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Electrocatalytic reactions of hydrogen peroxide at carbon paste electrodes modified by some metal hexacyanoferrates
Sensors and Actuators B 46 (1998) 236 – 241
Electrocatalytic reactions of hydrogen peroxide at carbon paste electrodes modified by some metal hexacyanoferrates Rasa Garjonyte; , Albertas Malinauskas * Institute of Chemistry, Gos' tauto Str. 9, LT-2600 Vilnius, Lithuania Received 6 October 1997; received in revised form 29 January 1998; accepted 2 February 1998
Abstract Carbon paste electrodes (CPE’s), modified by ferrous, cupric, cuprous, cobalt and nickel hexacyanoferrates, were shown to electrocatalyse the cathodic reduction of hydrogen peroxide. At pH 3.05 ferrous hexacyanoferrate (FHC) modified CPE shows a cathodic response of 0.2 mA/cm2 to 1 mM of peroxide added. By increasing the solution pH the response of this CPE diminishes drastically (by two orders of magnitude at pH 7.3). In contrast, the cupric hexacyanoferrate (CHC) modified CPE shows a high cathodic response in the whole pH range investigated, 3.05 – 7.3. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Carbon paste electrodes; Sensors; Hydrogen peroxide
1. Introduction The detection of hydrogen peroxide in aqueous solutions, provided by electrochemical means, plays a key role in bioelectroanalytical systems based on oxidase type enzymes. Since an early work of Guilbault and Lubrano on the detection of hydrogen peroxide by electrooxidation on platinum electrode at sufficiently high anodic potential (exceeding 0.6 V vs. Ag/AgCl reference electrode) [1], a great number of papers was published with an attempt to use this detection system in combination with a lot of known oxidases. Later, Wang et al. modified this detection system by the use of platinum-, palladium- and ruthenium-modified carbon paste electrodes (CPE’s), enabling the amperometric detection of this analyte at a constant potential of 0.8 V vs. Ag/AgCl at a detection limit as low as 5× 10 − 6 M [2]. However, the most biological liquids to be analysed contain some substances which are able to discharge on electrode at high anodic potential used in hydrogen peroxide detection at platinum or related materials. Therefore, it is impossible to discriminate the electrode response arising from hydrogen peroxide, e.g. produced in an enzyme-catalysed reaction, from anodic noise
* Corresponding author. 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00123-3
current arising from the contaminants. Thus, a great deal of attention was paid to avoid the influence of the reductants present in an analyte solution. One of the possible approaches is to use selective electrocatalysts, i.e. substances which lower an overpotential of hydrogen peroxide electrooxidation to an appropriate level, that prevents the discharge of the contaminants at the electrode potential applied. Among such substances, Prussian blue (ferric hexacyanoferrate) is a suitable one. It is well known that thin films of Prussian blue can be obtained on electrode surface either by chemical deposition [3,4], or electrodeposition [5–8]. The deposited films of Prussian blue can be easily reduced at a potential below ca. 0.2 V vs. SCE [8], yielding Prussian white (or Everitt salt). The latter was reported to be able to reduce hydrogen peroxide in an electrochemical system [9]. Based on Prussian blue electrodeposited on a glassy carbon electrode, Karyakin et al. developed an enzyme sensor suitable for glucose determination both by anodic oxidation and cathodic reduction of hydrogen peroxide, produced in an enzyme-catalysed reaction [10]. For the cathodic reduction, an electrode potential of 0.18 V vs. Ag/AgCl in a buffered solution of pH 6.0 was used [10]. Application of such a low electrode potential enabled the anodic discharge of many contaminants usually present in analyte solutions to be avoided.
R. Garjonyte; , A. Malinauskas / Sensors and Actuators B 46 (1998) 236–241
The modified CPEs present a very useful tool in electroanalysis because a wide variety of substances, including electrocatalysts, can be used as the components of carbon paste [11,12]. Prussian blue was included in a carbon paste and hydrogen peroxide was detected at resulting CPE by direct current voltammetry (reduction) at −0.4 V vs. SCE [13]. However, among a lot of well known mixed valence metal compounds including hexacyanoferrate complexes only a limited number of compounds, such as ruthenium purple or osmium purple [13], next to Prussian blue, were tested as the components of catalytically active CPEs. The aim of the present work was to investigate the electrocatalytic reactions of hydrogen peroxide at CPEs modified by some metal hexacyanoferrates.
2. Experimental All chemicals were of analytical grade. Metal hexacyanoferrates were obtained by mixing the aqueous solutions containing ferrous, copper, cobalt or nickel sulphates with a nearly equimolar quantity of aqueous solution of potassium ferricyanide. Cuprous hexacyanoferrate was prepared by dissolving cuprous oxide in excess of concentrated hydrochloric acid and reacting then with a solution of potassium ferricyanide. The precipitates were collected, rinsed with water and dried. Carbon paste was prepared by carefully mixing the dispersed graphite powder with metal hexacyanoferrates at a varying weight ratio. Then, an inert binder consisting of the mixture of hydrocarbons of the fraction C16 –C26 was added at a quantity of 0.3 ml to 1 g of graphite–hexacyanoferrate mixture and intimately mixed to obtain homogeneous mixture. The prepared paste was packed into an electrode body, consisting of a plastic tube of ca. 2.5 mm2 inner diameter and arranged with a copper wire serving as an electrode contact. All solutions used contained 0.1 M KCl. A solution of pH 3.05 contained additionally 1 mM HCl and solutions of nearly neutral pH contained additionally 0.01 M phosphate buffer, adjusted to an appropriate pH value. Electrochemical measurements were carried out in a thermostated at 25°C three-electrode cell containing a CPE as a working electrode, a glassy carbon counterelectrode and a saturated Ag/AgCl reference electrode. During the experiments, the solution in the cell was continuously stirred with a magnetic stirrer. A SVA-1 model potentiostat connected to a X – Y – t plotter was used. All potential values are quoted versus saturated Ag/AgCl reference electrode.
