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Electroanalytical determination of Bisphenol A: Investigation of electrode surface fouling using various carbon materials
Journal of Electroanalytical Chemistry 789 (2017) 58–66
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Electroanalytical determination of Bisphenol A: Investigation of electrode surface fouling using various carbon materials Abdelghani Ghanam, Abdellatif Ait Lahcen, Aziz Amine ⁎ Laboratoire Génie des Procédés & Environnement, Faculté des Sciences et Techniques, Hassan II University of Casablanca, B.P. 146, Mohammedia, Morocco
a r t i c l e
i n f o
Article history: Received 26 October 2016 Received in revised form 21 January 2017 Accepted 16 February 2017 Available online 20 February 2017 Keywords: Bisphenol A Electrode fouling Tween 20 Carbon paste electrodes Electrochemical detection
a b s t r a c t In this work, we report a study of electrode surface fouling due to phenols oxidation which considered being a major problem for the electrochemical study of phenols. Thus, Bisphenol A (BPA) was selected as model analyte in this study. Indeed, the products of its oxidation block the active surface area of the electrode and thus decreasing the electrochemical signal. Several parameters affecting this problem were investigated during this work such as the choice of the electrochemical analysis technique, the carbon material brand, the concentration of BPA, the temperature, the pH of the supporting electrolyte and the addition of Tween 20. Indeed, the optimization of these parameters has reduced significantly the electrode fouling problem and increased sensitivity. A comparative study of the analytical performance of several electrochemical carbonaceous paste electrodes based on graphite, carbon black, multiwall carbon nanotubes and glassy carbon to quantify BPA at low concentration levels was performed. The oxidation current intensity was proportional to the concentration of BPA within a concentration range from 1 μM to 16 μM in the presence of Tween 20. The limits of detection (LOD) obtained values were comprised between 0.12 μM and 0.8 μM for all tested electrodes. The carbon black paste electrode and the conventional carbon paste electrode were found to be the most sensitive electrodes towards BPA determination in presence of Tween 20. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Bisphenol A (BPA)is an intermediate chemical, used widely in the synthesis of polycarbonate and epoxy resins, also combined with other materials to fabricate plastics such as plastic food container, food can linings and water bottles [1,2]. The main risk posed by BPA is that it can be released into food and drinking water from various food contact materials [3]. BPA is considered as one of the most important disrupting chemicals (EDCs) that has attracted more attention due to its high toxicity. It can cause several health diseases such as cancers, heart disease and development problems [4,5]. In the last decade, the development of analytical methods for the determination of BPA compound in environmental compartments has become important. Since then, several analytical methods were reported for its determination in real samples at low concentration levels [6–10]. The electroanalytical methods based on sensors and biosensors offered various advantages towards BPA detection due to their high sensitivity, selectivity, low cost and rapid response [11–18]. However, the oxidation of BPA molecule is an irreversible process and the products of its oxidation cause the surface fouling of the electrodes. This phenomenon is considered as the main problem occurred during the electrochemical oxidation of ⁎ Corresponding author. E-mail address: [email protected] (A. Amine).
http://dx.doi.org/10.1016/j.jelechem.2017.02.026 1572-6657/© 2017 Elsevier B.V. All rights reserved.
phenols in general and especially for BPA study due to the electropolymerization of phenolic compounds [19–22]. The formation of polymeric product after BPA oxidation blocks the active surface area of the electrodes and delays important electrodes processes [23–25]. The phenols passivation process is dependent on the operational conditions which makes a better understanding of their deactivation mechanism of great interest for electroanalysis. In order to avoid and reduce this problem, several approaches were widely used to prevent the electrode surface fouling phenomena such as the use of conducting polymers [26–29] and the ionic liquids [30–32]. Because of their lipophilic properties, Surfactant agents were widely used in electroanalysis in order to enhance the sensitivity and selectivity [33–36]. They can modify the electrochemical process of some organic substances such as charge transfer coefficients, diffusion coefficient, and electrochemical signal and also modify the stability of intermediate electrogenerated by the oxidation of phenolic compounds [33–36]. Furthermore, it was reported that cationic surfactants such as Cetyltrimethylammonium Bromide (CTAB) added to the supporting electrolyte plays a role as surface antifouling [37–40]. In this paper we report a study of surface fouling phenomena occurred after Bisphenol A (BPA) oxidation on carbon electrodes. Several experimental parameters including concentration of nonionic surfactant Tween 20 were optimized in order to detect low level of BPA without loss of sensitivity after several measurements.
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2. Experimental
2.5. Solutions
2.1. Chemicals & reagents
Standard stock solution of (5 · 10−3 M) of Bisphenol A was prepared by dissolving the appropriate amount in 0.1 M of NaOH and diluted as required using phosphate buffer pH 7.0 (0.1 M). The stock solution was stored at + 4 °C. Phosphate buffer solution 0.1 M was prepared using KH2PO4 and Na2HPO4 dissolved in distilled water. BrittonRobinson buffer prepared from a mixture of acids, 0.04 M H3BO3, 0.04 M CH3COOH, and 0.04 M H3PO4and the pH was adjusted as desired with 0.2 M of NaOH.
Bisphenol A (BPA) ≥ 99.0%, mineral oil, and solvents were of analytical grade and purchased from Sigma-Aldrich. Carbon Black grade N220 used in this study were obtained from Cabot Corporation (Ravenna, Italy). Graphite b 0.1 mm, and Multiwall Carbon Nanotubes (MWCNTs) ≥ 98% carbon basis, O.D × L 10 nm ± 1 nm × 4.5 nm × 3– ~6 μm, TEM were purchased from Sigma-Aldrich. Tween 20 was purchased from Fisher scientific (USA). Sodium Dodecyl Sulphate (SDS), (1-Hexadecyl) trimethylammonium bromide (CTAB) were purchased from Loba Chemie (Mumbai, India).Triton X-100 was purchased from Sigma Chemical Company. H3PO4, Na2HPO4 and KH2PO4 were purchased from Solvachim (Morocco). Boric acid was purchased from Breckland Scientific (UK). CH3COOH was purchased from Loba Chemie (India). All reagents used in this study were of analytical grade. The distilled water was used throughout the experiments. 2.2. Apparatus Cyclic voltammetry (CV), Linear sweep Voltammetry (LSV), Squarewave voltammetry (SWV), Differential Pulse Voltammetry (DPV) and Amperometry measurements were carried out using an electrochemical instrument PalmSens (BV Houten-Netherlands) connected to a computer and controlled by software named PsTrace3.0. An electrochemical cell containing three electrodes system was used. Glassy carbon electrode and paste electrodes prepared using various carbon materials were used as working electrode with a diameter of 3 mm. The reference electrode was Ag/AgCl electrode (saturated with KCl) and a bare of stainless steel as the counter electrode, were employed. The pH values of the solutions were measured with OHAUS instrument (STARTER 2100) pH meter. The effect of temperature was performed with HAAKE FISONS DC3 apparatus. All the experiments were performed at room temperature.
