Universal analytical modeling of coated plasmonic particles

Abstract

From a structural point of view, plasmonic nanoparticles are always at least two-layer structures with a dielectric layer of stabilizing, targeting, fluorescent, Raman, or other functional molecules. To optimize the optical properties of such bioconjugates, one needs efficient analytical models based on simple physical ideas and with reasonable accuracy comparable to rigorous numerical methods requiring significant computer resources. Recently, we suggested an analytical approach based on a combination of the modal expansion method (MEM) and the dipole equivalence method (DEM). Here, we extend the MEM+DEM approach for particles of various shapes and orientations. To illustrate the possibilities of our method, we calculate extinction and scattering spectra of gold and silver nanorods, nanodiscs, triangle nanoprisms, bicones, and bipyramids with a dielectric coating thickness from 0 to 100 nm. For the main plasmonic peaks, we found excellent agreement between our analytical method and numerical simulations performed with COMSOL, except for some disagreement between MEM and COMSOL solutions for bicones with small rounding radii. To illustrate the application of our method, we prepared 12 AuNR samples using a chemical etching with fine tuning of the LPR over the 1020-580 nm range. By including the CTAB shell in the simulation model, we achieved excellent agreement between the calculated and measured dependence of the LPR peak position on the AuNR aspect ratio. In summary, our method uses the MEM parameters of the original metal particles and does not require additional numerical calculations to build analytical models for bilayered or multilayered conjugates. Due to the simplicity and reasonable accuracy analytical models, they can help apply machine learning to predict various plasmonic responses such as light absorption, scattering, SERS, and metal-enhanced fluorescence.