Binding site control in a layer-by-layer deposited chitosan–carbon nanoparticle film electrode

Abstract

Thin chitosan–carbon nanoparticle films (ca. 2 nm average thickness increase per layer) are assembled onto tin-doped indium oxide (ITO) electrode substrates in a layer-by-layer deposition process employing carbon nanoparticles of ca. 8 nm average diameter and an aqueous solution of chitosan (poly-D-glucosamine, low molecular weight, 75–85% deacetylated). Chitosan introduces amine/ammonium functionalities which are employed for the immobilization of redox systems (i) via physisorption of indigo carmine and (ii) via chemisorption of 2-methyleneanthraquinone. The number of binding sites within the chitosan–carbon nanoparticle film is controlled by changing the thickness of the film deposit or by changing the chitosan content, which is varied by changing the chitosan concentration during layer-by-layer deposition. Voltammetric characteristics and stability of the chemisorbed and physisorbed redox systems are reported as a function of pH. The physisorbed redox system is expelled from the film at a pH consistent with the pK_A of chitosan (approximately 6.5). However, the 2-methyleneanthraquinone redox system remains stable even in alkaline media and only a minor inflection in the plot of midpoint potentials vs. pH indicates the film deprotonation process at the pK_A of chitosan.