Multi-pulse galvanostatic preparation of nanostructured bismuth film electrode for trace metal detection

Accepted Manuscript Title: Multi-pulse galvanostatic preparation of nanostructured bismuth film electrode for trace metal detection Authors: T. Zidariˇc, V. Jovanovski, E. Menart, M. Zorko, M. Kolar, M. Veber, S.B. Hoˇcevar PII: DOI: Reference:

S0925-4005(17)30169-7 http://dx.doi.org/doi:10.1016/j.snb.2017.01.162 SNB 21687

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

4-11-2016 18-1-2017 25-1-2017

Please cite this article as: T.Zidariˇc, V.Jovanovski, E.Menart, M.Zorko, M.Kolar, M.Veber, S.B.Hoˇcevar, Multi-pulse galvanostatic preparation of nanostructured bismuth film electrode for trace metal detection, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.01.162 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Multi-pulse galvanostatic preparation of nanostructured bismuth film electrode for trace metal detection T. Zidariča, V. Jovanovskia, E. Menarta, M. Zorkob, M. Kolarc, M. Veberc, S.B. Hočevara,* a

Department of Analytical Chemistry, National Institute of Chemistry, Hajdrihova 19, SI1000 Ljubljana, Slovenia

b

Department of Materials Chemistry, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia c

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, Ljubljana, Slovenia

*Corresponding author: Samo B. Hočevar Department of Analytical Chemistry National Institute of Chemistry Hajdrihova 19 SI-1000 Ljubljana Slovenia Phone: +386 1 4760 230 Fax: +386 1 4760 300 E-mail: [email protected]

HIGHLIGHTS - nanostructured bismuth film electrode (nsBiFE) exhibits excellent sensitivity for measuring trace heavy metal ions - nsBiFE is characterized by good repeatability and functional stability - multi-pulse galvanostatic protocol enables favorable control of bismuth film deposition

ABSTRACT The preparation of nanostructured bismuth film electrode (nsBiFE) via ex-situ multi-pulse galvanostatic deposition protocol is presented. The nsBiFE was prepared on a glassy carbon substrate electrode and studied for anodic stripping voltammetric detection of trace metal ions. Several important parameters were examined and optimized, such as the composition of plating solution, pulse deposition current, pulse duration (pulse deposition time and relaxation time) etc. The nsBiFE exhibited excellent sensitivity associated with well-defined and reproducible signals of both test analytes, i.e. Cd(II) and Pb(II), accompanied with low background contribution. It revealed good linear behaviour in the examined concentration range of 20 - 100 µg L-1 along with remarkable low limits of detection, i.e. 0.4 µg L-1 for Cd(II) and 0.1 µg L-1 for Pb(II) after 300 s accumulation, and good repeatability with RSDs of 5.1 % and 3.8 % for Cd(II) and Pb(II), respectively.

Keywords: nanostructured bismuth film electrode; multi-pulse galvanostatic deposition; anodic stripping voltammetry; lead; cadmium; trace metal detection

1. Introduction

Since 2000, bismuth film electrode (BiFE) has been present in the field of electrochemical stripping analysis as an attractive non-toxic alternative to the commonly used mercury analogues which have dominated in trace metal analysis during last six decades [1-4]. The most significant advantage of bismuth-based electrodes, apart from their exceptional electroanalytical performance, is their environmentally friendly character due to low toxicity of bismuth and its salts. The insensitivity to dissolved oxygen, relatively wide operational potential window approaching that of mercury electrode, well-defined stripping signals of several (heavy) metal ions along with favourable selectivity and sensitivity contributed to broader utilization of bismuth electrodes [5]. The introduction of these electrodes has inspired several groups to further investigate their interesting electroanalytical characteristics and to expand their applicability toward other inorganic and organic analytes [5]. Hitherto, several types of bismuth electrodes have been introduced and studied, most of them prepared in-situ or ex-situ on different substrates, such as glassy carbon, carbon fibre, carbon paste, boron doped diamond, graphite, gold, platinum, etc. [1, 6-10]. Bismuth electrode can be prepared also via integrating bismuth powder or bismuth salts into (conductive) pasting material, via sputtering bismuth onto a suitable substrate (e.g. onto screen-printed electrode), or prepared as

