Optical Constants of Noble Metals at the Nanoscale within the Framework of the Drude Free-Electron Conduction Model: Implications for Liquid Crystal Sensing

The determination of optical constants of noble metals that govern the characteristics of the system has been found to be extremely important to retrospect the observed optical properties from theoretical perspectives to excavate the light–matter interaction at the bottom. Numerous experimental and theoretical approaches, often, followed by fitting through a specified model have been adopted in the literature to evaluate the optical constants either at the bulk, thin film, or nanoscale dimensions. Bulk optical constants have, often, been used for simulation of the optical extinction of noble metal clusters of arbitrary sizes. In 1900, Paul Drude proposed his model of free-electron conduction in a metal that allows expressing the plasmonic characteristics as a function of the common observables. Noble metals, like copper, silver, and gold, at the nanoscale dimension exhibit a characteristic strong absorption band in the UV–vis–NIR spectral region that can be ascribed to the localized surface plasmon resonance (LSPR) that is specific to nanostructures because of geometrical confinement effects of the free electrons. The spectral position and magnitude of the LSPR are, explicitly, governed by the density of conduction electrons, the effective electron mass, and the shape and the size of the charge distribution that can, solely, be attributed to the dielectric properties of both the materials and the surrounding medium. Prudent advances in the synthetic strategies have opened up avenues to achieve desired nanostructures with similar morphologies and stabilizing ligand shells dispersed under analogous conditions. The exquisite sensitivity of the plasmonic response under varieties of microenvironmental conditions could be employed to determine the optical constants of the corresponding metallic nanostructures. A comparative account of the plasmonic sensitivity of materials that requires the determination of the dielectric constant at the nanoscale dimension has been elucidated.