Controlled assembly of Janus nanoparticles.

Janus nanoparticles were prepared by interfacial ligand exchange reactions of octanethiolate-protected gold (AuC8) nanoparticles with 3-mercapto-1,2-propanediol (MPD) at the air/water interface. AFM and TEM measurements showed that the resulting particles formed stable aggregates in water with dimensions up to a few hundred nanometers, in sharp contrast to the original AuC8 particles and bulk-exchange counterparts where the aggregates were markedly smaller. Consistent behaviors were observed in dynamic light scattering measurements. FTIR measurements of solid films of the nanoparticles suggested that the octanethiolate ligands were mostly of trans conformation, whereas the MPD ligands exhibited gauche defects as a consequence of the hydrogen-bonding interactions between the hydroxyl moieties of adjacent ligands. Raman spectroscopic measurements in an aqueous solution of pyridine showed that the pyridine ring breathing modes remained practically unchanged and the intensity profiles indicated minimal interactions between pyridine and the gold cores within the three nanoparticle ensembles. However, water bending vibrational features were found to be enhanced substantially with the addition of Janus nanoparticles, which was ascribed to the formation of clusters of water molecules that were trapped within the nanoparticle ensembles. No apparent enhancement was observed with the AuC8 or bulk-exchange particles.