Wireless Electrosampling of Heavy Metals for Stripping Analysis with

Letter pubs.acs.org/ac

Wireless Electrosampling of Heavy Metals for Stripping Analysis with Bismuth-Based Janus Particles Hanna Sopha,†,‡ Jérome Roche,† Ivan Švancara,‡ and Alexander Kuhn*,† †

Université de Bordeaux, ISM, UMR 5255, ENSCBP, 33607 Pessac, France Department of Analytical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic

‡

ABSTRACT: In this contribution, a wireless method for the electrolytic sampling of heavy metals at special bismuth-modified particles is presented. For the first time, glassy carbon beads were asymmetrically modified with bismuth using bipolar electrochemistry. The resulting chemically asymmetric beads, so-called Janus particles, could be employed for the wireless electroaccumulation of heavy metal ions in the bismuth film. The qualitative and quantitative aspects of this concept have been studied by using anodic stripping voltammetry with Cd2+ and Pb2+ as the model ions. Different experimental and instrumental parameters have been optimized; among others, the concentration of Bi3+ ions, the deposition conditions for the bismuth-film, and the accumulation time of the target heavy metals. The developed concept could be applied to the transfer of heavy metal ions from a solution into a confined space without need to electrically connect the collector (electrode), thus representing an interesting new approach for trace metal analysis in small volumes. external electrical field applied between two feeder electrodes. Although the conducting object is not directly connected to the electrical circuit, there exists a polarization effect with respect to the surrounding solution. Under these conditions, the object behaves like anode and cathode at the same time; hence, it is called bipolar electrode (BPE). If the applied electric field is strong enough, electrochemical redox reactions can be driven at the extremities of the conductor. In a first approximation, the polarization voltage ΔV [V] is directly proportional to the electrical field ε [V m−1] and the length d [m] of the BPE:11

T

he spatially localized analysis of heavy metal ions is a topic of increasing interest in the view of mapping their concentration distribution in various environments.1−3 This could be done by using microelectrode arrangements for the accumulation and subsequent electrochemical stripping analysis.4 However, such electrodes need to be properly connected to an external power supply during the deposition and the stripping step, which makes parallelization, for example, in the form of electrode arrays, rather disadvantageous due to the expensive equipment. Recently, the group of Crooks has suggested the use of bipolar electrochemistry for simultaneous control of hundreds or thousands of electrodes for analytical purposes due to the wireless character of the setup. It has been demonstrated that this approach allows an unprecedented degree of parallelization and simplification of the analytical task, especially for electrocatalysis-assisted detection.5,6 Herein, we describe a complementary concept, also based on bipolar electrochemistry, where heavy metal ions are electrolytically accumulated, in a wireless arrangement, on individual particles and then analyzed by stripping voltammetry. As a first step, this requires the modification of the particles with a layer of bismuth that acts as the host matrix for the reduced heavy metals.7 Such a layer can be generated in a straightforward way, either separately or in situ during the accumulation step.8 In the present work bipolar electrochemistry was chosen for the bismuth deposition, giving rise to intrinsically formed Janus particles (with a different composition at both hemispheres), which then serve as the proper analytic tool. Bipolar electrochemistry is already known for several decades,9,10 and its application at the micrometer scale has first been demonstrated by Fleischmann et al. in the mid 1980s.11 This special electrochemical concept uses an electronic conductor, located in an electrolyte solution and exposed to an © 2014 American Chemical Society

(1)

ΔV = εd

From eq 1, it is obvious that higher electric fields have to be applied for BPE with smaller dimensions. More details about bipolar electrochemistry can be found in the literature.12−14 In recent years, bipolar electrochemistry has attracted considerable interest for the asymmetric modification of objects and the production of Janus particles. Furthermore, bipolar electrochemistry has been used in the fields of material sciences,15−20 development of swimmers,21−25 electronics,26 corrosion tests,27 and, most importantly, also for analytical purposes, such as the performance evaluation of catalysts,5 separations,28 or sensing.29,30 In this work, bipolar electrochemistry is employed for the first time for the wireless preparation of bismuth-coated particles and their subsequent use as sensing site for the detection of heavy metal ions. Since their introduction in the year 2000,31 bismuth-based electrodes (BiEs) have been established as an alternative of Received: September 9, 2014 Accepted: October 21, 2014 Published: October 21, 2014 10515

