Amphiphilic hyperbranched copolymers bearing a hyperbranched core and a dendritic shell as novel stabilizers rendering gold nanoparticles with an unprecedentedly long lifetime in the catalytic reduction of 4-nitrophenol

A series of organo-soluble gold nanoparticles (AuNPs) were prepared through the reduction of HAuCl4 by NaBH4 in the presence of amphiphilic hyperbranched copolymers bearing a hydrophilic hyperbranched polyethylenimine (PEI) core and a hydrophobic dendritic shell. For comparison, the corresponding analogues with a PEI core and a linear shell were also used to prepare the organo-soluble AuNPs. All the obtained AuNPs were characterized by transmission electron microscopy. It was found that under a high feed ratio of polymer to HAuCl4, the average diameter of the obtained small AuNPs was similar, and was independent of the shell morphology of the hyperbranched polymeric stabilizers. Reducing the feed ratio of polymer to HAuCl4 led to the formation of larger AuNPs and irregularly shaped AuNP aggregates. The stability evaluation on the basis of two-month shelf-storage demonstrated that the obtained AuNPs were stable under shelf-storage regardless of whether the shell of the hyperbranched polymeric stabilizer was dendritic or linear. These organo-soluble AuNPs could be used as efficient catalysts for the biphasic catalytic reduction of 4-nitrophenol by NaBH4, and could also be conveniently recovered and reused. The stabilizers with a dendritic shell endowed the AuNP catalyst with a higher performance than the corresponding stabilizers with a linear shell, including: (1) a higher catalytic rate; (2) the ability to be recovered and reused more times; (3) a maximal turnover number of around 23 000, which is unprecedented in the catalytic reduction of 4-nitrophenol.