3. Results and discussion 3.1. Electrochemical behavior of Bisphenol A The electrochemical behavior of BPA was investigated at a conventional carbon paste electrode in a phosphate buffer solution pH 7.0 containing 50 μM of BPA as illustrated in Fig. 1. The voltammograms shows a clear and highly intense oxidation peak of BPA at 0.580 V (vs. Ag/AgCl) reference electrode. No reduction peak was observed in the reverse potential scanning, indicating that the electrochemical process of BPA was completely an irreversible reaction. 3.2. Analytical performance of a conventional carbon paste electrode towards Bisphenol A determination Square-wave voltammetry (SWV) was used to investigate the linearity of BPA oxidation peak in the concentrations range 1 μM to 50 μMas shown in Fig. 2. The results obtained showed that the current intensity of BPA oxidation (Ipa) increases proportionally with the concentration of BPA until 4 μM. Beyond this value, the curve was not linear, and the oxidation peak potential (Epa) shifts to the positive values of potential. These results indicate that the oxidation of BPA may probably causes the formation of a non-conductive film leading to the electrode surface fouling. Thus, the study of the parameters affecting the electrode surface fouling is of importance.
2.3. Preparation of electrodes The paste electrodes used in this study were prepared according to the previous published works [41,42]. Carbon Black or MWCNTs were pure and they were not mixed with graphite powder. Briefly, the carbon particles (graphite powder, CB and MWCNT) were mixed separately with the mineral oil using a pestle and mortar to form a homogeneous carbonaceous paste. The resulting pastes were packed into the well of the working electrode with 3 mm of diameter and then the surface polished on a print paper to give a smooth surface before use. The Glassy carbon electrode (3 mm diameter) was polished using a solution of alumina slurries of (0.3 μm and 0.05 μm) before each use, then washed with distilled water, and finally was sonicated for 5 min in ethanol/ water solution.
3.3. Study of electrode surface fouling The Fig. 3 shows a successive cyclic voltammograms in the range + 0.2 V to + 1.0 V at a conventional CPE in phosphate buffer pH 7.0 containing 50 μM of BPA using a potential sweep repeated six
2.4. Electrochemical analysis techniques CV measurements were carried out by scanning the potential in the range + 0.0 V to 1.2 V, with a potential step of 10 mV and 100 mV/s as scan rate. LSV measurements were recorded by applying a sweep potential from + 0.0 V to + 1.0 V with a potential step of 10 mV at a scan rate of 100 mV/s. DPV measurements were performed by applying a sweep potential from 0.0 V to +1.0 V at pulse amplitude of 30 mV and pulse width 0.1 s with a scan rate of 10 mV/s. SWV measurements were performed by scanning in the potential range from (+ 0.0 V) to (+ 1.0 V) vs. Ag/AgCl reference electrode at the pulse amplitude, 10 mV with the frequency 8 Hz. Amperometric measurements of BPA were carried out under stirring solution at the applied potential +610 mV using a conventional carbon paste electrode.
Fig. 1. Cyclic voltammograms of 50 μM BPA in a phosphate buffer solution pH 7.0 (0.1 M) at a conventional carbon paste electrode. Scan rate 100 mV/s.
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These results confirm clearly the presence of surface fouling phenomena due to the formation of polymeric phenolic film after the initial BPA oxidation which adsorbs on the active surface of the electrode. The polymeric film blocks the access of BPA to the active surface of the electrode and thus decreases the signal of oxidation [22,30]. The oxidation current signal disappears completely from the third scan, indicating the complete deactivation of the electrode surface (electrode fouling). In order to find the similar BPA oxidation peaks current after the initial scan, it is necessary to make a mechanical polishing the electrode surface between two consecutive scans [43]. 3.4. Evaluation of the electrode surface fouling Several parameters affect the electrode surface fouling such as the concentration of BPA, the electrochemical analysis technique, the pH of the supporting electrolyte and the effect of the surfactant agent. Therefore, the study of the influence of those parameters was presented below. 3.4.1. Effect of Bisphenol A concentration The effect of phenolic compound concentration on the electrode fouling was investigated using a conventional CPE in a solution of phosphate buffer pH 7.0 containing 2 μM, 10 μM and 50 μM of BPA. The results obtained (Fig. 4) showed a decrease of the current intensity of the oxidation peak of BPA during the six successive SWV scans. It can be seen clearly from the results that the electrode fouling depends on BPA concentration and the fouling is reduced significantly at low concentration of 2 μM of BPA [44,45].
Fig. 2. A) Square-wave voltammograms in a phosphate buffer (0.1 M) pH 7.0 at the conventional CPE in the concentration range 1–50 μM BPA. B) Calibration curve of BPA in the concentration range 1–50 μM.
times at a scan rate 100 mV/s. The stirring was carried out at open circuit potential for 1 min between two consecutive scans. The obtained results showed that the intensity of the oxidation peak of BPA decreases cycle after cycle and the potential shifts to the positive values of potential.
Fig. 3. Successive cyclic voltammograms at a conventional carbon paste electrode in a solution of phosphate buffer pH 7.0 (0.1 M) containing 50 μM BPA. Scan rate 100 mV/s.
3.4.2. Influence of electrochemical analysis technique The effect of the electrochemical analysis technique on the electrode surface fouling was carried out for 2 μM of BPA using SWV and DPV as electrochemical analysis techniques. The results presented in Fig. 5 showed that the electrochemical analysis technique has also an effect on the electrode surface fouling. The reduction of the signal increases between the first and sixth scan when working with the DPV. Although, DPV is a sensitive technique as indicated by the magnitude of the value of the first peak. It may generate a high amount of oxidation products which reduce highly the oxidation signal after the first peak (Fig. 5B). However, in case of SWV analysis technique, the magnitude signal of the first peak was low which leads to a production of small amount of oxidation products. Therefore, the SWV technique was less sensitive to the fouling phenomena (Fig. 5A). The results obtained are consistent with those illustrated in Section 3.4.1. Effect of Bisphenol A concentration. 3.4.3. Effect of pH The effect of the pH values on the voltammetric behavior of BPA at a conventional CPE was investigated in the pH range from 3.0 to 10.0 of Britton-Robinson buffer as presented in Fig. 6A. The results showed that the highest oxidation peak current of BPA value was obtained at pH 7.0 with pH values further increasing, the oxidation peak current decreased significantly. Hence, a phosphate buffer solution at pH 7.0 was selected for further experimental investigations. The Fig. S1 presents the relationship between the oxidation peak potential of BPA and the pH values which found to be linear with a regression equation Epox (mV) = −57 pH+ 927 (R2 = 0.990). These results confirm that the oxidation process of BPA was proton dependent and the electron transfer was followed by the transfer of an equal number of protons. In parallel, the effect of the electrode surface fouling was investigated at a conventional CPE in different Britton-Robinson buffer solutions varying the pH values from 3 until 10 as indicated in Fig. 6B. The obtained results indicate that the fouling is reduced at a low pH values and this could be due to the participation of protons in the oxidation reaction of BPA. In the low pH values, a small current of the oxidation peak is observed (Fig. 6A) which implies a small appearance of the phenoxy radicals.
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Fig. 5. Six successive scans of 2 μM BPA in a phosphate buffer pH 7.0 (0.1 M) at a conventional CPE using: (A) SWV, (B) DPV.
Fig. 4. Six progressive square-wave voltammograms at a conventional CPE for A) 2 μM of BPA, (B) 10 μM BPA and (C) 50 μM of BPA.
