One spectroscopic technique for the detection of cancer is based on surface-enhanced Raman scattering (SERS). The method uses metallic nanoparticles as novel optical contrast agents or molecular probes to detect the Raman signal, or "molecular fingerprint", emanating from the cancer gene of interest. By modelling the nanoparticle's plasmonic interaction with the incident probing laser signal, the detection sensitivity can be optimized and enhanced for the given material, incident laser wavelength and nanoparticle shape and size.

Researchers at Duke University, North Carolina, US, have been investigating the accuracy of the finite element method (FEM) with respect to an analytic solution based on Mie theory, to study plasmonic properties of silica-silver core-shell nanoparticles with a dimension of 20 to 100 nm. Plasmonics refers to the study of enhanced electromagnetic properties of metallic nanostructures. The term is derived from plasmons, the quanta associated with longitudinal waves propagating in matter through the collective motion of large numbers of electrons.

The team's work shows that when compared with the Mie theory, the FEM accurately solves the near-field plasmonic behaviour of silver nanoshells, in both frequency and spatial domains, for dipole positions of interest: inside the core, inside the shell and in the surrounding medium. The power spectra were evaluated by integrating the Poynting vector over a sphere enclosing the shell. The quasi-static approximation, valid for nanoparticles much smaller than the excitation wavelength, was shown to rapidly break down as the nanoshell size increased, whereas the FEM faithfully reproduced the Mie solution. The FEM's tetrahedral meshing enabled the curved boundaries of the nanoshell to be properly discretized such that the sharp discontinuities in the electric field at the nanoshell core-shell-medium boundaries were solved to a high spatial resolution.

The results pave the way for confident use of the FEM for modelling sophisticated geometries of nanostructures, such as nanoparticle arrays or nanoparticle aggregates, for use as SERS nanoprobes in medical diagnostics and imaging.

Full details can be found in the journal Nanotechnology.