Radiative Relaxation of Gold Nanorods Coated with Mesoporous Silica with Different Porosities upon Nanosecond Photoexcitation Monitored by Time-Resolved Infrared Emission Spectroscopy.

Gold nanorods (AuNRs) have been widely used in photothermal conversion, and a coating of silica (SiO 2 ) provides higher thermal stability, better biocompatibility, and versatile chemical functionalization. In this work, two gold nanorods coated with surfactant-templated mesoporous silica layers of the same thickness but different porosities, and thus different specific surface areas, were prepared. Upon irradiation with 1064 nm nanosecond pulsed laser, the transient infrared emissions of AuNR@SiO 2 enveloped the stretching mode of the Si-O-Si bridge (1000-1250 cm -1 ), the bending mode of adsorbed H 2 O (1600-1650 cm -1 ) within the mesoporous silica layer, and blackbody radiation, in terms of an underlying broad band (1000-2000 cm -1 ) probed with a step-scan Fourier transform spectrometer. The mesoporous silica shell and the adsorbed H 2 O gained populations of their vibrationally excited states, and the whole AuNR@SiO 2 was heated up via the photothermal energy of the core AuNRs. An average temperature after 5-10 μs within 80% of the emission intensity was ca. 200 °C. The decay of the emission at 1000-1250 and 1500-1750 cm -1 was both accelerated, and the blackbody radiation components were negatively correlated with the porosity of the mesoporous silica layer. Higher porosity of the mesoporous silica layer was associated with more effective depopulation of the vibrationally excited states of the silica layers on the AuNRs via the nonradiative thermal conduction of the adsorbed H 2 O, since H 2 O has a larger thermal conduction coefficient than that of silica, in concomitance with the accelerated emission kinetics. This work unveils the roles of the porosity, capping materials, and entrapping molecules of a core-shell nanostructure during the thermalization after photoexcitation.