Simulation guided design of silver nanostructures for plasmon-enhanced fluorescence, singlet oxygen generation and SERS applications.

Plasmonic nanostructures such as gold and silver could alter the intrinsic properties of fluorophores, photosensitizers or Raman reporters in their close vicinity. In this study, we have conducted systematic simulations to provide insight for the design of silver nanostructures with appropriate geometrical features for metal-enhanced fluorescence (MEF), metal-enhanced singlet oxygen generation (ME-SOG) and surface-enhanced Raman scattering (SERS) applications. The size-dependent optical properties and electric field enhancement of single and dimeric nanocubes were simulated. The extinction spectra of silver nanocubes were analysed by the multipole expansion method. Results show that a suitable size of Ag nanocubes for MEF and ME-SOG can be selected based on their maximum light scattering yield, the excitation and emission wavelengths of a particular fluorophore/photosensitizer and their maximum spectral overlap. Simulations of the 'hot-spot' or gap distance between two silver nanocubes with different configurations (i.e., face-to-face, edge-to-edge and corner-to-corner) were also performed. A direct correlation was found between the size and enhanced electric field around the Ag nanocubes simulated under 15 common Raman laser wavelengths from the UV to near-infrared region. The maximum SERS enhancement factor can be achieved by selecting the silver nanocubes with the right orientation, suitable edge length and gap distance that give the highest electric field at a specific Raman laser wavelength. It was also found that the higher order of silver nanostructures, e.g., trimer and tetramer, can lead to better enhancement effects. These simulation results can serve as generic guidelines to rationally design metal-enhancement systems including MEF, ME-SOG and SERS for different application needs without cumbersome optimization and tedious trial-and-error experimentation.