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3. Results and discussion Fig. 1 shows cyclic voltammograms of CPEs, modified by ferrous and cupric hexacyanoferrates (FHC and CHC, respectively). In an anodic potential scan, a well defined wave with E1/2 value of ca. 0.2 V is observed for FHC. In the back potential scan, a cathodic wave at nearly same potential is seen. The E1/2 values obtained coincide with known values, e.g. a midpoint value of 0.43 V versus NHE was reported earlier [6] and a value of 0.2 V versus SCE was obtained from cyclic voltammetry of electrodes modified by a thin layer of Prussian blue [5,8]. As compared with thin deposited films of Prussian blue, both the anodic and cathodic maxima of Fig. 1 appear more flattened and less reversible, obviously because of the non-uniform nature of the particles of electroactive substance included into a conducting graphite matrix. Electrode processes which occur at CHC, appear from Fig. 1 to be less reversible, as in case of FHC, although even in thin films of copper hexacyanoferrate these redox processes seem to be more complex [14]. When held at a sufficiently negative potential where the reduced form of FHC, i.e. Prussian white, prevails, the modified CPE shows a cathodic response to added hydrogen peroxide which reaches a maximum value in about 10 or several tens of seconds (Fig. 2). On adding hydrogen peroxide in a concentration of 1 mM, a cathodic current density of about 0.2 mA/cm2 is obtained for a solution of pH 3.05, other conditions being as in Fig. 2. Karyakin et al. obtained the cathodic current response of 10 − 6 A/cm2 on addition of 10 − 6 M of hydrogen peroxide [10] by the use of FHC film modified electrode. This means that FHC-modified CPE shows high current response to added peroxide
Fig. 1. Cyclic voltammograms of CPE, modified by FHC (1) and CHC (2) in a solution containing 0.1 M KCl and 1 mM HCl, pH 3.05. The modified CPEs contained hexacyanoferrates and graphite, mixed at a weight ratio 3:7. Potential sweep rate 100 mV/s.
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2K2FeII[FeII(CN)6]+ H2O2 + 2H +
Fig. 2. The response of FHC-modified CPE on hydrogen peroxide at various pH values (as indicated) at an electrode potential of − 0.2 V. Hydrogen peroxide was added at the time intervals of 45 s to a total concentration 2.8, 5.6 and 8.4 mM (as indicated by the arrows). The modified CPE contained FHC and graphite, mixed at a weight ratio 3:7. The electrode was kept at a working potential for 20 min before hydrogen peroxide addition.
which is only about five times lower than that of a FHC-film sensor. A cathodic peak current density of FHC-modified CPE without peroxide present in solution, obtained at potentiodynamic conditions as in Fig. 1, is around 6 mA/cm2 and exceeds by about 30 times the response obtained on addition of 1 mM peroxide under potentiostatic conditions. This means that the peroxide-induced cathodic response is only observable if the electrode is held at cathodic potential for a definite period of time, sufficient to reduce cathodically an entire layer of FHC placed at the electrode/electrolyte interface. Under the conditions chosen, it took several tens of minutes to obtain a very low level of background current sufficient for measurement of the peroxide-induced cathodic response on a millimolar concentration scale. It is seen from Fig. 2 that no linear dependence of electrode current (or response) on hydrogen peroxide concentration is observed in the millimolar concentration range. It may be supposed that electrocatalytic reduction of hydrogen peroxide at FHC-modified CPE proceeds via at least two stages, one of them being oxidation of FHC by hydrogen peroxide and the other consisting of the electrochemical reduction of an oxidised form of FHC formed in preceding stage. As in the case of Prussian blue, these processes can be expressed as follows:
= 2KFeIII[FeII(CN)6]+ 2H2O+ 2K +
(1)
KFeIII[FeII(CN)6]+ K + + e − = K2FeII[FeII(CN)6]
(2)
The non-linear response of CPE to hydrogen peroxide concentration indicates that at least one of these processes becomes rate-limiting under the conditions used. In the case of a slow chemical oxidation of FHC by peroxide (Eq. (1)), i.e. fast electroreduction of the oxidised form of FHC formed in the preceding stage, linear dependence of cathodic current on analyte concentration should be observed. In the opposite case, if fast chemical oxidation (Eq. (1)) is followed by relatively slow electrochemical reduction (Eq. (2)), the saturation of the electroactive electrode layer by the oxidised form of FHC should be achieved at a sufficiently high peroxide concentration. This would lead to a hyperbolic dependence of the electrode current on analyte concentration. It might be concluded, based on the observed current-concentration dependence, that a rate-limiting process should be rather electrochemical reduction of oxidised form of FHC than the chemical oxidation step Eq. (1). Such a conclusion may be also indirectly supported by the observed slow relaxation of background current on holding the electrode at the cathodic potential where the reduction of FHC, initially present in oxidised form, occurs. Several reasons can be responsible for this, e.g. a low electric conductivity of the carbon paste or a slow charge transport within the microcrystals of FHC which is known to be of a semiconductor nature. By varying the electrode potential, drastic changes in the electrode response are observed (Fig. 3). The response obtained at an electrode potential of − 0.2 V is approximately double that obtained at 0.0 V. Also the response increases with an increase