Chiroptically Active Plasmonic Nanoparticles Having Hidden Helicity and Reversible Aqueous Solvent Effect on Chiroptical Activity. Issue 42 (5th September 2016) - Liu, Junjun
Abstract : 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. Abstract : Silver nanoparticles (AgNPs) fabricated by glancing angle deposition have hidden helicity with P (helical pitch) < d (wire diameter). This leads to intrinsic chiroptical activity characterized by circular dichroism (CD) that barely shifts but logarithmically rises in CD amplitude with increasing nominal P, and has a reversible aqueous solvent effect on CD since the chiral AgNP arrays are hydrophilic and have low array porosity.