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Highly ordered macroporous poly-3,4-ortho-xylendioxythiophene electrodes as a sensitive analytical tool for heavy metal quantification Gerardo Salinas, Bernardo Antonio Frontana-Uribe, Stephane Reculusa, Patrick Garrigue, and Alexander Kuhn Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03779 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018
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Analytical Chemistry
Highly ordered macroporous poly-3,4-ortho-xylendioxythiophene electrodes as a sensitive analytical tool for heavy metal quantification Gerardo Salinas,†‡ Bernardo A. Frontana-Uribe,*‡§ Stéphane Reculusa,†|| Patrick Garrigue,†|| Alexander Kuhn*†|| †
Univ. Bordeaux, ISM, UMR 5255, Bordeaux INP, Site ENSCBP, F 33607 Pessac, France. Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM, Toluca 50200, Estado de México. México. § Instituto de Química, Universidad Autónoma de México, Circuito Exterior, Ciudad Universidad 04510, CDMX, México. || CNRS, ISM, UMR 5255, F-33400 Talence, France. ‡
ABSTRACT: Highly ordered macroporous electrodes of the conducting polymer poly-3,4-ortho-xylendioxythiophene (PXDOT) are presented as a sensitive analytical tool for heavy metal ion quantification due to a controlled gain in electroactive area. They were designed by using colloidal crystal templates. A direct correlation between the final number of porous layers and the deposition charge (Qd), employed for electropolymerization is observed. All the electrodes exhibit a surface-templated structure due to an interaction between the radical cation, formed during the electropolymerization, and the surface groups of the silica beads. The voltamperometric response of the macroporous PXDOT electrodes shows a rather fast electron transfer with ∆Ep values between 70 mV and 110 mV. Square wave anodic stripping voltammetric (SWASV) analysis of Cu2+ as a representative heavy metal ion shows a linear response in the sub-ppm range. As a model application the efficient quantification of Cu2+ in a commercial Mezcal sample is validated by the standard addition method and the results correlate in a very satisfying way with the values obtained by atomic absorption spectroscopy.
Chemically modified electrodes with conducting polymers are interesting alternatives for heavy metal ion quantification.13 They allow avoiding the use of hazardous electrode materials and reduce the preconcentration times, due to the possible surface enrichment facilitated by the formation of coordination bonds between the electron-rich atoms within the monomeric structure and the metal ions.2 Furthermore, the natural porosity of the conducting polymers generates an increase in the electroactive area, which results in well-defined stripping peaks and leads to an improvement in sensitivity compared to classic electrodes. Polypyrrole (PPy),4-6 polyaniline (PANI),7,8 polythiophene (PTh)9-11 and poly-3,4-ethylendioxythiophene (PEDOT),12 deposited on flat electrodes, have been used to quantify several heavy metal ions in aqueous solutions by differential pulse and square wave anodic stripping voltammetry. Most of these electrodes require complexing agents inside the polymer matrix, as dopants or covalently immobilized, to improve the selectivity and sensitivity. A promising approach to increase the electroactive area is the use of highly ordered macroporous electrodes.13 The formation of macroporous electrodes of PPy, PANI and PTh on flat electrodes, using colloidal crystal templates of polystyrene beads, has been previously reported.14,15 However, such sophisticated architectures have never been used for the analysis of metal ions. Heim et al. generated PPy electrodeposits on Au sheets coated with silica beads of different diameters using the LangmuirBlodgett (LB) technique.16 Ordered structures with interconnected pores and uniform thickness were obtained. In general, the formation of ordered porous structures within conducting polymers improves specific properties of these materials, due to efficient ion transport inside the polymer matrix and an
increase of contact surface, which allows their possible use as electrodes for electroanalytical quantification of heavy metal ions.17 In this work we present the formation of highly ordered macroporous electrodes made out of poly-3,4-orthoxylendioxythiophene (PXDOT), by using the LB technique and their possible application in the quantification of metal ions. This polymer was chosen due to its excellent stability and electrochemical performance in hydroalcoholic mixtures.18 Cu2+ was used as a representative model heavy metal ion19 to evaluate the performance of the highly ordered macroporous PXDOT electrode. High concentrations of this metal ion may cause serious damage to human health involving liver and brain damage,20,21 and therefore its efficient quantification is important. The silica template was formed by transferring 20 layers of silica particles (d = 600 nm) onto the surface of a gold wire (d = 0.25 mm), after initial O2 plasma cleaning, by means of the LB technique.16,22 Oxidation of XDOT23 (5 mM in a 0.1 M LiClO4/ACN solution) occurs via an irreversible process on the surface of the templated Au electrode, showing in the reverse cycle of the voltamperogram the typical trace-cross effect observed during the electrochemical polymerization of π-conjugated systems (Figure S1).24 A potential of 1.29 V vs Ag/AgCl was selected to perform the potentiostatic electropolymerization, as this value allows a homogeneous growth inside the template. The potentiostatic electropolymerization of XDOT was performed at different values of deposition charge (Qd; from 2 mC to 15 mC) (Figure 1). At the first stage of electropolymerization (t < 5 s) the decrease in current reflects the effect of the double layer charging and the beginning
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Analytical Chemistry of the nucleation process. Subsequently, the current increases up to a constant value, where the polymer starts infiltrating the template. None of the plots shows the characteristic current oscillations observed for potentiostatic metal deposition.25,26
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time / s Figure 1. Potentiostatic electropolymerization of 5 mM XDOT in an 0.1 M LiClO4/ACN solution, Ed = 1.29 V vs Ag/AgCl, at different Qd (from 2 mC to 15 mC) using an Au wire (d = 0.25 mm) covered with 20 layers of silica beads (d = 600 nm) as working electrode. The first two curves (2mC and 4 mC) are almost not visible because of an overlap with the 6mC curve.
