Chemically modified electrodes Electroactive layers

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

Such lECMEs (immobilized enzyme chemically modified electrodes) can be used both as potentiometric and amperometric sensors [136]. The covalent bonding of enzyme causes the optimal orientation of electroactive enzyme centers toward the electrode surface. Direct transfer of electrons between bound enzyme and the carbon of an lECME was proved [137, 138]. The large active surface and very thin enzyme layer of lECMEs are the reasons for higher sensitivity, lower detection limit, broader linear concentration range and faster response than in the case of other enzyme sensors. 

The apparent concentration of electroactive sites in the polymer is often as high as 0.1-5 M. Compared with coatings of monolayers of low molecular weight compounds the electrochemical response is larger and easier to observe. The type and also thickness of the layer may influence the rate, intercalation of ions, chemical reactivity and velocity of the electrochemical process. This must be considered in preparing polymer-modified electrodes. 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

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