Azobenzene-containing molecularly imprinted polymer microspheres with photoresponsive template binding properties

Abstract

The first successful preparation of azobenzene (azo)-containing molecularly imprinted polymer (MIP) microspheres with photoresponsive template binding properties is described. A methacrylate azo functional monomer with a pyridine group was used for this purpose, and its good solubility in acetonitrile allowed the implementation of molecular imprinting viaprecipitation polymerization, leading to azo-containing MIP microspheres (number-average diameter = 1.33 μm, polydispersity index = 1.15) with obvious molecular imprinting effects towards the template 2,4-dichlorophenoxyacetic acid (2,4-D), rather fast template rebinding kinetics, and appreciable selectivity over structurally related compounds. The binding association constant K_a and apparent maximum number N_max for high-affinity sites of the imprinted polymer in the dark environment were determined by Scatchard analysis to be 2.3 × 10⁴ M⁻¹ and 10.0 μmol g⁻¹, respectively. Most importantly, the binding affinity of the imprinted sites in azo-containing MIP microspheres was found to be photoresponsive towards the template, which decreased upon UV light irradiation (as revealed by the resulting lower K_a value for high-affinity sites and reduced specific bindings), whereas it could be recovered during the subsequent thermal (or visible light-induced) back-isomerization. Furthermore, this photoregulation process proved to be highly repeatable under photoswitching conditions.