Lithographically fabricated disposable bismuth-film electrodes for the trace determination of Pb(II) and Cd(II) by anodic stripping voltammetry

Electrochimica Acta 53 (2008) 5294–5299

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Lithographically fabricated disposable bismuth-film electrodes for the trace determination of Pb(II) and Cd(II) by anodic stripping voltammetry Christos Kokkinos a , Anastasios Economou a,∗ , Ioannis Raptis b , Constantinos E. Efstathiou a a b

Laboratory of Analytical Chemistry, Department of Chemistry, University of Athens, 157 71 Athens, Greece Institute of Microelectronics, NCSR “Demokritos”, PO Box 60228, Aghia Paraskevi, Athens 153 10, Greece

a r t i c l e

i n f o

Article history: Received 8 January 2008 Received in revised form 20 February 2008 Accepted 23 February 2008 Available online 4 March 2008 Keywords: Stripping voltammetry Bismuth-film electrode Photolithography Sputtering Trace metal analysis

a b s t r a c t This work reports the photolithographic fabrication of disposable bismuth-film electrodes (BiFEs) using a thin-film deposition approach. The deposition of the bismuth layer was carried out by sputtering of metallic bismuth on a silicon substrate while the exact geometry of the BiFEs was produced by photolithography. The utility of these sensors was tested for the simultaneous trace determination of Cd(II) and Pb(II) by square wave anodic stripping voltammetry (SWASV). Using the selected conditions, the limits of detection were 0.5 ␮g l−1 for Pb(II) and 1 ␮g l−1 for Cd(II) at a preconcentration time of 4 min. The interference caused by Cu(II) was alleviated by the addition of ferrocyanide in the sample solution. Finally, the proposed BiFEs were successfully applied to the determination of Cd and Pb in a phosphate fertilizer and a river water sample. These sensors offer wide scope for trace metal analysis in terms of mass-production of mercury-free disposable sensors with performance comparable to their mercury counterparts. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction Anodic stripping voltammetry (ASV) has proved a powerful technique for the determination of trace metals in samples of environmental, clinical and industrial origin [1]. Mercury film electrodes (MFEs) and the hanging mercury drop electrode (HMDE) have been traditionally used in ASV, based on the ability of mercury to form amalgams with many heavy metals. However, the increased risks associated with the use, manipulation and disposal of metallic mercury or mercury salts have led to the search for alternative more environmentally friendly electrode materials. In 2000, a new type of electrode, the bismuth-film electrode (BiFE) – consisting of a thin film of bismuth deposited on a carbon substrate – has been proposed as an alternative to mercury electrodes in ASV [2]. Over the last few years, various new types of bismuth and bismuth-modified electrodes have been developed exhibiting comparable performance to MFEs in ASV [3,4]. The reported methods for preparing an electrode with a bismuth active surface have been either to electroplate metallic bismuth on a conductive material from a Bi(III) solution [3,4] or to modify the bulk or the surface of a solid electrode with metallic Bi or a Bi(III) compound [5–8]. By far the most widely used method for the preparation of BiFEs is electroplating which is usually carried out potentiostatically (in situ or ex situ) [3,4] and, occasionally, galvanostatically [9], by potential cycling [9] or by pulsed amperometry [10]. However, electroplat-

ing involves the use of Bi(III) salts, complicates the experimental procedure and requires a conductive substrate. Recently, our group has introduced a novel method for the formation of a BiFE based on sputtering a silicon substrate with bismuth [11]. The application of such a thin-film approach for the fabrication of BiFEs offers some distinct advantages compared to traditional electroplating. In particular, the use of Bi(III) salts is avoided, the experimental procedure is simplified, a conductive substrate is not necessary, the fabrication parameters (electrode geometry and bismuth-film thickness) can be precisely controlled while, more importantly, mass-production of inexpensive and disposable devices is possible. In the prototype version of sputtered BiFEs, individual electrodes were cut at the desired size from a silicon wafer sputtered with bismuth [11]. In this work, a more refined and precise procedure for the fabrication of sputtered BiFEs is presented, based on photolithography. The proposed photolithographic approach allows the rapid and inexpensive mass-production of bismuth-based sensors with precisely controlled geometry. Moreover, the experimental conditions for the determination of Pb(II) and Cd(II) at trace levels were studied in detail and the method developed was applied to the analysis of real samples. 2. Experimental 2.1. Chemicals and reagents

