPE/PANI-PDTDA sensor, the percentage recoveries for the fourmetal ions ranged between 58% and 87% at the differentsites.

Acknowledgments

This research was supported by funding received from theCSIR’s parliamentary grant (PG) process, the Focus Area

and RSA/Spain Bilateral Programmes of the National Re-search Foundation (NRF) of South Africa. The assistanceof CSIR colleagues in the collection of samples is also ac-knowledged.

References

[1] Khezri, B.; Amini, M.K.; Firooz, A.R. An optical chemical sensorfor mercury ion based on 2-mercaptopyrimidine in PVC membrane.Anal. Bioanal. Chem. 2008, 390, 943–1950.

[2] Senthilkumar, S.; Saraswathi, R. Electrochemical sensing of cad-mium and lead ions at zeolite-modified electrodes: Optimiza-tion and field measurements. Sens. nd Actuat. B 2009, 141,65–75.

[3] Abdollah, Y.; Fatemeh, P. Highly selective sensing of mercury (II) bydevelopment and characterization of a PVC-based optical sensor.Sens. Actuat. B 2009, 138, 467–473.

[4] Li, Y.; Liu, X.; Zeng, X.; Liu, Y.; Liu, X.; Wei, W.; Luo, S.Simultaneous determination of ultra-trace lead and cadmiumat a hydroxyapatite-modified carbon ionic liquid electrode bysquare-wave stripping voltammetry. Sens. Actuat. B 2009, 139,604–610.

[5] Liu, S.; Yuan, L.; Yue, X.; Zheng, Z.; Tang, Z. 2008. Review paper.Recent Advances in Nanosensors for Organophosphate PesticideDetection. Adv. Powder Techn. 2008, 19, 419–441.

[6] Hildebrandt, A.; Bragos, R.; Lacorte, S.; Marty, J.L. Performanceof a portable biosensor for the analysis of organophosphorus andcarbamate insecticides in water and food. Sens. Actuat. B 2008, 133,195–201.

[7] Raman, B.; Meier, D.C.; Evju, J.K.; Semancik, S. Designing andoptimizing microsensor arrays for recognizing chemical hazards incomplex environments. Sens. Actuat. B 2009, 137, 617–629.

[8] Somerset, V.; Leaner, J.; Mason, R.; Iwuoha, E.; Morrin, A. Devel-opment and application of a poly(2,2′-dithiodianiline) (PDTDA)-coated screen-printed carbon electrode in inorganic mercury deter-mination. Electrochim. Acta 2010, 55, 4240–4246.

[9] Maiti, J.; Pokhrel, B.; Boruah, R.; Dolui, S.K. Polythiophene basedfluorescence sensors for acids and metal ions. Sens. Actuat. B 2009,141, 447–451.

[10] Stozhko, N.Y.; Malakhova, N.A.; Fyodorov, M.V.; Brainina, K.Z.Modified carbon-containing electrodes in stripping voltammetry ofmetals Part I. Glassy carbon and carbon paste electrodes. J SolidState Electrochem. 2008, 12, 1185–1204.

[11] Yantasee, W.; Deibler, L.A.; Fryxell, G.E.; Timchalk, C.; Lin, Y.2005. Screen-printed electrodes modified with functionalized meso-porous silica for voltammetric analysis of toxic metal ions. Elec-trochem. Comm. 2005, 7, 1170–1176.

[12] Gornall, D.D.; Collyer, S.D.; Higson, S.P.J. Investigations into theuse of screen-printed carbon electrodes as templates for electro-chemical sensors and sonochemically fabricated microelectrode ar-rays. Sens. Actuat. B 2009, 141, 581–591.

[13] Fanjul-Bolado, P.; Hernandez-Santos, D.; Lamas-Ardisana, P.J.;Martın-Pernia, A.; Costa-Garcia, A. Electrochemical characteri-zation of screen-printed and conventional carbon paste electrodes.Electrochim. Acta 2008, 53, 3635–3642.

[14] Mojica, E-RE.; Merca, F.E. 2005. Anodic stripping voltammetricdetermination of mercury (ii) using lectin-modified carbon pasteelectrode. J. Appl. Sci. 2005, 5(8), 1461–1465.

[15] Raoof, J-B.; Ojani, R.; Beitollahi, H. Electrocatalytic determinationof ascorbic acid at chemically modified carbon paste electrode with2, 7-bis (Ferrocenyl ethynyl) fluoren-9-one. Int. J. Electrochem. Sci.2007, 2, 534–548.

[16] Arrigan, D.W.M. Voltammetric determination of trace metals andorganics after accumulation at modified electrodes. Analyst 1994,119, 1953–1966.

Dow

nloa

ded

by [

Uni

vers

ity o

f So

uth

Car

olin

a ]

at 0

5:45

07

Oct

ober

201

3

32 Somerset et al.

[17] Shams, E.; Torabi, R. Determination of nanomolar concentrationsof cadmium by anodic-stripping voltammetry at a carbon pasteelectrode modified with zirconium phosphated amorphous silica.Sens. Actuat. B 2006, 117, 86–92.