Thus, the fouling was not important at low pH. Despite the fouling problem may be reduced significantly at acidic pH (Fig. 6B) it cannot be the best choice for work due to the low sensitivity towards BPA oxidation compared to PH 7.0 measurements (Fig. 6A). 3.4.4. Effect of the carbon material The effect of the carbon material used for the preparation of carbon paste electrodes on deactivation of electrode surface was investigated
using graphite, carbon black and multiwall carbon nanotubes as illustrated in Fig. 7. The measurements were performed in a phosphate buffer solution pH 7.0 in the presence of 10 μM of BPA. The results showed that the paste electrodes based on graphite showed a significant reduction of fouling phenomena compared to the others prepared based on CB and CNTs. This approach depends on the physicochemical nature of the surface such as hydrophobicity and the particle size. Indeed, the use of carbon nanoparticles (Carbon Black, Multiwall Carbon Nanotubes) offer a large specific surface area for the adsorption of the analyte on the electrode surface which implied the easy formation of phenoxy radicals that can be polymerized onto the electrode surface. Thus, the fouling phenomena was increased. However in the case of paste electrodes based on graphite which has a higher particles size (0.1 mm) and low specific surface area the fouling phenomena was reduced. Thus, the nature of the electrode affects the fouling problem.
3.4.5. Effect of the temperature on electrode fouling It was reported [46] that the temperature influence the electrode surface fouling, therefore, a study of its effect on BPA oxidation was investigated at a conventional CPE in a solution of phosphate buffer pH 7.0, (0.1 M) using LSV as indicated in Fig. S2. The experiments were performed at 28.4 °C and 38.4 °C. The obtained results indicates that the oxidation peak signal was reduced by 60% at the sixth scan at 38.4 °C compared to the one performed at 28.4 °C that was reduced by
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Fig. 6. A) Effect of B-R buffer solution pH on the oxidation of BPA at a conventional CPE, B) dependence of BPA % signal reduction pH values.
100%.Hence, at high temperature value the electrode passivation was reduced. 3.5. Effect of the surfactant agents on the electrode surface fouling The cationic (CTAB), anionic (SDS) and nonionic (Tween 20, Triton X-100) surfactants were used to evaluate both the fouling problem and the electrode sensitivity. Taking into account that the sensitivity of the electrode towards the oxidation peak of 20 μM BPA in absence of surfactants which is equal to 0.087 μA/μM, it appears clearly from the results shown in Table 1 that non-ionic and cationic surfactants increase the sensitivity. However, the anionic surfactant decreases it. In the other side, the fouling was observed with all surfactants used at low concentration but highly diminished at high concentration except in the case of cationic surfactant CTAB. The large value of RSD% (20.1) obtained with Triton X-100 imposes the selection of the non-ionic surfactant tween 20 for the rest of the work. The Fig. 8 presents the percentages of reduction of BPA oxidation signal between the first and sixth scan over the potential range (+0.2 to + 1.0 V) depending on the concentration of Tween 20 surfactant agent in a phosphate buffer solution 0.1 M pH 7.0. The stirring was carried out at open circuit potential for a minute between each scan. The results obtained show that there is a significant reduction of the electrode surface fouling in accordance with the increase of the concentration of Tween 20. The optimum value of the reduction of electrode
Fig. 7. Successive square-wave voltammograms in phosphate buffer solution pH 7.0, (0.1 M) containing 10 μM of BPA at (A) conventional CPE, (B) CBPE; (C) CNTPE.
surface fouling was found in presence of 800 μM of Tween 20 which is the value of choice for the further experiments. The Fig. 9 below shows a cyclic voltammograms in presence and in absence of Tween 20 in phosphate buffer solution at a conventional CPE. The six successive scans showed that in presence of 800 μM of Tween 20 in phosphate buffer solution pH 7.0 the fouling phenomena was reduced and the oxidation signal of BPA was stabilized which confirms the above results. However, in absence of Tween 20, a
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Table 1 Study of the effect of different surfactants on the electrode surface fouling in terms of sensitivity, reproducibility and % of signal reduction. Surfactant
Tween 20 Triton X-100 CTAB SDS a
100 μM
800 μM
Sensitivitya (μA/μM)
RSD % of sensitivity
% of signal reduction between 1st and 6th scan
Sensitivitya (μA/μM)
RSD % of sensitivity
% of signal reduction between 1st and 6th scan
0.2 0.4 1.0 0.04
2.6 2.8 5.8 1.1
40.8 40.8 58.3 63.7
0.1 0.2 0.2 0.1
14.3 20.1 27.5 9.7
5.1 1.2 41.4 7.3
Average of three oxidation peak currents divided by the concentration of BPA used.
dramatically decrease of PBA oxidation signal was observed. Thus, the presence of surfactants in electrolyte solution could desorb the BPA oxidation product adsorbed on electrode surface and thus decreases strongly the fouling phenomena. 3.6. Amperometric study of Bisphenol A The amperometric measurement of 10 μM of BPA using a conventional carbon paste electrode was tested in the presence and in the absence of Tween 20 surfactant. The Fig. 10 illustrates the amperograms responses towards BPA with the surface of conventional CPE at a constant applied potential 610 mV. It can be seen that the conventional CPE has a better analytical performance for the detection of BPA and the electrochemical signal is more stable in the presence of Tween 20 which avoids the surface fouling phenomena. The signal background and noise increase in presence of Tween 20 at conventional CPE. As indicated in Fig. S3, the same experiments were performed at a carbon black paste electrode (CBPE) and showed the stable and low background in presence of Tween 20. Thus, it can be concluded from the above results that Tween 20 is efficient for reduction of surface fouling.
from these results that this electrode performed in this conditions could be of interests for total phenolic compounds determination in waste water and other environmental compartment. 3.7.2. Analytical performance of various carbon based sensors towards Bisphenol A detection SWV analysis technique was found to be more sensitive and less exposed to fouling phenomena. Therefore, under optimum condition, the analytical performance of Glassy carbon electrode, Carbon Black paste electrode, Conventional carbon paste electrode and Multiwall carbon nanotubes paste electrode towards BPA determination were studied and investigated. The Table 2 below presents the summarized data obtained for various tested electrodes in the presence and in the absence
3.7. Electrochemical detection of BPA using carbon based sensors 3.7.1. Determination of BPA using linear sweep voltammetry The Fig. 11A illustrates the linear sweep voltammograms obtained for BPA in the concentration range 10–400 μM in a solution of phosphate buffer pH 7.0, 0.1 M. The relationship between the current signal intensity and the concentration of BPA was linear with a regression equation of I (μA) = 0.127 C (μM) + 8.343 as indicated in Fig. 11B. The limit of detection (LOD) and the limit of quantification (LOQ) calculated were equals 3.5 μA and 11.8 μM respectively. It can be concluded
Fig. 8. % of reduction of BPA oxidation signal between the 1st and 6th scan at a conventional CPE as a function of concentration of Tween 20 in a phosphate buffer 0.1 M, pH 7.0.
Fig. 9. Successive cyclic voltammograms of 25 μM BPA at a conventional carbon paste electrode in a phosphate buffer solution pH 7.0. Scan rate 100 mV/s: (A) in presence of 800 μM of Tween 20, (B) without Tween 20.
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Fig. 10. Amperometric response of 10 μM of BPA at conventional CPE A) in the presence of Tween 20 B) in absence of Tween 20. The applied potential 610 mV.vs (Ag/AgCl).
of Tween 20 surfactant. The obtained results in the absence of Tween 20 in phosphate buffer show a low linear range of BPA concentration and all of the electrode surfaces were blocked by the formation of polyphenols films. However in case of the presence of Tween 20, which playing the role of inhibitor of electrode fouling, the linear range was extended to high concentration. Furthermore, all tested electrodes were repeatable for three consecutive calibration curves allowing to reach a low limits of detection comprised between 0.12 μM and 0.8 μM. The Conventional CPE and CBPE were found the most sensitive towards BPA detection. It can be seen as indicated in the Table 2 that in presence of surfactant, the CNTPE doesn't show any oxidation signal towards BPA in the range of concentration of 1–20 μM BPA. This result leads us to study electrochemical oxidation of other electroactive compounds in presence of Tween 20 as indicated below.
