Synthesis and characterisation of dual plasmonic gold nanostars as high-performance surface-enhanced Raman spectroscopy substrate

Synthesising gold nanoprobes in the near infrared (NIR) region is of particular interest in developing nanosensors due to the minimal light attenuation from biomolecules. Here, the controlled synthesis and tunability of gold nanostars' two distinct localised surface plasmon resonances (LSPRs) at around 700 and 1100 nm is reported. By using UV-Vis-NIR absorption measurements and finite-difference time-domain calculations, the induction of the LSPR and the multipolar nature of the resonances have been investigated experimentally and theoretically. Simulation results demonstrate that large electric fields are confined at the tips of the branches, where the LSPR can be induced specifically by controlling the polarisation of the incident electric field. The surface-enhanced Raman scattering (SERS) capability of these dual plasmonic gold nanostars (DPGNS) has also been demonstrated using a Raman reporter, diethylthiatricarbocyanine iodide and high SERS enhancement factor (EF) of 2 x 10(7) is obtained with 785 nm excitation. With ease of synthesis, LSPR at NIR and high SERS EF, DPGNS demonstrated the capability to be an effective SERS substrate and the potential to elicit the highest SERS EF ever reported for gold nanoparticles, with further longer wavelength excitations at and beyond 1064 nm.