A study of Nafion-coated bismuth-film electrodes for the

determination of trace metals by anodic stripping voltammetry

Georgia Kefala, Anastasios Economou* and Anastasios Voulgaropoulos

Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University of

Thesssaloniki, 541 24 Thessaloniki, Greece. E-mail: aeconomo@chem.auth.gr;

Fax: 10030 2310 997719; Tel: 10030 2310 997728

Received 2nd April 2004, Accepted 10th August 2004

First published as an Advance Article on the web 6th September 2004

This work reports on the fabrication, characterization and applications of Nafion-coated bismuth-film

electrodes (NCBFE’s) for the determination of trace metals by anodic stripping voltammetry (ASV). A NCBFE

was typically prepared by first applying a 5 ml drop of a 1% Nafion solution onto the surface of a glassy-

carbon rotating-disk electrode. After evaporation of the solvent, the Bi film was plated on the electrode in situ

(i.e. by spiking the sample with 1000 mg l21 of Bi(III) and simultaneous electrolytic deposition of the metal ions

and bismuth film on the electrode surface at 21. 4 V) or ex-situ (i.e. by electrolytic deposition of the bismuth

film in a separate solution containing 1000 mg l21 of Bi(III), followed by the ASV measurement step in the

sample solution). Various fabrication and operational parameters were thoroughly investigated and discussed in

terms of their effect on the ASV signals. It was found that this voltammetric sensor was suitable for the

determination of metals at trace levels by square-wave ASV (SWASV) due to its multi-element detection

potential, improved analytical sensitivity, high resistance to surfactants, low cost, ease of fabrication,

robustness, speed of analysis and low toxicity (as compared to traditional mercury electrodes). In the presence

of 4 mg l21 of Triton X-100, the NCBFE afforded a 10-fold peak height enhancement for the Pb peak and a

14-fold enhancement for the Cd peak over a bare BFE while the determination of Zn was feasible only on the

NCBFE. The limits of detection (at a signal-to-noise ratio of 3) were 0.1 mg l21 for Cd and Pb and 0.4 mg l21

for Zn for a deposition time of 10 min. Finally, the electrode was applied to different real samples (tap-water,

urine and wine) for the analysis of trace metals with satisfactory results.

Introduction

Anodic stripping voltammetry (ASV) has been established as apowerful technique for the determination of trace metals.1

Mercury, in the form of the hanging mercury-drop electrode(HMDE) and the mercury-film electrode (MFE), has been thetraditional electrode material for performing ASV measure-ments. Mercury possesses invaluable properties for electro-analysis such as its high cathodic hydrogen overpotential andits ability to form amalgams with many heavy metals.However, the considerable toxicity of mercury has led somecountries to completely ban its use2 and, as a result, alternativeelectrode materials are sought for use in stripping analysis.Chemically-modified,3 boron-doped diamond4 and screen-printed5 electrodes as well as noble metals6,7 have beeninvestigated for this purpose. Lately, a new type of electrode,the bismuth-film electrode (BFE), consisting of a thin bismuthfilm electroplated on an inert substrate, has been introducedsince it has been shown to possess performance comparableto MFE’s in ASV.8–11 The advantageous properties ofbismuth are attributed to its ability to form ‘‘fused’’ or ‘‘low-temperature’’ alloys with heavy metals12,13 facilitating thenucleation process during accumulation of heavy metal ionsand leading to sensitivity similar to MFE’s. The variousoperational parameters related to stripping analysis on BFE’shave been studied in earlier work.8,9,11

One of the most serious interferences in ASV, especially onMFE’s, arise from various surface-active compounds thatadsorb on the electrode causing fouling of its surface.14 Toalleviate this problem, different schemes, based on themodification of the electrode surface with a suitable perm-selective membrane, have been investigated. The principle ofthis strategy is that the bulkier surfactant species are

mechanically prevented from reaching the electrode surfaceby hindering their diffusion through the permselective mem-brane; on the other hand, the smaller metal cations can easilydiffuse through the membrane and ultimately reach theelectrode surface. Different materials have been used as perm-selective membranes on MFE’s such as cellulose acetate,15

poly-ester sulfonic acid,16 poly-(3-methylthiophene),17 poly-pyrrole,18 tosflex,19 zeolites,20 divinylbenzene-crosslinked poly-styrene,21 PTFE22 and omega-mercaptocarboxylic acid.23

Nafion, first introduced in ASV measurements of trace metalsby Hoyer et al.,24 is a perfluorosulfonate cation-exchangepolymer with ideal properties as a permselective membrane (i.e.

it is electroinactive, chemically inert, hydrophilic and insolublein water). It has found widespread application in conjunctionwith MFE’s in ASV in order to increase the electrode’sresistance against fouling by surface-active compounds.25–28

