Surface-enhanced Raman scattering (SERS) from gold and silver nanoparticles suspended in solution enables a more quantitative level of analysis relative to SERS from aggregated nanoparticles and roughened metal substrates. This is due to the more predictable and consistent near field enhancement regions created by isolated nanoparticles, and to averaging over the many nanoparticles that diffuse through the excitation beam during the measurement. However, we find that localized heating of the solution by the focused excitation leads to thermophoresis which alters the nanorod concentration in the focal volume and therefore impacts quantitative analysis. Since many phenomena may impact the Raman signal, we record both the Rayleigh and Raman scattering from gold nanoparticle solutions. This allows us to distinguish molecular processes from depletion of nanoparticles in the excitation beam. We observe that the concentration of nanorods can deplete to less than 50% of its original value over 100 second timescale, which are consistent with a thermophoretic effect driving nanoparticles from the beam spot. We also find that the particle motion drives convection within the sample cell that further contributes to signal instabilities.
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Directional Raman scattering spectra of metal–sulfur bonds at smooth gold and silver substrates
The gold–sulfur (Au–S) and silver–sulfur (Ag–S) bonds are integral to the surface modification of metal films with alkanethiol monolayers. Although the metal–sulfur bond can be characterized with surface‐enhanced Raman spectroscopy (SERS) at roughened metal films, some applications require or perform better when using a smooth metal surface, which is not suitable for SERS signal enhancement. Directional‐surface‐plasmon‐coupled Raman scattering (directional Raman scattering) is an approach to measure metal–sulfur bonds on smooth metal films with sub‐monolayer sensitivity. The metal–sulfur bonds formed from a benzenethiol monolayer on smooth planar gold or silver films are observed in the directional Raman scattering spectra between 240 and 270 cm−1; the signal‐to‐noise ratio of the Au–S Raman peak is 60. Importantly, the directional Raman scattering signal measured with smooth metal surfaces can be simply modeled and easily compared across many samples. Directional Raman scattering can also be measured at roughened metal films, which makes it applicable for many analyses.
more » « less- NSF-PAR ID:
- 10362435
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Journal of Raman Spectroscopy
- Volume:
- 52
- Issue:
- 7
- ISSN:
- 0377-0486
- Page Range / eLocation ID:
- p. 1246-1255
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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