Local Enhancement of Lipid Membrane Permeability Induced by Irradiated Gold Nanoparticles

Photothermal therapies are based on the optical excitation of plasmonic nanoparticles in the biological environment. The effects of the irradiation on the biological medium depend critically on the heat transfer process at the nanoparticle interface, on the temperature reached by the tissues, as well as on the spatial extent of temperature gradients. Unfortunately, both the temperature and its biological effects are difficult to be probed experimentally at the molecular scale. Here, we approach this problem using nonequilibrium molecular dynamics simulations. We focus on photoporation, a photothermal application based on the irradiation of gold nanoparticles by single, short-duration laser pulses. The nanoparticles, stably bound to cell membranes, convert the radiation into heat, inducing transient changes of membrane permeability. We make a quantitative prediction of the temperature gradient around the nanoparticle upon irradiation by typical experimental laser fluences. Water permeability is locally enhanced around the nanoparticle, in an annular region that extends only a few nanometers from the nanoparticle interface. We correlate the local enhancement of permeability at the nanoparticle-lipid interface to the temperature inhomogeneities of the membrane and to the consequent availability of free volume pockets within the membrane core.