Colloidal stability evolution and completely reversible aggregation of gold nanoparticles functionalized with rationally designed free radical initiators

A series of molecular adsorbates having various chain lengths of terminal poly(ethylene glycol methyl ether) (PEG) moieties, thiol head groups, and intervening free radical initiator moieties was used to functionalize the surface of gold nanoparticles (AuNPs). The bulky PEG groups stabilized the functionalized AuNPs by providing steric hindrance against AuNP aggregation, such aggregation being a major problem in the modification and manipulation of metal nanoparticles. UV-vis spectroscopy was used to evaluate the stability of the adsorbate-functionalized AuNPs as a function of AuNP size (similar to 15, 40, and 90 nm in diameter) and PEG chain length (Mn 350, 750, and 2,000). The longer PEG chains (Mn 750 and 2,000) afforded stability to AuNPs with smaller gold cores (similar to 15 and 40 nm in diameter) for up to several days without any marked aggregation. In contrast, the adsorbate-functionalized AuNPs with the largest gold cores (similar to 90 nm) were noticeably less stable than those with the smaller gold cores. Importantly, the adsorbate-functionalized AuNPs could be isolated in solvent-free dried form and readily dispersed in aqueous buffer solution (both acidic and basic) and various organic solvents (protic and aprotic). This isolation-redispersion (i.e., aggregation/deaggregation) process was completely reversible. The chemisorption of the PEG-terminated initiator on the surface of the AuNPs was verified by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). As a whole, the strategy reported here affords colloidally stable, free radical initiator-functionalized AuNPs and offers a promising general method for encapsulating metal nanoparticles within polymer shells.