Fiber fluorescence photo thermometry during magnetic heating reveals directional alignment of suspended nanoparticles

We developed highly sensitive camera-based fiber fluorescence photometry to measure temperature changes of magnetically heated nanoparticles. Temperature changes as small as 20 milli Kelvin were resolved with a temporal resolution of 5 ms, using common fluorophores. Suspensions of various nanoparticles, including synthesized magnetite and core-shell nanoparticles, engineered magnetoferritin and purified magnetosomes, as well as intact magnetotactic bacteria were heated in alternating magnetic fields (AMF). Using prior calibration, nanoparticle surface temperature changes were calculated from fluorescence intensity changes of the attached dyes. From nanoparticle surface temperatures, we determined the limits of concentration scaling of suspension temperature rise. We captured sudden (sub 5 ms) fluorescence changes, corresponding to AMF driven reversible nanoparticle alignment. The orientation of the AMF field lines determines the direction of the relative fluorescence changes. The rate of aggregation is modulated by changing the medium viscosity and depends on field strength and nanoparticle size. Our findings shed light on the implications of Specific Loss Power measurements and the limits of local heat confinement around freely diffusing nanoparticles.