Hybrids of Gold Nanoparticles with Core-Shell Hyperbranched Polymers: Synthesis, Characterization, and Their High Catalytic Activity for Reduction of 4-Nitrophenol

Hyperbranched core-shell structure can be constructed by the modification of hyperbranched polyethylenimine (HPEI) with different amide shells. Functionalized HPEI with acetic amide (HPEI-ACAm), propionic amide (HPEI-PRAm), butyric amide (HPEI-BUAm) and isobutyric amide (HPEI-IBAm) shells have been successfully prepared and used as protectors for gold nanoparticles (AuNPs). Novel AuNP composites were obtained through the non-covalent interaction between HPEI-XXAm and gold nanoparticles (XXAm represents ACAm, PRAm, BUAm or IBAm). The resulted AuNP composites can catalyze the reduction reaction of 4-nitrophenol by NaBH₄. Interestingly, the catalytic activity of the AuNPs mainly depends on the structure of protectors and the degree of carbon chain arrangement denseness, which should affect the diffusivity of the reactants. In addition, the order of reaction rate is HPEI10K-IBAm_0.80 > HPEI10K-ACAm_0.80 > HPEI10K-PRAm_0.82 > HPEI10K-BUAm_0.83. It was found that the increase of the concentrations of the capping HPEI-XXAm polymers can enhance both the reaction rate and the turnover frequency (TOF) values. Furthermore, the reaction rate was accelerated with increasing the reaction temperature for AuNPs-HPEI10K-ACAm_0.80 and AuNPs-HPEI10K-PRAm_0.82 systems. Interestingly, the reaction rate was accelerated with elevating reaction temperature at the beginning but reached a plateau or decreased sharply for AuNPs-HPEI10K-IBAm_0.80 and AuNPs-HPEI10K-BUAm_0.82 systems, owing to the thermoresponsivity of the corresponding AuNP composites. As a consequence, the catalytic activity could be controlled by adjusting the different shells of the hyperbranched polyethylenimine.