in cathode potential up to − 0.4 V (Fig. 3). Some cathodic response, although low in magnitude, can also be observed at positive potential values up to + 0.3 V. With a further positive shift in working potential the cathodic response disappears at high values, exceeding 0.4–0.5 V and the anodic response of CPE to hydrogen peroxide can be observed. Thus, the electrocatalysis of hydrogen peroxide oxidation also proceeds at FHC-modified electrode. This correlates with the data presented by Karyakin et al. [10]. However, the observed anodic response is much lower than the cathodic one, e.g. the anodic response of only 0.007 mA/cm2 is obtained at + 0.8 V for 1 mM of hydrogen peroxide. The electrode response depends also on the weight ratio of FHC and graphite used in the paste preparation. The authors obtained a maximum cathodic response at a weight fraction of FHC of 0.3 (Fig. 4). By lowering the FHC content, a decrease in response is observed, obviously because of the lower concentration
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of catalytically active material on the electrode surface. With the increase in FHC weight fraction a decrease in response is found which may be caused by lower paste conductivity due to the diminished graphite content. With an increase of the solution pH, the electrode response diminishes drastically (Fig. 2). In a buffered solution of pH 7.3, widely used in bioanalysis of metabolites, e.g. glucose, the response to hydrogen peroxide drops by two orders of magnitude compared with a solution of pH 3.05 (Fig. 4). Thus, a serious problem of low electrode response may arise by the use of FHC-modified CPE in bioanalysis, e.g. in combination with oxidase type enzymes. Therefore, an attempt should be made to choose other metal hexacyanoferrates as electrocatalysts for hydrogen peroxide detection at near neutral pH of analyte solution. Another electrocatalytic system investigated in the present work is CHC. In a solution of pH 3.05 the CHC-modified CPE shows a cathodic response to added hydrogen peroxide of nearly the same magnitude as in the case of the FHC-modified electrode (Fig. 5). However, contrary to FHC-modified CPE, the cathodic current becomes almost linear with the increase in hydrogen peroxide concentration. Assuming that electrocatalysis proceeds as in the case of the CHCmodified CPE, it may be concluded that neither chemical nor electrochemical process is rate-limiting in the cathodic reduction of peroxide in the concentration range investigated. This presents the advantage of CHC over FHC as an electrocatalyst. By increasing the pH of the solution no decrease in electrode response is ob-
Fig. 3. The dependence of FHC-modified CPE current, obtained after the addition of 2.8 mM hydrogen peroxide, on a holding time, provided at definite potential values (as indicated). The modified CPE contained FHC and graphite at a weight ratio 3:7. Measurements were made in a solution of 0.1 M KCl and 1 mM HCl, pH 3.05.
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Fig. 4. Top: Dependence of FHC-modified CPE current, obtained after addition of 2.8 mM hydrogen peroxide, on a weight fraction of FHC in the carbon paste. Measurements were made in a solution of 0.1 M KCl and 1 mM HCl, pH 3.05 at an electrode potential of −0.2 V. Bottom: Dependence of FHC-modified CPE current, obtained after addition of 2.8 mM hydrogen peroxide, on the solution pH. The modified CPE contained FHC and graphite at a weight ratio 3:7. Measurements were made at a potential of − 0.2 V.
served. Instead, the response slightly increases by a factor of 1.5–1.6 by changing the pH of the solution from 3.05 to 7.3 (Fig. 5). Thus, the CHC-modified CPE has another advantage over the FHC-modified CPE, since the cathodic response of the former is two orders of magnitude greater than the latter. Next to FHC and CHC, some other metal hexacyanoferrates were investigated as electrocatalysts in the hydrogen peroxide reduction and oxidation processes. These include cuprous, cobalt and nickel hexacyanoferrates. At pH 3.05 all these complexes, when used as components of CPEs, showed low cathodic response to peroxide (Fig. 6, top). As in the case of FHC, the response of these hexacyanoferrates was found to diminish with the increase of electrode potential. The response disappeared at electrode potentials exceeding 0.1–0.2 V. No anodic response at high positive potential values was observed for these hexacyanoferrates. However, in contrast to FHC, all other complexes investigated showed an increased response at
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higher pH values up to 7.3 (Fig. 6, bottom). Among all the complexes investigated, Cu(II) and Cu(I) hexacyanoferrates show the greatest cathodic response at physiological pH values. Apart from CHC-modified CPEs, the electrocatalytic reduction of hydrogen peroxide can be attained at a CHC layer, deposited on a conducting substrate, as in Ref. [10]. Fig. 7 shows the cathodic response to peroxide added on a copper electrode containing a deposited layer of CHC. Such a layer can be obtained by a simple anodic treatment of the copper wire electrode in a solution containing potassium ferricyanide, since copper ions generated by the anodic dissolution of copper substrate, react with the ferricyanide ions that are present in the electrolyte, forming a thin layer of CHC. As compared with CHC-modified CPE, in the present case a much stronger growth of electrode response by holding it under cathodic potential is observed, the CHC-covered electrode prepared in such a way shows high cathodic response to hydrogen peroxide even at physiological pH values.
