Preparation of Poly(N-isopropylacrylamide)-Monolayer-Protected Gold Clusters: Synthesis Methods, Core Size, and Thickness of Monolayer

The preparation of poly(N-isopropylacrylamide)-monolayer-protected clusters (PNIPAM-MPC) of gold nanoparticles was carried out in a homogeneous phase using three methods, in which three types of PNIPAM ligands were employed. The first type was comprised of PNIPAMs with narrow molar mass distributions, synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization and thus bearing a dithiobenzoate at the chain end. These polymers were used directly to passivate the gold nanoparticles upon the Schiffrin reaction in a one-pot synthesis. The second type of ligand was derived from the first one through hydrazinolysis, and they therefore contained a thiol end group. The third type of ligand was PNIPAMs obtained through conventional radical polymerization, postmodified to contain thiol end groups. The PNIPAM-MPCs were characterized by high-resolution transmission electron microscopy, UV−vis spectroscopy, and dynamic light scattering. The one-pot synthesis utilizing the ligands of the first type turned out to be a simple and facile method compared with the other two ways, with which the size of the gold nanoparticles can be easily manipulated mainly by adjusting the molar ratios of PNIPAM/HAuCl₄. PNIPAM is a more efficient ligand to stabilize the gold nanoparticles in water and in organic solvents than alkanethiols. The surface density of PNIPAM chains ranged from 1.8 to 2.5 chain/nm², which is much lower than that typical for alkanethiols. The thickness of a PNIPAM monolayer bound to the gold core is somewhat larger than the size of the random coil of the corresponding free PNIPAM in aqueous solution, which suggests that the conformation of a PNIPAM chain bound to the gold core is extended.