Investigation of local field enhancement near plain and shell-coated gold nanospheres for the optimization of surface enhanced spectroscopy

In this paper we perform a detailed and systematic investigation of electromagnetic field localization and enhancement, at different excitation wavelengths in the 520-640 nm domain, near spherical gold nanoparticles (AuNSs) of different sizes, using Finite-Difference Time-Domain (FDTD) simulations. We provide clear evidence of the size-dependent local electromagnetic field distribution and plasmon-dependent near-field intensity at the surface of plain and DNA-mimicking shell-coated individual AuNSs. This represents a crucial aspect which needs to be taken into consideration in the optimization of platforms based on AuNSs for plasmon-enhanced spectroscopies. Our set of FDTD simulations reveal useful insights regarding the extent of the spectral red-shift of the maximum electromagnetic field enhancement position relative to the localized surface plasmon resonance (LSPR) band, alongside an interesting AuNS size-dependent field enhancement variation in the 520-640 nm excitation range. Finally, we correlate some of the main important theoretical findings from FDTD simulations with experimental data from Surface Enhanced Raman Spectroscopy (SERS) and Metal Enhanced Fluorescence (MEF) assays based on particular types of plain and shell-coated AuNSs.