Chemical derivatization of electrode surfaces with derivatives of N,N,N′,N′-tetraalkyl-1,4-benzenediamine

Two N,N,N′,N′-tetraalkyl-1,4-benzenediamine surface derivatizing reagents have been synthesized and used to derivatize Au, Pt, SnO₂, and n-Si electrodes. The two new reagents are N,N-dimethyl-N′-ethyl-N′-(trimethoxysilyl-4-butyl)-1,4-benzenediamine (I) and N,N,N′,N′-tetrakis-(trimethoxysilyl-3-propyl)-1,4-benzenediamine (II). Either reagent can be used to modify electrode surfaces with greater than monolayer quantities (>10^-8 mol/cm²) of electroactive material that has redox behavior consistent with a surface-bound N,N,N′,N′-tetraalkyl-1,4-benzenediamine derivative. Oxidation of the surface-bound material from I or II occurs chemically reversibly at ca. +0.1 V vs. SCE in MeOH or CH₃CN electrolyte solution and the oxidation is accompanied by large visible spectral changes studied quantitatively using derivatized, optically transparent SnO₂ electrodes. The visible spectral changes are similar to those found for the solution species I and II upon oxidation in terms of molar absorptivity (∈≈12,000 M^-1 cm^-1) and absorption maxima (ca. 620, 580, and 530 nm). The redox potentials, E^o′, for the surface-bound reagents are within 100 mV of the solution species. Additionally, the E^o′ for the surface-bound materials in aqueous media are pH dependent as for solution species; at pH=7, the E^o′=+0.04V vs. SCE. Derivatized electrodes are fairly rugged in MeOH, CH₃CN, or H₂O electrolyte solutions, provided potentials negative of that necessary to effect the second oxidation are employed; the second oxidation is observed at ca. +0.7 V vs. SCE in CH₃CN electrolyte solution for either surface-bound or solution redox material. The two-electron oxidized species is very hydrolytically unstable. Electrodes modified with I or II show improved voltammetric response to horse heart ferri-and ferrocytochrome c, E^o′=+0.02 V vs. SCE, at pH=7 compared to the naked electrodes, consistent with the expectation that the surface-bound form of I or II should be a redox mediator for such a biological reagent. Also, derivatized electrodes show improved response for the oxidation of ascorbic acid; the rate constant for oxidation of ascorbic acid by the one-electron oxidized species on the electrode is ca. 10² M^-1 s^-1. Unfortunately, the derivatized the electrodes do not seem sufficiently durable to be useful in long term practical applications.