[18] Mousavi, M.F.; Rahmani, A.; Golabi, S.M.; Shamsipur, M.;Sharghi, H. Differential pulse anodic stripping voltammetricdetermination of lead(II) with a 1,4-bis(prop-2-enyloxy)-9,10-anthraquinone modified carbon paste electrode. Talanta 2001, 55,305–312.

[19] Mikysek, T.; Svancara, I.; Vytras, K.; Banica, F.G. Functionalisedresin-modified carbon paste sensor for the voltammetric determi-nation of Pb(II) within a wide concentration range. Electrochem.Comm. 2008, 10, 242–245.

[20] Shahrokhian, S.; Kamalzadeh, Z.; Bezaatpour, A.; Boghaei, D.M.Differential pulse voltammetric determination of N-acetylcysteineby the electrocatalytic oxidation at the surface of carbon nanotube-paste electrode modified with cobalt salophen complexes. Sens. Ac-tuat. B 2008, 133, 599–606.

[21] Mashhadizadeh, M.H.; Eskandari, K.; Foroumadi, A.; Shafiee,A. Copper(II) modified carbon paste electrodes based on self-assembled mercapto compounds-gold-nanoparticle. Talanta 2008,76, 497–502.

[22] Cesarino, I.; Marino, G.; do Rosario Matos, J.; Cavalheiro, E.T.G.Using the rganofunctionalised SBA-15 nanostructured silica as acarbon paste electrode modifier: determination of cadmium ionsby differential anodic pulse stripping voltammetry. J. Brazil. Chem.Soc. 2007, 18(4): 810–817.

[23] Huang, L.-M.; Wen, T.-C.; Yang, C.-H. Electrochemical copoly-merization of aniline and 2,2’-dithiodianiline. Mater. Chem. Phys.2002, 77, 434–441.

[24] Chen, S-Z.; Wen, T-C.; Gopalan, A. Electrosynthesis and character-ization of a conducting copolymer having S-S links. Synth. Metals2003, 132, 133–143.

[25] Somerset, V.S.; Klink, M.J.; Baker, P.G.L.; Iwuoha, E.I.Acetylcholinesterase-polyaniline biosensor investigation oforganophosphate pesticides in selected organic solvents. J. Environ.Sci. Health B 2007, 42, 297–304.

[26] McCormac, T.; Cassidy, J.F.; Crowley, K.; Trouillet, L.; Lafolet, F.;Guillerez, S. Electrochemical characterisation of an Os (II) conju-gated polymer in aqueous electrolytes. Electrochim. Acta 2006, 51,3484–3488.

[27] Berbejillo, J.; Liaz, J.; Cerda, M.F.; Martins, M.E.; Mendez, E.Topographic characterization of disposable carbon pencil modifiedelectrodes. Portugal. Electrochim. Acta 2004, 22, 375–385.

[28] Bertoncello, P.; Ugo, P. Preparation and Voltammetric Character-ization of Electrodes Coated with Langmuir-Schaefer UltrathinFilms of Nafion R©. J. Brazil. Chem. Soc. 2003, 14(4): 517–522.

[29] Cesarino, I.; Marino, G.; do Rosario Matos, J.; Cavalheiro, E.T.G.Evaluation of a carbon paste electrode modified with organofunc-tionalised SBA-15 nanostructured silica in the simultaneous deter-mination of divalent lead, copper and mercury ions. Talanta 2008,75, 15–21.

[30] Crew, A.; Cowell, D.C.; Hart, J.P. Development of an anodic strip-ping voltammetric assay, using a disposable mercury-free screen-printed carbon electrode, for the determination of zinc in humansweat. Talanta 2008, 75, 1221–1226.

[31] Zhao, W.; Ge, P-Y.; Xu, J-J.; Chen, H-Y. Catalytic deposition of pbon regenerated gold nanofilm surface and its application in selectivedetermination of Pb2+. Langmuir 2007, 23, 8597–8601.

[32] Somerset, V.; Leaner, J.; Mason, R.; Iwuoha, E.; Morrin, A. Deter-mination of inorganic mercury using a polyaniline and polyaniline-methylene blue coated screen-printed carbon electrode. Int. J. Env-iron. Anal. Chem. 2009, 90(9), 671–685.

[33] Laschi, S.; Palchetti, I.; Mascini, M. Gold-based screen-printed sen-sor for detection of trace lead. Sens. & Actuat. B 2006, 114, 460–465.

[34] Rico, A.G.; Olivares-Marın, M.; Gil, E.P. Modification of carbonscreen-printed electrodes by adsorption of chemically synthesizedBi nanoparticles for the voltammetric stripping detection of Zn(II),Cd(II) and Pb(II). 2009. doi:10.1016/j.talanta.2009.07.039.

[35] Kadara, R.O.; Tothill, I.E. Development of disposable bulk-modified screen-printed electrode based on bismuth oxide for strip-ping chronopotentiometric analysis of lead (II) and cadmium (II)in soil and water samples. Anal. Chim. Acta 2008, 623, 76–81.

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