In Situ Measurement of Magnetization Relaxation of Internalized Nanoparticles in Live Cells

Magnetization relaxation mechanisms strongly influence how magnetic nanoparticles respond to high-frequency fields in applications such as magnetic hyperthermia. The dominant mechanism depends on the mobility of the particles, which will be affected in turn by their microenvironment. In this study AC susceptometry was used to follow the in situ magnetic response of model systems of blocked and superparamagnetic nanoparticles, following their cellular internalization and subsequent release by freezethaw lysis. The AC susceptibility signal from internalized particles in live cells showed only Neel relaxation, consistent with measurements of immobilized nanoparticle suspensions. However, Brownian relaxation was restored after cell lysis, indicating that the immobilization effect was reversible and that nanoparticle integrity was maintained in the cells. The results presented demonstrate that cellular internalization can disable Brownian relaxation, which has significant implications for designing suitable nanoparticles for intracellular hyperthermia applications. Further to this, the results highlight the possibility that particles could be released in reusable form from degrading cells following hyperthermia treatment, and subsequently reabsorbed by viable cells.