∗ Corresponding author. E-mail address: [email protected] (A. Economou). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.02.079

All the chemicals were of analytical grade. Doubly distilled water was used throughout. Working metal ion solutions were

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Fig. 1. Steps for the photolithographic fabrication of the Bi-sputtered sensors. (a) Photoresist on Si wafer, (b) mask on photoresist, (c) exposure to light, (d) development, (e) Bi sputtering, (f) photoresist lift-off, (g) photoresist on wafer, (h) mask on photoresist, (i) exposure to light and (j) development.

prepared from 1000 mg l−1 atomic absorption standard solutions after appropriate dilution with water. The stock supporting electrolyte solution was 1 mol l−1 acetate buffer (pH 4.5). A 10−2 mol l−1 solution of K4 [Fe(CN)6 ] and 1000 mg l−1 solutions of Triton X-100, cetyltrimethylammonium bromide (CDAB) and sodium dodecyl sulphate (SDS) were prepared in water. 2.2. Fabrication of the electrodes A schematic diagram of the procedure for the preparation of the sensor is illustrated in Fig. 1. Silicon wafers (3 in diamater, 500 ␮m in thickness) were covered with a layer of SiO2 1080 nm thick by means of wet thermal oxidation. The wafer was spin-coated with a layer of photoresist (1 ␮m thickness) (Fig. 1(a)) and the shape of the electrodes was defined by photolithography (Fig. 1(b)–(d)). Then, bismuth was sputtered on the wafer at a nominal thickness of 400 nm from a Bi target (99.9% purity, Williams Advanced Materials, Buffalo, NY) at a constant current of 7 mA using a thin-film deposition system (CV401, Cooke Vacuum Products, South Norwalk, CT) (Fig. 1(e)). The electrodes were patterned by a lift-off process of the polymer (Fig. 1(f)). A second photolothographic step allowed to define the sensing area and to isolated it from the grip area (Fig. 1(f)–(j)). The final diameter of the working electrodes was 5 mm. Electrical contact of the sensor to the potentiostat was made with a crocodile clip.

2.3. Apparatus A home-made potentiostat was interfaced to a Pentium PC through a multi-function interface card (6025 E PCI, National Instruments, TX). SWASV were carried out by purpose-developed application programmes developed in LabVIEW 7.1 [12]. Experiments were carried out in a standard electrochemical cell equipped with a Ag/AgCl (saturated KCl) reference electrode and a Pt counter electrode. 2.4. Experimental procedure 2.4.1. Sample preparation Ten millilitres of the certified river water (TM-23.3, NWRI, Canada) was buffered to pH 4.5 using ammonia solution, the solution was spiked with ferrocyanide ions to a final concentration of 0.02 mmol l−1 and the solution was subjected to analysis as described in the next section with no further treatment. 0.7 g of the phosphate fertilizer sample was dissolved in concentrated HNO3 and boiled almost to dryness. The residue was reconstituted to 25 ml of 1 mol l−1 acetate buffer (pH 4.5), the solution was filtered through a 0.45 ␮m filter and made up to 250 ml with doubly distilled water. A 10.0 ml portion of the sample was spiked with ferrocyanide ions to a final concentration of 0.02 mmol l−1 and was analysed as described in the next section.

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Fig. 2. DC voltammograms taken on the photolithographically fabricated Bisputtered electrode in different media: (i) 0.1 mol l−1 ammonia buffer (pH 9.5); (ii) 0.1 mol l−1 acetate buffer (pH 4.5); (iii) 0.1 mol l−1 HNO3 (pH 1). Scan rate 50 mV s−1 .

