Direct Probing of Fe3O4 Nanoparticle Surface Temperatures during Magnetic Heating: Implications for Induction Catalysis

Abstract

The presence of a temperature gradient between the magnetic nanoparticle (NP) surface and the bulk medium during induction heating has been observed across several fields. While no noticeable increase in bulk temperature is observed, biological (DNA denaturation, tumor apoptosis) and chemical (bond cleavage) evidence indicates high temperatures near/at the NP surface. Unfortunately, current methods for temperature probing rely on bulk temperature measurements (fiber-optic IR probes) or are limited by thermal stability and spatial resolution (organic molecules). To further the understanding of magnetic heating as a driving force in catalysis, as well as drug delivery/hyperthermia treatments, a more accurate description of the nanoparticle surface temperature is needed. This work uses inorganic luminescent probes in direct contact with the particle surface, entailing the deposition of YVO4:Eu3+ around a Fe3O4|SiO2 structure, to measure the local temperature. The luminescent response is calibrated in situ via a controlled temperature stage to extract the field-dependent heating. The luminescent probe results in a high spatial resolution (<5.5 nm) with temperatures up to 64 °C higher than standard fiber-optic probes. The direct contact between the photoluminescence (PL) probe and Fe3O4 allows for ballistic transport and improved temporal resolution, mimicking an adiabatic system (negligible long-range heat dissipation). Other advantages include avoiding measurements in liquid media, where the distance between the heat source and the probe cannot be controlled, adding to the uncertainty of the temperature measurement due to changes in colloidal anisotropy (which changes with the heating profile) of the magnetic cores and surface quenching of the luminescent signal.