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Electrochemical characterization of glassy carbon electrodes modified by resol mixtures
Journal of Electroanalytical Chemistry 510 (2001) 115– 119 www.elsevier.com/locate/jelechem
Electrochemical characterization of glassy carbon electrodes modified by resol mixtures C.D. Garcı´a, C.P. De Pauli, P.I. Ortiz * INFIQC, Departamento de Fisicoquı´mica, Facultad de Ciencias Quı´micas, Uni6ersidad Nacional de Co´rdoba-Ciudad Uni6ersitaria, 5000 Cordoba, Argentina Received 13 July 2000; received in revised form 28 December 2000; accepted 12 May 2001
Abstract The present work aims to obtain more insights into the effects of the electrochemical polymerization of a 4-hydroxybenzaldehyde +formaldehyde (resol) mixture on the electrochemical properties of modified glassy carbon electrodes. Different redox couples, having a permanent positive charge (Fe-phenathroline), a permanent negative charge (K3Fe(SNC)6) and either a positive or negative charge, depending on solution pH (L-Dopa), have been analyzed. Cyclic voltammetry and impedance spectroscopy experiments were performed at different deposited polymer charge and pH values for the proposed redox systems and the comparison with polished electrodes is also analyzed. Electrophoretic mobilities were investigated and acid– base titrations were also carried out in order to prove that the surface is negatively charged. Electrochemical measurements showed an important electrostatic effect between the electrode surface and the couples analyzed. This effect plays an important role on the electrode response when used for analytical purposes. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Polymer modified electrodes; Resol mixtures; Surface characterization
1. Introduction Glassy carbon has been well established for many years as an electrode material. Structurally, it is a compact solid, mechanically stable, impermeable to gases and fluids and chemically resistant over a broad range of situations [1]. In addition, the low cost, great applicability and low background currents over a wide potential range, have driven years of research, particularly in electroanalysis and electrosynthesis, in order to obtain the relationship between surface structure and electrochemical activity. Specifically, adsorption, electron transfer kinetics and surface stability are the most important topics in this field [2,3]. Due to the electrochemical reactivity of carbon surfaces, these electrodes can be modified with a variety of materials such as metals [4], colloidal particles [5], proteins [6,7] or whole cells [8]. However, in some cases this capability can represent an inconvenience, as in the electrochemical determination of phenolic compounds, * Corresponding author. Fax: + 54-351-433-4188. E-mail address: [email protected] (P.I. Ortiz).
because it involves an accumulation of oxidation products on the working electrode surface, and the concomitant loss of activity [9]. In order to solve this problem, surface modifications have been proposed. Some polymeric coatings like poly(3-methylthiophene) [10] have been suggested for use as modifiers that improve the response or increase the electrode lifetime. Phenol+ formaldehyde mixtures synthesized in alkaline media are highly crosslinked materials which are called resol mixtures [11]. They not only preserve the electrode surface from the polymerization of phenol oxidation products, but also improve the electron transfer rate and increase the effective area, lifetime and electrochemical response [12,13]. In previous papers we have reported the effects of resol coating on electrode activity for amperometric determination of phenolic compounds [13,14]. The aim of the present study is to provide information about the effects of electrochemical polymerization of 4-hydroxybenzaldehyde+ formaldehyde resol mixtures on the electrochemical properties of modified glassy carbon electrodes. Different redox couples, having a permanent positive charge (Fe-phenathroline
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(FePh)), a permanent negative charge (K3Fe(SNC)6) and either a positive or negative charge depending on solution pH (L-Dopa (Dop)), have been analyzed. Cyclic voltammetry and impedance spectroscopy experiments were performed at different deposited polymer charge and pH values for the proposed redox systems and the comparison with polished electrodes was also analyzed. In order to complement the electrochemical results, other physicochemical properties from resol mixtures were evaluated by electrophoretic mobility measurements and acid– base titrations.
2.3. Solutions and reagents Aqueous buffer solution (0.01 M total phosphate+ 0.1 M sodium chloride) was used as supporting electrolyte. All the solutions were prepared using purified water (Milli Q, Millipore System) and analytical or HPLC grade reagents. Solutions for the redox couples were prepared from the following reagents: Fe-phenathroline by mixing Fe(NO3)3 (Anedra) and 1,10-phenanthroline-monohydrate (Merck) in a 1:1 ratio, K3Fe(SNC)6 (Mallinckrodt) and L-Dopa (L-b-3,4-dihydroxy-phenylalanine) (Sigma).
