Spontaneous formation of gold nanoparticles in poly(ethylene oxide)-poly(propylene oxide) solutions: Solvent quality and polymer structure effects

We report here on the effects that the solution properties of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers have on the reduction of hydrogen tetrachloroaurate(III) hydrate (HAuCl ₄·3H ₂O) and the size of gold nanoparticles produced. The amphiphilic block copolymer solution properties were modulated by varying the temperature and solvent quality (water, formamide, and their mixtures). We identified two main factors, (i) block copolymer conformation or structure (e.g., loops vs entanglements, nonassociated polymers vs micelles) and (ii) interactions between AuCl ₄ ^- ions and block copolymers (attractive ion-dipole interactions vs repulsive interactions due to hydrophobicity), to be important for controlling the competition between the reactivities of AuCl ₄ ^- reduction in the bulk solution to form gold seeds and on the surface of gold seeds (particles) and the particle size determination. The particle size increase observed with increased temperature in aqueous solutions is attributed to enhanced hydrophobicity of the block copolymer, which favors AuCl ₄ ^- reduction on the surface of seeds. The lower reactivity and higher particle sizes observed in formamide solutions are attributed to the shielding of ion-dipole interaction between AuCl ₄ ^- ions and block copolymers by formamide, which overcomes the beneficial effects of formamide on the block copolymer conformation (lower micelle concentration).