Metal porphyrins, chemically oxidized surface

One such reaction that has been studied is the electrocatalytic reduction of oxygen directly to water.25,27 The electrocatalysts for this process are often based on metal porphyrins and phthalo-cyanins. Thus a graphite electrode whose surface was modified by the irreversible adsorption of a cofacial dicobalt porphyrin dimer was able to reduce oxygen under conditions where the reduction did not occur on the bare electrode itself. Similarly, a catalytic chemically modified electrode for the oxidation of chloride to chlorine has been prepared28 where the active catalyst was reported to be a ruthenium dimer, [(bipy)2(OH)RulvORuvO(bipy)2]4+, which was reduced to the corresponding [Rum-RuIV] dimer during the reaction. 

Using a bare metal, carbon, or porphyrin electrode, it is impossible to differentiate nitrite produced electrochemically than from that derived from chemical NO oxidation. Therefore, a barrier has to be placed between the electrode and the analyte to prevent access of nitrite to the electrode surface. This has been achieved in the porphyrinic sensor by depositing a layer of cation exchanger (Nafion), which repels negatively charged species such as N02 (Fig. 2a). Nafion also prevents the formation of N02 in the film after one electron oxidation of NO to NO" ". In the Clark-type probe, the analyte solution containing NO and nitrite are separated from the inner electrode solution by a gas-permeable membrane. 

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