The polymerization plateau currents decrease with increasing deposition charge, because all electropolymerizations were performed in the same solution, therefore the current decays due to the slow decrease of the XDOT concentration. The SEM analysis of the electrodes provides more information about the electrodeposition process (Figure 2). The cross-
section view allows confirming the formation of a wellordered macroporous structure for Qd ≥ 4 mC (Figure S2). As can be seen from these results, for each additional 2 mC of Qd, a full new porous layer is obtained. These results allow correlating the number of layers and the Qd values, and thus a perfect control of the film thickness. The surface of all electrodes presents gaps between the hexagonal structures formed by the pores of the PXDOT film (red circles in Figure 2; Figure S3) and interconnection points between neighboring cavities (blue circles in Figure 2), characteristic properties of a surfacetemplated structure.13 According to Bartlett et al this is caused by an electrostatic interaction between the radical cation, formed during the anodic electropolymerization, and the negatively charged surface groups of the silica beads which guide the growth of the PXDOT.14 The electrochemical behavior of the macroporous electrodes in a monomer-free 0.1 M LiClO4/ACN solution is characteristic for a rather fast electron transfer system, indicated by ∆Ep values between 70 mV and 110 mV (Figure 3)27 in comparison with the slow electron transfer observed for polybithiophene (PBTh) and PEDOT (∆Ep = 350 mV and 570 mV respectively) (Figure S4). This behavior has been attributed to the presence of the xylen-unit attached to the polymer chain, which generates an internal order in the oligomeric chains.28 No significant change of the peak potential was measured (less than 10 mV) for all the PXDOT electrodes, therefore, the same type of oligomeric chains seems to be favored. All the electrodes show a reversible charge/discharge process in ACN (Qc/Qa > 0.98). The linear trend obtained for the plot of |ip| vs Qd is in agreement with the formation of macroporous structures and the efficient control of the film thickness (see inset of Figure 3).
Figure 2. SEM images of (a) a cross-section of a highly ordered macroporous PXDOT electrode obtained with a Qd of 15 mC, and (b) the top view of a macroporous PXDOT electrode obtained with a Qd of 8 mC. Both electrodes were grown following the conditions indicated in Figure 1. Insert shows a close-up of the gaps (red) and the interconnection points (blue).
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tequila and mezcal.40-42 However, some of these methods present drawbacks, such as the use of toxic electrode materials like mercury, the requirement for relatively long preconcentration times or the use of expensive and complicated pretreatment procedures. Furthermore, the efficient quantification of copper directly in mezcal is limited by the complex organic matrix of the samples.43 Therefore, the analysis requires the use of a 10 % dilution (with a 0.1 M LiClO4/H2O and AcOH/AcONa (0.05 M/0.008 M) buffer solution).
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Potential / V vs Ag/AgCl Figure 3. Potentiodynamic characterization of highly ordered macroporous PXDOT electrodes, obtained with different Qd values, in a monomer-free 0.1 M LiClO4/ACN solution, v = 25 mV/s (insert shows the plot of |ip| vs Qd for a series of macroporous PXDOT electrodes).