An additional advantage associated with the use of Nafion-coated MFE’s (NCMFE’s) is the improvement in terms ofrobustness towards mechanical damage (due to handling,electrode rotation or ultrasonic treatment).29

However, despite the scope of BFE’s for trace ASV analysis,the effect of surfactants on this type of electrodes has hardlybeen studied. The only investigation on this subject concludedthat BFE’s are, in general, at least as susceptible to the presenceof surfactants as MFE’s.30 In principle, it can only be assumedthat the surface modification of a BFE with a Nafion film,resulting in a Nafion-coated BFE (NCBFE), would confer thesame advantages in terms of selectivity and robustness as withMFE’s. An initial attempt has been reported employing apolymeric coating on a BFE for the analysis of Pb and Cd byASV but the utility of this approach was limited by a decreasedresponse for Cd and limited separation between the Pb and Cdpeaks.30 From these initial studies, it is clear that the field ofD

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polymer-coated BFE’s is still largely unexplored and that amore systematic study is required. Furthermore, the actualperformance of such a modified voltammetric sensor in realsamples has not been verified so far.

In this work we performed a thorough study of thefabrication and operational parameters affecting the perfor-mance of a NCBFE for the detection and quantification of Zn,Cd and Pb by square wave ASV (SWASV). Among the initialparameters investigated were the Nafion film thickness, thebismuth film thickness, the preconcentration time, the mass-transfer conditions and the SW parameters. The performanceof the sensor, in terms of peak height suppression and responsestability, was investigated in solutions containing ‘‘model’’surfactant compounds. The selected conditions were used toapply the NCBFE to the determination of trace metals intap-water, urine and wine samples.

Experimental

Chemicals

All the chemicals were of analytical grade and purchased fromMerck (Darmstadt, Germany) unless stated otherwise. Nafion(5% w/v solution in a mixture of water and lower alcohols) waspurchased from Aldrich (St. Louis, Missouri); more diluteNafion solutions were prepared after dilution with absoluteethanol. De-ionized water was used throughout. Workingmetal ion solutions were prepared from 1000 mg l21 atomicabsorption standard solutions after appropriate dilution withde-ionized water. A 1 mol l21 acetate buffer stock solution (pH4.5), prepared by mixing the appropriate amounts of glacialCH3COOH and NaOH, was used as the supporting electrolyte.Plating stock solution containing 100 mg l21 Bi(III) and Hg(II)were prepared in 0.1 mol l21 acetate buffer. Solutions con-taining 1000 mg l21 of Triton X-100 (BDH, Poole, UK) andgelatin were prepared in de-ionized water.

Apparatus

SWASV measurements were performed with a home-madepotentiostat interfaced to a PC though a 6025E PCI multi-purpose interface card (National Instruments, Austin, TX).The experimental sequence was fully automated and controlledby the PC using a control application developed in LabVIEW5.1 (National Instruments) as reported previously.31 An addi-tional programme was developed to calculate peak heights,peak positions and peak widths.

The voltammetric cell was a standard 50 ml glass vial(Metrohm, Switzerland) equipped with a Ag/AgCl referenceelectrode and a Pt counter electrode. An electrode rotator(Metrohm 628-10) was used during the preconcentration andcleaning steps. The glassy-carbon working electrode (3 mm indiameter) was from Metrohm.

For electrothermal atomic absorption spectrometry(ETAAS) measurements, a Perkin Elmer 5100 AAS spectro-meter (CT, USA) was used in combination with a Perkin Elmer5100 ZL furnace module. A JEOL 840A scanning electronmicroscope (SEM) (JEOL, Peabody, MA) operating at 20 kVand a Leica MZ6 optical microscope (Leica Microsystems AG,Wetzlar, Germany) equipped with a JVC TK-61381 DigitalVideo Camera were used for the microscopy studies.

Experimental procedure

Sample preparation. Tap-water was collected from taps inour laboratory. For the determination of Pb(II), the depositionpotential was set to 21.2 V. To determine Pb(II) on theNCBFE, 10 ml of the tap water, 2 ml of the 1 mol l21 acetatebuffer and 8 ml of de-ionized water were placed in the cell andthe analysis was carried out as described below with an in-stu

plated BFE. To determine Pb(II) on the bare BFE, 18 ml of thetap water and 2 ml of the 1 mol l21 acetate buffer were placed inthe cell and the analysis was carried out as described belowwith an in-stu plated BFE. To determine Zn(II) on the NCBFEor the bare BFE, 2 ml of the tap water, 2 ml of the 1 mol l21

acetate buffer and 8 ml of de-ionized water were placed in thecell and the analysis was carried out as described below.

Urine was supplied from volunteers in our laboratory. 5 mlof urine, 2 ml of the 1 mol l21 acetate buffer and 13 ml of de-ionized water were placed into the cell and the analysis carriedout as described below using a preplated BFE except that thedeposition potential was set to 21.2 V.