a bismuth bulk electrode [11-13]. Considering the electrochemical preparation of bismuth film electrode, there are three general approaches by which bismuth film can be electrochemically deposited onto selected substrate electrode. The ex-situ protocol involves a pre-plating step for depositing bismuth film from a separate plating solution; in this case the BiFE can be used for a series of measurements; hence, the overall functional stability of the electrode (electrochemical and mechanical) is more important than in the case of the in-situ preparation, when the bismuth film is deposited simultaneously with target analyte(s), e.g. metal ion(s), during the electrochemical accumulation step from the measurement solution containing surplus Bi(III), and is electrochemically dissolved/stripped off each time at the end of the stripping step [1, 6]. Considering the in-situ plating, a general rule is that Bi(III) concentration should be at least 10-times higher in comparison to the expected analyte concentration to avoid the saturation of bismuth film [7]. Apart from several excellent characteristics of the in-situ prepared BiFE, it is obvious that this approach cannot be used for direct measurements in sea, lakes, rivers, in-vivo and when the addition of Bi(III) into the sample solution is not possible or not desirable. The third method of bismuth film formation is based on modifying the bulk of, e.g. carbon paste or screen-printed electrode, with a selected bismuth compound, for example Bi2O3, which is reduced to metallic bismuth film during the electrochemical accumulation step [11, 14].

It is well-known that the nature of metal deposit (size, morphology, defects) influences physico-chemical and consequently electroanalytical performance and stability of electrochemical sensor. Several studies, but related particularly to the semiconductor research, have shown that the use of reductive current during metal-film deposition can affect the morphology in terms of particle size, distribution etc., and in this perspective, the multi-pulse galvanostatic deposition can be advantageous approach, since the crucial steps in nanocrystal formation, i.e. the nuclei formation and nuclei growth, can be controlled via electrochemical plating parameters; fine grained films with a higher uniformity can be obtained after adjusting selected variables, e.g. pulse waveform, pulse current height, on/off time of pulse cycle etc. [15]. To the best of our knowledge, there has been a single study on a pulse current preparation of bismuth film, but using polycrystalline copper substrate electrode, revealing its poor electroanalytical operation for detecting even 50 g L-1 Pb(II) [16].

In this paper we have demonstrated for the first time the application of a multi-pulse galvanostatic deposition protocol for preparation of a nanostructured bismuth film electrode

(nsBiFE) on a glassy carbon substrate; after the optimization of several important parameters influencing the electroanaytical operation, the nsBiFE revealed excellent performance for detecting trace metal ions in combination with anodic stripping voltammetry.

2. Experimental

2.1. Reagents and solutions Standard stock solutions of Bi(III), Cd(II), Cu(II), Pb(II) and Zn(II) (1000 mg L-1 in 2 - 3 % HNO3) were obtained from Merck and diluted as required. All chemicals used in this study were of analytical grade purity. Acetate buffer solution (0.1 M, pH 4.5) served as the supporting electrolyte and was prepared by mixing appropriate amounts of acetic acid and sodium acetate trihydrate. Water used throughout the work was purified via an Elix 10/MilliQ Gradient unit (Milipore, Bedford, USA).

2.2. Preparation of nsBiFE

Unless stated otherwise, the nsBiFEs were prepared ex-situ by multi-pulse galvanostatic deposition onto a thoroughly cleaned glassy carbon substrate electrode from a 0.1 M acetate buffer plating solution (pH 4.5) containing usually 20 mg L-1 Bi(III) and 40 mg L-1 NaBr as the auxiliary ligand. The multi-pulse galvanostatic deposition protocol consisted of 50 cycles; each cycle encompassed two steps, i.e. pulse deposition time (ton of usually 5 s) and relaxation time (toff of usually 2 s), i.e. the total time of pulse deposition current (ton) = 250 s. The optimized pulse deposition current of -100 µA was applied to the working electrode without stirring the solution. After electrochemical deposition, the nsBiFE was thoroughly rinsed with water and was ready to use.

2.3. Apparatus

For electrochemical deposition of bismuth films and anodic stripping voltammetric (ASV) measurements a modular electrochemical workstation Autolab (Eco Chemie, Utrecht, The Netherlands) equipped with PGSTAT12 and ECD modules and driven by GPES 4.9 software (Eco Chemie) was used. A glassy carbon substrate electrode modified with nanostructured bismuth film was employed as the working electrode, an Ag/AgCl(KCl sat.) electrode as the

reference electrode and a platinum rod as the counter electrode. All experiments were carried out in a one-compartment electrochemical cell at room temperature (23 + 2oC). The morphology of a nanostructured bismuth film was examined using scanning electron microscope (Carl Zeiss, SUPRA 35 VP) combined with EDX module (Inca 400, Oxford Instruments).