2. Experimental
2.4. Electrochemical measurements and data analysis
2.1. Electrodes
Impedance spectroscopy (IS) and cyclic voltammetry (CV) experiments were carried out in a conventional three-electrode cell, using Zhaner (IM5) equipment. An ac signal of 10 mV in amplitude with a frequency range of 10 − 2 to 105 Hz was superimposed at the peak potential value of the oxidation redox couple. In order to obtain the assumed parameters on the proposed equivalent circuits, impedance spectra were simulated using a commercial program (‘Equivalent Circuit’ by Dr Bernard Boukamp from the University of Twente, The Netherlands). The assumed equivalent circuit is described in Fig. 1 where Rel, RCT, CDL, Rads, Qads correspond to the electrolyte resistance, charge transfer resistance, double layer capacity, adsorption resistance and adsorption capacitance, respectively [3].
Glassy carbon (MF-1000 Bioanalytical Systems) was used as the working electrode. Prior to the modification it was mechanically polished with diamond paste (0.5 mm, Metadi II, Buehler) and alumina solution (0.05 mm, Buehler) and rinsed with ethanol and water (Milli Q, Millipore System) (polished electrode, PE). Ag AgCl 3 M NaCl (RE-4 Bioanalytical Systems) and a platinum wire were used as reference and auxiliary electrodes, respectively.
2.2. Electrode modification 4-hydroxybenzaldehyde (4.0 g) (ICN) was mixed with 10 ml 37% formaldehyde solution (Baker) in 50 ml of phosphate buffer (pH 12, 0.01 M total concentration) and refluxed for 4 h. The resulting dark solution was treated with HCl until the resol mixture started to precipitate. The solid residue was filtered, washed with water and redissolved in 50 ml of 2.0 M NaOH solution resulting in the pre-polymer resol mixture. The polymerization was carried out potentiostatically ( + 1.2 V vs. the platinum electrode) immersing the PE in the pre-polymer alkaline solution and the charge (Q) was measured using an electronic coulometer. The polymer modified electrode (PME) was rinsed with water and stored in buffer solution (0.010 M total phosphate, pH 7.0) until use.
Fig. 1. Equivalent circuit used for the simulations including the following elements: electrolyte resistance (Rel), charge transfer resistance (RCT), double layer capacity (CDL), adsorption resistance (Rads) and adsorption capacitance (Qads).
2.5. Electrophoretic mobilities A Rank Bros. Mark II electrophoresis apparatus equipped with a cylindrical cell of 2 mm internal diameter was used, applying 40.8 V with platinum electrodes. The suspension of resol particles was prepared by dispersing a known amount of solid in 1.00× 10 − 2 M KCl aqueous solution and equilibrating it at different pH values (B 9.00). The pH solution must be lower than 9.00 because at higher pH values resol particles dissolve. All data are the average of 10 measurements and were performed at 29°C.
2.6. Acid–base titrations An Orion 960 pH meter equipped with a BN 9101 glass electrode, an Orion 900200 double-junction Ag AgCl KClsat reference electrode and an automatic burette were used. Titrations were carried out by dispersing a known amount of solid in 100 ml 1.00×10 − 2 M KCl (pH 3) aqueous solution and adding 0.100 ml aliquots of 0.112 M KOH until the resol mixture was completely dissolved (pH\10). The initial pH was adjusted by addition of HCl solution. All the measurements were performed at room temperature.
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Table 2 Calculated parameters of 3.00×10−3 M FePh in phosphate buffer at PE and PME from CV and IS for different Q and solution pH pH
Q/C cm−2
RCT/MV
Area/%
103 k0/cm s−1
2.95 2.95 2.95 2.95 2.95
0.0 0.0279 0.1860 0.4650 1.8900
5.4 3.5 2.7 2.3 1.1
100 52 107 148 155
2.6 7.9 8.3 7.9 7.5
2.15 2.95 4.05
0.2323 0.2323 0.2323
1.8 2.0 1.0
110 112 117
7.1 8.7 8.7
The kinetic behavior of the system was studied by analyzing the k0 value (see Tables 1–3). As the difference between the anodic and cathodic peak potentials (DEp) was, in all cases, greater than (0.059/n) V, quasireversible behavior can be assumed for all the couples. The degree of reversibility (c) can be obtained from the variation of DEp according to the following expression [15]: c =(24.4/DEp − 59.8)−0.03
Fig. 2. I/E potentiodynamic profiles for 3.00 × 10 − 3 M potassium ferricyanide (A), 3.00 × 10 − 3 M FePh (B) and 3.00 ×10 − 3 M Dop (C) at a PME (Q= 5.00 C cm − 2) in pH 7.00 buffer at 0.100 V s − 1.
