Thermophoresis of gold nanorods from surface enhanced Raman scattering and real-time Rayleigh scattering in solution

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.