Short Communication Overoxidized Polypyrrole=Cobalt Tetrasulfonated Phthalocyanine Modified Ultramicro-Carbon-Fiber Electrodes for the Electrooxidation of 2-Mercaptoethanol Miguel Gulppi, +,++,+++ Fethi Bedioui, ++ and Jose ´ H. Zagal* + + Departamento de Quı ´mica de los Materiales, Facultad de Quı ´mica y Biologı ´a, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile; e-mail: jzagal@lauca.usach.cl ++ Laboratoire d’Electrochimie et Chimie Analytique, UMR CNRS-ENSCP n  7575, Ecole Nationale Superie ´ure de Chimie de Paris, 11, Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France +++ On leave from: Departamento de Quı ´mica de los Materiales, Facultad de Quı ´mica y Biologı ´a, Universidad de Santiago de Chile, Santiago, Chile Received: November 2, 2000 Final version: December 22, 2000 Abstract We report on the catalytic electrooxidation of 2-mercaptoethanol on ultramicro-carbon-fiber electrode (UMCF) modified with overoxidized polypyrrole-doped cobalt-tetrasulfonated phthalocyanine (CoTSPc) film. The easily prepared electrodes are stable and do not passivate after repetitive use. The sensitivity of the modified UMCF electrode towards the amperometric detection of 2-mercaptoethanol was tested with differential pulse voltammetry (DPV) and differential normal pulse amperometry (DNPA). A calculated detection limit as low as 8610 5 M was obtained by DPV. Our results show that UMCF electrodes are promising for the detection of low concentrations of thiols as mass transport limitations are minimized. Keywords: Mercaptoethanol electrooxidation, Carbon microfiber electrodes, Cobalt tetrasulfophthalocyanine, Polypyrrole modified electrodes The detection of thiol-derivated substances is an important field of research as they can be present as contaminants in fuels [1] and biological fluids [2] and they may be useful as markers of food deterioration [3, 4]. During the last decade, several authors [5–17] have reported the catalytic electrooxidation of various thiols on electrodes modified with metallophthalocyanines. In general, electrodes of this type can be used as sensors [15–25] and various configurations have been developed such as carbon paste [15–17] and screen-printed carbon electrodes [19–25] containing cobalt phthalocyanines and derivatives. The advantage of the reported cobalt phthalocyanine-based electrodes is that the thiols (namely 2-mercaptoethanol, L-cysteine, reduced gluthathione etc.) can be monitored at considerable lower potentials compared with the nonmodified electrodes. Superior selectivity and sensitivity were also achieved. We and others have reported that much advantage can be obtained from the activity of cobalt phthalocyanines if they are electropolymerized forming conductive films that coat an electrode surface [14, 18], or if they are dispersed within a conductive polymeric matrix [12, 13]. In addition, such an approach should allow the conception of various shapes and geometrical designs of electrode surfaces and a better control of their modification. In this work, we combine for the first time the use of electrochemically formed polypyrrole-doped cobalt tetra- sulfonated phthalocyanine (CoTSPc) with ultramicro-carbon-fiber electrode configuration (UMCF) to achieve the electrocatalytic oxidation of 2-mercaptoethanol (2-ME) in alkaline aqueous solu- tion and provide a miniaturized electrochemical sensor that is easily fabricated from low cost materials, providing an ultramicroprobe that may be developed for applications in biological systems. We have selected 2-ME as the examined analyte target since its elec- trochemistry is well developed in the literature [5–18, 25]. Electrochemical formation of polypyrrole-doped CoTSPc on UMCF was performed by anodic oxidation of 1 mM CoTSPc tetrasodium salt þ0.1 M pyrrole in aqueous solution by one potential scan between 0.2 and 0.7V, at room temperature, starting at the lower potential limit. The incorporation of CoTSPc within the polypyrrole matrix (PPy) is based on the ion exchange properties of the oxidized polymer [26]: the multianionic tetra- sulfonated cobalt phthalocyanine molecules are introduced into polypyrrole film as counterions (or ‘‘doping’’ ions) during the electrochemical growth of the polymer matrix. Note that in this operation, only CoTSPc was used as supporting electrolyte. The obtained film is denoted as PPy=CoTSPc. In order to avoid having a large electrical charge in the potentiodynamic response of the polypyrrole-based electrodes which creates a large background current that hinder the characterization of the electrocatalytic effects of the complex, the polymer was overoxidized by succes- sive potential scans between 0.2 and 0.4 V in 0.1 M NaOH (in the absence of pyrrole monomers) until the total suppression of its electroactivity. The obtained modified UMCF electrode is then denoted as OPPy=CoTSPc. In general, these electrodes are much more stable than those obtained by simple adsorption of phtha- locyanine monolayers on carbon or graphite surfaces since the catalyst is not lost after repetitive measurements [11, 12, 14]. First, we have assessed this approach by using a planar disk glassy carbon electrode (GC). In Figure 1A, curve I shows the cyclic voltammogram of the GC=PPy=CoTSPc modified GC electrode (obtained by anodic oxidation of 1 mM CoTSPc tetra- sodium salt þ0.1 M pyrrole in aqueous solution by two potential scans between 0.2 and 1.0V, at room temperature) in 0.1M NaOH aqueous solution. Curve II shows the cyclic voltammo- gram of the same electrode after the overoxidation of the poly- pyrrole matrix as indicated above. In Figure 1B, curve I illustrates the response of the GC=OPPy=CoTSPc upon the addition of 2- ME to the electrolytic solution (9 mM) while curve II exhibits the cyclic voltammogram of GC=OPPy electrode prepared by anodic oxidation of 0.1 M pyrrole in aqueous solution containg 20 mM sodium p-toluenesulfonic acid as supporting electrolyte (instead 1136 Electroanalysis 2001, 13, No. 13 # WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1040-0397/01/1309–1136 $17.50þ.50=0