Diminishing Cohesion of Chitosan Films in Acidic Solution by Multivalent Metal Cations

Chitosan is a natural polymer with good biocompatibility, biodegradability, and bioactivity that has great potential for biomedical and industrial applications. Like other natural sugar-based polymers, chitosan molecules own versatile adhesion abilities to bind with various surfaces, owing to multiple functional moieties contained in the chain. To develop the promising biomaterials based on the chitosan chemistry, it is fundamentally important to figure out its adhesion mechanism under a certain condition, which leaves us numbers of open questions. In this work, we characterized the chitosan films adsorbed on a mica substrate in acidic solution and investigated the effects of multivalent salts on the cohesive behaviors of the films by means of the surface forces apparatus. The results showed that the cohesion capacities of chitosan films were reduced to around 30% of their original states after the addition of 10^-7 M LaCl₃ into 150 mM acetic acid, which could be partially recovered by holding the films at the contact position for a longer time. Surprisingly, the cohesion loss in the films exhibited the dependence on the properties of the metal cations including valance and concentration. The topography of the chitosan-coated surface also showed obvious aggregation in the presence of submicromolar of the salts. Here, we attributed these phenomena regarding cohesion loss to the mechanisms involved in the absorption of metal cations by the chitosan chains, which not only consumed the binding sites but also induced conformation change in the polymer network. Our findings may offer a suggestion for the production of chitosan-based materials to notice the potential impacts of ultralow concentrated salts that are usually neglected even under acidic conditions.