UV Circular Dichroism of Molecular Films on Plasmonic Nanostructures: A Comparative Case Study with On-Resonant Aluminum and Off-Resonant Silver Nanoparticles

Abstract

Electronic absorption bands of important biomolecules and pharmaceutical compounds lie in the ultraviolet (UV) between 180 and 280 nm and thus in a spectral range that does not overlap with the localized surface plasmon resonances (LSPRs) of conventional gold (Au) or silver (Ag) nanoantennas. Aluminum (Al) nanostructures support resonances in UV, and there is significant interest in utilizing UV-resonant nanostructured Al substrates for enhancing the sensitivity of chiroptical spectroscopies such as circular dichroism (CD). In this study, we compare the CD of a chiral molecular film on Al and Ag substrates and evaluate the role of inherent and induced absorptive CD as well as of scattering CD in the UV. The CD signal of the test molecule 2,2′-bis(di-p-tolylphosphino)-1,1′-binaphthyl (Tol-BINAP) was measured on large random arrays of Al and Ag nanoparticles (NPs) generated by hole-mask colloidal lithography (HCL). Al NPs provide size-tunable quadrupole and dipole resonances overlapping with molecular absorption bands in the UV, while Ag NPs lack plasmon resonances in this range. The CD signal on the Al NP substrate was increased by up to 154% compared to Ag NP controls. Using Poynting’s theorem generalized to chiral media, we decompose the CD into scattering and absorptive components. The chiral film induces an asymmetric scattering response in resonant achiral Al nanocylinders, dominating the total CD. In contrast, the CD in off-resonant Ag nanocylinders arises from asymmetric absorption. The induced and inherent absorptive CD components have opposite signs and partially cancel each other.