Preparation of Poly(N-isopropylacrylamide)-Monolayer-Protected Gold Clusters: Synthesis Methods, Core Size and Thickness of Monolayer
Markus Nuopponen, Heikki Tenhu,
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 included PNIPAMs with narrow molar mass distributions, synthesized by the 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, UV-vis spectroscopy, and by dynamic light scattering. The electron microscopy revealed that the one-pot synthesis utilizing the ligands of the first type is a simple and facile method, compared with the other two ways and that the size of the gold nanoparticles can be easily manipulated mainly by adjusting the molar ratios of PNIPAM / HAuCl4 in the one-pot synthesis. PNIPAM is a more effective 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 (nm2)–1, 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 somewhat extended.
gold nanoparticle, monolayer-protected cluster (MPC), gold cluster, poly(N-isopropylacrylamide), RAFT polymerization