Fig. 11. A) Linear sweep voltammograms of (a) 10 μM; (b) 20 μM; (c) 50 μM; (d) 100 μM; (e) 150 μM; (f) 200 μM; (g) 250 μM; (h) 300 μM; (i) 350 μM and (j) 400 μM of BPA in a phosphate buffer pH 7.0, 0.1 M in presence of 800 μM of Tween 20 at a conventional CPE. B) Calibration curve in the range of 10–400 μM of BPA.
supplementary material (Figs. S4, S5 and S6) that even in the presence of Tween 20 the electrochemical background increase significantly for each scan and no response observed for ascorbic acid. However for 100 μM of catechol, dopamine and BPA a signal of oxidation was observed but at a very high background was obtained. These results indicate that the Tween 20 adsorbs on the surface of CNTPE which blocks the access to the active surface area. The hydrophilic part of Tween 20
Table 2 Summary of data obtained using various kinds of carbon based sensors in the presence and in the absence of Tween 20 for BPA determination. Electrode
3.7.3. Evaluation of CNTPE surface fouling As explained above, the presence of Tween 20 had a significant effect on both the sensitivity of the electrode, the repeatability of the measurements and on the electrochemical behavior of BPA. However, in case of paste electrodes based on CNTs nanoparticles the presence of Tween 20 surfactant blocks the active area of the electrode. Ascorbic acid, Dopamine and catechol are well known as highly electroactive molecules. Therefore, a study of their oxidation at a CNTPE in the presence of 800 μM of Tween 20in a phosphate buffer pH 7.0 was performed. The obtained results show as indicated in
CPEa CBPEb CNTPEc GCEd a b c d
In absence of Tween 20
In the presence of Tween 20
Linear range (μM)
LOD (μM)
LOQ (μM)
Sensitivity (nA/μM)
Linear range (μM)
LOD (μM)
LOQ (μM)
Sensitivity (nA/μM)
0.5–6 0.5–4 1–4 1–8
0.36 0.8 0.7 0.8
1.22 2.6 2.4 2.7
74.1 20.2 45.7 20
1–12 1–16 – 1–16
0.12 0.30 – 0.7
0.4 0.99 – 2.4
81.8 76.3 – 10
Conventional carbon paste electrode. Carbon black paste electrode. Multiwall carbon nanotubes paste electrode. Glassy carbon electrode.
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always remains in the aqueous phase while the tails hydrocarbon (hydrophobic portion) may react physically with the surface of the nanotube and adsorb irreversibly [47,48]. 4. Conclusions In this work the electrode surface fouling due to the oxidation of BPA was electrochemically investigated. The effect of several parameters such as the nature of the electrode, the pH of the electrolytic medium, the concentration of BPA and the effect of the addition of surfactant agent (Tween 20) were studied. It was revealed that the addition of Tween 20 has a significant role in reducing the problem of poisoning of the active surface of the electrode. Glassy carbon electrode and paste electrodes based on graphite, MWCNTs and carbon black were used to study their electrochemical behavior towards BPA oxidation. The paste electrodes based on graphite and carbon black showed the best results in terms of sensitivity, detection limit and resistance to electrode fouling particularly in presence of surfactant Tween 20. A concentration as low as 0.12 μM of BPA was successfully detected. Acknowledgements The work presented in this paper was supported by Center SISA from Hassan II University of Casablanca (Morocco). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2017.02.026. References [1] C.A. Staples, P.B. Dome, G.M. Klecka, S.T. Oblock, L.R. Harris, A review of the environmental fate, effects, and exposures of bisphenol A, Chemosphere 36 (1998) 2149–2173. [2] Y. Zhang, Y. Cheng, Y. Zhou, B. Li, W. Gu, X. Shi, Y. Xian, Electrochemical sensor for bisphenol A based on magnetic nanoparticles decorated reduced graphene oxide, Talanta. 107 (2013) 211–218. [3] P. Deng, Z. Xu, Y. Kuang, Electrochemically reduced graphene oxide modified acetylene black paste electrode for the sensitive determination of bisphenol A, J. Electroanal. Chem. 707 (2013) 7–14. [4] X. Wang, H. Zeng, Y. Wei, J.M. Lin, A reversible fluorescence sensor based on insoluble β-cyclodextrin polymer for direct determination of bisphenol A (BPA), J. Sens. Actuators B Chem. 114 (2006) 565–572. [5] L.N. Vandenberg, R. Hauser, M. Marcus, N. Olea, W.V. Welshons, Human exposure to bisphenol A (BPA), Reprod. Toxicol. 24 (2007) 139–177. [6] C. Lu, J. Li, Y. Yang, J.M. Lin, Determination of bisphenol A based on chemiluminescence from gold (III)–peroxymonocarbonate, Talanta 82 (2010) 1576–1580. [7] G. Gatidou, N.S. Thomaidis, A.S. Stasinakis, T.D. Lekkas, Simultaneous determination of the endocrine disrupting compounds nonylphenol, nonylphenolethoxylates, triclosan and bisphenol A in wastewater and sewage sludge by gas chromatography–mass spectrometry, J. Chromatogr. A 1138 (2007) 32–41. [8] A. Kim, C.R. Li, C.F. Jin, K.W. Lee, S.H. Lee, K.J. Shon, N.G. Park, D.K. Kim, S.W. Kang, Y.B. Shim, J.S. Park, A sensitive and reliable quantification method for bisphenol A based on modified competitive ELISA method, Chemosphere 68 (2007) 1204–1209. [9] W. Zhou, C. Sun, Y. Zhou, X. Yang, W. Yang, A facial electrochemical approach to determinate bisphenol A based on graphene-hypercrosslinked resin MN202 composite, Food Chem. 158 (2014) 81–87. [10] E. Ferrer, E. Santoni, S. Vittori, G. Font, J. Manes, G. Sagratini, Simultaneous determination of bisphenol A, octylphenol, and nonylphenol by pressurised liquid extraction and liquid chromatography-tandem mass spectrometry in powdered milk and infant formulas, Food Chem. 126 (2011) 360–367. [11] H.S. Yin, Y.L. Zhou, S.Y. Ai, Preparation and characteristic of cobalt phthalocyanine modified carbon paste electrode for bisphenol A detection, J. Electroanal. Chem. 626 (2009) 80–88. [12] Y. Tan, J. Jin, S. Zhang, Z. Shi, J. Wang, J. Zhang, W. Pu, C. Yang, Electrochemical determination of bisphenol A using a molecularly imprinted chitosan-acetylene black composite film modified glassy carbon electrode, Electroanalysis 28 (2016) 189–196. [13] M. Han, Y. Qu, S. Chen, Y. Wang, Z. Zhang, M. Ma, Z. Wang, G. Zang, C. Li, Amperometric biosensor for bisphenol A based on a glassy carbon electrode modified with a nanocomposite made from polylysine, single walled carbon nanotubes and tyrosinase, Microchim. Acta 180 (2013) 989–996. [14] Y. Lin, K. Liu, C. Liu, L. Yin, Q. Kang, L. Li, B. Li, Electrochemical sensing of bisphenol A based on polyglutamic acid/amino-functionalised carbon nanotubes nanocomposite, Electrochim. Acta 133 (2014) 492–500.