3. Results and discussion Fig. 2 shows the potentiodynamic I/E profiles for the three redox couples analyzed with a PME (deposited charge 5.00 C cm − 2) at pH 7.00. In all cases only one anodic current peak during the positive scan and the corresponding reduction peak in the negative sweep are observed. Table 1 Calculated parameters of 3.00×10−3 M potassium ferricyanide in phosphate buffer at PE and PME from CV and IS for different Q and solution pH Q/C cm−2
RCT/MV
Area/%
7.2 7.2 7.2 7.2 7.2 7.2
0.0 0.05 0.14 0.48 1.51 3.12
0.054 0.890 0.187 0.059 0.053 0.032
100 79 93 130 213 203
2.95 7.2 10.5
3.12 3.12 3.12
0.032 0.051 0.053
242 203 204
pH
102 k0/cm s−1 1.64 0.796 1.32 2.01 2.15 3.43 13.7 3.43 3.43
Therefore the peak shape and associated parameters are conveniently expressed by another parameter \ (\=c/p1/2) where the oxidation and reduction processes are considered simultaneously and from which the k0 value can be calculated [15]. k0 = \(D(nF/RT)6)1/2 where D is the diffusion coefficient of the electroactive species (cm2 s − 1), n the number of electrons transferred, F the Faraday constant, R the gas constant, T the temperature and 6 the sweep rate (mV s − 1). The three redox couples showed a linear Ip versus 6 1/2 dependence, indicating that they are diffusion controlled. As described earlier (Section 2) two different electrode pretreatments were analyzed, mechanical polishTable 3 Calculated parameters of 3.00×10−3 M Dop in phosphate buffer at PE and PME from CV and IS for different Q and solution pH pH
Q/C cm−2
RCT/KV
Area/%
7.2 7.2 7.2 7.2 7.2 7.2 7.2
0.0 0.0033 0.019 0.056 0.25 1.47 3.21
49.1 51.0 59.1 24.0 10.7 4.11 4.19
100 88 99 139 174 191 201
2.9 4.5 7.2
0.056 0.056 0.056
0.42 0.42 4.19
228 215 139
a
Reversible behavior.
102 k0/cm s−1 1.02 0.455 1.48 4.63 a a a
99.6 4.53 4.63
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Fig. 3. Cyclic voltammograms for 3.00 × 10 − 3 M K3Fe(SNC)6 for (— — ) a polished electrode and ( ) a polymer modified electrode at 0.100 V s − 1.
ing and polymer modification. Fig. 3 shows a typical I/E profile using 3.00× 10 − 3 M Fe(SCN)36 − /4 − as the redox couple and buffer solution (0.01 M total phosphate concentration+0.1 M NaCl, pH 4.10) as supporting electrolyte at 0.100 V s − 1. As can be observed for a PME, a clear current increase in the whole voltammogram is obtained when compared to the PE. The other couples analyzed showed similar behavior (not shown). From these experiments it can be concluded that a change in the electrochemical response between the PE and PME is observed. At low deposited charges, there is a decrease in the current peak values, while for higher charge values, better electrochemical responses were obtained, showing an increase in the peak current and a decrease in the peak potential values. This behavior can be explained by assuming that at low charges, molecules arrange parallel to the surface and less active sites are available. However, for higher charges there is a polymer rearrangement producing an active site enrichment similar to the electrode activation (not shown). Similar behavior was observed by cyclic voltammetry for all the couples analyzed (not shown). The electrode active area was calculated from Ip according to the following equation:
Tables 1– 3 correspond to potassium ferricyanide, FePh and Dop solutions respectively and summarize the results obtained for the active area and k0 (cm s − 1) by cyclic voltammetry and the charge transfer resistance (RCT) by impedance spectroscopy as a function of deposited charge and solution pH. In the case of Dop, after the initial decrease, the system becomes reversible, not allowing the calculation of k0 (marked in the table). For reactions showing less reversible behavior, the kinetic process becomes more important, which means that k0 should be lower and RCT higher. This is the observed behavior for the couples analyzed (Tables 1–3), where important changes during the first modification steps take place in a similar way as described for the Ip versus Q dependence. Further important behavior that can be observed in Tables 1–3, is the solution pH effect. Ferricyanide is a molecule with a permanent negative charge, so for alkaline pH values, an RCT increase and k0 decrease is observed accordingly; the voltammetric anodic current peak has a lower value. FePh showed an opposite effect, that is, an RCT decrease and a k0 and Ip increase, for higher pH values due to the permanent positive charge. On the other hand, Dop is a neutral molecule that becomes negatively charged as the pH increases, so no changes were observed for pH values lower than pKa and similar behavior to that described for ferricyanide was observed for higher pH values.