2.4. Measurement procedures

All ASV measurements were carried out in a 0.1 M acetate buffer solution containing different amounts (in µg L-1 range) of test metal analytes in the presence of dissolved oxygen. Unless stated otherwise, an accumulation potential of -1.2 V was applied to the working electrode for 120 s under stirring conditions. After the accumulation step, the stirring was stopped, and following 15 s equilibration period, the anodic stripping voltammogram was recorded by applying a positive-going square-wave voltammetric stripping scan from -1.2 to 0.4 V with a frequency of 25 Hz, a potential step of 4 mV and an amplitude of 25 mV.

3. Results and discussion

Initial investigations tackled the comparison of the newly developed nsBiFE, prepared ex-situ via multi-pulse galvanostatic deposition, with its conventional ex-situ (using constant potential) and in-situ prepared counterparts (BiFEs), under the same measurement conditions by recording anodic stripping voltammograms (ASVs) of low concentration levels, i.e. 5 µg L-1, of Cd(II) and Pb(II). As evident from Fig. 1, the nsBiFE and both BiFEs yielded welldeveloped and separated current responses of both test metal analytes being the signal of Cd(II) shifted slightly toward less negative potentials in the case of nsBiFE; the signals obtained at nsBiFE were notably higher than those recorded at the conventional ex-situ and in-situ prepared BiFEs, i.e. 20 % and 34 % increase versus ex-situ prepared BiFE, and 30 % and 184 % increase versus in-situ prepared BiFE for Cd(II) and Pb(II), respectively. In addition, the Cd(II)/Pb(II) signal ratios are similar in the case of both ex-situ prepared electrodes

compared

to

the

in-situ

prepared

accumulation/stripping patterns.

Fig. 1.

analogue

reflecting

different

Aimed at developing the nsBiFE with enhanced electrochemical characteristics, several key parameters were further investigated and optimized. As expected, the electroanalytical performance of nsBiFE was affected by the concentration of Bi(III) in the plating solution. A concentration range of 10 - 50 mg L-1 Bi(III) was examined to find the optimal composition (for comparison, one measurement was carried out also in the absence of Bi(III)); these concentration values also correspond to the concentrations exhibiting optimal performance in previous studies related to conventionally ex-situ prepared BiFEs on different substrates [2, 3]. It was found that the plating solution containing 10 mg L-1 Bi(III) provided relatively good ASV signals of both test analytes; however, and as illustrated in Fig. 2, the ASV responses of both test metal ions were the highest at Bi(III) concentration of 20 mg L-1 and then decreased with increasing concentration of Bi(III). Regarding anodic stripping voltammetric detection at the conventionally (using constant potential) ex-situ prepared BiFEs, usually higher concentrations of Bi(III) were recognized as the most favourable [2, 3]. On the other hand, and as also reported in the literature, concentrations of Bi(III) higher than 50 mg L-1 affect the performance through increased thickness of bismuth film, which as a consequence, might exhibit lower physical stability and/or decreased conductivity [2, 3, 8]. In our study, at Bi(III) concentrations higher than 20 mg L-1, besides the decreased signals of both analytes, the change of Cd(II)/Pb(II) signal ratio was observed, implying that both Cd(II) and Pb(II) are relatively strongly, but rather differently influenced by the concentration of Bi(III) in the plating solution; we can assume that the application of multi-pulse galvanostatic deposition mode resulted in a formation of thinner and uniform, and thus more stable nanostructured bismuth film at lower concentrations of Bi(III); therefore, a concentration of 20 mg L-1 Bi(III) was chosen as optimal and was employed in all further studies. Fig. 2.

Considering the stability and overall electroanalytical performance of nsBiFE, it was observed that these characteristics were dependent on the addition of NaBr as the auxiliary ligand during formation of a nanostructured bismuth film. It was also previously reported that selected (inorganic or organic) additives commonly exhibit an important role in the electrodeposition of metal films onto different conductive substrates, e.g. the complexation of bismuth with bromide ions improves the deposition of bismuth film onto a carbon substrate electrode [6, 16, 17]. Accordingly, the effect of different concentrations of bromide upon the ASV signals of Cd(II) and Pb(II) was explored and is depicted in Fig. 3, which shows that the

amount of ca. 40 mg L-1 NaBr in the plating solution provided the highest ASV responses of both test analytes. Similarly as in the previous study shown in Fig. 2, a rather different influence on Cd(II) and Pb(II) could be observed. The beneficial effect of NaBr was monitored also through repeatability; namely, the addition of 40 mg L-1 NaBr, i.e. the ratio of 1:2 with respect to c(Bi(III))/c(NaBr), resulted in a well-reproducible response of nsBiFE without any memory effect over several repetitive measurements.