Fig. 6. Top: Dependence of modified CPE current, obtained after the addition of 2.8 mM hydrogen peroxide, on electrode potential, obtained for CPE, containing FHC (1), cuprous (2), cupric (3), cobalt (4) and nickel (5) hexacyanoferrates at a weight ratio of hexacyanoferrates to graphite 3:7, in a solution of 0.1 M KCl, containing 1 mM HCl, pH 3.05. The electrodes were kept at a working potential for 20 min before hydrogen peroxide addition. Bottom: Dependence of modified CPE current, obtained after addition of 2.8 mM hydrogen peroxide, on a solution pH, obtained for CPE’s, indicated above. Measurements were made at a potential of −0.4 V, the electrodes were kept at this potential for 20 min before hydrogen peroxide addition.
4. Conclusions
Fig. 5. The response of CHC-modified CPE on hydrogen peroxide at various pH values (as indicated), at electrode potential of −0.4 V. Hydrogen peroxide was added at the time intervals of 45 s to a total concentration 2.8, 5.6 and 8.4 mM (as indicated by arrows). The modified CPE contained CHC and graphite in a weight ratio 3:7. The electrode was kept at a working potential for 20 min before hydrogen peroxide addition.
CPEs, modified by some metal hexacyanoferrates, were shown to be sensitive for hydrogen peroxide determination. The active material for these electrodes can be prepared by simple mixing of graphite powder, an inert hydrocarbon binder and a metal hexacyanoferrate at appropriate proportions. It was shown, that FHC modified CPE exhibits a relatively high cathodic response to peroxide. However, the sensitivity of FHCmodified CPE drops drastically with the increase of solution pH from 3.05 to 7.3. Thus, this electrode cannot be used successfully as a sensing element for
R. Garjonyte; , A. Malinauskas / Sensors and Actuators B 46 (1998) 236–241
Fig. 7. The dependence of the current of a modified copper wire electrode, obtained after addition of 2.8 mM hydrogen peroxide, on a holding time. The modified copper wire electrode was prepared by applying a potential of +0.2 V for 5 min in a solution containing 0.1 M KCl, 1 mM HCl and 20 mM potassium ferricyanide. Measurements were made in a solution of pH 7.3 at a potential − 0.2 V (1) and in solution of pH 3.05 at a potential − 0.4 V (2).
biosensors based on oxidase type enzymes which produce hydrogen peroxide in an enzyme-catalyzed reaction operating at near neutral solution pH. In contrast, CPEs based on cuprous, cupric, cobalt and nickel hexacyanoferrates, were shown to retain their sensitivity to hydrogen peroxide even at neutral pH values. Among these, CHC shows the greatest cathodic response at physiological pH values.
References [1] G.G. Guilbault, G.J. Lubrano, An enzyme electrode for the amperometric determination of glucose, Anal. Chim. Acta 64 (1973) 439 – 455. [2] J. Wang, N. Naser, L. Angnes, H. Wu, L. Chen, Metal-dispersed carbon paste electrodes, Anal. Chem. 64 (1992) 1285–1288. [3] V.D. Neff, Electrochemical oxidation and reduction of thin films of Prussian blue, J. Electrochem. Soc. 125 (1978) 886–887. [4] D. Ellis, M. Eckhoff, V.D. Neff, Electrochromism in the mixedvalence hexacyanides. 1. Voltammetric and spectral studies of the oxidation and reduction of thin films of Prussian blue, J. Phys. Chem. 85 (1981) 1225–1231. [5] K. Itaya, H. Akahoshi, S. Toshima, Electrochemistry of Prussian blue modified electrodes: an electrochemical preparation method,
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J. Electrochem. Soc. 129 (1982) 1498 – 1500. ˇ ukauskaite, A. Malinauskas, P. Dobrovolskis, Elec[6] N. Z trochromic systems. 4. Prussian blue modified electrodes, Liet. TSR Mokslu Akad. Darb., Ser. B 2 (1985) 8 – 15 (In Russian). [7] B.J. Feldman, O.R. Melroy, Ion flux during electrochemical charging of Prussian blue films, J. Electroanal. Chem. 234 (1987) 213 – 227. [8] P.J. Kulesza, K. Doblhofer, The membrane properties of Prussian blue films on electrodes, J. Electroanal. Chem. 274 (1989) 95 – 105. [9] K. Itaya, N. Shoji, I. Uchida, Catalysis of the reduction of molecular oxygen to water at Prussian blue modified electrodes, J. Am. Chem. Soc. 106 (1984) 3423 – 3429. [10] A.A. Karyakin, O.V. Gitelmacher, E.E. Karyakina, Prussian blue-based first generation biosensor. A sensitive amperometric electrode for glucose, Anal. Chem. 67 (1995) 2419 – 2423. [11] K. Kalcher, J.-M. Kauffmann, J. Wang, I. Svancara, K. Vytras, C. Neuhold, Z. Yang, Sensors based on carbon paste in electrochemical analysis: a review with particular emphasis on the period 1990 – 1993, Electroanalysis 7 (1995) 5 – 22. [12] L. Gorton, Carbon paste electrodes modified with enzymes, tissues and cells, Electroanalysis 7 (1995) 23 – 45. [13] A. Boyer, K. Kalcher, R. Pietsch, Voltammetric behavior of perborate on Prussian blue-modified carbon paste electrodes, Electroanalysis 2 (1990) 155 – 161. ˇ ukauskaite, A. Malinauskas, P. Dobrovolskis, Elec[14] N. Z trochromic systems. 6. Electrodes modified by copper hexacyanoferrate, Liet. TSR Mokslu Akad. Darb., Ser. B 2 (1986) 33 – 40 (In Russian).
