Modified carbon nanoparticle-chitosan film electrodes: Physisorption versus chemisorption

Surface functionalised carbon nanoparticles of ca. 8 nm diameter co-assemble with chitosan into stable thin film electrodes at glassy carbon surfaces. Robust electrodes for application in sensing or electrocatalysis are obtained in a simple solvent evaporation process. The ratio of chitosan binder backbone to carbon nanoparticle conductor determines the properties of the resulting films. Chitosan (a poly-d-glucosamine) has a dual effect (i) as the binder for the mesoporous carbon composite structure and (ii) as binding site for redox active probes. Physisorption due to the positively charged ammonium group (pK_A ≈ 6.5) occurs, for example, with anionic indigo carmine (a reversible 2e^--2H⁺ reduction system in aqueous media). Chemisorption at the amine functionalities is demonstrated with 2-bromo-methyl-anthraquinone in acetonitrile (resulting in a reversible 2e^--2H⁺ anthraquinone reduction system in aqueous media). Redox processes within the carbon nanoparticle-chitosan films are studied and at sufficiently high scan rates diffusion of protons (buffer concentration depended) is shown to be rate limiting. The chemisorption process provides a much more stable interfacial redox system with a characteristic and stable pH response over a pH 2-12 range. Chemisorption and physisorption can be employed simultaneously in a complementary binding process.