Fig. 3.

The preparation of bismuth film via multi-pulse galvanostatic deposition greatly depends on the applied pulse deposition current (Ion) and also its duration, i.e. pulse deposition time (ton) and relaxation time, i.e. the time between pulses (toff). For example, in the field of semiconductor development, it has been generally recognized that pulse electrodeposition yields a finer grained and more homogeneous surface of the metal deposit. By changing pulse parameters, the metal ion concentration at the electrode/solution interface can be controlled [15]; thus, the investigation of pulse deposition current in the range of -10 to -500 µA was carried out with respect to the electroanalytical performance of nsBiFE. As illustrated in Fig. 4, the signals of 10 µg L-1 Cd(II) and Pb(II) increased with increasing cathodic pulse deposition current up to ca. -100 µA, and then attenuated. According to the observations of different research groups related to semiconductor studies, for a fixed ton and toff, the use of increasing (to a certain limit) cathodic/reductive pulse deposition current (Ion) improved the nucleation, thus favouring the nuclei formation rather than crystal growth, i.e. a higher density of nuclei results in a better coverage of the electrode surface [15, 18]; it has also been shown that the deposit morphology strongly depends on the galvanostatic deposition parameters - it evolves from a powdery (larger deposits) to a metallic appearance when the pulse deposition current decreases, i.e. at less negative cathodic current. In addition, if the pulse deposition current is too low, the film peels off the substrate presumably as a consequence of hydrogen evolution at the electrode surface. On the other hand, if the pulse deposition current increases negatively over a certain limit, the deposits become more powdery with decreased uniformity accompanied with lower stability. From this perspective we can assert that in our case, by applying a pulse deposition current more negative than -100 µA, a similar effect might develop resulting in lower electroanalytical performance.

Fig. 4.

Since the multi-pulse galvanostatic deposition provides a favourable route of controlling the bismuth film and hence the sensor's electroanalytical characteristics, a further study was carried out through varying the parameters, such as pulse deposition time (ton) and relaxation time (toff) at fixed pulse deposition current (Ion); the effect of ton is depicted in Fig. 5A for 20 µg L-1 Cd(II) and Pb(II), whereas Fig. 5B shows the effect of t off upon the ASV responses of 40 µg L-1 Cd(II) and Pb(II). Relatively strong effect of pulse deposition time (ton) upon the electrode performance was observed, i.e. the ASV signals for both test metal ions first increased with increasing ton up to ca. 7 s and then started to decrease. While ton of 7 s exhibited slightly higher response of Cd(II), a shorter ton of 5 s provided improved repeatability; thus, the ton of 5 s was chosen as the optimum. The signal attenuation at higher ton (˃7 s) might be induced by less homogenous, more powdery and thicker deposits, similarly as those obtained via constant current (or constant potential) deposition; obviously, and as already observed by other authors in the field of semiconductor development, lower ton yields higher degree of nucleation (larger number of short pulses), thus providing finer grains together with more uniform coverage and enhanced stability [15]. At the same time too short ton resulted in decreased ASV signals due to lower surface coverage. Fig. 5. On the other hand, toff exhibited somewhat lower effect upon the ASV operation of nsBiFE. The highest deposition/nucleation rate can be maintained only for a fraction of time due to instant and strong decrease of the bismuth ion concentration in the vicinity of the electrode surface which leads then to a diffusion-controlled process. During the relaxation time (toff), i.e. when the pulse current is switched off, metal ions diffuse from the bulk solution, thus replenishing their amount at the electrode surface providing the initial conditions and consequently uniform nucleation. Fig. 5B reveals that toff of 2 s assured the highest responses of Cd(II) and Pb(II); in contrast, prolonged toff led to attenuated signals, probably due to a longer exposure of bismuth film to the plating solution at zero current, resulting in the commencement of dissolution and/or oxidation processes.

