In Situ Formation of Dual-Phase Thermosensitive Ultrasmall Gold Nanoparticles

A novel method for the in situ synthesis of dual-phase thermosensitive ultrasmall gold nanoparticles (USGNPs) with diameters in the range of 1-3 nm was developed by using poly(N-isopropylacrylamide)-block-poly(N-phenylethylenediamine methacrylamide) (PNIPAM-b-PNPEDMA) amphiphilic diblock copolymers as ligands. The PNPEDMA block promotes the in situ reduction of gold precursors to zero-valent gold and subsequently binds to the surface of gold nanoparticles, while PNIPAM acts as a stabilizing and thermosensitive block. The as-synthesized USGNPs stabilized by a thermosensitive PNIPAM layer exhibit a sharp, reversible, clear-opaque transition in aqueous solution between 30 and 38 C. An unprecedented finding is that these USGNPs also show a reversible soluble-precipitate transition in nonpolar organic solvents such as chloroform at around 0 C under acidic conditions. Two phases, one formula: A facile method for preparing highly stabilized ultrasmall gold nanoparticles (USGNPs) with thermosensitive properties was developed. Amphiphilic poly(N-isopropylacrylamide)-block-poly(N-phenylethylenediamine methacrylamide) diblock copolymers (PNIPAM-b-PNPEDMA) act as both reductant and stabilizer in the single-step synthesis of USGNPs from AuCl₄^- ions in aqueous solution without any additional reductants.