Effect of local heating on the SERS efficiency of optically trapped prismatic nanoparticles

Local heating of laser trapped metallic nanoparticles results in a gradual decrease in surface-enhanced Raman scattering (SERS) which can be attributed to both photophysical and chemical processes. Initially trapping results in a dramatic increase in Raman signal due to improved excitation and collection efficiencies in the trapping volume, followed by a gradual decay to a stable asymptotic level approximately 20% of the initial maximum value. The underlying processes leading to this decrease in SERS signal include photothermal-induced changes in dielectric properties of the particles and medium, and adsorption/desorption kinetics of the surface molecules. The relatively long apparent decay time seems to suggest that either process occurs on surprisingly long timescales for the thiol capped nanoprisms reported here. Here we present a simple semiempirical description of the observed time dependence of the single nanoparticle Raman intensities simultaneously accounting for both processes. Time series data of total integrated Raman signal supports a simple steady-state thermal equilibrium model which yields an equilibrium temperature change for the particle of T = 205 +/- 7 K and the best-fit time constant of T = 0.10 +/- 0.05 s, smaller than obtained by other more empirical models. The quality of the fit suggests that desorption kinetics dominate, and that the observed decay cannot be fully explained using a single (average) desorption free energy (Delta G). This work demonstrates the dynamic process of SERS from optically trapped prismatic nanoparticles, the utility of single particle measurements, and the effect of local heating on observed Raman scattering efficiency.