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obtain bismuth films with different thickness on the surface of the GC beads. Cadmium and lead were accumulated in the bismuth film from freshly prepared aqueous solutions (obtained by diluting the above-mentioned stock solutions in HCl) by applying a potential of 1 kV. Square-wave voltammetric measurements of cadmium and lead were performed as follows: The bismuth-modified carbon beads containing the accumulated heavy metals were attached to a carbon paste electrode, and the potential was scanned from −1.2 to −0.3 V using a frequency of 25 Hz, a potential step of 4 mV, and an amplitude of 50 mV.

ecologically questionable mercury electrodes, and nowadays, they can be found in many laboratories worldwide.7 Such electrodes have been used in many different configurations during the last years, as in situ- or ex situ-prepared bismuth films on different (usually, carbonaceous) substrates,32−36 sputtered37 and compact bismuth electrodes,38 and the bulkmodified variants with admixed bismuth precursors (i.e., bismuth oxide,39 bismuth powder,40 or bismuth-containing ion-associates41). Most of these configurations have been applied for the analysis of various inorganic42−44 and organic species.45−47 In this work, bipolar electrochemistry is for the first time employed for the deposition of bismuth films onto glassy carbon beads as supporting material. Since this substrate is used as BPE, it is located in the solution without any direct electrical connection. Furthermore, heavy metal ions as the target analytes are accumulated wirelessly in the bismuth film, the efficiency of the deposition process being ensured by the formation of the respective BiAMeB alloys,7 which is a process being often compared to the well-known and highly efficient amalgamation at mercury-based electrodes. The modified glassy carbon beads are able to collect the metal ions from the solution at their cathodically polarized side. Finally, the beads are attached to a conventional carbon paste electrode via their uncovered moieties in order to ensure a good electrical contact, and voltammetry as the technique of choice is employed for the determination of the heavy metal ions.

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RESULTS AND DISCUSSION The principles of the bismuth film preparation and the following determination of Cd2+ and Pb2+ ions with bipolar electrochemistry are illustrated in Figure 1. In the first step,

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EXPERIMENTAL SECTION Apparatus. The bipolar electrodeposition of bismuth, as well as of the test metal analytes, cadmium and lead, was carried out using a high voltage generator (Heinzinger PNC 10000200). A gold electrode and a platinum mesh were used as the feeder cathode and anode, respectively. The glassy carbon beads (GC beads, 500−1000 nm; purchased from Alfa Aesar) and the degree of their coverage/modification with bismuth were studied using scanning electron microscopy (SEM, model TM-1000; Hitachi, Japan). For voltammetric measurements, a modular electrochemical system AUTOLAB (model PGSTAT 12; Ecochemie, Utrecht, Holland) was employed driven by GPES software (same manufacturer). A conventional threeelectrode configuration was employed with the modified GC beads attached to a carbon paste electrode (CPE) as the working electrode, Ag/AgCl/KCl(sat.) as the reference (from BAS, USA), and a platinum rod as counter electrode. The carbon paste was prepared by thoroughly mixing carbon powder (natural graphite of the “CR-5″ type;48 Maziva Týn, Czech Republic) with 30% (w/w) paraffin oil (Merck) using a pestle and mortar. This mixture was filled into a pipet tip and electrically connected to the potentiostat. Reagents and Solutions. Bi(NO3)3 pentahydrate (purity: 98+%) was obtained from Fluka and dissolved in DMSO (Sigma-Aldrich). Stock solutions of 1 mM Cd(II) and Pb(II) were prepared from appropriately weighted amounts of CdCl2 (99.99%, Acros Organics) and Pb(CH3COO)2 (99+ %, SigmaAldrich) in 0.01 M HCl. A solution of 0.1 M acetate buffer was used as the supporting electrolyte for voltammetric measurements. Procedures. If not stated otherwise, bismuth was deposited by applying a potential of 1 kV to a 1 mM Bi(NO3)3 solution in DMSO for 20 s. After the first deposition, the used Bi(NO3)3 solution was exchanged with a fresh one and, if necessary, the deposition procedure was repeated several times in order to

Figure 1. Schematic illustration of the bipolar electrodeposition of (a,b) a bismuth film on a glassy carbon bead and (c) cadmium and lead on the bismuth film modified glassy carbon bead.

