XPS investigation on the chemical structure of a very thin, insulating, film synthesized on platinum by electropolymerization of o-aminophenol (oAP) in aqueous solution at neutral pH

Previous investigations on o-aminophenol (oAP) electropolymerization by cyclic voltammetry (CV), onto platinum, in aqueous solution (phosphate buffer) at neutral pH, have shown the reproducible formation of an insulating film of inherent adherence and thinness, poly(o-aminophenol) (PoAP). The applicability of this produced film as a permselective membrane for biosensors was demonstrated, as well as the influence of the platinum surface state on its self-limited growth and morphology. Based on these findings, we have continued our investigation on insulating PoAP by combining compositional (XPS) and electrochemical (CV) analyses. To evaluate the finite film on its compositional homogeneity and stability, we have highlighted,in turn, different analyzed depths by angle-resolved XPS, repeated experiments and performed stability test in ultra-high vacuum. This investigation, combined with an accurate curve-fitting procedure, has allowed the estimation of the PoAP thickness and the assignment of chemical states for determining its polymerization sequence. We show that PoAP thickness is about 7 nm; its repeat formula comprises alternating quinoneimine and oAP units, including approximately one water molecule, and the polymer chains are terminated with carbonyl-type groups.