Chiroptically Active Plasmonic Nanoparticles Having Hidden Helicity and Reversible Aqueous Solvent Effect on Chiroptical Activity

The geometrical prerequisite for forming a helix is P (helical pitch) > d (wire diameter). Limited by the current development of nanofabrication techniques, it is difficult to minimize d and consequently P to the sub-10 nm molecule-comparable scale, preventing the study of chiral plasmonics at dimensions approaching the physical limit. Herein, glancing angle deposition is operated at substrate temperature of 0 °C and high speed of substrate rotation to generate silver nanoparticles (AgNPs) with nominal P < d. The AgNPs have intrinsic chiroptical activity characterized by circular dichroism (CD), originating from the hidden helicity. With increasing P from 3 to 66 nm, the plasmonic mode barely shifts but shows a logarithmic increase in CD amplitude. Immersing AgNPs in water causes the plasmonic mode to redshift and rise in CD amplitude, i.e., a water effect on chiroptical activity. Hydrophilic AgNP arrays with low array porosity show a reversible water effect, but hydrophobic Ag nanospiral arrays with P > d and high array porosity have an irreversible water effect. This work introduces a cost-effective, facile approach to minimize P to sub-10 nm at a regular substrate temperature, paving the way to study chiral plasmonics approaching the physical limit and exploit chirality-related bioapplications typically operated in aqueous solutions to tackle significant health and environmental problems.