2.4.2. Measurement procedure The sensor was immersed in the sample solution and electrolytic preconcentration of Pb(II) and Cd(II) was carried out at −1.20 V for a predefined time period in stirred solution. After the accumulation, the stirring was stopped, the solution was allowed to settle for 10 s and a square wave voltammetric scan was applied to the working electrode from −1.20 to −0.30 V while the voltammogram was recorded. Then, the electrode was cleaned from traces of remaining target metals for 20 s at −0.30 V under stirring. 3. Results and discussion 3.1. Initial characterisation of the BiFEs The polarization window of the sputtered BiFE was examined in different supporting electrolytes by DC voltammetry by scanning the potential in the range −1.40 to 0.00 V in different media: 0.1 mol l−1 HNO3 (pH 1), 0.1 mol l−1 acetate buffer (pH 4.5) and 0.1 mol l−1 ammonia buffer (pH 9.5) (Fig. 2). All voltammograms displayed a flat middle potential region. At more negative potentials (cathodic side) the reduction of hydrogen ions caused an increase in the background current. The same was caused at more positive potentials (anodic side) by the oxidation of bismuth. The most extended cathodic range was obtained in more alkaline media, while more acidic media led to an extension of the anodic range. The useful potential windows were: −0.35 to −1.40 V in 0.1 mol l−1 ammonia buffer (pH 9.5) (Fig. 2(i)), −0.24 to −1.1 V in 0.1 mol l−1 acetate buffer (pH 4.5) (Fig. 2(ii)) and −0.05 to −0.90 V in 0.1 mol l−1 HNO3 (pH 1) (Fig. 2(iii)). These results were in agreement with the values reported for electroplated BiFEs and metallic bismuth electrodes [13–15]. For the determination of Cd(II) and Pb(II), strongly acidic media were avoided to prevent excessive hydrogen evolution that could interfere with the deposition process and alkaline media were avoided because of the limited anodic range that would cause difficulties in the determination of Pb (since in alkaline media bismuth oxidizes at more negative potential). Therefore, 0.1 mol l−1 acetate buffer (pH 4.5) was selected as the supporting electrolyte. The redox behaviour of the sputtered BiFE was investigated by cyclic voltammetry in 0.1 mol l−1 acetate buffer (pH 4.5) in the potential range −1.00 to +0.40 V (Fig. 3(a)). In the first forward cathodic scan recorded on a fresh electrode (Fig. 3(a)-(i)), an oxidation peak with peak potential at +0.08 V was obtained due to the oxidation of the bismuth on the electrode surface while in the reverse scan a wide reduction peak with peak potential at −0.48 V was obtained due to the reduction of the Bi(III) ions generated in