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The redox response of Cu stripping in aqueous media, as a function of the thickness of the macroporous electrodes was evaluated. The Cu2+ quantification was carried out using square wave anodic stripping voltammetry (SWASV). The square wave pulse diagram allows an efficient elimination of the capacitive current, providing good voltammetric data with an enhanced sensitivity.29-31 The relatively high scan rate (v = f ∆Es)32 allows to minimize problems associated with the adsorption of organic molecules onto the electrode surface.31 SWASV of an aqueous solution containing different Cu2+ concentrations (from 0.1 ppm to 0.7 ppm) using PXDOT macroporous electrodes synthesized with an increasing Qd shows a well-defined characteristic Cu oxidation peak at potentials between -0.010 and 0.021 V vs Ag/AgCl (Figure S5), potentials where the polymer presents sufficient conductivity.28 The half-wave widths (W1/2 measured for the last standard addition) are in the range of 60 to 75 mV for the electrodes. Plots of the peak current as a function of the Cu2+ concentration reveal a linear correlation (r2 = 0.99) for macroporous PXDOT electrodes obtained with Qd ≤ 6 mC. The sensitivity was determined for all the electrodes, showing the highest value, without loss of linearity, for the PXDOT film obtained at Qd = 6 mC (m = 60.89 µA/ppm) (Figure 4a). Sensitivity and linearity achieved for Cu2+ quantification significantly improved compared to those obtained with flat PEDOT electrodes in aqueous solutions.12 A decrease in sensitivity was observed for electrodes synthesized with a Qd > 6 mC. This may be attributed to the short accumulation time, which doesn’t allow the copper ions to penetrate the whole polymer matrix and benefit of the increase in surface area. In order to test the macroporous electrodes in the frame of a real application, the quantification of copper in mezcal, a typical Mexican alcoholic beverage, was performed. The presence of metallic ions, such as copper, is a common phenomenon, mainly caused by the use of metallic equipment during the distillation and/or fermentation process.33 The presence of copper in whiskey, rum, wine, brandy, chachaza and tequila34 is a main problem in the alcoholic beverage industry. Stripping voltammetry and polarography have been used to quantify metals in different alcoholic beverages,35-39 such as
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Analytical Chemistry
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Potential / V vs Ag/AgCl Figure 4. SWASV for successive addition of Cu2+ obtained with a macroporous PXDOT electrode synthesized with a Qd of 6 mC, (a) 0.1 M LiClO4/H2O, AcOH/AcONa (0.05 M/0.008 M) buffer, and (b) 10 % mezcal 0.1 M LiClO4/H2O, AcOH/AcONa (0.05 M/0.008 M) buffer. For all the experiments, Ed = -0.3 V, td = 120 s, f = 15 Hz, ∆ES = 4 mV, ESW = 50 mV as analytical parameters. The accumulation of the metal ion was carried out under stirring. Insert shows a plot of peak current vs concentration of added copper.
SWASV of a 10 % mezcal dilution using a PXDOT macroporous electrode synthesized with a Qd of 6 mC showed a well-defined characteristic Cu oxidation peak at 0.063 V vs Ag/AgCl (Figure 4b) and a W1/2 of 120 mV. Comparing the response of the macroporous PXDOT electrode in aqueous buffer solution with the one in 10% mezcal dilution, the oxidative stripping peak shows a potential shift of 66 mV and a broader peak. This behavior can be attributed to the presence of organic molecules, which adsorb on the electrode surface, and may affect electron transfer.44 The standard addition method (from 0.05 ppm to 0.3 ppm) was selected to carry out
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the quantification of Cu2+. Plotting the peak current as a function of Cu2+ concentration leads to a linear response (r2 = 0.99) with the regression equation i = 15.78 µA + 54.42 µA/ppm [Cu2+]. Using this equation, a copper concentration of 2.90 ppm can be calculated, which is in extremely good agreement with the copper content obtained by atomic absorption spectroscopy (2.97 ppm). This copper concentration is comparable to those measured in other distilled alcoholic beverages such as vodka, cachaça, gin, and tequila using carbon paste modified electrodes.45 In conclusion these results demonstrate that the presented approach allows determining with a high sensitivity metal ions such as copper even in complex matrices as the one of mezcal. The engineering of a macroporous PXDOT architecture constitutes therefore a fast and low-cost electroanalytical technique with a non-toxic electrode material, which might outperform spectroscopic methods, having high operation and maintenance costs.46-48
EXPERIMENTAL SECTION Chemicals and Materials LiClO4 (Aldrich, 99.9%), glacial acetic acid (Scharlau, reagent grade), sodium acetate (Acros Organics, 99 %), Au wires (d = 0.25 mm, Alfa Aesar, 99.9%) were used as received. All the solutions were prepared with deionized water (MilliQ DirectQ®). The 3,4-ortho-xylendioxythiophene was synthetized in our laboratory following a published methodology for the synthesis of 3,4-alkoxythiophenes.23 The LB film formation and the deposition of the LB films onto cylindrical substrates were carried out as described previously.16,22 A single compartment three-electrode cell, equipped with a templated or modified Au wire working electrode, a cylindrical Pt mesh as auxiliary electrode, and an Ag/AgCl (in 3M KCl) reference electrode (Bioanalytical Systems), was used. All electrochemical experiments were performed with a µAutolab type III potentiostat (Metrohm). For all the SWASV experiments, Ed = -0.3 V, td = 120 s, f = 15 Hz, ∆ES = 4 mV, ESW = 50 mV as analytical parameters. SEM experiments were carried out using a Hitachi TM-1000 tabletop microscope.
ASSOCIATED CONTENT Supporting Information Potentiodynamic, SWASV experiments and additional SEM images of the surface and the cross-section view of all the macroporous electrodes. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected];
[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
ACKNOWLEDGMENT This study was funded by CONACYT (scholarships 288088 and 291212 awarded to the doctoral student GSS and support to the project 179356).
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