Behaviors of H2TPP and CoTPPCl in Nafion® film and the catalytic activity for nitric oxide oxidation

The behavior of meso-tetraphenylporphyrin (H₂TPP) and its Co(III)-complex (CoTPPCl) in Nafion® film has been studied by cyclic voltammetry and visible spectroscopy. Buffer titration of H₄TPP²⁺ in the film showed only the dication–free-base equilibrium (H₄TPP²⁺H₂TPP + 2H⁺) and the apparent equilibrium constant, pK_3,4, was determined to be 4.12. Insertion of cobalt(III) into the H₂TPP core in the film has been realized at room temperature by immersion of H₂TPP-containing Nafion® film into CoCl₂ solution. The electrocatalytic activity of the resulting film toward NO oxidation at a glassy carbon (GC) electrode has been monitored in comparison with other Nafion® films doped with H₂TPP or free CoCl₂. A GC electrode coated with Nafion® film doped with CoTPPCl exhibited the highest catalytic activity, confirming the involvement of the Co site in a catalytic cycle of NO oxidation. In amperometric detection of NO in phosphate buffer solution at pH 7.4 this electrode demonstrated a linear response to the NO concentration increment. The detection limit at a signal-to-noise ratio of 3 was 1.2 nM.