3.1. Electrophoretic mobility measurements Electrophoretic (v) mobilities were measured in KCl and the values obtained show that the polymer has a negative charge that increases as the solution pH becomes more alkaline. The isoelectric point was estimated by extrapolating the v versus pH curve [16], and
Ip = −2.69×10 − 5n 3/2AD0[O]6 1/2 Ip being in A, n is the number of electrons transferred, A is the electrode active area measured in cm − 2, D0 is the diffusion coefficient in cm2 s − 1, [O] is the concentration in the bulk in mol cm − 3 and 6 is the sweep rate in mV s − 1. The electroactive area obtained was analyzed as a function of the deposited charge assuming that the calculated areas for polished electrodes are 100% of the active area.
Fig. 4. Surface charge expressed in mmol g − 1 as a function of solution pH performed in 1.00 × 10 − 3 M KCl solution.
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119
the value obtained was ca. 2.4. More negatively charged particles were observed when v was measured in phosphate buffer solution. This can be explained by assuming that some specific interactions between phosphate ions and resol particles take place.
nipulation of these parameters contributes to the expansion of the applicability of this type of modification.
3.2. Acid–base titrations
The authors acknowledge the Alexander von Humboldt Foundation for the donation of the Zahner instrument and CONICET, CONICOR and SeCyT-UNC for financial support. CDG thanks CONICET for a fellowship granted. The authors also wish to thank Dr Marcelo J. Avena for helpful assistance.
In order to confirm the hypothesis of a negatively charged polymer, acid– base potentiometric titrations were performed [16]. Fig. 4 shows the dependence of surface charge as a function of pH. The surface charge, expressed as mmol of unit charge, was calculated using a spreadsheet provided by Vermeer [17]. As can be observed, a quasi-constant dependence is obtained at pH B6.50 that increases linearly to values more than 10 times greater for higher pH values.
4. Conclusions Protonation/deprotonation reactions on the acid– base groups lead to typical titration curves and are responsible for the development of electrical charges in the resol chain. The assumption that the polymer is negatively charged was confirmed by electrophoretic mobility measurements and acid– base titrations giving results that are in good agreement. Electrochemical measurements showed an important electrostatic interaction between the electrode surface and the couples analyzed, which plays an important role in the electrode response when used for analytical purposes. Positive, neutral and negatively charged molecules can be oxidized at PME; however, better responses were observed with positive couples at high pH values and negative couples at low pH values. Higher active area values were observed with increased deposited charges, whereas RCT decreased. Maximum responses were obtained for high electrode active areas and by setting the correct pH value. Ma.
