pH- and Anion-Responsive Poly(1-vinylimidazole) Opal Films for Smart Sensing

Materials containing imidazole have been used as promising substances in the fields of life sciences, environmental science, and electrochemistry. In this study, tailored core–shell particles that respond to acidic solutions and fluorine-containing hydrophobic anions were synthesized through starved-feed emulsion polymerization. Imidazole, which responds to proton acids and hydrophobic anions, was incorporated as a functional moiety into the shell of the particles. The soft and viscoelastic matrix was composed of the copolymer, poly((n-butyl acrylate)-co-(1-vinylimidazole)), allowing for control of the hydrodynamic diameter of the core–shell particles due to the balance between hydrophilic and hydrophobic properties. The size comparison of monodisperse particles in the colloid state was investigated using dynamic light scattering (DLS) and transmission electron microscopy (TEM). Changes in the glass transition temperature, depending on the copolymer ratio, were calculated using the Fox equation. The particles were melt-sheared after extrusion to produce viscoelastic opal films, arranging the particles into colloidal crystal stacks showing vivid structural colors. The optical features changed in response to acidic solutions and hydrophobic anions and were examined using in situ ultraviolet–visible (UV–vis) spectroscopy. The degree of hydrophilicity of the film was compared through contact angle measurements. The manufactured smart opal film can be applied as an affordable sensor that exhibits optical color changes in response to acidic pH and hydrophobic anions.