the forward scan. In a second cyclic scan immediately following the first scan (Fig. 3(a)-(ii)), both the oxidation and the reduction peaks greatly decreased in height. Subsequent scans were erratic indicating that the surface of the electrode was affected and that the conductivity of the electrode was lost. This behaviour was elucidated by comparing the surface structures of an unused electrode and of an electrode that had been subjected to two cyclic scans by using optical microscopy (Fig. 3(b)). The unused electrode exhibited a smooth bismuth surface with only tiny defects (scratches) (Fig. 3(b)-(i)) whereas after two cyclic voltamogramms significant areas of the electrode surface were depleted of bismuth exposing the underlying silicon dioxide layer (Fig. 3(b)-(ii)). These micrographs indicate that the bismuth layer did not undergo uniform oxidation but the oxidation process occurred rather patchily and selectively at particular areas of the electrode. As a result, after each anodic scan, the surface of the bismuth film (which, it must be stressed, coincides with the surface area of the electrode since there is not conductive support) was further fragmented and reduced so that only a percentage of the stripped Bi(III) ions was able to replate during the reverse cathodic scan. This process eventually led to depletion of bismuth from large areas of the electrode, fragmentation of the continuity of the bismuth film and loss of conductivity. This experiment indicates that it is of utmost importance to maintain the electrode at potentials more negative than the oxidation potential of bismuth otherwise oxidation of the bismuth coating will occur accompanied by deterioration of the sensor’s performance. It was found that maintaining the electrode at potentials more negative than −0.30 V in 0.1 mol l−1 acetate buffer (pH 4.5) was a sufficient precaution to preserve the integrity of the bismuth coating. The sputtered BiFE was compared to an electrolytically plated BiFE and an MFE, in terms of their respective stripping response for Cd and Pb using square-wave ASV (SWASV). Comparative SWASV traces taken on the different electrodes are illustrated in Fig. 4. Differences in peak potentials have been observed earlier between electrolytically plated BiFEs and MFEs [16]: the peak potentials of Pb and Cd were generally shifted to more negative values (with the Cd peak exhibiting the more significant shift) at the BiFE (Fig. 4(ii)) compared to the MFE (Fig. 4(i)), resulting in better separation between the Cd and Pb peaks on bismuth electrodes. This effect has not been accounted for so far but is probably related to the fact that the oxidation of the accumulated metals is energetically favoured at bismuth electrodes and therefore occurs at more negative potentials compared to mercury electrodes. On the sputtered BiFE (Fig. 4(iii)), equally sharp and well-defined stripping peaks for both Cd and Pb were obtained, indicating that this type of electrtode is well suited to SWASV detection of these cations. Moreover, on the sputtered BiFE, the Pb and Cd peaks potential were further cathodically shifted compared to the electrolytically plated film electrodes. Another observation was that at the BiFEs (both ex situ plated and sputtered), the sensitivity for Pb was higher than for Cd whereas similar sensitivities ate observed at the MFE. The possibility of carry-over effects was investigated by performing an anodic scan from −1.20 to −0.30 V immediately after each preconcentration/stripping cycle. The appearance of weak Cd and Pb peaks indicated that, with no provision for electrode cleaning, traces of the accumulated metals remained within the bismuth film after the stripping step. In order to quantitatively oxidize the remaining metals, a short cleaning step at −0.30 V for 20 s was introduced between successive stripping cycles which was effective in eliminating any carry-over effects. 3.2. Effect of the preconcentration potential, preconcentration time and square-wave parameters The effect of the deposition potential in the range −0.50 to −1.20 V on the stripping current of Cd and Pb is illustrated in

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Fig. 3. (a) Cyclic voltammograms (CVs) taken on the photolithographically fabricated Bi-sputtered electrode in 0.1 mol l−1 acetate buffer (pH 4.5): (i) first scan; (ii) second scan. Scan rate 50 mV s−1 . (b) Optical micrographs of the sensor: (i) before use; (ii) after two CV scans.

Fig. 5(a). For both Cd and Pb, the stripping peak current was zero at more positive potentials (as the potential was not sufficiently negative to initiate reduction of Cd(II) and Pb(II)), then started to increase as the deposition potential became more negative (kinetically controlled region) and leveled-off at the most negative potentials (mass-transfer control region). The expected sigmoidal shape was strictly obeyed by the Cd stripping current

Fig. 4. Comparative SWASV voltammograms of a solution containing 40 ␮g l−1 each of Cd(II) and Pb(II) on: (i) a preplated MFE on glassy carbon; (ii) a preplated BiFE on glassy carbon; (iii) a photolithographically fabricated Bi-sputtered electrode. Supporting electrolyte: 0.1 mol l−1 acetate buffer (pH 4.5); deposition potential: −1.20 V; deposition time: 120 s; frequency: 50 Hz; pulse height: 40 mV; step increment: 4 mV.

while there was a deviation in the case of Pb. The Pb stripping current increased in the range −0.50 to −0.80 V (where deposition of Cd had not started yet). At the deposition range −0.90 V (i.e. the potential at which deposition of Cd started) to −1.00 V there was a drop in the rate of the Pb current increase which was attributed to a competitive effect of Cd for access to surface sites. This explanation was corroborated by the fact that in solutions containing only Pb(II), the curve of Pb peak height vs. deposition potential was sigmoidal. At deposition potentials more positive than −1.10 V (mass-transfer control region for both metals), the stripping peak currents for Cd and Pb leveled-off and achieved their maximum values. Therefore, a deposition potential of −1.20 V was selected for further work. The electrolytic preconcentration time was studied in the range 15–330 s for a solutions containing ␮g l−1 each of Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5). The plots of the stripping peak height vs. time were rectilinear: the peak heights increased sharply with the deposition time at low depositions times while they started to level-off as the deposition time increased, as shown in Fig. 5(b). Essentially, the stripping response exhibited negligible increase for preconcentration times higher than 4 min which was considered the longest practical time for a satisfactory compromise between high sensitivity and short analysis times. The SW parameters affecting the response were the SW frequency, the SW step increment and the SW pulse height and were investigated using a solution containing 20 ␮g l−1 each of Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5). The effect of the frequency was studied in the range 12.5–200 Hz. As the frequency increased, the peak potentials shifted to the anodic direction (presumably as the oxidation of the metals became less reversible at