Bottled white wine was purchased from a local supermarket.The wine samples were acidified to pH 1.5 and left for 24 h inorder for metals to be released from the complexing ligandsexisting in wine. For the analysis, 5 ml of wine, 2 ml of the1 mol l21 acetate buffer and 13 ml of de-ionized water wereplaced into the cell and the analysis carried out as describedbelow using a preplated BFE except that the depositionpotential was set to 21.2 V.

Nafion film preparation. The glassy carbon electrode waspolished with a water slurry of 0.3 mm Al2O3, rinsed withethanol and water and dried. A 5 ml drop of the Nafion solutionwas placed on the electrode surface followed by a 5 ml drop ofDMF and the solvents were left to evaporate at room tem-perature for 10 min. Then, the polymer membrane was curedwith a hot air stream from a heat-gun for 1 min and left to coolto room temperature before being used.

Measurement procedure. For preplating the substrate with abismuth film, the Nafion-covered electrode was immersed in a1000 mg l21 Bi(III) plating solution and the Bi film was formedby holding the working electrode potential at 21.0 V underrotation of the electrode for 2 min. Then, the electrodes wereimmersed into the sample solution and deposition of the metalswas carried out at 21.4 V under rotation for a carefully definedperiod of time followed by a 10 s rest period. The voltammo-gram was recorded between 21.4 and 20.4 V by applying a SWwaveform and the electrode was cleaned from residual metalsfor 30 s at 20.4 V under rotation.

In-situ bismuth films were prepared by spiking the samplewith 1000 mg l21 Bi(III) and simultaneously depositing thebismuth film and the metals on the surface of the electrodeat 21.4 V under rotation of the electrode for a carefully definedperiod of time followed by a 10 s rest period. The voltammo-gram was recorded between 21.4 and 0 V by applying a SWwaveform and the electrode was cleaned from residual metalsand the bismuth film for 30 s at 0.0 V under rotation. In-situmercury films were prepared by spiking the sample with5,000 mg l21 Hg(II) and simultaneously depositing mercury andthe metals on the surface of the electrode at –1.4 V underrotation for a carefully defined period of time followed by a 10 srest period. The voltammogram was recorded between –1.4 and10.6 V by applying a SW waveform and the electrode wascleaned from residual metals and the mercury film for 30 s at10.6 V under rotation of the electrode. All measurements werecarried out without prior deoxygenation of the solutions.

Results and discussion

Nafion and bismuth film deposition

The Nafion film can be deposited on the electrode surface byspread-coating,24 dip-coating,32 spin-coating,33 electrochemicalplating34 or electrostatic spraying.25 In this work, the simplerspread-coating approach was adopted involving the applica-tion of a drop of the Nafion solution on the electrode surfacefollowed by a drop of casting solvent (DMF); the latter has

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been shown to improve the stability and properties of thepolymeric film.35 Assuming a uniform distribution of theNafion film on the electrode surface, the average thickness ofthe Nafion film, lNa, can be calculated using the formula:

lNa~mNa

pR2dNa

where mNa is the mass of the Nafion attached to the electrodesurface, dNa is the density of the Nafion film (1.58 g cm23)36

and R is the electrode radius (1.5 mm). By using a 5 ml drop ofNafion solution containing 0.5, 1 and 2.5% w/v of Nafion, filmswere produced with thicknesses of 2.2, 4.5 and 11.2 mm,respectively. In the following sections, NCBFE’s fabricatedwith these thicknesses are referred to as low thickness NCBFE(LTNCBFE’s), medium thickness NCBFE’s (MTNCBFE’s)and high thickness NCBFE’s (HTNCBFE’s), respectively.

In a way similar to the preparation of MFE’s, the bismuthfilm can be plated on the Nafion-coated electrode by eitherin-situ deposition or preplating. For the usual in-situ depositionmethod under the conditions employed in these experimentsand assuming a uniform distribution of the bismuth film onthe electrode surface (which is only a rough approximation),the average bismuth film thickness, lBi, can be calculatedusing the equation:

lBi~mBi

pR2dBi

~QBiMBi

nFpR2dBi

where mBi is the amount of metallic bismuth deposited on theelectrode surface, QBi is the charge for bismuth deposition,MBi is the formal weight of bismuth (209 g mol21), n is thenumber of electrons per molecule of Bi during Bi(III) reduction(3 eq mol21), F is the Faraday constant (96,487 Cb eq21), R isthe electrode radius (1.5 mm) and dBi is the density of metallicBi (9.8 g cm23).

