Controllable Plasmonic Enhancement and Hot Spots of Metallic Nanoclusters Assembled by Green Fluorescent Proteins

Abstract

Biocompatible metallic nanoparticles in optical regime have been incessantly explored for the state of art technologies of non-invasive sensing, high resolution imaging and surface-enhanced Raman spectroscopy. We propose three-dimensional numerical modeling to unveil the physical phenomena of gold nanoclusters assembled by green fluorescent protein (GFP) with FDTD technique. GFPs over metallic nanoparticles are physically modeled as a nanometer cylindrical shape with estimated refractive index depending on the molecular concentration of buffer solution. Electromagnetic field distributions in the vicinity of metallic nanoparticles accompanying with GFPs exhibit localized hot spot intensity and locations as a function of gap length between nanoparticles, incident polarization state of light, and array types. Calculated spectra is verified with experimental data in terms of buffer concentration with and without gold nanoparticles. In addition, plasmonic tunability within visible range is observed in randomized hetero gold nanoclusters assembled by specific binding between split-GFP with 40 nm gold particle and M3 peptide with 20 nm gold particle, giving rise to major electric capacitive coupling. SERS enhancement factor relies on the coupling location of hetero gold nanoparticles. Interparticle electric couplings between 40 nm gold particles and 20 nm gold particles in randomized hetero gold nanoclusters produce localized hot spots in response to the interparticle distance, polarization orientation, and nanoparticle size. In case of gold nanoheptamer, which is also assembled by site-specific GFP binding, symmetric localized hot spots are observed corresponding to the couplings between center gold nanoparticle and surrounding gold nanoparticles. By the variation of surrounding gold nanoparticle size, the number and position of localized hot spots, which can be called Raman active site, is controllable. According to numerical analysis of GFP-driven gold nanocluster, tailored gold nanoclusters are offered.