Phase-Transition Temperature of Gold-Nanorod-Coated Nanodroplets to Microbubbles by Pulsed Laser

Previously, gold-nanorod-coated perfluorocarbon nanodroplets have been developed as light-activated on-demand drug delivery carriers. When gold nanorods on the perfluorocarbon nanodroplets resonate with a laser wavelength, plasmonic heat is generated and vaporizes the nanodroplets to gas bubbles. Optimal laser parameters such as pulse duration, pulse repetition frequency, and average power are critical to effectively trigger the phase transition of nanodroplets and allow for drug release. This study focused on determining the temperature of a gold-nanorod-coated perfluorocarbon nanodroplet during phase transition to a gas bubble using a femtosecond laser. Two integrated experimental and theoretical methods were explored. First, the theoretical temperature was determined by the Arrhenius equation and the time it took for the phase transition to occur, assuming the phase-transition process followed a first-order kinetic model. The activation energy and Arrhenius constant of the phase-transition process were obtained via light transmittance through a nanodroplet suspension at different temperatures. The time required for phase transition by a femtosecond laser was measured using an optical microscope. The second approach used a classical heat diffusion model. When the pulse peak energy was considered in the model, the temperature predicted matched the experimental observation of phase-transition temperature threshold, while the total energy value failed to predict the temperature threshold. The results suggest that the phase-transition mechanism is triggered by the vaporization of the nanodroplets via photothermal heating, which is influenced by the peak energy of the laser. It also indicates that optimal laser parameters can be determined by a simple calculation using the classical heat diffusion model and peak energy to control phase transition.