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Electroanalytical determination of Bisphenol A: Investigation of electrode surface fouling using various carbon materials Abdelghani Ghanam, Abdellatif Ait Lahcen, Aziz Amine ⁎ Laboratoire Génie des Procédés & Environnement, Faculté des Sciences et Techniques, Hassan II University of Casablanca, B.P. 146, Mohammedia, Morocco
a r t i c l e
i n f o
Article history: Received 26 October 2016 Received in revised form 21 January 2017 Accepted 16 February 2017 Available online 20 February 2017 Keywords: Bisphenol A Electrode fouling Tween 20 Carbon paste electrodes Electrochemical detection
a b s t r a c t In this work, we report a study of electrode surface fouling due to phenols oxidation which considered being a major problem for the electrochemical study of phenols. Thus, Bisphenol A (BPA) was selected as model analyte in this study. Indeed, the products of its oxidation block the active surface area of the electrode and thus decreasing the electrochemical signal. Several parameters affecting this problem were investigated during this work such as the choice of the electrochemical analysis technique, the carbon material brand, the concentration of BPA, the temperature, the pH of the supporting electrolyte and the addition of Tween 20. Indeed, the optimization of these parameters has reduced significantly the electrode fouling problem and increased sensitivity. A comparative study of the analytical performance of several electrochemical carbonaceous paste electrodes based on graphite, carbon black, multiwall carbon nanotubes and glassy carbon to quantify BPA at low concentration levels was performed. The oxidation current intensity was proportional to the concentration of BPA within a concentration range from 1 μM to 16 μM in the presence of Tween 20. The limits of detection (LOD) obtained values were comprised between 0.12 μM and 0.8 μM for all tested electrodes. The carbon black paste electrode and the conventional carbon paste electrode were found to be the most sensitive electrodes towards BPA determination in presence of Tween 20. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Bisphenol A (BPA)is an intermediate chemical, used widely in the synthesis of polycarbonate and epoxy resins, also combined with other materials to fabricate plastics such as plastic food container, food can linings and water bottles [1,2]. The main risk posed by BPA is that it can be released into food and drinking water from various food contact materials [3]. BPA is considered as one of the most important disrupting chemicals (EDCs) that has attracted more attention due to its high toxicity. It can cause several health diseases such as cancers, heart disease and development problems [4,5]. In the last decade, the development of analytical methods for the determination of BPA compound in environmental compartments has become important. Since then, several analytical methods were reported for its determination in real samples at low concentration levels [6–10]. The electroanalytical methods based on sensors and biosensors offered various advantages towards BPA detection due to their high sensitivity, selectivity, low cost and rapid response [11–18]. However, the oxidation of BPA molecule is an irreversible process and the products of its oxidation cause the surface fouling of the electrodes. This phenomenon is considered as the main problem occurred during the electrochemical oxidation of ⁎ Corresponding author. E-mail address: [email protected] (A. Amine).
http://dx.doi.org/10.1016/j.jelechem.2017.02.026 1572-6657/© 2017 Elsevier B.V. All rights reserved.
phenols in general and especially for BPA study due to the electropolymerization of phenolic compounds [19–22]. The formation of polymeric product after BPA oxidation blocks the active surface area of the electrodes and delays important electrodes processes [23–25]. The phenols passivation process is dependent on the operational conditions which makes a better understanding of their deactivation mechanism of great interest for electroanalysis. In order to avoid and reduce this problem, several approaches were widely used to prevent the electrode surface fouling phenomena such as the use of conducting polymers [26–29] and the ionic liquids [30–32]. Because of their lipophilic properties, Surfactant agents were widely used in electroanalysis in order to enhance the sensitivity and selectivity [33–36]. They can modify the electrochemical process of some organic substances such as charge transfer coefficients, diffusion coefficient, and electrochemical signal and also modify the stability of intermediate electrogenerated by the oxidation of phenolic compounds [33–36]. Furthermore, it was reported that cationic surfactants such as Cetyltrimethylammonium Bromide (CTAB) added to the supporting electrolyte plays a role as surface antifouling [37–40]. In this paper we report a study of surface fouling phenomena occurred after Bisphenol A (BPA) oxidation on carbon electrodes. Several experimental parameters including concentration of nonionic surfactant Tween 20 were optimized in order to detect low level of BPA without loss of sensitivity after several measurements.
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2. Experimental
2.5. Solutions
2.1. Chemicals & reagents
Standard stock solution of (5 · 10−3 M) of Bisphenol A was prepared by dissolving the appropriate amount in 0.1 M of NaOH and diluted as required using phosphate buffer pH 7.0 (0.1 M). The stock solution was stored at + 4 °C. Phosphate buffer solution 0.1 M was prepared using KH2PO4 and Na2HPO4 dissolved in distilled water. BrittonRobinson buffer prepared from a mixture of acids, 0.04 M H3BO3, 0.04 M CH3COOH, and 0.04 M H3PO4and the pH was adjusted as desired with 0.2 M of NaOH.
Bisphenol A (BPA) ≥ 99.0%, mineral oil, and solvents were of analytical grade and purchased from Sigma-Aldrich. Carbon Black grade N220 used in this study were obtained from Cabot Corporation (Ravenna, Italy). Graphite b 0.1 mm, and Multiwall Carbon Nanotubes (MWCNTs) ≥ 98% carbon basis, O.D × L 10 nm ± 1 nm × 4.5 nm × 3– ~6 μm, TEM were purchased from Sigma-Aldrich. Tween 20 was purchased from Fisher scientific (USA). Sodium Dodecyl Sulphate (SDS), (1-Hexadecyl) trimethylammonium bromide (CTAB) were purchased from Loba Chemie (Mumbai, India).Triton X-100 was purchased from Sigma Chemical Company. H3PO4, Na2HPO4 and KH2PO4 were purchased from Solvachim (Morocco). Boric acid was purchased from Breckland Scientific (UK). CH3COOH was purchased from Loba Chemie (India). All reagents used in this study were of analytical grade. The distilled water was used throughout the experiments. 2.2. Apparatus Cyclic voltammetry (CV), Linear sweep Voltammetry (LSV), Squarewave voltammetry (SWV), Differential Pulse Voltammetry (DPV) and Amperometry measurements were carried out using an electrochemical instrument PalmSens (BV Houten-Netherlands) connected to a computer and controlled by software named PsTrace3.0. An electrochemical cell containing three electrodes system was used. Glassy carbon electrode and paste electrodes prepared using various carbon materials were used as working electrode with a diameter of 3 mm. The reference electrode was Ag/AgCl electrode (saturated with KCl) and a bare of stainless steel as the counter electrode, were employed. The pH values of the solutions were measured with OHAUS instrument (STARTER 2100) pH meter. The effect of temperature was performed with HAAKE FISONS DC3 apparatus. All the experiments were performed at room temperature.
3. Results and discussion 3.1. Electrochemical behavior of Bisphenol A The electrochemical behavior of BPA was investigated at a conventional carbon paste electrode in a phosphate buffer solution pH 7.0 containing 50 μM of BPA as illustrated in Fig. 1. The voltammograms shows a clear and highly intense oxidation peak of BPA at 0.580 V (vs. Ag/AgCl) reference electrode. No reduction peak was observed in the reverse potential scanning, indicating that the electrochemical process of BPA was completely an irreversible reaction. 3.2. Analytical performance of a conventional carbon paste electrode towards Bisphenol A determination Square-wave voltammetry (SWV) was used to investigate the linearity of BPA oxidation peak in the concentrations range 1 μM to 50 μMas shown in Fig. 2. The results obtained showed that the current intensity of BPA oxidation (Ipa) increases proportionally with the concentration of BPA until 4 μM. Beyond this value, the curve was not linear, and the oxidation peak potential (Epa) shifts to the positive values of potential. These results indicate that the oxidation of BPA may probably causes the formation of a non-conductive film leading to the electrode surface fouling. Thus, the study of the parameters affecting the electrode surface fouling is of importance.
