Amphiphilic multiblock copolymer stabilized Au nanoparticles

Stable gold nanoparticles (AuNPs) are synthesized in N,N-dimethylformamide (DMF) by using amphiphilic multiblock copolymer (P4VP–PS–P4VP)n containing multiple trithiocarbonate moieties or amphiphilic triblock copolymer (HS–P4VP–PS–P4VP–SH) with thiol end groups as protecting agents. Only isolated gold nanoparticles are obtained, which indicate that the multiple functional groups of a single polymer chain are grafted at the same time to the same nanoparticle. The sizes of the AuNPs are slightly increased with raising the concentration of gold salt for a given concentration of the copolymers, leading to the red shift of the surface plasmon resonance of the AuNPs. Fourier transform infrared (FTIR) spectra and scanning force microscopy (SFM) images confirm that the AuNPs are indeed wrapped by the block copolymers. With the amphiphilic characteristic of the block copolymers, the AuNPs can be successfully transferred from DMF into aqueous solution. An unexpected red shift of the surface plasmon resonance (SPR) of the AuNPs after solvent transfer is observed, which is mainly attributed to the dense protecting polymer layers on the surfaces of the AuNPs in aqueous solutions. Both the size and structure of nanoparticles are maintained after solvent transfer. The AuNPs in aqueous solutions can be stable for a wide range of pH value up to pH ≈ 9. At strong basic condition with pH > 10, the AuNPs will aggregate. The effect of pH value on the stability of the AuNPs is reversible. The optical property of the AuNPs also slightly depends on the pH value.