Local Temperature Evolution during Nanoparticle Hyperthermia Probed by Fluorescence Thermometry

Abstract

Here we investigate the spatial and temporal evolution of temperature around heating nanoparticles held in varies spatial geometries. We compare isolated nanoparticles to ones aligned along an axis inside magnetotactic bacteria, to cell-bound ones, whereby the density is varied to change the particle-particle interaction. The nanoparticles or the cell membrane are labeled with fluorophores whose fluorescence intensity and lifetime are reduced with increasing temperature (TAMRA or Dylight for the particles, GFP tagged proteins for the cell). We use superparamagnetic nanoparticles which were heated by radio-frequency magnetic fields in a custom built set-up permitting simultaneous fluorescence imaging. We studied the differences in the nature of heat dissipation between particles bound to cell membranes and particles in suspension, undergoing Brownian motion. A comparative study of the geometric effects arising from particle arrangement and placement within the cells have also been studied. We find local heating to be strongly dependent on geometric particle arrangement, as well as on the relaxation mechanism of the nanoparticles, Brownian or Neel relaxation. To aid the understanding of the theoretical aspects of nanoscale heat transfer we incorporate simulations of the nanoparticle heating mechanism and heat dissipation in our study. Together, these results provide insights on efficient experimental designs for biomedical application of nanoparticle heating in-vivo.