The charge for, and the mass of, the bismuth depositioncould be the calculated by integrating the stripping peak ofbismuth. For a Bi film formed from a deoxygenated solutioncontaining 1000 mg l21 Bi(III) in the absence of Nafion, theamount of bismuth deposited on the electrode ranged from1.5 6 10210 mol (for a 60 s deposition) to 7.5 6 10210 mol (for300 s of deposition) and the average Bi film thickness on the

electrode was in the range 0.45–2.3 nm. In the presence of theNafion coating the amount of Bi deposited on the electrodesurface was less than the amount deposited on the uncoatedelectrode. The relative amounts of bismuth on the NCBFE anduncoated BFE electrodes for a typical deposition time of 120 swere estimated by integrating the bismuth stripping peaksfor the two electrodes: it was found that the LTNCBFE’s,MTNCBFE’s and HTNCBFE’s incorporated 30, 21 and 10%as much Bi as the uncoated BFE and the average bismuth filmthickness was 0.3 nm, 0.19 nm and 0.09 nm, respectively.

The diffusion layer thickness on the RDE, d, is given by theequation:

d~1:61D1=3v

{1=2n1=6

where D is the diffusion coefficient of Bi(III) (y1025 cm2 s21), vis the angular velocity of the RDE (52.3 rad s21 for a rotationspeed of 500 rpm) and v is the kinematic viscosity of thesolution (y1022 cm2 s21). The diffusion layer thickness wascalculated as 22 mm under the conditions of these experiments.

Under the typical experimental conditions used in this work,on the MTNCBFE the thickness of the Nafion film was one-fifth of the diffusion layer thickness. On the other hand, the Bifilm conformed to the definition of a true ‘‘thin film’’ by beingmuch thinner than the Nafion film. The surface of the electrodewas visualised by scanning electron microscopy (SEM) andoptical microscopy (Fig. 1). Optical microscopy showed thatafter coating the bare glassy-carbon electrode (Fig. 1(B)(a))with the Nafion film, the appearance of the electrode becamedull and concentric ring zones developed on the electrodesurface after the polymer film dried (Fig. 1(B)(b)). The bismuthdeposit on the Nafion-covered electrode was clearly visible as agrey deposit covering a large proportion of the electrodesurface (Fig. 1(B)(c)). The SEM image of the Nafion-coveredelectrode revealed the presence of the Nafion film thatdeveloped tiny cracks after evaporation of the solvent(Fig. 1(A)(b)). The SEM image of the bismuth coating onthe Nafion-covered electrode (Fig. 1(A)(c)) revealed that theaccumulated bismuth film was relatively, but not entirely,uniform in terms of thickness (isolated locations with higherthickness appeared as brighter patches in Fig. 1(A)(c)) and

Fig. 1 SEM (A) and optical microscopy (B) images of the surface of: (a) a glassy-carbon electrode; (b) a Nafion-covered glassy-carbon electrode,and; (c) a Nafion-covered glassy-carbon electrode plated with bismuth (electrode diameter 3 mm; bismuth was deposited from a 50 mg l21 Bi(III)solution for 2 min at 21.0 V).

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distribution over the surface (some parts of the electrode werenot coated with bismuth). It is interesting to note that there wasno indication of preferential deposition of bismuth in thecracks of the polymer film.

Comparison between the SWASV height on a bare BFE anda MTNCBFE indicated that on the MTNCBFE the sensi-tivities for Pb and Cd were enhanced by a factor of 2.5 and 3,respectively, while the sensitivity for Zn was similar on the twoelectrodes (Fig. 2 and Table 1). Examination of the forwardand reverse SW stripping currents on the MTNCBFE(Fig. 2(b)) showed that for Pb and Cd both the constituentcurrents were higher. This phenomenon has also been observedin the case of pulsed stripping techniques on NCMFE’s24,28 andcan be attributed to the more efficient replating of Pb and Cdat the end of the forward pulse as the Nafion film helpedconfine the cationic oxidized species close to the electrodesurface.24 On the contrary, on the bare BFE the oxidized

species were able to diffuse away from the electrode beforesignificant replating occurred. However, clearly this mechanismwas not operative in the case of Zn since the NCBFE and thebare BFE produced similar sensitivity. This may be due tothe lower reversibility of Zn (as indicated by the low values ofthe reverse current in Fig. 2), that caused Zn to be moreinsensitive to the signal-enhancing redox cycling mechanismthat occurred on the NCBFE. On the other hand, SWASVexperiments conducted with a Nafion film without a bismuthcoating indicated that weak stripping peaks (much lower inheight than in the presence of the bismuth film) were obtainedfor all three metal ions, in accordance with previous work onNCMFE’s.24 This could be explained by the fact that at pH 4.5used in this work, the sulfonate groups in the Nafion film werenegatively charged and, as a result, the polymeric membraneacted as a cation-exchanger facilitating the non-faradaicpreconcentration of metal cations.28,37,38 Therefore, a synergisticeffect of redox cycling and cation-exchanging preconcentration,both due to the presence of the Nafion film, may account for theincrease in sensitivity on the NCBFE over the bare BFE. In brief,these findings suggested that the sensitivity for the three metalsbenefited on the NCBFE in the following order: Cd w Pb w Zn.The peak enhancement observed on the NCBFE for Pb and Cdas compared to the bare BFE has significant implicationsregarding the analytical sensitivity of this type of electrode in theultra-trace determination of these metals, in addition to thetolerance to surfactants that will be discussed below.

