Effect of Polymer Chain Length, pH, and Temperature on the Preparation of Polymer-Gold Nanoconjugates and Their Application in Catalysis

Abstract

Gold nanoparticles (GNPs) have attracted enormous attention as efficient catalysts because of their high surface area and specific crystal facets. However, high-surface-energy-induced aggregation among nanoparticles results in a drop in catalytic efficiency, which is often prevented by conjugating the nanoparticles with polymers. In this direction, a series of pH- and thermoresponsive polymers, poly(N-vinylpyrrolidone)-block-poly(dimethylaminoethyl methacrylate) (PNVP-b-PDMAEMA), were synthesized and employed for preparing and/or stabilizing GNPs. The size and shape of the synthesized GNPs were influenced by the choice of the stabilizing polymer and the solution pH. Notably, GNPs could be synthesized without a reducing agent at higher pH values, whereas that was not possible at lower pH values. The catalytic efficiency of these polymer-gold nanoconjugates was assessed for the reduction of 4-nitrophenol, where a significant enhancement in catalytic efficiency was observed due to ionic interactions between the reactants and catalysts at low pH values. However, the amphiphilic behavior of the polymers under varying environmental conditions resulted in a non-Arrhenius-type catalytic response in response to temperature variation. At higher pH values, the polymers became hydrophobic, facilitating the recovery of the catalyst from the reaction medium. Overall, the dual-responsiveness of the polymer-gold nanoconjugates offered versatility in catalytic applications as well as advantages for catalyst recyclability.