Preparation and characterization of novel thermoresponsive gold nanoparticles and their responsive catalysis properties

Novel thermoresponsive gold nanoparticles (AuNPs) with lower critical solution temperature (LCST) were obtained through the non-covalent interaction between a thermoresponsive hyperbranched polyethylenimine with isobutyramide groups (HPEI-IBAm) and citrate-protected AuNPs. The LCSTs of the thermoresponsive AuNPs could be conveniently modulated over a broad range by altering the molecular weight of the HPEI core, the degree of substitution (DS) of the IBAm groups of the HPEI-IBAm polymers or the pH of the solution. The obtained thermoresponsive AuNPs could be used as recyclable responsive catalysts for the reduction reaction of 4-nitrophenol by NaBH₄. As far as the thermoresponsive catalysts were concerned, reducing the molecular weight of the HPEI core, lowering the DS values and increasing the concentrations of the capping HPEI-IBAm polymers or the gold resulted in the acceleration of the reaction. By choosing the right capping HPEI-IBAm polymers, the reaction was faster than that catalyzed by AuNPs without capping polymers. The reaction rate was accelerated by elevating the reaction temperature at first, but reached a plateau or decelerated upon raising the temperature close to the LCSTs of the thermoresponsive AuNPs catalysts. Moreover, the obtained thermoresponsive AuNP catalysts could be recovered by heating the temperature above their LCSTs and be recycled at least six times with more than 95% conversion.