Size Effects in Gold Nanorod Light-to-Heat Conversion under Femtosecond Illumination

The relative contributions of absorption vs scattering phenomena in plasmonic nanoparticles are pivotal to optimizing light-to-heat conversion efficiency, which is of particular interest in plasmonic photothermal therapy and targeted destruction of pathogenic cells, among other applications. The present study focuses on an explicit comparison of light-to-heat conversion efficiency in gold nanorods of identical aspect ratios but four different volumes, based on tunable ultrafast laser excitation, diffuse reflectance spectroscopy, and finite element simulations. Systematic analysis of photothermal properties under low-intensity femtosecond illumination reveals that larger-volume gold nanorods in colloidal solution have comparable performance to much smaller rods in overall photothermal conversion efficiency at identical optical density but behave quite differently from expectations based on diffuse reflectance spectroscopy. In addition, although the smallest rods exhibit equivalent photothermal conversion efficiencies to rods 10 times larger in volume, the temperature increase per rod is 4 times lower. These results show that the larger gold nanorods with high scattering still possess strong photothermal capabilities, which rival that of smaller rods on an ensemble level, and surpass small rods in both single-particle temperature increases and volume-normalized extinction.