Acknowledgements
References [1] P. Heiduschka, A.W. Munz, W. Go¨ pel, Electrochim. Acta 39 (1994) 2207. [2] A.I. Rosa, L. Earley, M.W. Lehmann, L.E. Welch, Talanta 46 (1998) 1507. [3] Q. Chi, W. Go¨ pel, T. Ruzgas, L. Gorton, P. Heiduschka, Electroanalysis 9 (1997) 357. [4] F. Rodriguez Nieto, R. Tucceri, D. Posadas, J. Electroanal. Chem. 383 (1995) 21. [5] P. Joo, Colloids Surf. A 141 (1998) 337. [6] M. El Rhazi, H. Randriamahazaka, J. Nigretto, Electroanalysis 9 (1997) 403. [7] M. Garguillo, A. Michael, Anal. Chim. Acta 307 (1995) 291. [8] E.S. Forzani, G.A. Rivas, V.M. Solı´s, J. Electroanal. Chem. 435 (1997) 77. [9] C. Fernandez, E. Chico, P. Yan˜ ez-Seden˜ o, J.M. Pingarro´ n, L.M. Polo, Analyst 117 (1992) 1919. [10] L. Agu¨ i, P. Yan˜ ez-Seden˜ o, J.M. Pingarro´ n, Electroanalysis 9 (1997) 468. [11] M. Grenier-Loustalot, S. Larroque, P. Grenier, Polymer 37 (1996) 639. [12] P. Abu Nader, P.I. Ortiz, H.A. Mottola, Anal. Chim. Acta 249 (1991) 395. [13] C.D. Garcı´a, P.I. Ortiz, Electroanalysis 10 (1998) 832. [14] C.D. Garcı´a, P.I. Ortiz, Anal. Sci. 15 (1999) 461. [15] C.M.A. Brett, A.M. Oliveira Brett, Electrochemistry, Principles, Methods and Applications, Oxford University Press, 1993, Ch. 9. [16] C.E. Giacommelli, M.J. Avena, C.P. De Pauli, J. Colloid Interface Sci. 188 (1997) 387. [17] A.W.P. Vermeer, personal communication, 2000.
Electrochemical characterization of glassy carbon electrodes modified by resol mixtures C.D. Garcı´a, C.P. De Pauli, P.I. Ortiz * INFIQC, Departamento de Fisicoquı´mica, Facultad de Ciencias Quı´micas, Uni6ersidad Nacional de Co´rdoba-Ciudad Uni6ersitaria, 5000 Cordoba, Argentina Received 13 July 2000; received in revised form 28 December 2000; accepted 12 May 2001
Abstract The present work aims to obtain more insights into the effects of the electrochemical polymerization of a 4-hydroxybenzaldehyde +formaldehyde (resol) mixture on the electrochemical properties of modified glassy carbon electrodes. Different redox couples, having a permanent positive charge (Fe-phenathroline), a permanent negative charge (K3Fe(SNC)6) and either a positive or negative charge, depending on solution pH (L-Dopa), have been analyzed. Cyclic voltammetry and impedance spectroscopy experiments were performed at different deposited polymer charge and pH values for the proposed redox systems and the comparison with polished electrodes is also analyzed. Electrophoretic mobilities were investigated and acid– base titrations were also carried out in order to prove that the surface is negatively charged. Electrochemical measurements showed an important electrostatic effect between the electrode surface and the couples analyzed. This effect plays an important role on the electrode response when used for analytical purposes. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Polymer modified electrodes; Resol mixtures; Surface characterization
1. Introduction Glassy carbon has been well established for many years as an electrode material. Structurally, it is a compact solid, mechanically stable, impermeable to gases and fluids and chemically resistant over a broad range of situations [1]. In addition, the low cost, great applicability and low background currents over a wide potential range, have driven years of research, particularly in electroanalysis and electrosynthesis, in order to obtain the relationship between surface structure and electrochemical activity. Specifically, adsorption, electron transfer kinetics and surface stability are the most important topics in this field [2,3]. Due to the electrochemical reactivity of carbon surfaces, these electrodes can be modified with a variety of materials such as metals [4], colloidal particles [5], proteins [6,7] or whole cells [8]. However, in some cases this capability can represent an inconvenience, as in the electrochemical determination of phenolic compounds, * Corresponding author. Fax: + 54-351-433-4188. E-mail address: [email protected] (P.I. Ortiz).
because it involves an accumulation of oxidation products on the working electrode surface, and the concomitant loss of activity [9]. In order to solve this problem, surface modifications have been proposed. Some polymeric coatings like poly(3-methylthiophene) [10] have been suggested for use as modifiers that improve the response or increase the electrode lifetime. Phenol+ formaldehyde mixtures synthesized in alkaline media are highly crosslinked materials which are called resol mixtures [11]. They not only preserve the electrode surface from the polymerization of phenol oxidation products, but also improve the electron transfer rate and increase the effective area, lifetime and electrochemical response [12,13]. In previous papers we have reported the effects of resol coating on electrode activity for amperometric determination of phenolic compounds [13,14]. The aim of the present study is to provide information about the effects of electrochemical polymerization of 4-hydroxybenzaldehyde+ formaldehyde resol mixtures on the electrochemical properties of modified glassy carbon electrodes. Different redox couples, having a permanent positive charge (Fe-phenathroline
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(FePh)), a permanent negative charge (K3Fe(SNC)6) and either a positive or negative charge depending on solution pH (L-Dopa (Dop)), have been analyzed. Cyclic voltammetry and impedance spectroscopy experiments were performed at different deposited polymer charge and pH values for the proposed redox systems and the comparison with polished electrodes was also analyzed. In order to complement the electrochemical results, other physicochemical properties from resol mixtures were evaluated by electrophoretic mobility measurements and acid– base titrations.
