Surface-catalyzed growth of pH-responsive copolymer thin films

Abstract

Polymer films that respond to pH have broad applications in chemical and biological sensors, smart membranes, and dynamic surfaces. We have engineered a new class of pH-responsive copolymer films on gold electrode surfaces by first developing a controlled, surface-catalyzed copolymerization to prepare a copolymer film consistent with poly(methylene-co-ethyl acetate) and subsequently hydrolyzing the ester side chains to carboxylic acids (denoted as PM-CO2H). The effects of polymer composition and thickness on pH response were investigated by electrochemical impedance spectroscopy, reflectance-absorption infrared spectroscopy, and spectroscopic ellipsometry. At a 1% - 4% molar acid content, the copolymer film exhibits a five order of magnitude change in its resistance to ion transport over 2 – 3 pH units. The onset pH at which this response initiates can be tailored from pH 5 to 10 by decreasing the acid content in the film from 4% to 1%. All films exhibit rapid ionization but gradual protonation. The increase of acid content within the films accelerates both processes, especially the protonation process. At the same acid content, thinner films require much less time to protonate, consistent with a diffusion-limited process, although film thickness in the sub-100 nm regime has no measurable effect on the rates of ionization. Thus, based on acid content and thickness, the range and rate of the pH-induced response can be tailored. These films are particularly interesting for pH-dependent separations and as chemically sensitive thin films. For separations, these super-responsive films can be grown as ultrathin skins on nanoporous membranes to control the passage of ions in a pH-specific manner. To provide sensitivity over a broader range of pH for sensor applications, the acid groups can be further modified to create tertiary amino side groups that become protonated and charged as pH is reduced, opposite to the response of PM-CO2H.