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Fig. 5. (a) Effect of the deposition potential on the peak heights of Pb () and Cd () for a solution containing 40 ␮g l−1 of Pb(II) and Cd(II) on a photolithographically fabricated Bi-sputtered electrode. Conditions as in Fig. 4. (b) Effect of the deposition time on the peak heights of Pb (\) and Cd () for a solution containing 20 ␮g l−1 of Cd(II) and Pb(II) on a photolithographically fabricated Bi-sputtered electrode. Conditions as in Fig. 4.

higher frequencies) while the peak heights for Pb and Cd increased up to 100 Hz. Besides, at higher frequencies the separation of the Cd and Pb peaks decreased and the background at the anodic side of the Pb peak deteriorated. The effect of the SW pulse height was studied in the range from 10 to 80 mV. The peak potentials were shifted to the cathodic direction and the peak heights increased significantly upon increase of the pulse height. However, the background deteriorated and the peak widths increased at higher pulse heights. The SW step increment was investigated in the range of 1–16 mV. Interestingly, the Pb peak height was unaffected by the variation in the step increment whereas the Cd peak showed a modest increase. In addition, the widths of the Cd and Pb peaks increased excessively and the peak potentials shifted to the anodic direction at increasing step increments. As a conclusion, the recommended SW settings are: SW frequency: 25–50 Hz (selected 50 Hz); SW pulse height: 40–80 mV (selected 40 mV); SW step increment: 4 mV (selected 4 mV). 3.3. Calibration features Calibration was for Pb(II) and Cd(II) was performed on the sputtered BiFEs at different deposition times. As expected and demonstrated in Fig. 5(b), higher sensitivities were obtained for longer deposition times but at the expense of the linear range. A 120 s deposition time offered a good compromise between sensitivity, analysis time and linear range and was selected for most of the experiments. A series of stripping voltammograms in the range of 5–45 ␮g l−1 of Pb(II) and 10–90 ␮g l−1 of Cd(II) using a 120 s accumulation time is illustrated in Fig. 6 (the respective calibration graphs are shown as an inset in Fig. 6). The analytical sensitivities were Pb: 0.75 ␮A ␮g−1 l (R2 = 0.996) and Cd: 0.32 ␮A ␮g−1 l (R2 = 0.994). On the same sensor, the relative standard deviations were 4.4% for Pb and 3.7% for Cd at the 20 ␮g l−1 level (n = 15) with a preconcentration time of 120 s. The same sensor could be used for at least 15–20 preconcentration/stripping cycles without loss of sensitivity allowing operation in the semi-disposable mode. The limits of detection, calculated at a signal-to-noise ratio of 3 at a preconcentration time of 4 min, were 0.5 ␮g l−1 for Pb(II) and 1 ␮g l−1 for Cd(II).

Fig. 6. A series of voltammograms taken on a photolithographically fabricated Bisputtered electrode for increasing concentrations of Cd(II) and Pb(II). From below: blank and nine successive additions of 5 ␮g l−1 of Pb(II) and 10 ␮g l−1 of Cd(II). (the calibration graph is shown as the inset (Pb: ; Cd: )). Conditions as in Fig. 4.

main surfactant classes on the stripping response of Pb and Cd on the sputtered BiFEs. Both Triton X-100 and CDAB caused significant suppression of the Cd and Pb peak heights even at low surfactant concentration (Fig. 7) while the effect of SDS (not shown) was negligible. Triton X-100 is known to possess strong surface adsorptive properties while the strong surface activity of CDAB is attributed to the electrostatic attraction between its positive charge and the

3.4. Interference study BiFEs, in common with MFEs, are prone to interference form surface-active compounds that adsorb on the electrode and cause deactivation of its surface [17–20]. Triton X-100 (a non-ionic surfactant), CDAB (a cationic surfactant) and SDS (an anionic surfactant) were selected as “model” compounds to study the effect of the

Fig. 7. Effect of the presence of Triton X-100 (—) and CETAB (- - -) on the peak heights of Pb () and Cd () for a solution containing 10 ␮g l−1 each of Cd(II) and Pb(II) on a photolithographically fabricated Bi-sputtered electrode. Conditions as in Fig. 4.

