Large distribution in the Donnan potential of hexacyanoferrate anions permeating in and partially dissolving (PAH-HA)n polyelectrolyte multilayer films

We investigate herein the electrochemical behavior of hexacyanoferrate anions in polyelectrolyte multilayer (PEM) films made from poly(allylamine) (PAH) as a polycation and from hyaluronic acid (HA) as a polyanion as a function of the ionic strength and the concentration of the redox probe. Our data show that the film undergoes a partial dissolution in the presence of hexacyanoferrate in the 0.1-1 mM concentration range. The fraction of the film undergoing dissolution is however dependant on the number of deposition steps and hence on the film thickness. Upon contact with the redox probe containing solutions, for films made from 10 layer pairs, cyclic voltammogram experiments show that there are at least 3 oxidation peaks reflecting the heterogeneity of the binding sites for hexacyanoferrate anions. This constitutes the major and original finding of this work. Upon rinsing with buffer only the strongly bound anions remain in the film and in the presence of 0.05 M supporting electrolyte, the amount of hexacyanoferrate retained in the film is almost independent of its solution concentration, reflecting a high affinity binding isotherm. The hexacyanoferrate containing (PLL-HA)(10) films display also a markedly different surface morphology and roughness with respect to the pristine films. The distribution in the Donnan potential of the redox probes and the partial but almost concentration independent film dissolution point to a large heterogeneity in the binding sites of ferrocyanide with the HA-PAH matrix. This heterogeneity as well as the partial film dissolution can be totally suppressed upon exposure of the film to a carbodiimide/N-hydroxysulfosuccinimide containing solution, allowing for the formation of amide bonds between the amino groups of PAH and the carboxylates of HA. We rationalize all our findings in the framework of the 3 zone model of PEM films.