Janus nanoparticles: reaction dynamics and NOESY characterization

Janus nanoparticles were prepared by taking advantage of interfacial ligand exchange reactions of hydrophobic hexanethiolate-protected gold nanoparticles with hydrophilic 2-(2-mercaptoethoxy)ethanol (MEA). A monolayer of the particles was first formed at the air-water interface by the Langmuir technique and then deposited onto a substrate surface by the Langmuir-Blodgett method. The particle monolayer was then immersed into an aqueous solution of MEA for different periods of time. It was found that the exchange reactions occurred but were limited only to the top face of the nanoparticles and the reaction reached equilibrium in about 8 h. The resulting particles exhibited amphiphilic characters as confirmed by contact angle and UV-visible, FTIR and NMR spectroscopic measurements. Of these, the structural discrepancy between the Janus nanoparticles and bulk-exchanged particles was clearly manifested, in particular, by NOESY NMR measurements.