The peak positions of the three metals were also shifted tothe cathodic direction on the MTNCBFE as compared to thebare BFE (by 24, 34 and 28 mV for Pb, Cd and Zn, respec-tively) (Table 1). This shift was indicative of coulombicinteractions between the cation-exchanging Nafion film andthe accumulated metals that affect the redox potential of thelatter as mentioned earlier in the case of NCMFE’s.24

The effect of the thickness of the Nafion film on the strippingresponse for Zn, Cd and Pb with in situ Bi plating is illustratedin Fig. 3(a) and Table 1. The peak currents were highest for theMTNCBFE followed by the LTNCBFE and the HTNCBFE.The loss in sensitivity for very thick Nafion films has beenobserved earlier in conjunction with NCMFE’s.24 Thisphenomenon could be explained by the fact that high thicknessNafion films displayed large cracks due to contractive forceswithin the film. Tiny cracks also developed on the MTNCBFEwhen seen at high magnification (as illustrated in the SEMimage in Fig. 1(A)(b)) but those on the HTNCBFE were muchmore prominent and clearly visible even at low magnification.These openings in the polymer structure allowed the oxidizedspecies to diffuse away from the electrode before significantreplating occurred so that the redox cyclic mechanism dis-cussed earlier in conjunction with the medium thickness Nafionfilm was impaired.24 The presence of Nafion also affected thepeak widths, in general causing an increase in the peak widthscompared to the bare BFE. The Zn exhibited the highest degreeof widening whereas the Cd and Pb peaks were only slightlyaffected by the thickness of the Nafion film (Table 1). However,no overlap between the Cd and Pb peak was observed for the

Fig. 2 Comparative SWASV signals and the constituent differential,forward and reverse currents (Id, I f and Ir, respectively) on: (a) a bareBFE, and; (b) a MTNCBFE for a solution containing 25 mg l21 each ofZn(II), Cd(II) and Pb(II) in 0.1 mol l21 acetate buffer (pH 4.5);deposition potential: 21.4 V; rotation speed: 500 rpm; deposition time:120 s; frequency 25 Hz; pulse height: 40 mV; step increment: 2 mV; Bifilm plated in-situ (Bi(III) concentration: 1,000 mg l21).

Table 1 Effect of the Nafion film thickness on the SWASV peak currents, peak potentials and peak widths for a solution containing 25 mg l21 eachof Zn(II), Cd(II) and Pb(II) in 0.1 mol l21 acetate buffer (pH 4.5)

Zn Cd Pb

ip/mA Ep/mV W1/2/mV ip/mA Ep/mV W1/2/mV ip/mA Ep/mV W1/2/mV

BFE 9.1 21.150 51 13.8 20.778 42 17.9 20.548 41LTNCBFE 12.3 21.172 76 30.2 20.802 44 35.9 20.568 42MTNCBFE 13.7 21.178 81 40.5 20.812 45 44.3 20.572 43HTNCBFE 4.8 21.162 87 24.6 20.808 45 29.7 20.568 45a Conditions as in Fig. 2.

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NCBFE’s as reported in previous work.30 On the contrary, theseparation between the Cd and Pb peaks on the MTNCBFEwas excellent and much better than on the analogousMTNCMFE as shown in Fig. 3(b). Comparing theMTNCBFE and the MTNCMFE in terms of peak heights,the sensitivity for Cd and Pb was similar on the two electrodeswhile the sensitivity for Zn was worse on the MTNCBFE, asindeed observed in the case of bare BFE’s and MFE’s.8

The nominal thickness of the Bi film could be controlled byvarying the Bi(III) concentration in the sample. The effect of theBi film thickness on the SWASV response of Cd, Pb and Znwas investigated on a MTNCBFE plated in-situ with Bi(III)concentrations ranging from 250 to 10,000 mg l21. For a 120 sdeposition time, the average thickness of the Bi film rangedfrom 0.045 to 1.8 nm, respectively. For Bi(III) concentrationslower than 1,000 mg l21, no definite trend was observed, mostlikely because only small amounts of Bi were plated on theelectrode (as, indeed, indicated by the very weak bismuth peaksin the respective voltammograms). For Bi(III) concentrationshigher than 1,000 mg l21, the Pb and Cd peaks exhibited adecrease with increasing Bi film thickness. The Zn peak wasmaximum at 1,000 mg l21 of Bi(III) and, also, started decreasingat higher concentrations. The results, indicated that the bestcombination of sensitivity, peak sharpness and backgroundcontribution (especially close to the Zn peak) was obtained fora Bi(III) concentration of 1000 mg l21.

