Origin of additional factors measured by fluorescent nanothermometers in living cells

Determine the origin and physical mechanism of the additional factors measured by fluorescent nanothermometers in living cells that are distinct from temperature defined under local thermal equilibrium, as evidenced by seconds-scale signal components that do not follow thermal conduction and appear alongside the rapid temperature response during controlled intracellular heating.

Background

Fluorescent nanothermometry has reported pronounced intracellular temperature inhomogeneities, leading to controversy with heat conduction predictions in aqueous-like intracellular environments. Using label-free mid-infrared photothermal optical diffraction tomography (MIP-ODT), the authors measure intracellular thermal diffusivity close to that of water, contradicting the hypothesis of significantly slowed intracellular conduction.

Direct comparison between MIP-ODT and a fluorescent polymeric thermometer (FPT) under identical conditions shows that fluorescent nanothermometers exhibit not only an instantaneous, step-like temperature change consistent with rapid thermal diffusion but also an additional slowly varying signal over seconds. This slow component does not match expectations from thermal conduction and suggests sensitivity to factors beyond temperature under local thermal equilibrium, prompting an explicit open question regarding their origin.