Size Effect on SERS of Gold Nanorods Demonstrated via Single Nanoparticle Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) has attracted tremendous interest as a label-free highly sensitive analytical method. For optimization of SERS activity, it is highly important to systematically investigate the size effect of nanoparticles on the SERS enhancement, which appears to be challenging in experiment, as the localized surface plasmon resonance (LSPR) of nanoparticles also changes with the change of the particle size. This challenge can be overcome by utilizing the unique property of gold nanorods, whose LSPR wavelength can be controlled to be the same by properly choosing the size and aspect ratio of the nanorods. We obtained the correlated SEM images, scattering spectra, and SERS spectra on a home-built single nanoparticle spectroscopy system and systematically investigate the size effect on SERS of individual gold nanorods using the adsorbed malachite green isothiocyanate (MGITC) molecule as the probe molecule. The dark field scattering intensity was found to increase with the increase of the size of nanoparticles, whereas the SERS intensity increases with the decrease of the size as a result of the stronger lightning rod effect and weaker radiation damping. We further explored the size-dependent effect for the coupled nanorod dimer system. The SERS activity was also found to increase with a decrease of the particle size when the excitation is close to the LSPR wavelength. Understanding of the size effect on the local field enhancement may help to design and fabricate SERS substrate and TERS tips with high SERS activity.