Cellular Thermometry Considerations for Probing Biochemical Pathways

Local Temperature Increments and Induced Cell Death in Intracellular Magnetic Hyperthermia

The generation of temperature gradients on nanoparticles heated externally by a magnetic field is crucially important in magnetic hyperthermia therapy. But the intrinsic low heating power of magnetic nanoparticles, at the conditions allowed for human use, is a limitation that restricts the general implementation of the technique. A promising alternative is local intracellular hyperthermia, whereby cell death (by apoptosis, necroptosis, or other mechanisms) is attained by small amounts of heat generated at thermosensitive intracellular sites. However, the few experiments conducted on the temperature determination of magnetic nanoparticles have found temperature increments that are much higher than the theoretical predictions, thus supporting the local hyperthermia hypothesis. Reliable intracellular temperature measurements are needed to get an accurate picture and resolve the discrepancy. In this paper, we report the real-time variation of the local temperature on γ-Fe₂O₃ magnetic nanoheaters using a Sm³⁺/Eu³⁺ ratiometric luminescent thermometer located on its surface during exposure to an external alternating magnetic field. We measure maximum temperature increments of 8 °C on the surface of the nanoheaters without any appreciable temperature increase on the cell membrane. Even with magnetic fields whose frequency and intensity are still well within health safety limits, these local temperature increments are sufficient to produce a small but noticeable cell death, which is enhanced considerably as the magnetic field intensity is increased to the maximum level tolerated for human use, consequently demonstrating the feasibility of local hyperthermia.

Challenges for optical nanothermometry in biological environments

Temperature monitoring is useful in medical diagnosis, and essential during hyperthermia treatments to avoid undesired cytotoxic effects. Aiming to control heating doses, different temperature monitoring strategies have been developed, largely based on luminescent materials, a.k.a. nanothermometers. However, for such nanothermometers to work, both excitation and emission light beams must travel through tissue, making its optical properties a relevant aspect to be considered during the measurements. In complex tissues, heterogeneity, and real-time alterations as a result of therapeutic treatment may have an effect on light-tissue interaction, hindering accuracy in the thermal reading. In this Tutorial Review we discuss various methods in which nanothermometers can be used for temperature sensing within heterogeneous environments. We discuss recent developments in optical (nano)thermometry, focusing on the incorporation of luminescent nanoparticles into complex in vitro and in vivo models. Methods formulated to avoid thermal misreading are also discussed, considering their respective advantages and drawbacks.