2.3. Solutions and reagents Aqueous buffer solution (0.01 M total phosphate+ 0.1 M sodium chloride) was used as supporting electrolyte. All the solutions were prepared using purified water (Milli Q, Millipore System) and analytical or HPLC grade reagents. Solutions for the redox couples were prepared from the following reagents: Fe-phenathroline by mixing Fe(NO3)3 (Anedra) and 1,10-phenanthroline-monohydrate (Merck) in a 1:1 ratio, K3Fe(SNC)6 (Mallinckrodt) and L-Dopa (L-b-3,4-dihydroxy-phenylalanine) (Sigma).
2. Experimental
2.4. Electrochemical measurements and data analysis
2.1. Electrodes
Impedance spectroscopy (IS) and cyclic voltammetry (CV) experiments were carried out in a conventional three-electrode cell, using Zhaner (IM5) equipment. An ac signal of 10 mV in amplitude with a frequency range of 10 − 2 to 105 Hz was superimposed at the peak potential value of the oxidation redox couple. In order to obtain the assumed parameters on the proposed equivalent circuits, impedance spectra were simulated using a commercial program (‘Equivalent Circuit’ by Dr Bernard Boukamp from the University of Twente, The Netherlands). The assumed equivalent circuit is described in Fig. 1 where Rel, RCT, CDL, Rads, Qads correspond to the electrolyte resistance, charge transfer resistance, double layer capacity, adsorption resistance and adsorption capacitance, respectively [3].
Glassy carbon (MF-1000 Bioanalytical Systems) was used as the working electrode. Prior to the modification it was mechanically polished with diamond paste (0.5 mm, Metadi II, Buehler) and alumina solution (0.05 mm, Buehler) and rinsed with ethanol and water (Milli Q, Millipore System) (polished electrode, PE). Ag AgCl 3 M NaCl (RE-4 Bioanalytical Systems) and a platinum wire were used as reference and auxiliary electrodes, respectively.
2.2. Electrode modification 4-hydroxybenzaldehyde (4.0 g) (ICN) was mixed with 10 ml 37% formaldehyde solution (Baker) in 50 ml of phosphate buffer (pH 12, 0.01 M total concentration) and refluxed for 4 h. The resulting dark solution was treated with HCl until the resol mixture started to precipitate. The solid residue was filtered, washed with water and redissolved in 50 ml of 2.0 M NaOH solution resulting in the pre-polymer resol mixture. The polymerization was carried out potentiostatically ( + 1.2 V vs. the platinum electrode) immersing the PE in the pre-polymer alkaline solution and the charge (Q) was measured using an electronic coulometer. The polymer modified electrode (PME) was rinsed with water and stored in buffer solution (0.010 M total phosphate, pH 7.0) until use.
Fig. 1. Equivalent circuit used for the simulations including the following elements: electrolyte resistance (Rel), charge transfer resistance (RCT), double layer capacity (CDL), adsorption resistance (Rads) and adsorption capacitance (Qads).
2.5. Electrophoretic mobilities A Rank Bros. Mark II electrophoresis apparatus equipped with a cylindrical cell of 2 mm internal diameter was used, applying 40.8 V with platinum electrodes. The suspension of resol particles was prepared by dispersing a known amount of solid in 1.00× 10 − 2 M KCl aqueous solution and equilibrating it at different pH values (B 9.00). The pH solution must be lower than 9.00 because at higher pH values resol particles dissolve. All data are the average of 10 measurements and were performed at 29°C.
2.6. Acid–base titrations An Orion 960 pH meter equipped with a BN 9101 glass electrode, an Orion 900200 double-junction Ag AgCl KClsat reference electrode and an automatic burette were used. Titrations were carried out by dispersing a known amount of solid in 100 ml 1.00×10 − 2 M KCl (pH 3) aqueous solution and adding 0.100 ml aliquots of 0.112 M KOH until the resol mixture was completely dissolved (pH\10). The initial pH was adjusted by addition of HCl solution. All the measurements were performed at room temperature.
