Multivalent-ion-mediated stabilization of hydrogen-bonded multilayers

Hydrogen-bonding interactions are an important alternative to electrostatic interactions for assembling multilayer thin films of uncharged components. Herein, a new method is reported for rendering such films stable at pH values close to physiological conditions. Multilayer films based on hydrogen bonding are assembled by the alternate deposition of poly[(styrene sulfonic acid)-co-(maleic acid)] (PSSMA) and poly(N-isopropylacrylamide) (PNiPAAm) at pH 2.5. The use of PSSMA results in multi-layers that contain free styrene sulfonate groups, as these moieties do not interact with the PNiPAAm functional groups. Subsequent infiltration of a multivalent ion (Ce⁴⁺ or Fe ³⁺) leads to an increase in the total film mass, with little impact on the film morphology, as determined by using atomic force microscopy. To examine the film stability, the resulting films have been exposed to elevated pH (7.1). While there is substantial swelling of the multilayers (25 % and 55 % for Ce⁴⁺- and Fe³⁺-stabilized films, respectively), film loss is negligible. This provides a stark contrast with non-stabilized films, which disassemble almost immediately upon exposure to pH7.1. This method represents a simple and effective strategy for stabilizing hydrogen-bonded structures non-covalently. Further, the multivalent ions also render the films responsive to changes in the local redox environment, as demonstrated by film disassembly after exposure of Fe³⁺-treated films to iodide solutions.