Reexamination of Surface-Enhanced Raman Scattering from Gold Nanorods as a Function of Aspect Ratio and Shape

It is now believed that the near-resonance excitation of plasmonic nanoparticles is necessary to increase the Raman signal of nearby molecules. Consequently, for surface-enhanced Raman scattering (SERS) applications, researchers seek to synthesize rationally designed nanoparticles with plasmon resonances (PRs) close to the excitation wavelength. However, existing experiments show contradicting results for the dependence of the SERS enhancement on the PR wavelength. Here, we used the etching method to prepare a set of Au nanorod (AuNR) colloids with a decreasing aspect ratio. The shape morphology of the AuNRs and their concentration and width were kept constant, while the plasmon resonance was progressively decreased from 925 to 650 nm. The AuNRs were functionalized with 1,4-nitrobenzenethiol (NBT), and SERS spectra of the colloids were measured under 785 nm laser excitation. The nanorod concentration (∼7 × 1010 mL–1) was quantified by the atomic absorption spectroscopy and spectrophotometry combined with TEM statistical data and T-matrix simulations. The number of adsorbed NBT molecules per one nanorod (∼104) corresponded to the effective footprint of ∼0.55 nm2 and was close to the monolayer packing density with the topological polar surface area of NBT at 0.468 nm2. The plasmon peak position correlated weakly with the SERS response; specifically, the ratio between the SERS intensities for on- and off-resonance excitation was below 1.5. This observation contradicts the current understanding of the electromagnetic contribution to the SERS signal. In particular, our simulations agreed with the experimental data for plasmon resonance wavelengths of 785–925 nm, but for shorter wavelengths the simulations predicted an order-of-magnitude decrease in the averaged enhancement factor. In contrast to this finding, the shape morphology strongly affected the SERS response. Specifically, when the initial cigarlike AuNRs were further overgrown to yield dumbbell morphology, their SERS intensity increased 5-fold. Finally, we show that the SERS background spectra can be attributed to both the photoluminescence from AuNR ensemble and the elastic light scattering of a very weak laser background by the same AuNR ensemble.