Voltammetry at polymer-modified stationary and rotating microelectrodes. Application to determination of electron-transfer rates at polymer/solution interfaces

Voltammetric behavior of rotating disk and band microelectrodes with critical dimensions of 0.25-25 μm has been examined in solutions containing electroactive species. The microelectrode is fabricated to be in the plane but about 4 mm off the rotation axis of an insulating rotating disk. Such electrodes exhibit mass transport limited current densities which are 5-30 times higher than their macroscopic counterparts. This enhanced mass transport rate follows from equations previously derived for rotating ring electrodes. Rotating microelectrodes were functionalized with polymers derived from N,N′-bis((p-tri-methoxysilyl)benzyl)-4,4′-bipyridinium (I, [(BPQ²⁺)_n]_surf), N,N′-bis((trimethoxysilyl)propyl)-4,4′-bipyridinium (II, [(PQ²⁺)]_surf), and 1,1′-bis[N-(3-(triethoxysilyl)propyl)carboxamide]cobalticenium (III, [Co(CpR)₂ ⁺]_n,Surf). Steady-state currents, measured as a function of rotation rate for thermodynamically favored, polymer-mediated reductions of Ru(NH₃)₆ ³⁺, Co(bpy)₃ ³⁺, and Cr³⁺-phenanthroline complexes, are in quantitative agreement with theory for such polymer-mediated redox processes. Values of rate constants for the polymer/redox couple electron-exchange reactions are found to be in the range 3 × 10⁵ to >4 × 10⁷ M^-1 s^-1. Rough agreement was found between measured rate constants and those calculated from the Marcus outer-sphere cross-reaction relation, although the calculated values were systematically higher.