To provide additional insights into the multi-pulse galvanostatic deposition, the SEM investigation was carried out, as depicted in Fig. 6; the corresponding images of ex-situ electroplated BiFE (at constant potential) and of nsBiFE revealed that pulse current led to

reduction in grain size of bismuth particles. In addition, the so-prepared bismuth particles covered the substrate electrode surface more densely and evenly; the difference is apparently reflected also through improved anodic stripping voltammetric performance of nsBiFE, as already shown in Fig. 1. Fig. 6.

We also explored the operation of nsBiFE for measuring separately Cd(II) or Pb(II) before and after addition of the other one in its ten-fold excess. Fig. 7A depicts the anodic stripping voltammograms of 20 µg L-1 Pb(II) before and after addition of 200 µg L-1 Cd(II), whereas Fig. 7B shows the signals of 20 µg L-1 Cd(II) before and after addition of 200 µg L-1 Pb(II) in the measurement solution; evidently, there is practically no effect after the addition of the second analyte. Likewise, the addition of 50 µg L-1 Zn(II) resulted in very well-developed ASVs for all three metal analytes without any cross-interference; however, and as anticipated, after the addition of 50 µg L-1 Cu(II), the signals of Zn(II) and Cd(II) were considerably supressed, whereas the signal of Pb(II) decreased by ca. 65 % (not shown). The detrimental effect of Cu(II) can be partially mitigated by the addition of excess Ga(III) in the measurement solution.

Fig. 7. In the case of simultaneous detection of 20 g L-1 Tl(I) together with 10 g L-1 Cd(II) and Pb(II), we observed an overlap of Tl(I) and Cd(II) ASV signals. After addition of 0.01 M EDTA (with chelating affinity toward Cd(II) and Pb(II)) to the measurement solution, the nsBiFE exhibited again a well-defined ASV signal of Tl(I), whereas the signals of other two metal ions were completely suppressed. We also examined the detection of 20 g L-1 In(III) before and after addition of Cd(II) and Pb(II). After addition of 10 g L-1 Cd(II) and Pb(II), we observed a strong overlap between In(III) and Cd(II) which precluded their simultaneous detection; the addition of KBr did not significantly improve the separation.

The sensor's stability was examined via its exposure to ambient air; the measurements were carried out using freshly prepared sensor, then after one hour, and after 72 hours of exposure. The signals of 50 g L-1 Cd(II) and Pb(II) obtained at the same nsBiFE in this time span have proven relatively good electroanalytical stability, as presented in Table 1. After one hour

exposure to ambient air, the sensor's performance did not change considerably with respect to both test metal ions, and after 72 hours, the signal of Cd(II) remained almost unchanged, whereas the signal of Pb(II) increased for ca. 24 %. This rather surprising phenomenon might be attributed to consecutive surface oxidation of bismuth film at ambient air followed by its electrochemical re-reduction during ASV measurements which might alter the accumulation pattern of the analytes due to slightly changed bismuth film deposit. However, this peculiarity should not present a problem, since standard addition protocol is usually used in such measurements. Table 1 caption is : The sensor's performance after its exposure to ambient air.

freshly prepared after 1 hour after 72 hours

Cd(II) / area [µC] 0,198 0,191 0,197

Pb(II) / area [µC] 0,152 0,140 0,189

Table 1. The nsBiFE was further investigated by following ASV signals of Cd(II) and Pb(II) while increasing their concentrations in 10 µg L-1 steps, as shown in Fig. 8. The ASVs recorded in combination with 120 s accumulation time exhibited well-developed signals over a flat background along with a linear behaviour in the examined concentration range of 20 - 100 µg L-1 with correlation coefficients r2 of 0.996 and 0.992 for Cd(II) and Pb(II), respectively. In addition, an overlap of calibration curves can be observed due to different sensitivities towards Cd(II) and Pb(II); the overlap can be detected at concentration levels of ca. 40 µg L-1 depending on the applied conditions. Slight shifts of ASV signals toward less negative potentials with increasing concentrations of both metal analytes can be observed implying on attractive forces within the metal deposit; the effect is somewhat more pronounced in the case of Pb(II) [19]. The novel nsBiFE was characterized by good repeatability with RSDs of 5.1 % for Cd(II) and 3.8 % for Pb(II) associated with 120 s accumulation time (c = 10 µg L-1); such performance over a continuous 45 min operation reflected favourable stability of the nanostructured bismuth film sensor. The calculated limits of detection based on 3σ criterion for the 1 µg L-1 data point and after 300 s accumulation were 0.4 µg L-1 for Cd(II) and 0.1 µg L-1 for Pb(II). In the case of using extended deposition times, even lower limits of detection can be expected. Importantly, the nsBiFE did not display any memory effects when examining test metal ions.

