Abstract Carbon paste electrodes, modified by Cu2O or CuO, were prepared and tested as sensors for hydrogen peroxide in aqueous solutions. They show a ...
Fresenius J Anal Chem (1998) 360 : 122–123 – © Springer-Verlag 1998
Rasa Garjonyte · Albertas Malinauskas
Amperometric sensor for hydrogen peroxide, based on Cu2O or CuO modified carbon paste electrodes Received: 18 April 1997 / Accepted: 24 June 1997 Abstract Carbon paste electrodes, modified by Cu2O or CuO, were prepared and tested as sensors for hydrogen peroxide in aqueous solutions. They show a cathodic response to the analyte ranging from 0.45 to 0.14 mA/cm2 for 1 mmol/L hydrogen peroxide for solutions of pH 5.2 and 7.3, respectively, at a working potential of –0.4 V vs. Ag/AgCl. The cathodic operation mode used diminishes or excludes the possibility of anodic discharge of contaminants usually present in biological fluids, and enables the use of the sensors in bioanalytical systems based on enzymes.
binder, consisting of a mixture of aliphatic hydrocarbons of the fraction C16-C26, was added in a quantity of 0.3 mL to 1 g of graphite-oxide mixture, and carefully mixed to obtain a homogeneous mixture. The resulting paste was placed into an electrode body, consisting of a plastic tube of ca. 2.5 mm i.d., and arranged with a copper wire serving as an electric contact. Electrochemical measurements were performed in a thermostated (25° C) three-electrode cell, fitted with a magnetic stirrer and containing a carbon paste electrode as a working electrode, a glassy carbon counter-electrode, and a saturated Ag/AgCl reference electrode. An electrochemical system SVA1 connected to an X-Y-t plotter, was used. All solutions contained 0.1 mol/L KCl and were in some cases buffered with 0.01 mol/L phosphate buffer to final pH-values of 6.1 and 7.3.
Results and discussion The Cu2O modified carbon paste electrode (CPE) shows an amperometric cathodic response to hydrogen peroxide. With a fixed peroxide concentration, the electrode response depends on both the electrode potential applied and the pH of the solution. When held at a constant potential for a definite time, the electrode response increases in magnitude (top of Fig. 1). At
Introduction Amperometric detectors of hydrogen peroxide are of great importance in bioanalytical systems based on enzyme-catalysed reactions. Usually a platinum anode is used in such detectors, enabling an electrooxidation of hydrogen peroxide to proceed at a potential exceeding + 0.65 V vs. Ag/AgCl. As a consequence, an anodic oxidation of some contaminants usually present in an analyte solution. e.g. ascorbate and uric acid, occurs and interferes with the electrooxidation of hydrogen peroxide generated in an enzyme reaction. In order to avoid the influence of this interference, a number of modifications of the basic peroxide detector was proposed. Karyakin et al. [1] used a Prussian Blue coated inert electrode, which permits the detection of hydrogen peroxide at much lower potential values, where an interference from contaminants is negligible. Also, the incorporation of Prussian Blue into a carbon paste electrode yielded a peroxide-sensitive amperometric detector, operating at a cathodic reduction of peroxide at -0.4 V vs. SCE [2]. Some other electrocatalysts, which allow to lower the operation potential of hydrogen peroxide detection, were incorporated into carbon paste [3] and used in bioanalytical systems [4]. However, we showed [5] that the sensitivity of a Prussian Blue based peroxide detector is drastically diminished with extending the pH from slightly acidic to neutral values, the latter being most important in bioanalysis. Therefore, it seems to be important to search for other electrocatalysts, suitable for use in neutral solutions. In this paper we report a new hydrogen peroxide detector, based on Cu2O or CuO, incorporated into a carbon paste.
Experimental Carbon paste was prepared by mixing finely dispersed Cu2O or CuO with graphite at a weight ratio of 3 : 7. Then an inert
R. Garjonyte · A. Malinauskas (Y) Institute of Chemistry, Gostauto Str. 9, LT-2600 Vilnius, Lithuania
Fig. 1 Top: Dependence of cathodic current obtained after addition of 2.8 mmol/L hydrogen peroxide on holding time; electrode potential –0.4 V vs. Ag/AgCl in solutions of various pH for Cu2O modified CPE. Bottom: Dependence of cathodic (hollow circles, electrode operated at –0.4 V) and anodic current (full circles, electrode operated at + 0.6 V) on hydrogen peroxide concentration in a pH 7.3 solution. The electrode was held at the operating potential for 10 min before an analyte was added
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near neutral pH, an almost constant value of electrode response is attained after a holding time of 20 min. At lower pH-values the cathodic current, generated by the electrode, becomes greater. At pH 5.2, a cathodic current density of 0.45 mA/cm2 was obtained for 1 mmol/L hydrogen peroxide at a potential of -0.4 V, whereas an increase of pH up to 7.3 causes a diminished current density of 0.14 mA/cm2. Under closely similar conditions, the iron (II) hexacyanoferrate modified CPE showed a cathodic current density of 0.04 and 0.002 mA/cm2 for solutions of pH 5.3 and 7.3, respectively. At lower cathodic potential values the electrode response to hydrogen peroxide drops down. At -0.2 V, a ca. 20-fold decrease of the cathodic current density is observed and no response at 0.0 V was detected. At positive potential values, no current response at + 0.2 V was generated; however, at more positive potential, an anodic current was generated after an addition of hydrogen peroxide. With a holding time of 20 min at an electrode potential of + 0.6 V, an anodic current density of 0.035 mA/cm2 for 1 mmol/L analyte concentration was attained. However for practical applications in biosensors, the use of these electrodes, operated in an anodic mode, seems to be questionable when compared to the cathodic operation mode, because a high anodic potential (+ 0.6 V or more) should cause well known problems, associated with an electrooxidation of contaminants present in the analyte solutions.
