Synergistic Bonding of Poly(N-isopropylacrylamide)-Based Hybrid Microgels and Gold Nanoparticles Used for Temperature-Responsive Controllable Catalysis of p-Nitrophenol Reduction

Stimuli-responsive hybrid nanoparticles used for controllable catalysis have been attracting increasing attention. This study aims to prepare hybrid microgels with excellent temperature-sensitive colorimetric and catalytic properties through combining the surface plasmon resonance properties of gold nanoparticles (AuNPs) with the temperature-sensitive properties of poly(N-isopropylacrylamide) (PNIPAM)-based microgels. Microgels with hydroxy groups (MG-OH) were prepared by soap-free emulsion polymerization, using N-isopropylacrylamide as the main monomer, hydroxyethyl methylacrylate as the functional monomer, N,N'-methylene bisacrylamide as the crosslinker, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride as an initiator to ensure the microgels are positively charged. Furthermore, chemical modification on the surface of MG-OH was carried out by 3-mercaptopropyltriethoxysilane to obtain thiolated microgels (MG-SH). Two kinds of hybrid nanoparticles, AuNPs@MG-OH and AuNPs@MG-SH, were self-assembled, through electrostatic interaction between positive MG-OH and negative citrate-stabilized AuNPs as well as through synergistic bonding of electrostatic interaction and Au-S bonding between positive MG-SH and negative AuNPs. The morphology, stability, temperature-sensitive colorimetric properties, and catalytic properties of hybrid microgels were systematically investigated. Results showed that although both AuNPs@MG-OH and AuNPs@MG-SH exhibit good temperature-sensitive colorimetric properties and controllable catalytic properties for the reduction reaction of p-nitrophenol, AuNPs@MG-SH with synergistic bonding has better stability and higher catalytic performance than AuNPs@MG-OH. This work has good competitiveness against known PNIPAM-based materials and may provide an effective method for preparing smart catalysts by self-assembly with stimuli-responsive polymers, which has a great potential application for catalyzing a variety of reactions.