2.3. Preparation of electrodes The paste electrodes used in this study were prepared according to the previous published works [41,42]. Carbon Black or MWCNTs were pure and they were not mixed with graphite powder. Briefly, the carbon particles (graphite powder, CB and MWCNT) were mixed separately with the mineral oil using a pestle and mortar to form a homogeneous carbonaceous paste. The resulting pastes were packed into the well of the working electrode with 3 mm of diameter and then the surface polished on a print paper to give a smooth surface before use. The Glassy carbon electrode (3 mm diameter) was polished using a solution of alumina slurries of (0.3 μm and 0.05 μm) before each use, then washed with distilled water, and finally was sonicated for 5 min in ethanol/ water solution.
3.3. Study of electrode surface fouling The Fig. 3 shows a successive cyclic voltammograms in the range + 0.2 V to + 1.0 V at a conventional CPE in phosphate buffer pH 7.0 containing 50 μM of BPA using a potential sweep repeated six
2.4. Electrochemical analysis techniques CV measurements were carried out by scanning the potential in the range + 0.0 V to 1.2 V, with a potential step of 10 mV and 100 mV/s as scan rate. LSV measurements were recorded by applying a sweep potential from + 0.0 V to + 1.0 V with a potential step of 10 mV at a scan rate of 100 mV/s. DPV measurements were performed by applying a sweep potential from 0.0 V to +1.0 V at pulse amplitude of 30 mV and pulse width 0.1 s with a scan rate of 10 mV/s. SWV measurements were performed by scanning in the potential range from (+ 0.0 V) to (+ 1.0 V) vs. Ag/AgCl reference electrode at the pulse amplitude, 10 mV with the frequency 8 Hz. Amperometric measurements of BPA were carried out under stirring solution at the applied potential +610 mV using a conventional carbon paste electrode.
Fig. 1. Cyclic voltammograms of 50 μM BPA in a phosphate buffer solution pH 7.0 (0.1 M) at a conventional carbon paste electrode. Scan rate 100 mV/s.
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These results confirm clearly the presence of surface fouling phenomena due to the formation of polymeric phenolic film after the initial BPA oxidation which adsorbs on the active surface of the electrode. The polymeric film blocks the access of BPA to the active surface of the electrode and thus decreases the signal of oxidation [22,30]. The oxidation current signal disappears completely from the third scan, indicating the complete deactivation of the electrode surface (electrode fouling). In order to find the similar BPA oxidation peaks current after the initial scan, it is necessary to make a mechanical polishing the electrode surface between two consecutive scans [43]. 3.4. Evaluation of the electrode surface fouling Several parameters affect the electrode surface fouling such as the concentration of BPA, the electrochemical analysis technique, the pH of the supporting electrolyte and the effect of the surfactant agent. Therefore, the study of the influence of those parameters was presented below. 3.4.1. Effect of Bisphenol A concentration The effect of phenolic compound concentration on the electrode fouling was investigated using a conventional CPE in a solution of phosphate buffer pH 7.0 containing 2 μM, 10 μM and 50 μM of BPA. The results obtained (Fig. 4) showed a decrease of the current intensity of the oxidation peak of BPA during the six successive SWV scans. It can be seen clearly from the results that the electrode fouling depends on BPA concentration and the fouling is reduced significantly at low concentration of 2 μM of BPA [44,45].
Fig. 2. A) Square-wave voltammograms in a phosphate buffer (0.1 M) pH 7.0 at the conventional CPE in the concentration range 1–50 μM BPA. B) Calibration curve of BPA in the concentration range 1–50 μM.
times at a scan rate 100 mV/s. The stirring was carried out at open circuit potential for 1 min between two consecutive scans. The obtained results showed that the intensity of the oxidation peak of BPA decreases cycle after cycle and the potential shifts to the positive values of potential.
Fig. 3. Successive cyclic voltammograms at a conventional carbon paste electrode in a solution of phosphate buffer pH 7.0 (0.1 M) containing 50 μM BPA. Scan rate 100 mV/s.
3.4.2. Influence of electrochemical analysis technique The effect of the electrochemical analysis technique on the electrode surface fouling was carried out for 2 μM of BPA using SWV and DPV as electrochemical analysis techniques. The results presented in Fig. 5 showed that the electrochemical analysis technique has also an effect on the electrode surface fouling. The reduction of the signal increases between the first and sixth scan when working with the DPV. Although, DPV is a sensitive technique as indicated by the magnitude of the value of the first peak. It may generate a high amount of oxidation products which reduce highly the oxidation signal after the first peak (Fig. 5B). However, in case of SWV analysis technique, the magnitude signal of the first peak was low which leads to a production of small amount of oxidation products. Therefore, the SWV technique was less sensitive to the fouling phenomena (Fig. 5A). The results obtained are consistent with those illustrated in Section 3.4.1. Effect of Bisphenol A concentration. 3.4.3. Effect of pH The effect of the pH values on the voltammetric behavior of BPA at a conventional CPE was investigated in the pH range from 3.0 to 10.0 of Britton-Robinson buffer as presented in Fig. 6A. The results showed that the highest oxidation peak current of BPA value was obtained at pH 7.0 with pH values further increasing, the oxidation peak current decreased significantly. Hence, a phosphate buffer solution at pH 7.0 was selected for further experimental investigations. The Fig. S1 presents the relationship between the oxidation peak potential of BPA and the pH values which found to be linear with a regression equation Epox (mV) = −57 pH+ 927 (R2 = 0.990). These results confirm that the oxidation process of BPA was proton dependent and the electron transfer was followed by the transfer of an equal number of protons. In parallel, the effect of the electrode surface fouling was investigated at a conventional CPE in different Britton-Robinson buffer solutions varying the pH values from 3 until 10 as indicated in Fig. 6B. The obtained results indicate that the fouling is reduced at a low pH values and this could be due to the participation of protons in the oxidation reaction of BPA. In the low pH values, a small current of the oxidation peak is observed (Fig. 6A) which implies a small appearance of the phenoxy radicals.
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Fig. 5. Six successive scans of 2 μM BPA in a phosphate buffer pH 7.0 (0.1 M) at a conventional CPE using: (A) SWV, (B) DPV.
Fig. 4. Six progressive square-wave voltammograms at a conventional CPE for A) 2 μM of BPA, (B) 10 μM BPA and (C) 50 μM of BPA.