The CPE’s prepared show a near-linear dependence of the cathodic response to hydrogen peroxide added, whereas no linear dependence is observed for the same CPEs operating in the anodic mode (bottom of Fig. 1). Closely similar results were also obtained for CuO modified CPEs; however, the reproducibility in this case was worse. The results obtained show that the detection of hydrogen peroxide is possible in slightly acidic and neutral solutions on Cu2O modified CPE, operating in a cathodic mode at a potential of -0.4 V, where an anodic discharge of many contaminants, usually present in biological samples, is restricted. The results also show that, as compared with iron (II) hexacyanoferrate modified CPE, the Cu2O based sensor possesses a higher electric response, especially in pH 7.3 buffered solution commonly used in bioanalysis.
References 1. Karyakin AA, Gitelmacher OV, Karyakina EE (1995) Anal Chem 67 : 2419 2. Boyer A, Kalcher K, Pietsch R (1990) Electroanalysis 2 : 155 3. Kalcher K, Kaufmann J-M, Wang J, Svancara I, Vytras K, Neuhold C, Yang Z (1995) Electroanalysis 7 : 5 4. Gorton L (1995) Electroanalysis 7 : 23
Fresenius J Anal Chem (1998) 360 : 123–125 – © Springer-Verlag 1998
E. Mauerhofer · S. J. Reddy
Determination of the stoichiometry of mixed microcrystals KxCsyZnCl4 using instrumental neutron activation analysis Received: 26 May 1997 / Revised: 1 July 1997 / Accepted: 2 July 1997 Abstract Instrumental neutron activation analysis (INAA) has been employed as an absolute method for the determination of the stoichiometry of mixed microcrystals KxCsyZnCl4 with a weight ranging between 20 and 50 µg. The reliability of the method has been checked with the pure substances KCl, NaCl, CsCl and RbCl, for which the mean value of the ratio Cl/X was found to be 1.04 (3).
Introduction Stoichiometry exerts a very important influence on the material properties and hence plays a significant role in the fabrication
E. Mauerhofer (Y) Institut für Kernchemie, Johannes-Gutenberg Universität, D-55099 Mainz, Germany S. J. Reddy Department of Chemistry, S.V. University, Tirupati-517502, India
of semiconducting devices [1]. Significant changes in the electrical and optical properties have been noticed due to deviations from theoretical compositions in the case of crystals [2–4]. The properties of CuInSe2 films were reported to substantially differ for Cu- and In-rich regions. Cu-rich films were observed to be of the p-type [5] and In-rich films greatly enhanced the conduction band [6]. Enhanced research activity in fundamental and applied aspects in several important fields, such as superconductivity studies, requires knowledge of the exact stoichiometry of the compound. Development of a reliable and precise method for the direct determination of the stoichiometric composition of crystals deserves attention. Marienko [7] and Kurumato et al. [8] used a coulometric method for the determination of gallium arsenide crystals. Electrochemical techniques such as DC polarography, cyclic voltammetry and chronoamperometry have been demonstrated to be useful in the determination of the stoichiometry of thin films of binary semiconductor materials such as CdSe, CdTe, InSe and InTe. Potentiometric titration has been employed by Lanza and Rossi for checking the stoichiometry of the superconductor YBa2Cu3Oy [10]. The stoichiometry of lanthanum strontium manganates has been determined by using a wet-chemical method [11]. The coulometric method was used for the determination of the stoichiometry of the CuInS2 semiconductor material [12]. The composition of CuInS2 was identified by precise X-ray diffraction measurements [3]. Basically, most of physical methods used for the analysis are matrix dependent and rely on a comparison with analyzed standards or require standards for correction of the instrumental parameters. The results of the analysis of thin films by different methods like electron probe microanalysis (EPMA) with wave (WDX) or energy dispersion (EDX) analyzers, Rutherford back scattering (RBS), Auger electron spectroscopy (AES) and secondary mass-spectrometry (SNMS) were reported to show significant deviations from each other [14].
