Hydrophobicity effects in iron polypyridyl complex electrocatalysis within Nafion thin-film electrodes

Four polypyridyl redox catalysts Fe(bp)₃ ²⁺, Fe(ph)₃ ²⁺, Fe(dm)₃ ²⁺, and Fe(tm)₃ ²⁺ (with bp, ph, dm, and tm representing 2,2′-bipyridine, 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine, and 3,4,7,8-tetramethyl-1,10-phenanthroline, respectively) are investigated for the electrocatalytic oxidation of three analytes (nitrite, arsenite, and isoniazid). The poly-pyridyl iron complex is exchanged into a Nafion film immobilized on a glassy carbon electrode, which is then immersed in 0.1 M Na₂SO₄. Cyclic voltammetry is employed for the evaluation of the mechanism and estimation of kinetic parameters. The electrocatalytic behaviour going from low to high substrate concentration is consistent with the Albery-Hillman cases of "LEty" switching to "LEk" (changing from the first order in the substrate to half order in the substrate), denoting a process that occurs in a reaction zone close to the electrode surface with diffusion of charge (from the electrode surface into the film) and of anionic or neutral analyte (from the Nafion-solution interface into the film). The relative hydrophobicity of the iron polypyridyl catalyst within the film is shown to affect both the diffusion of charge/electrons and analyte within the film with Fe(tm)₃ ²⁺ providing the mildest catalyst. All three analytes, nitrite, isoniazid, and arsenite, exhibit linear calibration ranges beneficial for analytical applications in the micro-molar to the milli-molar range.