Thus, the fouling was not important at low pH. Despite the fouling problem may be reduced significantly at acidic pH (Fig. 6B) it cannot be the best choice for work due to the low sensitivity towards BPA oxidation compared to PH 7.0 measurements (Fig. 6A). 3.4.4. Effect of the carbon material The effect of the carbon material used for the preparation of carbon paste electrodes on deactivation of electrode surface was investigated
using graphite, carbon black and multiwall carbon nanotubes as illustrated in Fig. 7. The measurements were performed in a phosphate buffer solution pH 7.0 in the presence of 10 μM of BPA. The results showed that the paste electrodes based on graphite showed a significant reduction of fouling phenomena compared to the others prepared based on CB and CNTs. This approach depends on the physicochemical nature of the surface such as hydrophobicity and the particle size. Indeed, the use of carbon nanoparticles (Carbon Black, Multiwall Carbon Nanotubes) offer a large specific surface area for the adsorption of the analyte on the electrode surface which implied the easy formation of phenoxy radicals that can be polymerized onto the electrode surface. Thus, the fouling phenomena was increased. However in the case of paste electrodes based on graphite which has a higher particles size (0.1 mm) and low specific surface area the fouling phenomena was reduced. Thus, the nature of the electrode affects the fouling problem.
3.4.5. Effect of the temperature on electrode fouling It was reported [46] that the temperature influence the electrode surface fouling, therefore, a study of its effect on BPA oxidation was investigated at a conventional CPE in a solution of phosphate buffer pH 7.0, (0.1 M) using LSV as indicated in Fig. S2. The experiments were performed at 28.4 °C and 38.4 °C. The obtained results indicates that the oxidation peak signal was reduced by 60% at the sixth scan at 38.4 °C compared to the one performed at 28.4 °C that was reduced by
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Fig. 6. A) Effect of B-R buffer solution pH on the oxidation of BPA at a conventional CPE, B) dependence of BPA % signal reduction pH values.
100%.Hence, at high temperature value the electrode passivation was reduced. 3.5. Effect of the surfactant agents on the electrode surface fouling The cationic (CTAB), anionic (SDS) and nonionic (Tween 20, Triton X-100) surfactants were used to evaluate both the fouling problem and the electrode sensitivity. Taking into account that the sensitivity of the electrode towards the oxidation peak of 20 μM BPA in absence of surfactants which is equal to 0.087 μA/μM, it appears clearly from the results shown in Table 1 that non-ionic and cationic surfactants increase the sensitivity. However, the anionic surfactant decreases it. In the other side, the fouling was observed with all surfactants used at low concentration but highly diminished at high concentration except in the case of cationic surfactant CTAB. The large value of RSD% (20.1) obtained with Triton X-100 imposes the selection of the non-ionic surfactant tween 20 for the rest of the work. The Fig. 8 presents the percentages of reduction of BPA oxidation signal between the first and sixth scan over the potential range (+0.2 to + 1.0 V) depending on the concentration of Tween 20 surfactant agent in a phosphate buffer solution 0.1 M pH 7.0. The stirring was carried out at open circuit potential for a minute between each scan. The results obtained show that there is a significant reduction of the electrode surface fouling in accordance with the increase of the concentration of Tween 20. The optimum value of the reduction of electrode
Fig. 7. Successive square-wave voltammograms in phosphate buffer solution pH 7.0, (0.1 M) containing 10 μM of BPA at (A) conventional CPE, (B) CBPE; (C) CNTPE.
surface fouling was found in presence of 800 μM of Tween 20 which is the value of choice for the further experiments. The Fig. 9 below shows a cyclic voltammograms in presence and in absence of Tween 20 in phosphate buffer solution at a conventional CPE. The six successive scans showed that in presence of 800 μM of Tween 20 in phosphate buffer solution pH 7.0 the fouling phenomena was reduced and the oxidation signal of BPA was stabilized which confirms the above results. However, in absence of Tween 20, a
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Table 1 Study of the effect of different surfactants on the electrode surface fouling in terms of sensitivity, reproducibility and % of signal reduction. Surfactant
Tween 20 Triton X-100 CTAB SDS a
100 μM
800 μM
Sensitivitya (μA/μM)
RSD % of sensitivity
% of signal reduction between 1st and 6th scan
Sensitivitya (μA/μM)
RSD % of sensitivity
% of signal reduction between 1st and 6th scan
0.2 0.4 1.0 0.04
2.6 2.8 5.8 1.1
40.8 40.8 58.3 63.7
0.1 0.2 0.2 0.1
14.3 20.1 27.5 9.7
5.1 1.2 41.4 7.3
Average of three oxidation peak currents divided by the concentration of BPA used.
dramatically decrease of PBA oxidation signal was observed. Thus, the presence of surfactants in electrolyte solution could desorb the BPA oxidation product adsorbed on electrode surface and thus decreases strongly the fouling phenomena. 3.6. Amperometric study of Bisphenol A The amperometric measurement of 10 μM of BPA using a conventional carbon paste electrode was tested in the presence and in the absence of Tween 20 surfactant. The Fig. 10 illustrates the amperograms responses towards BPA with the surface of conventional CPE at a constant applied potential 610 mV. It can be seen that the conventional CPE has a better analytical performance for the detection of BPA and the electrochemical signal is more stable in the presence of Tween 20 which avoids the surface fouling phenomena. The signal background and noise increase in presence of Tween 20 at conventional CPE. As indicated in Fig. S3, the same experiments were performed at a carbon black paste electrode (CBPE) and showed the stable and low background in presence of Tween 20. Thus, it can be concluded from the above results that Tween 20 is efficient for reduction of surface fouling.
from these results that this electrode performed in this conditions could be of interests for total phenolic compounds determination in waste water and other environmental compartment. 3.7.2. Analytical performance of various carbon based sensors towards Bisphenol A detection SWV analysis technique was found to be more sensitive and less exposed to fouling phenomena. Therefore, under optimum condition, the analytical performance of Glassy carbon electrode, Carbon Black paste electrode, Conventional carbon paste electrode and Multiwall carbon nanotubes paste electrode towards BPA determination were studied and investigated. The Table 2 below presents the summarized data obtained for various tested electrodes in the presence and in the absence
3.7. Electrochemical detection of BPA using carbon based sensors 3.7.1. Determination of BPA using linear sweep voltammetry The Fig. 11A illustrates the linear sweep voltammograms obtained for BPA in the concentration range 10–400 μM in a solution of phosphate buffer pH 7.0, 0.1 M. The relationship between the current signal intensity and the concentration of BPA was linear with a regression equation of I (μA) = 0.127 C (μM) + 8.343 as indicated in Fig. 11B. The limit of detection (LOD) and the limit of quantification (LOQ) calculated were equals 3.5 μA and 11.8 μM respectively. It can be concluded
Fig. 8. % of reduction of BPA oxidation signal between the 1st and 6th scan at a conventional CPE as a function of concentration of Tween 20 in a phosphate buffer 0.1 M, pH 7.0.
Fig. 9. Successive cyclic voltammograms of 25 μM BPA at a conventional carbon paste electrode in a phosphate buffer solution pH 7.0. Scan rate 100 mV/s: (A) in presence of 800 μM of Tween 20, (B) without Tween 20.
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Fig. 10. Amperometric response of 10 μM of BPA at conventional CPE A) in the presence of Tween 20 B) in absence of Tween 20. The applied potential 610 mV.vs (Ag/AgCl).
of Tween 20 surfactant. The obtained results in the absence of Tween 20 in phosphate buffer show a low linear range of BPA concentration and all of the electrode surfaces were blocked by the formation of polyphenols films. However in case of the presence of Tween 20, which playing the role of inhibitor of electrode fouling, the linear range was extended to high concentration. Furthermore, all tested electrodes were repeatable for three consecutive calibration curves allowing to reach a low limits of detection comprised between 0.12 μM and 0.8 μM. The Conventional CPE and CBPE were found the most sensitive towards BPA detection. It can be seen as indicated in the Table 2 that in presence of surfactant, the CNTPE doesn't show any oxidation signal towards BPA in the range of concentration of 1–20 μM BPA. This result leads us to study electrochemical oxidation of other electroactive compounds in presence of Tween 20 as indicated below.