Biographies Rasa Garjonyte was born in 1959 in Kaunas, Lithuania. In 1982 she graduated from the Vilnius University. Since 1982 she worked in the Department of Organic Chemistry at Institute of Chemistry, Vilnius. Since 1996 she is a Ph.D. student. Albertas Malinauskas was born in 1952 in Vilnius, Lithuania. In 1975 he received his M.S. degree from the Vilnius University and in 1979 he obtained a Ph.D. equivalent from Moscow University. In 1994 he was awarded a Habil.Dr. from Institute of Chemistry, Vilnius. He is currently head of a Department of Organic Chemistry at Institute of Chemistry, Vilnius. He published over 100 scientific papers and 26 patents. His current interests include conducting polymers, organic electrochemistry, spectroelectrochemistry and biosensors.
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Electrocatalytic reactions of hydrogen peroxide at carbon paste electrodes modified by some metal hexacyanoferrates Rasa Garjonyte; , Albertas Malinauskas * Institute of Chemistry, Gos' tauto Str. 9, LT-2600 Vilnius, Lithuania Received 6 October 1997; received in revised form 29 January 1998; accepted 2 February 1998
Abstract Carbon paste electrodes (CPE’s), modified by ferrous, cupric, cuprous, cobalt and nickel hexacyanoferrates, were shown to electrocatalyse the cathodic reduction of hydrogen peroxide. At pH 3.05 ferrous hexacyanoferrate (FHC) modified CPE shows a cathodic response of 0.2 mA/cm2 to 1 mM of peroxide added. By increasing the solution pH the response of this CPE diminishes drastically (by two orders of magnitude at pH 7.3). In contrast, the cupric hexacyanoferrate (CHC) modified CPE shows a high cathodic response in the whole pH range investigated, 3.05 – 7.3. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Carbon paste electrodes; Sensors; Hydrogen peroxide
1. Introduction The detection of hydrogen peroxide in aqueous solutions, provided by electrochemical means, plays a key role in bioelectroanalytical systems based on oxidase type enzymes. Since an early work of Guilbault and Lubrano on the detection of hydrogen peroxide by electrooxidation on platinum electrode at sufficiently high anodic potential (exceeding 0.6 V vs. Ag/AgCl reference electrode) [1], a great number of papers was published with an attempt to use this detection system in combination with a lot of known oxidases. Later, Wang et al. modified this detection system by the use of platinum-, palladium- and ruthenium-modified carbon paste electrodes (CPE’s), enabling the amperometric detection of this analyte at a constant potential of 0.8 V vs. Ag/AgCl at a detection limit as low as 5× 10 − 6 M [2]. However, the most biological liquids to be analysed contain some substances which are able to discharge on electrode at high anodic potential used in hydrogen peroxide detection at platinum or related materials. Therefore, it is impossible to discriminate the electrode response arising from hydrogen peroxide, e.g. produced in an enzyme-catalysed reaction, from anodic noise
* Corresponding author. 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00123-3
current arising from the contaminants. Thus, a great deal of attention was paid to avoid the influence of the reductants present in an analyte solution. One of the possible approaches is to use selective electrocatalysts, i.e. substances which lower an overpotential of hydrogen peroxide electrooxidation to an appropriate level, that prevents the discharge of the contaminants at the electrode potential applied. Among such substances, Prussian blue (ferric hexacyanoferrate) is a suitable one. It is well known that thin films of Prussian blue can be obtained on electrode surface either by chemical deposition [3,4], or electrodeposition [5–8]. The deposited films of Prussian blue can be easily reduced at a potential below ca. 0.2 V vs. SCE [8], yielding Prussian white (or Everitt salt). The latter was reported to be able to reduce hydrogen peroxide in an electrochemical system [9]. Based on Prussian blue electrodeposited on a glassy carbon electrode, Karyakin et al. developed an enzyme sensor suitable for glucose determination both by anodic oxidation and cathodic reduction of hydrogen peroxide, produced in an enzyme-catalysed reaction [10]. For the cathodic reduction, an electrode potential of 0.18 V vs. Ag/AgCl in a buffered solution of pH 6.0 was used [10]. Application of such a low electrode potential enabled the anodic discharge of many contaminants usually present in analyte solutions to be avoided.
R. Garjonyte; , A. Malinauskas / Sensors and Actuators B 46 (1998) 236–241
The modified CPEs present a very useful tool in electroanalysis because a wide variety of substances, including electrocatalysts, can be used as the components of carbon paste [11,12]. Prussian blue was included in a carbon paste and hydrogen peroxide was detected at resulting CPE by direct current voltammetry (reduction) at −0.4 V vs. SCE [13]. However, among a lot of well known mixed valence metal compounds including hexacyanoferrate complexes only a limited number of compounds, such as ruthenium purple or osmium purple [13], next to Prussian blue, were tested as the components of catalytically active CPEs. The aim of the present work was to investigate the electrocatalytic reactions of hydrogen peroxide at CPEs modified by some metal hexacyanoferrates.
