Behavior of metallic electrodes modified with a polymer derived from 4-( β-trimethoxysilyl) ethylpyridine : Charge transport, pH sensitivity, and anion selectivity

Abstract

Pt electrodes can be modified with 4-( β-trimethoxysilyl)ethylpyridine to produce a surface-bound polymer, (SiPy) n , yielding a pH sensitive, anion selective interface when Γ py≥10 −7 mol/cm 2. At pH≥6 Pt/[(SiPy) n ] surf. electrodes are insulated from a large variety of solution redox species, giving ∼1% of the current observed at a naked surface. Below pH 5 anions such as M(CN) y x− (M=Fe, Ru, Mo, Co; y=6 or 8; x=3 or 4), naphthoquinone sulfonate, indigo carmine and I − may be firmly electrostatically bound in the [(SiPyH +) n ] surf. polymer. For pH<5 the Pt/[(SiPyH +) n ] surf. electrodes show good response to the negatively charged electroactive species, but do not show significant response to redox active cations such as Co(phen) 3 3+/2+ or Ru(NH 3) 6 3+/2+. Detailed study of the response at pH<5 to the Fe(CN) 6 3−/4− couple has been made. Rotating disk studies and potential step chronoamperometry establish charge transport through the polymer as the rate limiting step for Fe(CN) 6 4− oxidation at modified surfaces with Γ py≥10 −7 mol/cm 2. The limiting current at a rotating disk electrode increases with ionic strength over the range of μ=0.4 M to μ=1.4 M. The diffusion coefficient for charge transport within the polymer loaded with Fe(CN) 6 4− is D E=9×10 −9 cm 2/s, in pH 2.5, μ=0.4 M ClCH 2COO −, 0.5 M KCl solution.