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

Abstract

Four polypyridyl redox catalysts Fe(bp)₃²⁺, Fe(ph)₃²⁺, Fe(dm)₃²⁺, and Fe(tm)₃²⁺ (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)₃²⁺ 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.