Temperature dependence of localised surface plasmon resonance effect on photoisomerisation of azobenzene units in Ag/poly(N-isopropylacrylamide)-blockpoly{6-[4-(4-methylphenyl-azo)phenoxy] hexylacrylate} composite particles

A series of Ag/poly(N-isopropylacrylamide)-block-poly{6-[4-(4-methylphenyl-azo)phenoxy] hexylacrylate} (PNIPAM-b-PAzoM) composite particles (CPAs) with varying concentrations of Ag nanoparticles are prepared by self-assembly of PNIPAM-b-PAzoM in a mixture of H₂O and THF at first, and then by chemical reduction of AgNO₃ in the suspension of the self-assembled particles. The distance between Ag nanoparticles and azobenzene units is determined by the concentration of Ag nanoparticles and temperature because hydrophilic poly(N-isopropylacrylamide) (PNIPAM) chains are thermosensitive and adsorbed with Ag nanoparticles. Both trans-to-cis and cis-to-trans photoisomerisations of the CPAs are investigated at different temperatures and concentrations of Ag nanoparticles. Experimental results reveal that, at room temperature, both rates of the trans-to-cis and the cis-to-trans photoisomerisations are decreased by the localised surface plasmon resonance of Ag nanoparticles with experimental concentrations lower than 0.04 mmol/l. With the temperature increasing and gradually closing to the low critical solution temperature of PNIPAM chains the rate of the trans-to-cis photoisomerisation can be found to be enhanced gradually within the same concentration range of Ag nanoparticles. However, the rate of the cis-to-trans photoisomerisation is little affected. Detailed analysis shows that this effect is related to the overlap of azobenzene absorption and Ag nanoparticle absorption.