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Table 2 Calculated parameters of 3.00×10−3 M FePh in phosphate buffer at PE and PME from CV and IS for different Q and solution pH pH
Q/C cm−2
RCT/MV
Area/%
103 k0/cm s−1
2.95 2.95 2.95 2.95 2.95
0.0 0.0279 0.1860 0.4650 1.8900
5.4 3.5 2.7 2.3 1.1
100 52 107 148 155
2.6 7.9 8.3 7.9 7.5
2.15 2.95 4.05
0.2323 0.2323 0.2323
1.8 2.0 1.0
110 112 117
7.1 8.7 8.7
The kinetic behavior of the system was studied by analyzing the k0 value (see Tables 1–3). As the difference between the anodic and cathodic peak potentials (DEp) was, in all cases, greater than (0.059/n) V, quasireversible behavior can be assumed for all the couples. The degree of reversibility (c) can be obtained from the variation of DEp according to the following expression [15]: c =(24.4/DEp − 59.8)−0.03
Fig. 2. I/E potentiodynamic profiles for 3.00 × 10 − 3 M potassium ferricyanide (A), 3.00 × 10 − 3 M FePh (B) and 3.00 ×10 − 3 M Dop (C) at a PME (Q= 5.00 C cm − 2) in pH 7.00 buffer at 0.100 V s − 1.
3. Results and discussion Fig. 2 shows the potentiodynamic I/E profiles for the three redox couples analyzed with a PME (deposited charge 5.00 C cm − 2) at pH 7.00. In all cases only one anodic current peak during the positive scan and the corresponding reduction peak in the negative sweep are observed. Table 1 Calculated parameters of 3.00×10−3 M potassium ferricyanide in phosphate buffer at PE and PME from CV and IS for different Q and solution pH Q/C cm−2
RCT/MV
Area/%
7.2 7.2 7.2 7.2 7.2 7.2
0.0 0.05 0.14 0.48 1.51 3.12
0.054 0.890 0.187 0.059 0.053 0.032
100 79 93 130 213 203
2.95 7.2 10.5
3.12 3.12 3.12
0.032 0.051 0.053
242 203 204
pH
102 k0/cm s−1 1.64 0.796 1.32 2.01 2.15 3.43 13.7 3.43 3.43
Therefore the peak shape and associated parameters are conveniently expressed by another parameter \ (\=c/p1/2) where the oxidation and reduction processes are considered simultaneously and from which the k0 value can be calculated [15]. k0 = \(D(nF/RT)6)1/2 where D is the diffusion coefficient of the electroactive species (cm2 s − 1), n the number of electrons transferred, F the Faraday constant, R the gas constant, T the temperature and 6 the sweep rate (mV s − 1). The three redox couples showed a linear Ip versus 6 1/2 dependence, indicating that they are diffusion controlled. As described earlier (Section 2) two different electrode pretreatments were analyzed, mechanical polishTable 3 Calculated parameters of 3.00×10−3 M Dop in phosphate buffer at PE and PME from CV and IS for different Q and solution pH pH
Q/C cm−2
RCT/KV
Area/%
7.2 7.2 7.2 7.2 7.2 7.2 7.2
0.0 0.0033 0.019 0.056 0.25 1.47 3.21
49.1 51.0 59.1 24.0 10.7 4.11 4.19
100 88 99 139 174 191 201
2.9 4.5 7.2
0.056 0.056 0.056
0.42 0.42 4.19
228 215 139
a
Reversible behavior.
102 k0/cm s−1 1.02 0.455 1.48 4.63 a a a
99.6 4.53 4.63
118
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Fig. 3. Cyclic voltammograms for 3.00 × 10 − 3 M K3Fe(SNC)6 for (— — ) a polished electrode and ( ) a polymer modified electrode at 0.100 V s − 1.