bismuth was deposited on GC beads used as BPE (steps a and b). The Bi3+ ions dissolved in the electrolyte solution are reduced to metallic bismuth at the cathodically polarized side of the bead. DMSO was used as the electrolyte of choice due to its high resistance; hence, almost all the current is passing through the BPE.18 After the deposition of bismuth onto the surface of the beads, the solution containing bismuth was removed and replaced with an aqueous solution containing model analyte ions, i.e., Cd2+ and/or Pb2+. The other experimental conditions, namely, the separation distance of the feeder electrodes, as well as the position and orientation of the BPEs did not need to be altered (step c). During the application of an external field, the model ions were reductively accumulated in the bismuth film. Modification of the Glassy Carbon Beads with Bismuth. The theoretically needed potential for the modification with bismuth can be calculated, in a first-order approximation, from the difference of the standard potentials (vs SHE) of the reduction and oxidation reactions. At the cathodic side of the BPE, the Bi3+ ions are reduced to metallic bismuth Bi 3 + + 3e− ⇋ Bi 10516

E° = + 0.308 V

(2)

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model species of heavy metals.7 As already mentioned, Cd2+ and Pb2+ ions were accumulated onto the bismuth film by bipolar electrochemistry. Well-developed voltammetric signals could be obtained when Pb2+ ions were alone in solution (see Figure 3a). Lead was accumulated from a 5 μM Pb2+ solution

Simultaneously, an oxidation has to take place at the anodic side of the BPE in order to equilibrate the charge flow, in this case the oxidation of traces of water present in the DMSO solution 2H 2O → 4H+ + O2 + 4e−

E° = +1.23 V

(3)

Then, for the potential difference, one can calculate ΔVmin = |E Bi ° 3+/Bi0 − EO°2 /H2O| = 0.922 V

(4)

which is the minimum potential difference needed for the reactions to occur simultaneously. For the used GC beads with a diameter of ca. 1 mm, it can be estimated, when using eq 1, that the external electric field has to be ε=

0.922 V ΔV = = 922 Vm−1 d 0.001 m

(5)

Therefore, for a distance of 30 mm between the feeder electrodes, a minimal potential of ca. 28 V has to be applied to trigger the reactions at both sides of the BPE. However, this is a theoretical value that would cause reactions just at the extreme ends of the bead that are oriented towards the feeder electrodes, and therefore, in practice, significantly higher potentials have to be applied to cover approximately half of the BPE surface with bismuth. This phenomenon is illustrated in Figure 2. The SEM pictures show GC beads coated with a bismuth film after

Figure 3. Anodic stripping voltammetry of Cd and Pb after bipolar electrochemical accumulation at the bismuth-film modified glassy carbon bead(s). (a) Pb-signal for 5 μM Pb2+; accumulation time, 6 s (dotted line), 20 s (dashed line), and 60 s (solid line); (b) accumulation time of 60 s in 1 μM Pb2+ (dotted line), 2.5 μM Pb2+ (dashed line), and 5 μM Pb2+ (solid line); (c) Cd-signal for an accumulation time of 60 s in 1 μM Cd2+ (dotted line), 5 μM Cd2+ (dashed line), and 10 μM Cd2+ (solid line); (d) simultaneous detection of both metals, 5 μM Cd2+ + 5 μM Pb2+, accumulation time of 60 s, shown together with the baseline (dotted line). All voltammetric measurements are carried out in the SWASV mode with 0.1 M acetate buffer (pH 4.5).

into the bismuth film by using different deposition periods; namely, 6, 20, and 60 s. As can be seen, the signal of lead is increased when the deposition time is raised from 6 to 20 s; however, longer times do not result in a higher voltammetric signal. This might have two different reasons: (i) a possible saturation of the bismuth film with lead (similar to the saturation of a conventional BiFE) or (ii) the exhaustive deposition of all the lead available in the accumulation solution due to its relatively low concentration with respect to a high potential and the long deposition time. Moreover, redox transformations in bipolar electrochemistry are not taking place at the BPE alone, but also at the feeder electrodes. This might result in a fast depletion of Pb2+ ions in the electrolytic solution with just a small quantity accumulated in the bismuth film. The dependence of the height of the voltammetric signal on the lead concentration is depicted by Figure 3b. It is obvious that the peak height is increasing almost linearly with the lead concentration in the accumulation solution used herein, i.e., c(Pb) = 1, 2.5, and 5 μM. However, as quoted, only a fraction of the lead present is accumulated in the BPE, while most of the metal ends up on the feeder cathode. This problem can be circumvented using special separators, such as Nafion membranes, between the BPE and the feeder electrode. Also when the lead concentration in the test solution increases, some properties of the solution are changed (e.g., conductivity), which might result in a different deposition efficiency at the BPE and consequently in a slightly nonlinear calibration curve. In a similar way, when cadmium was the analyte of interest, the peak height for Cd correlates also with its concentration in the solution (Figure 3c) and with the deposition time, up to a

