  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 NP, in an annular
region that extends only a few nm far from the nanoparticle interface. We
correlate the local enhancement of permeability at the NP-lipid interface to
the temperature inhomogeneities of the membrane and to the consequent
availability of free volume pockets within the membrane core.