2. Experimental All chemicals were of analytical grade. Metal hexacyanoferrates were obtained by mixing the aqueous solutions containing ferrous, copper, cobalt or nickel sulphates with a nearly equimolar quantity of aqueous solution of potassium ferricyanide. Cuprous hexacyanoferrate was prepared by dissolving cuprous oxide in excess of concentrated hydrochloric acid and reacting then with a solution of potassium ferricyanide. The precipitates were collected, rinsed with water and dried. Carbon paste was prepared by carefully mixing the dispersed graphite powder with metal hexacyanoferrates at a varying weight ratio. Then, an inert binder consisting of the mixture of hydrocarbons of the fraction C16 –C26 was added at a quantity of 0.3 ml to 1 g of graphite–hexacyanoferrate mixture and intimately mixed to obtain homogeneous mixture. The prepared paste was packed into an electrode body, consisting of a plastic tube of ca. 2.5 mm2 inner diameter and arranged with a copper wire serving as an electrode contact. All solutions used contained 0.1 M KCl. A solution of pH 3.05 contained additionally 1 mM HCl and solutions of nearly neutral pH contained additionally 0.01 M phosphate buffer, adjusted to an appropriate pH value. Electrochemical measurements were carried out in a thermostated at 25°C three-electrode cell containing a CPE as a working electrode, a glassy carbon counterelectrode and a saturated Ag/AgCl reference electrode. During the experiments, the solution in the cell was continuously stirred with a magnetic stirrer. A SVA-1 model potentiostat connected to a X – Y – t plotter was used. All potential values are quoted versus saturated Ag/AgCl reference electrode.
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3. Results and discussion Fig. 1 shows cyclic voltammograms of CPEs, modified by ferrous and cupric hexacyanoferrates (FHC and CHC, respectively). In an anodic potential scan, a well defined wave with E1/2 value of ca. 0.2 V is observed for FHC. In the back potential scan, a cathodic wave at nearly same potential is seen. The E1/2 values obtained coincide with known values, e.g. a midpoint value of 0.43 V versus NHE was reported earlier [6] and a value of 0.2 V versus SCE was obtained from cyclic voltammetry of electrodes modified by a thin layer of Prussian blue [5,8]. As compared with thin deposited films of Prussian blue, both the anodic and cathodic maxima of Fig. 1 appear more flattened and less reversible, obviously because of the non-uniform nature of the particles of electroactive substance included into a conducting graphite matrix. Electrode processes which occur at CHC, appear from Fig. 1 to be less reversible, as in case of FHC, although even in thin films of copper hexacyanoferrate these redox processes seem to be more complex [14]. When held at a sufficiently negative potential where the reduced form of FHC, i.e. Prussian white, prevails, the modified CPE shows a cathodic response to added hydrogen peroxide which reaches a maximum value in about 10 or several tens of seconds (Fig. 2). On adding hydrogen peroxide in a concentration of 1 mM, a cathodic current density of about 0.2 mA/cm2 is obtained for a solution of pH 3.05, other conditions being as in Fig. 2. Karyakin et al. obtained the cathodic current response of 10 − 6 A/cm2 on addition of 10 − 6 M of hydrogen peroxide [10] by the use of FHC film modified electrode. This means that FHC-modified CPE shows high current response to added peroxide
Fig. 1. Cyclic voltammograms of CPE, modified by FHC (1) and CHC (2) in a solution containing 0.1 M KCl and 1 mM HCl, pH 3.05. The modified CPEs contained hexacyanoferrates and graphite, mixed at a weight ratio 3:7. Potential sweep rate 100 mV/s.
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2K2FeII[FeII(CN)6]+ H2O2 + 2H +
Fig. 2. The response of FHC-modified CPE on hydrogen peroxide at various pH values (as indicated) at an electrode potential of − 0.2 V. Hydrogen peroxide was added at the time intervals of 45 s to a total concentration 2.8, 5.6 and 8.4 mM (as indicated by the arrows). The modified CPE contained FHC and graphite, mixed at a weight ratio 3:7. The electrode was kept at a working potential for 20 min before hydrogen peroxide addition.
