Controllable Catalytic Activity of Temperature-Responsive Polymer Hybrid Microgels Designed Using a Gold Nanoparticle Monomer with Polymerizable Groups

Abstract

Gold nanoparticles (AuNPs) are representative nanomaterials in designing sensors, photothermal therapy agents, and catalysts owing to their unique optical, electrical, and catalytic properties. Temperature-responsive polymers (e.g., poly( N -isopropylacrylamide) (PNIPAAm)) undergo drastic changes in hydrophilicity and water solubility in response to a small temperature change, and their combination with AuNPs enables us to regulate the unique properties of AuNPs by temperature. Such polymer–AuNPs hybrids have been prepared by three methods: surface modification of AuNPs with polymers having thiol groups, controlled radical polymerization of functional monomer from a AuNP surface, and formation of AuNPs within polymeric self-assemblies and gels. Here, we report a simpler method by using a AuNP monomer with polymerizable groups that can be copolymerized with functional monomers, which were designed by introducing both polymerizable hydrophobic acryloyl and hydrophilic carboxy groups on the AuNP surface. The copolymerization of the AuNP monomer with N -isopropylacrylamide (NIPAAm) afforded PNIPAAm–AuNP hybrid microgels. The catalytic activity of the hybrid microgels in the reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a function of temperature was studied. The hybrid microgels catalyzed only the reduction at temperatures below the lower critical solution temperature (LCST) of PNIPAAm. Temperature dependence of their catalytic activity is attributed to a conformational change of PNIPAAm chains from random coil to globule structures. PNIPAAm chains expand at temperatures below the LCST, which allow the approach of 4-NP molecules to the AuNP surface and their reduction to 4-AP by the AuNPs’ catalytic activity. The shrunken PNIPAAm chains prevent the approach of the 4-NP molecules at temperatures above the LCST, thus suppressing the catalytic activity of the PNIPAAm–AuNP hybrid microgels. Reversible on–off regulation in the reduction reaction was achieved by the temperature-responsive conformation change in the PNIPAAm chains.

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