Ultrafast Thermometry of Gold Nanoparticles: Resolving Particle and Medium Temperature Dynamics via Transient Absorption Spectroscopy.

Metal nanoparticles and their environment can be locally heated on an ultrafast time scale using femtosecond pulsed illumination of their plasmon resonance, making them of interest for spatiotemporal temperature control. Here, we propose experimental approaches to obtain time-resolved particle and medium temperatures using gold nanoparticles. 23.5 and 39 nm nanoparticles dispersed in water and DMF:water mixture were heated and probed using transient absorption spectroscopy. Simulations indicate that the change in absorbance >10 ps after excitation arises from temperature-induced alterations in the dielectric functions of the particle and the medium. Thus, we measured the temperature-dependent absorbance spectra of nanoparticles, where the signal reflects the combined response of the particle and the medium to heating for a known temperature. We then disentangled the spectra obtaining the particle (Method 1) and the medium contributions (Method 2) to heating independently, followed by a consistency check between the two approaches (Method 3). Accordingly, the transient absorbance spectrum was resolved to extract particle and medium temperatures at each time delay. The resulting profiles are in line with each other, revealing temperature increases of ∼80 K for the particle and 5-15 K for the medium when excited at 400 nm with ∼4 J/m2 fluence. A faster particle temperature decay was observed with decreasing particle size and a faster medium temperature decay with increasing medium thermal diffusivity, in agreement with expectations. Overall, we demonstrate an experimental methodology for simultaneous determination of particle and medium temperatures under a spatiotemporal gradient which is relevant for studies with transient heating and nanoparticles as sensors.