UC Santa Cruz

Janus nanoparticles have been an interesting topic due to their dual functionality by the segregation of surface ligands, whereas chiral nanostructures have been attracting extensive interest in recent years primarily because of the unique materials properties that can be exploited for diverse applications. My research focus was based on chirality originated from the structural asymmetry of Janus nanoparticles and study the enantiomeric selectivity of these nanoparticles.

Gold Janus nanoparticles functionalized with hexanethiolates and 3-mercapto-1,2-propanediol segregated on the two hemispheres, self-assembled into vesicle-like, hollow nanostructures in both water or organic media, and exhibited apparent plasmonic circular dichroism (PCD) absorption in the visible range. The PCD signals were found to become intensified with increasing coverage of the 3-mercapto-1,2-propanediol ligands on the nanoparticle surface, which was attributed to the dipolar property of the structurally asymmetrical Janus nanoparticles. Theoretical simulations based on first principles calculations showed that when the nanoparticle dipoles self-assembled onto the surface of a hollow sphere, a vertex was formed which gave rise to the unique chiral characteristics. The resulting chiral nanoparticle vesicles could be exploited for the separation of optical enantiomers, as manifested in the selective identification and separation of D-alanine from the L-isomer.

The enantiomeric selectivity of Janus nanoparticles protected by hexanethiolates and 3-mercapto-1,2-propanediol was further studied using D,L-cysteine as the molecular probe. Experimental results demonstrate that D-cysteine was the preferred enantiomers entrapped within the nanoparticle emulsions, where the ensuing ligand exchange reaction was initially confined to the hydrophilic face of the Janus nanoparticles. This suggests that with a deliberate control of the reaction time, chiral Janus nanoparticles could be readily prepared by ligand exchange reactions even with a racemic mixture of ligands.

In a different study, gold Janus nanoparticles functionalized with poly(ethylene glycol) (PEG) ligands hexanethiolates was synthesized. Due to specific interaction of PEG with alkali metal ions, the Janus nanoparticles exhibited marked conformational changes forming organized ensembles in the presence of Na+ and K+, as manifested in dynamic light scattering, UV-vis absorption and TEM measurements, whereas no apparent variation was observed with bulk-exchange nanoparticles where the two types of capping ligands were homogeneously mixed on the nanoparticle surface or nanoparticles capped with the PEG ligands alone. The ion complexation was further probed in NMR measurements. Results from this study indicate that select doping of alkali metal ions into PEG-functionalized nanoparticles may be used for controlled assembly of the Janus nanoparticles.

In addition to Janus nanoparticles with segregated ligands, Janus Cu1.75S-Au nanostructure was synthesized using Langmuir-Blodgett method at room temperature through ion exchange of gold atoms onto Cu1.75S nanoparticles. Positive chiroptical response arose around 600 nm closed to gold SPR absorption region, whereas the Cu1.75S nanoparticles shown no chiroptical response. Upon etching away the Cu1.75S, the deposited gold nanostructure shown weak positive chiroptical response at the same region, indicating chiral origin was due to the asymmetric deposition of gold atoms. In addition, the chiroptical signal of asymmetric gold nanostructure was enhanced upon the formation of Cu1.75S-Au hybrid nanostructure.