Fig. 8.

In addition, we examined the potential applicability of this sensor via measuring Cd(II) and Pb(II) in a tap water sample. The sample was spiked with 10 g L-1 Cd(II) and Pb(II) and acidified with acetate buffer solution (50:50). The stripping analysis was carried out using standard addition method (3 standard additions) in the presence of dissolved oxygen without any other sample pretreatment. The sensor revealed excellent electroanalytical performance with calculated recoveries of 97.6 % for Cd(II) (r2 = 0.996) and 103.4 % for Pb(II) (r2 = 0.999).

4. Conclusions

The study of nanostructured bismuth film electrode (nsBiFE) prepared ex-situ by means of multi-pulse galvanostatic deposition is presented. The optimization of important parameters, such as pulse deposition current, pulse deposition time, pulse relaxation time, and the presence of auxiliary ligand, i.e. NaBr, resulted in nsBiFE which exhibited advantageous electroanalytical performance. Low limits of detection for test metal ions in a nanomolar range, i.e. 3.6 nM (0.4 µg L-1) for Cd(II) and 0.5 nM (0.1 µg L-1) for Pb(II) were obtained using anodic stripping voltammetric mode associated with accumulation time of 300 s. It can be concluded that the overall performance of nsBiFE suggests its potential application in trace metal analysis as another promising type of still increasing family of bismuth electrodes which deserves to be further explored.

Acknowledgements

This work was supported by the Slovenian Research Agency (ARRS), Program No. P1-0034.

Author Biographies Dr. Samo B. Hočevar received his PhD in 2002 from the University of Ljubljana, Faculty of Chemistry and Chemical Technology, Slovenia. Presently, he is the Head of the Department of Analytical Chemistry at the National Institute of Chemistry, Slovenia. His research interests include development, study, preparation, miniaturization and application of advanced electrochemical sensors.

Tanja Zidarič received her BSc in 2012 from the University of Ljubljana, Biotechnical Faculty, Slovenia. Presently, she is a PhD student at the National Institute of Chemistry, Department of Analytical Chemistry, Slovenia. Her research interests include development, study and preparation of electrochemical sensors.

Dr. Vasko Jovanovski received his PhD in 2007 from the University of Ljubljana, Faculty of Chemistry and Chemical Technology, Slovenia, and has 14 years of experience in studying ionic liquids and in advanced electroanalysis. Presently, he is a researcher at the National Institute of Chemistry, Department of Analytical Chemistry, Slovenia. Among others, his research interests include also development and study of electrochemical sensors.

Dr. Eva Menart received her PhD in 2013 from the University College London, UK and had since been a researcher at the National Institute of Chemistry, Department of Analytical Chemistry, Slovenia, until the end of 2016. Her main research interest is development of electrochemical sensors, recently focusing on gas sensing. Currently, she is employed in the National Museum of Slovenia.

Dr. Milena Zorko received her PhD in 2016 from the University of Ljubljana, Faculty of Chemistry and Chemical Technology, Slovenia. Presently, she is employed at the National Institute of Chemistry, Department of Materials Chemistry, Slovenia. Her research interests include the preparation and study of novel (nano)materials and their broader application in the fields of batteries, electrodes, textiles, etc.

Dr. Mitja Kolar received his PhD in 2001 from the University of Maribor, Faculty of Chemistry and Chemical Engineering, Slovenia. Presently, he is employed at the University of Ljubljana, Faculty of Chemistry and Chemical Technology, Laboratory for Analytical Chemistry, Slovenia, holding a position of Assistant Professor. Among his research interests are also analytical electrochemistry and development of electrochemical sensors.

Prof. Dr. Marjan Veber received his PhD in 1988 from the University of Ljubljana, Faculty of Natural Sciences and Technology, Slovenia. He was a Professor at the University of Ljubljana, Faculty of Chemistry and Chemical Technology, Laboratory for Analytical Chemistry, Slovenia. He was also the Head of the Laboratory for Analytical Chemistry, and has been retired since 2016. Among his broad research interests in analytical chemistry, he has great experience in advanced electroanalysis.