which is only about five times lower than that of a FHC-film sensor. A cathodic peak current density of FHC-modified CPE without peroxide present in solution, obtained at potentiodynamic conditions as in Fig. 1, is around 6 mA/cm2 and exceeds by about 30 times the response obtained on addition of 1 mM peroxide under potentiostatic conditions. This means that the peroxide-induced cathodic response is only observable if the electrode is held at cathodic potential for a definite period of time, sufficient to reduce cathodically an entire layer of FHC placed at the electrode/electrolyte interface. Under the conditions chosen, it took several tens of minutes to obtain a very low level of background current sufficient for measurement of the peroxide-induced cathodic response on a millimolar concentration scale. It is seen from Fig. 2 that no linear dependence of electrode current (or response) on hydrogen peroxide concentration is observed in the millimolar concentration range. It may be supposed that electrocatalytic reduction of hydrogen peroxide at FHC-modified CPE proceeds via at least two stages, one of them being oxidation of FHC by hydrogen peroxide and the other consisting of the electrochemical reduction of an oxidised form of FHC formed in preceding stage. As in the case of Prussian blue, these processes can be expressed as follows:
= 2KFeIII[FeII(CN)6]+ 2H2O+ 2K +
(1)
KFeIII[FeII(CN)6]+ K + + e − = K2FeII[FeII(CN)6]
(2)
The non-linear response of CPE to hydrogen peroxide concentration indicates that at least one of these processes becomes rate-limiting under the conditions used. In the case of a slow chemical oxidation of FHC by peroxide (Eq. (1)), i.e. fast electroreduction of the oxidised form of FHC formed in the preceding stage, linear dependence of cathodic current on analyte concentration should be observed. In the opposite case, if fast chemical oxidation (Eq. (1)) is followed by relatively slow electrochemical reduction (Eq. (2)), the saturation of the electroactive electrode layer by the oxidised form of FHC should be achieved at a sufficiently high peroxide concentration. This would lead to a hyperbolic dependence of the electrode current on analyte concentration. It might be concluded, based on the observed current-concentration dependence, that a rate-limiting process should be rather electrochemical reduction of oxidised form of FHC than the chemical oxidation step Eq. (1). Such a conclusion may be also indirectly supported by the observed slow relaxation of background current on holding the electrode at the cathodic potential where the reduction of FHC, initially present in oxidised form, occurs. Several reasons can be responsible for this, e.g. a low electric conductivity of the carbon paste or a slow charge transport within the microcrystals of FHC which is known to be of a semiconductor nature. By varying the electrode potential, drastic changes in the electrode response are observed (Fig. 3). The response obtained at an electrode potential of − 0.2 V is approximately double that obtained at 0.0 V. Also the response increases with an increase in cathode potential up to − 0.4 V (Fig. 3). Some cathodic response, although low in magnitude, can also be observed at positive potential values up to + 0.3 V. With a further positive shift in working potential the cathodic response disappears at high values, exceeding 0.4–0.5 V and the anodic response of CPE to hydrogen peroxide can be observed. Thus, the electrocatalysis of hydrogen peroxide oxidation also proceeds at FHC-modified electrode. This correlates with the data presented by Karyakin et al. [10]. However, the observed anodic response is much lower than the cathodic one, e.g. the anodic response of only 0.007 mA/cm2 is obtained at + 0.8 V for 1 mM of hydrogen peroxide. The electrode response depends also on the weight ratio of FHC and graphite used in the paste preparation. The authors obtained a maximum cathodic response at a weight fraction of FHC of 0.3 (Fig. 4). By lowering the FHC content, a decrease in response is observed, obviously because of the lower concentration
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of catalytically active material on the electrode surface. With the increase in FHC weight fraction a decrease in response is found which may be caused by lower paste conductivity due to the diminished graphite content. With an increase of the solution pH, the electrode response diminishes drastically (Fig. 2). In a buffered solution of pH 7.3, widely used in bioanalysis of metabolites, e.g. glucose, the response to hydrogen peroxide drops by two orders of magnitude compared with a solution of pH 3.05 (Fig. 4). Thus, a serious problem of low electrode response may arise by the use of FHC-modified CPE in bioanalysis, e.g. in combination with oxidase type enzymes. Therefore, an attempt should be made to choose other metal hexacyanoferrates as electrocatalysts for hydrogen peroxide detection at near neutral pH of analyte solution. Another electrocatalytic system investigated in the present work is CHC. In a solution of pH 3.05 the CHC-modified CPE shows a cathodic response to added hydrogen peroxide of nearly the same magnitude as in the case of the FHC-modified electrode (Fig. 5). However, contrary to FHC-modified CPE, the cathodic current becomes almost linear with the increase in hydrogen peroxide concentration. Assuming that electrocatalysis proceeds as in the case of the CHCmodified CPE, it may be concluded that neither chemical nor electrochemical process is rate-limiting in the cathodic reduction of peroxide in the concentration range investigated. This presents the advantage of CHC over FHC as an electrocatalyst. By increasing the pH of the solution no decrease in electrode response is ob-
Fig. 3. The dependence of FHC-modified CPE current, obtained after the addition of 2.8 mM hydrogen peroxide, on a holding time, provided at definite potential values (as indicated). The modified CPE contained FHC and graphite at a weight ratio 3:7. Measurements were made in a solution of 0.1 M KCl and 1 mM HCl, pH 3.05.
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Fig. 4. Top: Dependence of FHC-modified CPE current, obtained after addition of 2.8 mM hydrogen peroxide, on a weight fraction of FHC in the carbon paste. Measurements were made in a solution of 0.1 M KCl and 1 mM HCl, pH 3.05 at an electrode potential of −0.2 V. Bottom: Dependence of FHC-modified CPE current, obtained after addition of 2.8 mM hydrogen peroxide, on the solution pH. The modified CPE contained FHC and graphite at a weight ratio 3:7. Measurements were made at a potential of − 0.2 V.