Mass-transfer conditions

The effect of the preconcentration time on the SWASV peakheights of Zn, Cd and Pb is illustrated in Fig. 4(a). At shorter

preconcentration times, the stripping currents for the threemetals increased linearly with the deposition time since in thisrange the enhanced accumulation of the metals prevailed.However, as the deposition times increased, the thickness of thebismuth film also increased and its effect became more severe(tending to cause a decrease in the peak heights, as mentionedin the previous paragraph). Thus, the non-linearity at higherdeposition times was caused by a competition between twoeffects: the enhancing effect owing to the enhanced accumula-tion and the suppressing effect due to the thicker bismuth films.

For a membrane-covered rotating-disk electrode, the plot ofthe current versus the square root of the electrode rotationspeed gives information about the diffusion process to theelectrode surface, a linear plot implying that the overalldiffusion rate of the analyte to the electrode surface iscontrolled by convective mass-transfer.39 While its is knownthat the diffusion coefficients of metal cations in Nafion arelower than those in the aqueous phase,24,39,40 the effectivediffusion coefficients in the Nafion phase (i.e. the product of thediffusion coefficients in the Nafion phase with the partitioncoefficients between the aqueous and the polymeric phase)approach those in the aqueous phase.40 In this work, in therange of the available rotation speeds (500–3,000 rpm) the peakheights of the three metals increased linearly with the squareroot of the electrode rotation speed (Fig. 4(b)). This experimentsuggested that the deposition process was, indeed, diffusion-controlled in the range of the deposition conditions typicallyemployed in SWASV, in agreement with previous work withNCMFE’s.24,29 The same conclusions about mass-transfercontrol can be drawn for the deposition of bismuth itself,indicating that the same amount of bismuth is expected on the

Fig. 3 (a) Effect of the Nafion film thickness on the SWASV signal fora solution containing 25 mg l21 each of Zn(II), Cd(II) and Pb(II) in0.1 mol l21 acetate buffer (pH 4.5); (b) Comparison between aMTNCBFE and a MTNCMFE. Conditions as in Fig. 2 with Hg(II)concentration: 5,000 mg l21.

Fig. 4 Effect of: (a) the preconcentration time, and; (b) the electroderotation speed on the SWASV peak height for a solution containing25 mg l21 each of Zn(II) (+), Cd(II) (&) and Pb(II) (#) in 0.1 mol l21

acetate buffer (pH 4.5). Conditions as in Fig. 2

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NCBFE’s and the bare BFE’s. Yet, as mentioned previously,the amount of bismuth was always lower on the NCBFE’s anddecreased with increasing Nafion thickness. A likely explana-tion of this effect are steric effects: the physical presence of thepolymeric coating on the electrode surface caused a decrease inthe highly active surface area of the glassy carbon substrate,41

leading to a decrease of the metal deposited. Also, on thickerNafion films there is an increased likelihood that a significantamount of bismuth is reduced and deposited on the polymeritself; due to the weak adhesion forces between the polymer sub-strate and the metal, the bismuth film could be more susceptible tomechanical dislodgement during convective accumulation.

Square wave parameters

The square wave parameters studied were the SW frequency,the step height and the pulse height. Increasing the SWfrequency resulted in a moderate increase in the peak heights ofCd, Pb and Zn owing to the increase in the effective scan rate.Increasing the step increment did not cause a significantincrease in the Cd and Pb peak heights despite the drasticincrease in the effective scan rate while, at the same time, the Znpeak height exhibited a marked increase. It has been observedearlier that the SW peak current of reversible or quasi-reversible reactions may be insensitive to variations in the stepheight as opposed to the SW peak height of irreversiblereactions that exhibit significant enhancement upon increasingthe step increment.42 This observation supported the notionthat the oxidation of Zn was less reversible than the oxidationof Cd and Pb. The Zn, Cd and Pb peak positions were shiftedanodically by increasing either the scan increment or thefrequency. This shift was more significant for the Zn peak andthis observation further corroborated the earlier conclusionabout the low reversibility of Zn. The heights of all three metalsincreased almost linearly with increasing pulse height but theresolution between adjacent peaks deteriorated, as expected.The conditions selected for quantitative measurements at lowmetal concentrations were: SW frequency: 50 Hz; step height:4 mV, and; pulse height: 40 mV.

