Sep 1, 2010
Understanding gold and silver nanoshells: plasmonics analysis using finite element method and Mie theory
Nanoparticles of gold and silver could play an important role in helping to detect diseases such as cancer. This possibility has triggered the need to understand, characterize and optimize the physical properties of these nanomaterials.
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.
About the author
The study was conducted by members of the Fitzpatrick Institute of Photonics and Departments of Biomedical Engineering and Chemistry at Duke University, North Carolina, US. The research was part of projects funded by the NIH (R01 EB006201 and R01 ES014774). Chris Khoury is a PhD student in the Biomedical Department. Dr Stephen Norton is a physicist interested in the stimulation of plasmon resonances in nanostructures. Prof. Tuan Vo-Dinh is R Eugene and Susie E Goodson Distinguished Professor of Biomedical Engineering, Professor of Chemistry and Director of the Fitzpatrick Institute for Photonics at Duke University. His research is focused on the development of advanced technologies for the protection of the environment and the improvement of human health. His research activities involve nano-biophotonics, plasmonics, laser spectroscopy, molecular imaging, medical diagnostics, cancer detection, chemical sensors, biosensors, nanosensors and biochips.