Measurements of extreme orientation-dependent temperature increase around an irradiated gold nanorod

When irradiated at its resonance frequency, a metallic nanoparticle efficiently converts the absorbed energy into heat which is locally dissipated. This effect can be used in photothermal treatments, e.g., of cancer cells. However, to fully exploit the functionality of metallic nanoparticles as nanoscopic heat transducers, it is essential to know how the photothermal efficiency depends on parameters like size and shape. Here we present the measurements of the temperature profile around single irradiated gold nanorods and nanospheres placed on a biologically relevant matrix, a lipid bilayer. [1] We developed a novel assay based on molecular partitioning between two coexisting phases, the gel and fluid phase, within the bilayer. [2, 3] This assay allows for a direct measurement of local temperature gradients, an assay which does not necessitate any pre-assumptions about this system and is generally applicable to any irradiated nanoparticle system. The nanorods are irradiated with a tightly focused laser beam at a wavelength of 1064 nm where biological matter exhibits a minimum in absorption. By controlling the polarization of the laser light we show that the absorption of light by the nanorod and the corresponding dissipated heat strongly depends on the orientation of the nanorod with respect to the polarization. Finally, by comparing to spherical gold nanoparticles, we demonstrate how a change in shape, from spherical to rod like, leads to a dramatic enhancement of heating when using near infrared light.