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determination of Pb(II) and Cd(II) in the river water sample are illustrated in Fig. 8 (in which the respective standard additions plot is shown as an inset) The determined concentrations of Cd(II) and Pb(II) were 3.1 ± 0.3 and 2.7 ± 0.3 ␮g l−1 (n = 3) while the certified values are 3.2 and 2.5 ␮g l−1 , respectively. A second sample was a phosphate fertilizer sample in which Cd and Pb originate from the phosphate rock used as the raw material for the production of the fertilizer. The Cd and Pb content determined by the present method on the photolithographically fabricated Bi-sputtered electrode was 4.2 ± 0.4 and 3.1 ± 0.3 mg kg−1 , respectively (n = 3). The same analysis was also carried out by ASV on a mercury-coated wax-impregnated graphite electrode using a previously reported and validated method [25] with Cd and Pb content of 4.3 ± 0.3 and 3.3 ± 0.3 mg kg−1 . Fig. 8. Voltammograms for the determination of Pb(II) and Cd(II) in a certified river water sample on a photolithographically fabricated Bi-sputtered electrode. From below: sample and three standard additions of 5 ␮g l−1 Pb(II) and Cd(II). Conditions as in Fig. 4.

negatively charged electrode surface [17,20]. On the contrary, the negligible effect of the anionic surfactant SDS is accounted for by considering the electrostatic repulsion between the negatively charged surfactant species and the electrode surface [17,20]. The formation of intermetallic compounds is also considered a serious interference in the determination of Pb and Cd by ASV on BiFEs. The well-known interference of Cu(II) in the determination of Pb and Cd has been reported previously on electroplated BiFEs and has been attributed both to the formation of mixed compounds and on the undesired deposition of the target metals on electroplated Cu instead of Bi [21,22]. The interference of Cu(II) on the stripping peaks of Pb and Cd on the sputtered BiFEs was evident on the sputtered BiFEs even at a Cu(II)-to-Pb(II) or Cu(II)-to-Cd(II) concentration ratios 3:1 and was more severe as the Cu(II)-to-metal concentration ratio increased. However, this interference was conveniently and efficiently alleviated by the addition of ferrocyanide ions that form a stable complex with Cu(II), as suggested previously [23]. In our experiments, 0.02 mmol l−1 of ferrocyanide was sufficient to alleviate the interference by 200 ␮g l−1 of Cu(II) on 20 ␮g l−1 of Cd(II) and Pb(II) with no effect on the Cd and Pb peaks. It must be mentioned that, once Cu had deposited on the electrode, it was not possible to clean, and therefore to restore, the surface of the BiFE by electrochemical means (since Cu oxidizes at a more positive potential than Bi). Thus, ferrocyanide was not effective unless added in the solution prior to the accumulation step. In addition, the problem of peak overlap was investigated. It was found that only In(III) and Tl(I) produced stripping peaks at −0.79 and −0.73 V, respectively, that overlapped mainly with the Cd peak while the Pb peak was only slightly affected. Sn(II) has been reported as producing a stripping peak that overlaps with the Pb peak on electrolytically plated BiFEs [24] but no response was obtained on the sputtered BiFEs used in the present work. The presence of other common cations such as Mn(II), Co(II), Ni(II), Na(I), K(I), Ca(II), Mg(II), Zn(II), at a 10-fold excess over Cd(II) and Pb(II) did not affect the stripping peaks of Pb and Cd. 3.5. Applications BFEs were finally applied to the analysis of Pb and Cd in a certified river water sample and a phosphate fertilizer sample. In both cases ferrocyanide ions to were added to a final concentration of 0.02 mmol l−1 in the sample solutions to alleviate the interference by Cu(II). Representative stripping voltammograms for the