References [1] J. Wang, J. Lu, S.B. Hočevar, P.A.M. Farias, Bismuth-coated carbon electrodes for anodic stripping voltammetry, Anal. Chem. 72 (2000) 3218-3222. [2] A. Economou, Bismuth-film electrodes: recent developments and potentialities for electroanalysis, Trends in Anal. Chem. 24 (2005) 334-340. [3] J. Wang, Stripping analysis at bismuth electrodes: a review, Electroanalysis 17 (2004) 1341-1346. [4] G. Kefala, A. Economou, A. Voulgaroulos, M. Sofoniou, A study of bismuth-film electrodes for the detection of trace metals by anodic stripping voltammetry and their application to the determination of Pb and Zn in tapwater and human hair, Talanta 61 (2003) 603-610. [5] I. Švancara, C. Prior, S.B. Hočevar, J. Wang, A Decade with Bismuth-Based Electrodes in Electroanalysis, Electroanalysis 22 (2010 ) 1405-14020. [6] E.A. Hutton, S.B. Hočevar, B. Ogorevc, Ex situ preparation of bismuth film microelectrode for use in electrochemical stripping microanalysis, Anal. Chim. Acta 537 (2005) 285-292. [7] A. Królicka, R. Pauliukaitė, I. Švancara, R. Metelka, A. Bobrowski, E. Norkus, K. Kalcher, K. Vytřas, Bismuth-film-plated carbon paste electrodes, Electrochem. Commun. 4 (2002) 193-196. [8] D. Demetriades, A. Economou, A. Voulgaropoulos, A study of pencil-lead bismuth film electrodes for determination of trace metals by anodic stripping voltammetry, Anal. Chim. Acta 519 (2004) 167-172. [9] K.E. Toghill, G.G. Wildgoose, A. Moshar, C. Mulcahy, R.G. Compton, The fabrication and characterization of a bismuth nanoparticle modified boron doped diamond electrode and its application to the simultaneous determination of cadmium(II) and lead(II), Electroanalysis 20 (2008) 1731-1737. [10] S.B. Hočevar, B. Ogorevc, J. Wang, B. Pihlar, A Study on Operational Parameters for Advanced Use of Bismuth Film Electrode in Anodic Stripping Voltammetry, Electroanalysis 14 (2002) 1707-17012. [11] H. Sopha, L. Baldrianová, E. Tesařová, G. Grinciene, T. Weidlich, I. Švancara, S.B. Hoĉevar, A new type of bismuth electrode for electrochemical stripping analysis based on the

ammonium tetrafluorobismuthate bulk-modified carbon paste, Electroanalysis 22 (2010) 1489-1493. [12] N. Lezi, A. Economou, Voltammetric Determination of Neonicotinoid Pesticides at Disposable Screen-Printed Sensors Featuring a Sputtered Bismuth Electrode, Electroanalysis 27 (2015) 2313-2321. [13] R. Pauliukaite, S.B. Hočevar, B. Ogorevc, J. Wang, Characterization and applications of a bismuth bulk electrode, Electroanalysis 16 (2004) 719-723. [14] N. Lezi, A. Economou, P.A. Dimovasilis, P.N. Trikalitis, M.I. Prodromidis, Disposable screen-printed sensors modified with bismuth precursor compounds for the rapid voltammetric screening of trace Pb(II) and Cd(II), Anal. Chim. Acta 728 (2012) 1-8. [15] D. Del Frari, S. Diliberto, N. Stein, C. Boulanger, J.-M. Lecuire, Pulsed electrodeposition of (Bi1-xSbx)2Te3 thermoelectric thin films, J. Appl. Electrochem. 36 (2006) 449-454. [16] A.R. Rajamani, S. Jothi, M.D. Kumar, S. Srikaanth, M.K. Singh, G. Otero-Irurueta, D. Ramasamy, M. Datta, M. Rangarajan, Effects of additives on kinetics, morphologies and leadsensing property of electrodeposited bismuth films, J. Phys. Chem. C 120 (2016) 2239822406. [17] A. Królicka, A. Bobrowski, Bismuth film electrode for adsorptive stripping voltammetry – electrochemical and microscopic study, Electrochem. Commun. 6 (2004) 99-104. [18] V. Richoux, S. Diliberto, C. Boulanger, J. M. Lecuire, Pulsed electrodeposition of bismuth telluride films: Influence of pulse parameters over nucleation and morphology, Electrochim. Acta 52 (2007) 3053-3060. [19] V. Mirčeski, S.B. Hočevar, B. Ogorevc, R. Gulaboski, I. Drangov, Diagnostics of anodic stripping mechanisms under square-wave voltammetry conditions using bismuth film substrates, Anal. Chem. 84 (2012) 4429-4436.