Calibration parameters

For a deposition time of 180 s, the calibration curves for thethree metals were linear in the range 1–20 mg l21 and could berepresented by the equations:

IPb (mA) ~ (2.66 ¡ 0.042) (mA mg21 l)CPb (mg l21) 1 (0.26 ¡ 0.11) (mA)

ICd (mA) ~ (2.37 ¡ 0.046) (mA mg21 l)CCd (mg l21) 1 (0.23 ¡ 0.21) (mA)

IZn (mA) ~ (0.74 ¡ 0.017) (mA mg21 l)CZn (mg l21) 1 (0.17 ¡ 0.13) (mA)

The sensitivity of the analysis could be increased byemploying higher deposition times at the expense of narrowerlinear ranges; the sensitivity enhancement as a function ofthe deposition time could be estimated from Fig. 4(a). Theregression coefficients were 0.992, 0.997 and 0.995 and thecoefficients of variation were 3.1%, 3.7% and 3.5% for Zn, Pband Cd, respectively, at the 10 mg l21 level (n ~ 10). The limitsof detection were determined as the metal ion concentrationsproducing a signal-to-noise ratio of 3 and were calculated as0.1 mg l21 for Cd and for Pb and 0.4 mg l21 for Zn for adeposition time of 10 min.

Performance and stability of NCBFE in the presence ofsurfactants

MFE’s are particularly prone to interference from surfactantsthat adsorb and foul the electrode surface.14 The effect oftypical surfactants has also been investigated on BFE’s.30 Forthis work, Triton X-100 and gelatin were selected as ‘‘model’’non-ionic surfactants compounds. The effect of these surface-active compounds was compared on the MTNCBFE and thebare BFE under identical conditions (Fig. 5(A) and 5(B)).Triton X-100 caused more severe interference, in terms of peakheight suppression, than similar concentrations of gelatin on

Fig. 5 (a) Effect of different concentrations of: (A) gelatin, and (B) Triton X-100 on: (a) a bare BFE, and; (b) on a MTNCBFE on the SWASV peakheights for a solution containing 12.5 mg l21 each of Zn(II) (+), Cd(II) (&) and Pb(II) (#) in 0.1 mol l21 acetate buffer (pH 4.5). Conditions as in Fig. 2

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both the MTNCBFE and the bare BFE. On the other hand, theZn peak height was the most affected by the presence of bothsurfactants and it was completely suppressed on the bare BFEin the presence of 8 mg l21 of either Triton X-100 or gelatin.Comparing the MTNCBFE and the bare BFE, it was clearthat the former was much more tolerant to the presence of non-ionic surface-active compounds than the latter. This resistanceto surfactants was attributed to the polymeric film that formedan effective barrier for the transport of macromoleculesto the electrode surface. The electrode tolerance to surfac-tants increased in the order: LTNCBFE v MTNCBFE v

HTNCBFE, as expected. However, considering that the

HTNCBFE exhibited lower sensitivity than the MTNCBFE(as discussed earlier) and that the LTNCBFE was the mostsensitive to surfactants, the MTNCBFE was selected as the bestcompromise between sensitivity and resistance to interferents.In the presence of 4 mg l21 of Triton X-100, the MTNCBFEafforded a 10-fold peak height enhancement for the Pb peakand a 14-fold enhancement for the Cd peak over a bare BFEwhile the determination of Zn was feasible only on the NCBFEand not on the bare BFE (Fig. 6(a)). It is interesting to notethat, in the presence of 4 mg l21 Triton X-100, the Cd peak onthe bare BFE was shifted to the anodic direction while nosignificant changes in peak positions were observed on theNCBFE (Fig. 6(a)). Initial experiments with an anionicsurfactant (Aerosol OT) and a cationic surfactant (tetrabutyl-ammonium bromide) have indicated that the NCBFE is alsomore tolerant than the bare BFE. The long-term stability ofthe NCBFE in the presence of surfactants was tested for 20repetitive SWASV analysis cycles in a solution containing Cd,Pb and Zn in the presence of 8 mg l21 of gelatin; the long-termstability of the MTNCBFE was satisfactory (Fig. 6(b)) whileon the bare BFE, the peak heights exhibited a decreasing trenddue to the accumulating fouling of the electrode surface. Themechanical robustness of the NCBFE’s was excellent, espe-cially when compared with bare BFE’s. As far as the polymericcoating itself was concerned, a single Nafion membrane couldbe used for a few hours without apparent deterioration of thesignal quality although in samples containing surfactants it wasnecessary to form a new Nafion more frequently. Additionally,the Nafion coating provided good protection to the bismuthfilm from mechanical damage. The NCBFE’s could be wipedwith a tissue without removing the bismuth coating and couldwithstand high rotation speeds of the rotating electrodewithout deterioration of their performance.

Applications

The NCBFE’s were applied to the determination of metalsin different real samples. In all cases, the method of standardadditions was selected for quantification. Representativevoltammograms are illustrated in Fig. 7 and 8 and comparativedata obtained by SWASV and ETAAS for the determination ofPb are tabulated in Table 2.