4. Conclusions A new type of lithographically fabricated bismuth-sputtered disposable electrode is proposed for the determination of trace metals by ASV. The new BiFEs exhibit significant advantages compared to their electroplated counterparts: the experimental procedure is simplified (since the Bi electroplating step is not necessary), the waste is reduced (because no Bi(III) salts are used) and a conductive substrate is not necessary (since silicon acts as the Bi support). This approach allows strict control of the fabrication parameters and can lead to the mass-production of inexpensive and disposable devices. Different chemical and instrumental parameters (nature of the supporting electrolyte, deposition time, deposition potential, scanning waveform parameters and potential interferences) were investigated with the view to simultaneously determine trace Pb(II) and Cd(II) on these sensors by SWASV. The interference caused by Cu(II) was alleviated by the addition of ferrocyanide in the sample solution. Finally, the electrodes were successfully applied to the analysis of river water and a fertilizer sample. References [1] J. Wang, Stripping Analysis, VCH, Deerfield Beach, Florida, 1985. [2] J. Wang, J.M. Lu, S.B. Hocevar, P.A.M. Farias, B. Ogorevc, Anal. Chem. 72 (2000) 3218. [3] A. Economou, Trends Anal. Chem. 24 (2005) 334. [4] J. Wang, Electroanalysis 17 (2005) 1341. ¨ [5] M. Bukova, P. Grundler, G.U. Flechsig, Electroanalysis 17 (2005) 440. ˇ [6] S.B. Hocevar, I. Svancara, K. Vytras, B. Ogorevc, Electrochim. Acta 51 (2005) 706. ˇ [7] R. Pauliukaite, R. Metelka, I. Svancara, A. Krolicka, A. Bobrowski, K. Vytras, E. Norkus, K. Kalcher, Anal. Bioanal. Chem. 374 (2002) 1155. [8] S.M. Skogvold, O. Mikkelsen, K.H. Schroder, Electroanalysis 21 (2005) 1938. [9] R. Pauliukaite, C.M.A. Brett, Electroanalysis 17 (2005) 1354. [10] S. Legeai, S. Bois, O. Vittori, J. Electroanal. Chem. 591 (2006) 93. [11] C. Kokkinos, A. Economou, I. Raptis, C.E. Efstathiou, T. Speliotis, Electrochem. Commun. 9 (2007) 2795. [12] A. Economou, S.D. Bolis, C.E. Efstathiou, G. Volikakis, Anal. Chim. Acta 467 (2002) 179. ˇ [13] I. Svancara, M. Fairouz, Kh. Ismail, R. Metelka, K. Vytras, Sci. Pap. Univ. Pardubice Ser. A 9 (2003) 31. [14] E.A. Hutton, B. Ogorevc, S.B. Hoˇcevar, F. Weldon, M.R. Smyth, J. Wang, Electrochem. Commun. 3 (2001) 707. [15] R. Pauliukaite, S.B. Hocevar, B. Ogorevc, J. Wang, Electroanalysis 16 (2004) 719. [16] T. Demetriadis, A. Economou, A. Voulgaropoulos, Anal. Chim. Acta 519 (2004) 167. [17] S.B. Hocevar, B. Ogorevc, J. Wang, B. Pihlar, Electroanalysis 14 (2002) 1707. [18] J. Wang, R.P. Deo, S. Thongngamdee, B. Ogorevc, Electroanalysis 13 (2001) 1153. [19] C. Gouveia-Caridade, R. Pauliukaite, C.M.A. Brett, Electroanalysis 18 (2006) 854. [20] J. Jia, L. Cao, Z. Wang, Electroanalysis 19 (2007) 1845. [21] R.O. Kadara, I.E. Tothill, Anal. Bioanal. Chem. 378 (2004) 770. [22] R.O. Kadara, I.E. Tothill, Talanta 66 (2005) 1089. [23] K. Crowley, J. Cassidy, Electroanalysis 14 (2002) 1077. [24] C. Prior, G.S. Walker, Electroanalysis 18 (2006) 823. [25] M. Maroulis, A. Economou, A. Voulgaropoulos, Electroanalysis 19 (2007) 2149.