FIGURE CAPTIONS Fig. 1. ASVs of 5 µg L-1 Cd(II) and Pb(II) obtained at conventional ex-situ (dashed line) and in-situ (dotted line) prepared BiFEs, and at nsBiFE (solid line) in 0.1 M acetate buffer solution (pH 4.5) using accumulation potential of -1.2 V for 120 s. Plating conditions for exsitu prepared BiFE and nsBiFE: 0.1 M acetate buffer solution containing 20 mg L-1 Bi(III) and 40 mg L-1 NaBr; for BiFE: deposition potential of -1.2 V for 120 s; for nsBiFE: 50 pulses (ton of 5 s) of -100 µA alternating with 50 pulses (toff of 2 s) of zero current; for in-situ prepared BiFE: acetate buffer solution containing 1 mg L-1 Bi(III). Fig. 2. The effect of Bi(III) concentration in the plating solution upon ASV responses of 20 µg L-1 Cd(II) and Pb(II) at nsBiFE in 0.1 M acetate buffer solution (pH 4.5) using accumulation potential of -1.2 V for 60 s. Plating conditions: 0.1 M acetate buffer solution containing increasing levels of Bi(III) in 10 mg L-1 steps; 50 pulses (ton of 5 s) of -100 µA alternating with 50 pulses (toff of 2 s) of zero current. Fig. 3. The effect of NaBr concentration in the plating solution upon ASV responses of 40 µg L-1 Cd(II) and Pb(II) at nsBiFE in 0.1 M acetate buffer solution (pH 4.5) using accumulation potential of -1.2 V for 120 s. Plating conditions: 0.1 M acetate buffer solution containing 20 mg L-1 Bi(III) and increasing levels of NaBr in 10 mg L-1 steps. Other conditions are as in Fig. 2. Fig. 4. The effect of pulse deposition current upon ASV responses of 10 g L-1 Cd(II) and Pb(II) at nsBiFE. Plating conditions: 0.1 M acetate buffer solution containing 20 mg L-1 Bi(III) and 40 mg L-1 NaBr; 50 pulses (ton of 5 s) of different pulse deposition currents alternating with 50 pulses (toff of 2 s) of zero current. Other conditions are as in Fig. 3. Fig. 5. The effect of ton upon ASV responses of 20 g L-1 Cd(II) and Pb(II); plating conditions: 50 pulses of -100 µA with different ton alternating with 50 pulses (toff of 2 s) of zero current (A), and the effect of toff upon ASV responses of 40 g L-1 Cd(II) and Pb(II); plating conditions: 50 pulses (ton of 5 s) of -100 µA alternating with 50 pulses of zero current with different toff (B). Other conditions are as in Fig. 3. Fig. 6. SEM images of BiFE prepared with constant potential (left) and nsBiFE prepared via multi-pulse galvanostatic deposition (right). Plating conditions: 0.1 M acetate buffer solution containing 20 mg L-1 Bi(III) and 40 mg L-1 NaBr; for BiFE: deposition potential of -1.2 V for 120 s; for nsBiFE: 50 pulses (ton of 5 s) of -100 µA alternating with 50 pulses (toff of 2 s) of zero current. Fig. 7. ASVs of 20 µg L-1 Pb(II) before (solid line) and after (dashed line) addition of 200 µg L-1 Cd(II) (A), and ASVs of 20 µg L-1 Cd(II) before (solid line) and after (dashed line) addition of 200 µg L-1 Pb(II) (B) at nsBiFE in 0.1 M acetate buffer solution (pH 4.5) using accumulation potential of -1.2 V for 120 s. Plating conditions: 0.1 M acetate buffer solution

containing 20 mg L-1 Bi(III) and 40 mg L-1 NaBr; 50 pulses (ton of 5 s) of -100 µA alternating with 50 pulses (toff of 2 s) of zero current. Fig. 8. ASVs for increasing concentrations of Cd(II) and Pb(II) in 10 µg L-1 steps together with background (dashed line) at nsBiFE. Inset depicts corresponding calibration plots. Other conditions are as in Fig. 7.

LIST OF FIGURES

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.