Galvanic Replacement of Chiral Hosts for Generating Helical Nanoparticles with Engineerable Compositions and Optical Activity

Abstract

Metallic helices with a characteristic helical pitch (P) in the micro- or nano-scale have been proposed to develop diverse chirality-related primary applications. However, limit development of nanofabrication techniques leads to P > 20 nm; molecules are too small in size to effectively perceive the helical chirality, and such the dimensional mismatch will substantially prohibit the development of those applications. Currently, we devise glancing angle deposition (GLAD) with fast substrate rotation to successfully generate metallic helical nanoparticles (HNPs) with a sub-10-nm P. The sub-10-nm P is compatible with the molecular scale, such that we use the sub-10-nm-P HNPs to amplify optical activity of enantiomers that are grafted on the HNPs in roughly one order of magnitude, and to effectively mediate the enantioselective photocyclodimerization of 2-Anthracenecarboxylic acid. However, GLAD is operated at an extremely oblique deposition angle of > 85° with respect to the direction of substrate normal, resulting in an evidently low deposition yield given that most of evaporated atoms will not condense on the substrate. From the production cost point of view, low deposition yield tends to severely limit the application of such the technique to generate HNPs made of noble metals, which typically function as heterogeneous asymmetric catalysts. It is of practical demand on developing an economic method to produce noble metal HNPs with high production yield. In this talk, we will present a two-step fabrication of noble metal HNPs, composed of fast-substrate-rotation GLAD of HNPs made of low-cost metals (such as Cu, denoted as chiral host) and galvanic replacement of chiral hosts with noble metals (such as Ag, Au, Pt, and so on). Limit, controllable usage of noble metal salts in the galvanic replacement reaction not only reduces the production cost of noble metals, but also leads to generating alloy HNPs with tunable alloy composition. Systematic characterizations of nanostructure, material and optical activity are operated to study the galvanic replacement processes and dynamics. This work paves the way to generating novel asymmetric catalysts to operate heterogeneous asymmetric catalysis with high enantioselectivity, and to dramatically enhancing enantiodifferentiation by amplifying optical activity of enantiomers.