Fig. 11. A) Linear sweep voltammograms of (a) 10 μM; (b) 20 μM; (c) 50 μM; (d) 100 μM; (e) 150 μM; (f) 200 μM; (g) 250 μM; (h) 300 μM; (i) 350 μM and (j) 400 μM of BPA in a phosphate buffer pH 7.0, 0.1 M in presence of 800 μM of Tween 20 at a conventional CPE. B) Calibration curve in the range of 10–400 μM of BPA.
supplementary material (Figs. S4, S5 and S6) that even in the presence of Tween 20 the electrochemical background increase significantly for each scan and no response observed for ascorbic acid. However for 100 μM of catechol, dopamine and BPA a signal of oxidation was observed but at a very high background was obtained. These results indicate that the Tween 20 adsorbs on the surface of CNTPE which blocks the access to the active surface area. The hydrophilic part of Tween 20
Table 2 Summary of data obtained using various kinds of carbon based sensors in the presence and in the absence of Tween 20 for BPA determination. Electrode
3.7.3. Evaluation of CNTPE surface fouling As explained above, the presence of Tween 20 had a significant effect on both the sensitivity of the electrode, the repeatability of the measurements and on the electrochemical behavior of BPA. However, in case of paste electrodes based on CNTs nanoparticles the presence of Tween 20 surfactant blocks the active area of the electrode. Ascorbic acid, Dopamine and catechol are well known as highly electroactive molecules. Therefore, a study of their oxidation at a CNTPE in the presence of 800 μM of Tween 20in a phosphate buffer pH 7.0 was performed. The obtained results show as indicated in
CPEa CBPEb CNTPEc GCEd a b c d
In absence of Tween 20
In the presence of Tween 20
Linear range (μM)
LOD (μM)
LOQ (μM)
Sensitivity (nA/μM)
Linear range (μM)
LOD (μM)
LOQ (μM)
Sensitivity (nA/μM)
0.5–6 0.5–4 1–4 1–8
0.36 0.8 0.7 0.8
1.22 2.6 2.4 2.7
74.1 20.2 45.7 20
1–12 1–16 – 1–16
0.12 0.30 – 0.7
0.4 0.99 – 2.4
81.8 76.3 – 10
Conventional carbon paste electrode. Carbon black paste electrode. Multiwall carbon nanotubes paste electrode. Glassy carbon electrode.
A. Ghanam et al. / Journal of Electroanalytical Chemistry 789 (2017) 58–66
always remains in the aqueous phase while the tails hydrocarbon (hydrophobic portion) may react physically with the surface of the nanotube and adsorb irreversibly [47,48]. 4. Conclusions In this work the electrode surface fouling due to the oxidation of BPA was electrochemically investigated. The effect of several parameters such as the nature of the electrode, the pH of the electrolytic medium, the concentration of BPA and the effect of the addition of surfactant agent (Tween 20) were studied. It was revealed that the addition of Tween 20 has a significant role in reducing the problem of poisoning of the active surface of the electrode. Glassy carbon electrode and paste electrodes based on graphite, MWCNTs and carbon black were used to study their electrochemical behavior towards BPA oxidation. The paste electrodes based on graphite and carbon black showed the best results in terms of sensitivity, detection limit and resistance to electrode fouling particularly in presence of surfactant Tween 20. A concentration as low as 0.12 μM of BPA was successfully detected. Acknowledgements The work presented in this paper was supported by Center SISA from Hassan II University of Casablanca (Morocco). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2017.02.026. References [1] C.A. Staples, P.B. Dome, G.M. Klecka, S.T. Oblock, L.R. Harris, A review of the environmental fate, effects, and exposures of bisphenol A, Chemosphere 36 (1998) 2149–2173. [2] Y. Zhang, Y. Cheng, Y. Zhou, B. Li, W. Gu, X. Shi, Y. Xian, Electrochemical sensor for bisphenol A based on magnetic nanoparticles decorated reduced graphene oxide, Talanta. 107 (2013) 211–218. [3] P. Deng, Z. Xu, Y. Kuang, Electrochemically reduced graphene oxide modified acetylene black paste electrode for the sensitive determination of bisphenol A, J. Electroanal. Chem. 707 (2013) 7–14. [4] X. Wang, H. Zeng, Y. Wei, J.M. Lin, A reversible fluorescence sensor based on insoluble β-cyclodextrin polymer for direct determination of bisphenol A (BPA), J. Sens. Actuators B Chem. 114 (2006) 565–572. [5] L.N. Vandenberg, R. Hauser, M. Marcus, N. Olea, W.V. Welshons, Human exposure to bisphenol A (BPA), Reprod. Toxicol. 24 (2007) 139–177. [6] C. Lu, J. Li, Y. Yang, J.M. Lin, Determination of bisphenol A based on chemiluminescence from gold (III)–peroxymonocarbonate, Talanta 82 (2010) 1576–1580. [7] G. Gatidou, N.S. Thomaidis, A.S. Stasinakis, T.D. Lekkas, Simultaneous determination of the endocrine disrupting compounds nonylphenol, nonylphenolethoxylates, triclosan and bisphenol A in wastewater and sewage sludge by gas chromatography–mass spectrometry, J. Chromatogr. A 1138 (2007) 32–41. [8] A. Kim, C.R. Li, C.F. Jin, K.W. Lee, S.H. Lee, K.J. Shon, N.G. Park, D.K. Kim, S.W. Kang, Y.B. Shim, J.S. Park, A sensitive and reliable quantification method for bisphenol A based on modified competitive ELISA method, Chemosphere 68 (2007) 1204–1209. [9] W. Zhou, C. Sun, Y. Zhou, X. Yang, W. Yang, A facial electrochemical approach to determinate bisphenol A based on graphene-hypercrosslinked resin MN202 composite, Food Chem. 158 (2014) 81–87. [10] E. Ferrer, E. Santoni, S. Vittori, G. Font, J. Manes, G. Sagratini, Simultaneous determination of bisphenol A, octylphenol, and nonylphenol by pressurised liquid extraction and liquid chromatography-tandem mass spectrometry in powdered milk and infant formulas, Food Chem. 126 (2011) 360–367. [11] H.S. Yin, Y.L. Zhou, S.Y. Ai, Preparation and characteristic of cobalt phthalocyanine modified carbon paste electrode for bisphenol A detection, J. Electroanal. Chem. 626 (2009) 80–88. [12] Y. Tan, J. Jin, S. Zhang, Z. Shi, J. Wang, J. Zhang, W. Pu, C. Yang, Electrochemical determination of bisphenol A using a molecularly imprinted chitosan-acetylene black composite film modified glassy carbon electrode, Electroanalysis 28 (2016) 189–196. [13] M. Han, Y. Qu, S. Chen, Y. Wang, Z. Zhang, M. Ma, Z. Wang, G. Zang, C. Li, Amperometric biosensor for bisphenol A based on a glassy carbon electrode modified with a nanocomposite made from polylysine, single walled carbon nanotubes and tyrosinase, Microchim. Acta 180 (2013) 989–996. [14] Y. Lin, K. Liu, C. Liu, L. Yin, Q. Kang, L. Li, B. Li, Electrochemical sensing of bisphenol A based on polyglutamic acid/amino-functionalised carbon nanotubes nanocomposite, Electrochim. Acta 133 (2014) 492–500.
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