served. Instead, the response slightly increases by a factor of 1.5–1.6 by changing the pH of the solution from 3.05 to 7.3 (Fig. 5). Thus, the CHC-modified CPE has another advantage over the FHC-modified CPE, since the cathodic response of the former is two orders of magnitude greater than the latter. Next to FHC and CHC, some other metal hexacyanoferrates were investigated as electrocatalysts in the hydrogen peroxide reduction and oxidation processes. These include cuprous, cobalt and nickel hexacyanoferrates. At pH 3.05 all these complexes, when used as components of CPEs, showed low cathodic response to peroxide (Fig. 6, top). As in the case of FHC, the response of these hexacyanoferrates was found to diminish with the increase of electrode potential. The response disappeared at electrode potentials exceeding 0.1–0.2 V. No anodic response at high positive potential values was observed for these hexacyanoferrates. However, in contrast to FHC, all other complexes investigated showed an increased response at
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higher pH values up to 7.3 (Fig. 6, bottom). Among all the complexes investigated, Cu(II) and Cu(I) hexacyanoferrates show the greatest cathodic response at physiological pH values. Apart from CHC-modified CPEs, the electrocatalytic reduction of hydrogen peroxide can be attained at a CHC layer, deposited on a conducting substrate, as in Ref. [10]. Fig. 7 shows the cathodic response to peroxide added on a copper electrode containing a deposited layer of CHC. Such a layer can be obtained by a simple anodic treatment of the copper wire electrode in a solution containing potassium ferricyanide, since copper ions generated by the anodic dissolution of copper substrate, react with the ferricyanide ions that are present in the electrolyte, forming a thin layer of CHC. As compared with CHC-modified CPE, in the present case a much stronger growth of electrode response by holding it under cathodic potential is observed, the CHC-covered electrode prepared in such a way shows high cathodic response to hydrogen peroxide even at physiological pH values.
Fig. 6. Top: Dependence of modified CPE current, obtained after the addition of 2.8 mM hydrogen peroxide, on electrode potential, obtained for CPE, containing FHC (1), cuprous (2), cupric (3), cobalt (4) and nickel (5) hexacyanoferrates at a weight ratio of hexacyanoferrates to graphite 3:7, in a solution of 0.1 M KCl, containing 1 mM HCl, pH 3.05. The electrodes were kept at a working potential for 20 min before hydrogen peroxide addition. Bottom: Dependence of modified CPE current, obtained after addition of 2.8 mM hydrogen peroxide, on a solution pH, obtained for CPE’s, indicated above. Measurements were made at a potential of −0.4 V, the electrodes were kept at this potential for 20 min before hydrogen peroxide addition.
4. Conclusions
Fig. 5. The response of CHC-modified CPE on hydrogen peroxide at various pH values (as indicated), at electrode potential of −0.4 V. Hydrogen peroxide was added at the time intervals of 45 s to a total concentration 2.8, 5.6 and 8.4 mM (as indicated by arrows). The modified CPE contained CHC and graphite in a weight ratio 3:7. The electrode was kept at a working potential for 20 min before hydrogen peroxide addition.
CPEs, modified by some metal hexacyanoferrates, were shown to be sensitive for hydrogen peroxide determination. The active material for these electrodes can be prepared by simple mixing of graphite powder, an inert hydrocarbon binder and a metal hexacyanoferrate at appropriate proportions. It was shown, that FHC modified CPE exhibits a relatively high cathodic response to peroxide. However, the sensitivity of FHCmodified CPE drops drastically with the increase of solution pH from 3.05 to 7.3. Thus, this electrode cannot be used successfully as a sensing element for
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Fig. 7. The dependence of the current of a modified copper wire electrode, obtained after addition of 2.8 mM hydrogen peroxide, on a holding time. The modified copper wire electrode was prepared by applying a potential of +0.2 V for 5 min in a solution containing 0.1 M KCl, 1 mM HCl and 20 mM potassium ferricyanide. Measurements were made in a solution of pH 7.3 at a potential − 0.2 V (1) and in solution of pH 3.05 at a potential − 0.4 V (2).
biosensors based on oxidase type enzymes which produce hydrogen peroxide in an enzyme-catalyzed reaction operating at near neutral solution pH. In contrast, CPEs based on cuprous, cupric, cobalt and nickel hexacyanoferrates, were shown to retain their sensitivity to hydrogen peroxide even at neutral pH values. Among these, CHC shows the greatest cathodic response at physiological pH values.
References [1] G.G. Guilbault, G.J. Lubrano, An enzyme electrode for the amperometric determination of glucose, Anal. Chim. Acta 64 (1973) 439 – 455. [2] J. Wang, N. Naser, L. Angnes, H. Wu, L. Chen, Metal-dispersed carbon paste electrodes, Anal. Chem. 64 (1992) 1285–1288. [3] V.D. Neff, Electrochemical oxidation and reduction of thin films of Prussian blue, J. Electrochem. Soc. 125 (1978) 886–887. [4] D. Ellis, M. Eckhoff, V.D. Neff, Electrochromism in the mixedvalence hexacyanides. 1. Voltammetric and spectral studies of the oxidation and reduction of thin films of Prussian blue, J. Phys. Chem. 85 (1981) 1225–1231. [5] K. Itaya, H. Akahoshi, S. Toshima, Electrochemistry of Prussian blue modified electrodes: an electrochemical preparation method,
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Biographies Rasa Garjonyte was born in 1959 in Kaunas, Lithuania. In 1982 she graduated from the Vilnius University. Since 1982 she worked in the Department of Organic Chemistry at Institute of Chemistry, Vilnius. Since 1996 she is a Ph.D. student. Albertas Malinauskas was born in 1952 in Vilnius, Lithuania. In 1975 he received his M.S. degree from the Vilnius University and in 1979 he obtained a Ph.D. equivalent from Moscow University. In 1994 he was awarded a Habil.Dr. from Institute of Chemistry, Vilnius. He is currently head of a Department of Organic Chemistry at Institute of Chemistry, Vilnius. He published over 100 scientific papers and 26 patents. His current interests include conducting polymers, organic electrochemistry, spectroelectrochemistry and biosensors.
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