Electroactive poly(amino acids). Part 1. Modified electrodes from platinum with an adsorbed film of poly(Nε-4-nitrobenzoyl-L-lysine)

Abstract

Poly(N^ε-4-nitrobenzoyl-L-lysine) is obtained with average molecular weight (2.0 + 1.3)× 10⁵ by reaction between the monomer N-carboxyanhydride and sodium hydride. Platinum has been dipcoated with this material from dimethylacetamide solution, and the behaviour of the coating has been examined in acetonitrile by cyclic voltammetry and chronoamperometry. Surface coverage was in the range 28–5.4 nmol cm^–2 of 4-nitrobenzoyl residues. On repeated cyclic voltammetry the development of multiple peaks is observed. On allowing the electrode to rest, the original smooth cyclic voltammogram is restored. This process is ascribed to a reversible conformational change in the film. Under prolonged electrical stress the films age in a characteristic manner which is ascribed to separation between the film and the metal support. The polymer in solution adopts the α-helix structure and it is assumed to be adsorbed in this form onto platinum. A close-packed layer of α-helices has surface coverage of 0.66 nmol cm^–2 and so we estimate that our films range from ca. 40–10 layers thick.