In the case of tap-water, it would initially seem that the use ofa NCBFE would not confer any particular advantage, as theconcentration of surfactants in this medium was low. However,other benefits were derived from the use of a Nafion coating.First, the sensitivity of the analysis was increased for thedetermination of Pb and Cd on the NCBFE over the bare BFEas discussed earlier. This allowed more extensive dilution of thesample in order to avoid the interference from the large Zn

Fig. 6 (a) Comparative SWASV signals for a solution containing25 mg l21 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l21 acetate buffer(pH 4.5): (1) with no surfactant added, and; (2) in the presence of4 mg l21 Triton X-100, on: (A) a MTNCBFE, and; (B) a bare FBE;(b) stability of the SWASV peak heights on a MTNCBFE for a solutioncontaining 12.5 mg l21 each of Zn(II) (+), Cd(II) (&) and Pb(II) (#) inthe presence of 8 mg l21 gelatin. Conditions as in Fig. 2.

Fig. 7 SWASV signals for the determination of Pb in tap-water on: (a) a MTNCBFE, and; (b) a bare BFE. From below: sample and successivestandard additions of 0.8 mg l21 of Pb(II). Deposition potential: 21.2 V; deposition time: 180 s. Conditions as in Fig. 2.

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peak (as Zn existed at a 100-fold excess over Pb in tap-water)without loss of sensitivity. This advantage would be moresignificant if the sample contained Cd (in the case of our watersample, Cd was undetectable) since the Cd peak would severelyoverlap with the Zn peak. In addition, on the NCBFE the Pbpeak was better separated from the Bi peak since the bismuthpeak on this electrode was lower than the peak on the bare BFE(owing to the lower amount of bismuth preconcentrated on theNCBFE). Comparative voltammograms for the determinationof Pb(II) in tap-water on a MTNCBFE (Fig. 7(a)) and a bareBFE (Fig. 7(b)) demonstrate the better sensitivity and resolu-tion achieved with the former electrode. The results of theanalysis of Pb are shown in Table 2. For the determination ofZn (not shown), the sample was diluted 1:10 with deionisedwater and a short preconcentration time of 30 s was used. Asexpected from the previous study, the sensitivities of the bare

BFE and the NCBFE were similar and the Zn concentrationwas calculated as 265 mg l21 (bare BFE) and 282 mg l21

(NCBFE).An attempt to determine Pb and Cd in urine was also made.

For this analysis, it was found that a preplated MTNCBFEproduced better results than an in-situ plated MTNCBFE.However, the analysis of undiluted urine was not possible oneither the bare BFE or the NCBFE, so that the urine wasdiluted 1:4 with the acetate buffer (pH 4.5). Although urinecontains some potential ligands for Pb and Cd, it has beenshown that analysis at pH 4–6 reflects total metals.43,44 Theanalysis of even the diluted urine was not possible on a bareBFE since the presence of large concentrations of varioussurface-active compounds caused a continuous decrease ofthe metal peak heights upon continuous stripping cycles. Incontrast, analysis on the NCBFE produced a well-defined andstable peak for Pb while the concentration of Cd was below thelimit of detection (Fig. 8(a), lower trace). Standard additions oflow concentrations of Cd and Pb produced clear increase intheir respective peak heights (Fig. 8(a)). Although Cd wasundetectable in this sample, the recovery after spiking thesample with Cd was calculated as 96.5%.

Finally, the NCBFE was applied to the determination ofPb in wine (Fig. 8(b)). The wine sample was also diluted 1:4with the 1 mol l21 acetate buffer (pH 4.5) and a preplatedMTNCBFE was used. Again, Cd was undetectable but a well-defined peak was obtained for Pb (Fig. 8(b), lower trace).

Although a well-defined Zn peak was obtained in both urineand wine, the quantification of Zn in these samples provedunsuccessful only due to the large excess of Cu over Zn in thesesamples that caused the formation of a well-known inter-metallic Cu–Zn compound. This could be alleviated by theaddition of Ga(III) ions (that preferentially bind Cu and releaseZn).9

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Fig. 8 SWASV signals for the determination of Pb and Cd on aMTNCBFE in: (a) urine; from below: sample, sample with addition of0.3 mg l21 Cd(II), and, successive additions of 0.3 mg l21 Cd(II) andPb(II), and; (b) white wine; from below: sample and successive additionsof 2 mg l21 of Pb(II). Deposition potential: 21.2 V; deposition time:180 s; preplated Bi film. Conditions as in Fig. 2.

Table 2 Results for the determination of Pb in tap-water, urine andwhite wine by SWASV and ETAAS

Sample

SWASVa ETAASa

Added/mg l21

Found/mg l21 R%b

Found/mg l21 er

c

Tap-water — 2.6 ¡ 0.1 2.4 ¡ 0.2 8.31.6 4.1 ¡ 0.2 93.8

Urine — 1.5 ¡ 0.1 1.6 ¡ 0.1 26.31.2 2.8 ¡ 0.2 108

Wine — 21.2 ¡ 0.8 23.2 ¡ 1.9 28.68.0 28.9 ¡ 1.0 96.2

a Measurements were performed in triplicate. b Recovery. c Relativeerror.

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