We report a method to establish experimental conditions for surface-enhanced Raman scattering (SERS) spectroscopy in order to avoid thermal damage on heat-sensitive materials by monitoring thermal desorption of thiolate molecules from a metal surface. Thiolate molecules adsorbed on gold desorb at 60100°C, which corresponds to the denaturation or degradation temperature of heat-sensitive materials (e.g. DNA, proteins, polymers, and lipid membranes). By observing the change in Raman signal from the thiolates on SERS substrate, it is possible to estimate the maximum power of excitation laser required for the samples to reach their critical temperatures for thermal degradation.
Keywords:Surface-enhanced Raman scattering (SERS) spectroscopy | Temperature sensor | Thiol moleculeSurface-enhanced Raman scattering (SERS) spectroscopy is a powerful nanospectroscopic method to characterize molecular behaviors on metallic particles or nanostructures with ultrahigh sensitivity.13 SERS spectroscopy enables low-destructive and label-free measurements, while maintaining high sensitivity at a single-molecule level.4,5 Thus, SERS has been widely applied to surface-sensitive analyses in various fields such as chemistry, biology, and medicine.
68A significant increase in the local temperature of metallic nanostructures is a critical issue in SERS measurements of heatsensitive materials. 911 The increase in temperature originates mainly from Joule heating induced by enhanced plasmonic electron movements. The increase in temperature depends on several factors (e.g. the degree of plasmonic resonance, power density of the excitation laser, thermal capacity, conductivity of the surrounding medium, etc.). A previous report suggested that the local temperature may increase up to several hundred degrees Celsius during the measurements.12 Therefore, great care should be taken in establishing the experimental conditions for the measurements of heat-sensitive materials. In particular, a precise evaluation of the local temperature of metallic nanostructures is critically important.There are mainly two approaches to evaluate the local temperature: the ratio of Stokes to anti-Stokes Raman intensities of samples 13,14 and the scattering spectra of the metal nanostructures.12 Although these methods are useful for estimating local temperature over a wide range, the accuracy of the temperature evaluated by these approaches is limited to «20°C. In the case of biomolecules, e.g. proteins and double-stranded DNA helices, the critical temperatures for their conformational changes (denaturation and unfolding) are ca. 60°C. 15,16 The range of temperatures, at which conformational changes occur, is within «5°C.15,16 Therefore, there is a high demand for a new approach to evaluate the local temperature with the accuracy of several degrees in Celsius.Herein, we propose a new method for establishing experimental conditions of the laser power density, to avoid thermal damage by monitoring SERS signal from thiols. Temperatures for the thiols to desorb (cleavag...