ing and polymer modification. Fig. 3 shows a typical I/E profile using 3.00× 10 − 3 M Fe(SCN)36 − /4 − as the redox couple and buffer solution (0.01 M total phosphate concentration+0.1 M NaCl, pH 4.10) as supporting electrolyte at 0.100 V s − 1. As can be observed for a PME, a clear current increase in the whole voltammogram is obtained when compared to the PE. The other couples analyzed showed similar behavior (not shown). From these experiments it can be concluded that a change in the electrochemical response between the PE and PME is observed. At low deposited charges, there is a decrease in the current peak values, while for higher charge values, better electrochemical responses were obtained, showing an increase in the peak current and a decrease in the peak potential values. This behavior can be explained by assuming that at low charges, molecules arrange parallel to the surface and less active sites are available. However, for higher charges there is a polymer rearrangement producing an active site enrichment similar to the electrode activation (not shown). Similar behavior was observed by cyclic voltammetry for all the couples analyzed (not shown). The electrode active area was calculated from Ip according to the following equation:
Tables 1– 3 correspond to potassium ferricyanide, FePh and Dop solutions respectively and summarize the results obtained for the active area and k0 (cm s − 1) by cyclic voltammetry and the charge transfer resistance (RCT) by impedance spectroscopy as a function of deposited charge and solution pH. In the case of Dop, after the initial decrease, the system becomes reversible, not allowing the calculation of k0 (marked in the table). For reactions showing less reversible behavior, the kinetic process becomes more important, which means that k0 should be lower and RCT higher. This is the observed behavior for the couples analyzed (Tables 1–3), where important changes during the first modification steps take place in a similar way as described for the Ip versus Q dependence. Further important behavior that can be observed in Tables 1–3, is the solution pH effect. Ferricyanide is a molecule with a permanent negative charge, so for alkaline pH values, an RCT increase and k0 decrease is observed accordingly; the voltammetric anodic current peak has a lower value. FePh showed an opposite effect, that is, an RCT decrease and a k0 and Ip increase, for higher pH values due to the permanent positive charge. On the other hand, Dop is a neutral molecule that becomes negatively charged as the pH increases, so no changes were observed for pH values lower than pKa and similar behavior to that described for ferricyanide was observed for higher pH values.
3.1. Electrophoretic mobility measurements Electrophoretic (v) mobilities were measured in KCl and the values obtained show that the polymer has a negative charge that increases as the solution pH becomes more alkaline. The isoelectric point was estimated by extrapolating the v versus pH curve [16], and
Ip = −2.69×10 − 5n 3/2AD0[O]6 1/2 Ip being in A, n is the number of electrons transferred, A is the electrode active area measured in cm − 2, D0 is the diffusion coefficient in cm2 s − 1, [O] is the concentration in the bulk in mol cm − 3 and 6 is the sweep rate in mV s − 1. The electroactive area obtained was analyzed as a function of the deposited charge assuming that the calculated areas for polished electrodes are 100% of the active area.
Fig. 4. Surface charge expressed in mmol g − 1 as a function of solution pH performed in 1.00 × 10 − 3 M KCl solution.
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the value obtained was ca. 2.4. More negatively charged particles were observed when v was measured in phosphate buffer solution. This can be explained by assuming that some specific interactions between phosphate ions and resol particles take place.
nipulation of these parameters contributes to the expansion of the applicability of this type of modification.
3.2. Acid–base titrations
The authors acknowledge the Alexander von Humboldt Foundation for the donation of the Zahner instrument and CONICET, CONICOR and SeCyT-UNC for financial support. CDG thanks CONICET for a fellowship granted. The authors also wish to thank Dr Marcelo J. Avena for helpful assistance.
In order to confirm the hypothesis of a negatively charged polymer, acid– base potentiometric titrations were performed [16]. Fig. 4 shows the dependence of surface charge as a function of pH. The surface charge, expressed as mmol of unit charge, was calculated using a spreadsheet provided by Vermeer [17]. As can be observed, a quasi-constant dependence is obtained at pH B6.50 that increases linearly to values more than 10 times greater for higher pH values.
4. Conclusions Protonation/deprotonation reactions on the acid– base groups lead to typical titration curves and are responsible for the development of electrical charges in the resol chain. The assumption that the polymer is negatively charged was confirmed by electrophoretic mobility measurements and acid– base titrations giving results that are in good agreement. Electrochemical measurements showed an important electrostatic interaction between the electrode surface and the couples analyzed, which plays an important role in the electrode response when used for analytical purposes. Positive, neutral and negatively charged molecules can be oxidized at PME; however, better responses were observed with positive couples at high pH values and negative couples at low pH values. Higher active area values were observed with increased deposited charges, whereas RCT decreased. Maximum responses were obtained for high electrode active areas and by setting the correct pH value. Ma.
Acknowledgements
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