Figure 2. SEM images of bismuth-modified glassy carbon beads using bipolar electrochemistry: (a) 50% of the bead is covered with Bi; (b) only ca. 5% are modified. Experimental conditions: (a) 1 mM Bi(NO3)3 in DMSO, E = 1 kV, tDEP = 40 s; (b) 2 mM Bi(NO3)3 in DMSO, E = 0.1 kV, tDEP = 60 s.

applying different potentials between the feeder electrodes, namely, 0.1 and 1 kV. As can be seen, for 0.1 kV applied between the feeder electrodes, just a small part of about 5% of the surface of the GC bead is covered with bismuth (Figure 2b). However, when a high potential, i.e., 1 kV, is used, about 50% of the sphere is coated with a bismuth film (Figure 2a). The bismuth concentration in the electrolyte solution and the deposition time do not have a significant effect on the fraction of the BPE surface covered with metallic bismuth but may have an impact on the thickness of the deposit (not shown). The bismuth film-modified GC beads showed excellent properties for the determination of Cd2+ and Pb2+ ions, when the bismuth film was deposited from a 1 mM Bi(NO3)3 solution for 20 s. Voltammetric Measurements of Cadmium and Lead. The analytical performance of the bismuth-modified GC beads was examined in the square-wave anodic stripping mode with two typical metal ions, Cd2+ and Pb2+, frequently used as the 10517

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acknowledges financial support from the Institut Universitaire de France.

certain limit (not shown). However, the respective signals were satisfactory only for higher concentrations of cadmium, in this case 5 μM Cd2+, and similar observations were made for an increase in deposition time. Only tDEP > 10 s gives rise to welldeveloped voltammograms, while, for deposition of lead, a tDEP of 6 s is sufficient. Figure 3d confirms that a simultaneous determination of both model ions is feasible. In this experiment, the two signals of cadmium and lead were well-developed and sufficiently separated. However, when higher concentrations of Cd2+ and Pb2+ and longer accumulation periods have been used, the signals were affecting each other, especially those belonging to lead being generally larger.

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CONCLUSIONS In this article, bipolar electrochemistry has been shown to be a new attractive approach for the wireless preparation of bismuth films. The configuration of the so-obtained Janus particles has proven itself to represent a suitable host matrix for the wireless electrolytic accumulation of heavy metals, such as lead and cadmium, by bipolar electrochemistry. Subsequently these metals could be detected, also in parallel  using voltammetric methods. When a sufficiently high potential is applied between the feeder electrodes, bismuth films cover approximately 50% of the surface of the glassy carbon beads selected as the substrate of choice. This degree of coating was found to be ideal with respect to fixing the uncovered side of the beads to the collector electrode, thus enabling the subsequent stripping voltammetric detection. The bismuth films prepared in this way were studied by SEM, revealing smooth and uniform particles that could successfully be employed for the above-mentioned anodic stripping detection of the two model ions, Pb2+ and Cd2+. Despite the already quite satisfying analytical results, there is room for further performance improvement; for instance, (i) chemical pretreatment/modification of the GC-beads, (ii) use of other carbonaceous materials (see, e.g., refs 7 and 48−50 and literature therein), and (iii) optimization of the potential and its time of application in order to control the appropriate bismuth film deposition via its structural morphology (see a set of SEM images in ref 7). From an electroanalytical point of view, the BiFE in the bipolar electrode configuration does not need to be directly connected to a power supply, which allows employing many beads simultaneously as delocalized collectors of heavy metal ions, for example, to monitor actual concentration variations in a confined area.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 0033 5 40 00 65 73. Fax: 0033 5 40 00 27 17. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Education, Youth, and Sports of the Czech Republic (Project CZ.1.07/2.3.00/30.0021 “Enhancement of R&D Pools of Excellence at the University of Pardubice“) is gratefully acknowledged. J.R. and A.K. are grateful for financial support through the French research agency ANR under contract ANR-2011-EMMA-007-01. A.K. 10518

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