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 ways, in which three types of PNIPAM ligands were employed. The first type 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 ligands was derived from the first one through hydrazinolysis, and they therefore contained a thiol end group. The third type of ligands was PNIPAMs obtained through conventional radical polymerization, post-modified to contain thiol end groups. The PNIPAM-MPCs were characterized by high-resolution transmission electron microscopy, and by 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 3.6 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 corresponding free PNIPAM in aqueous solution, which suggests that the conformation of a PNIPAM chain bound to the gold core is to extended.