📝 SummarXiv

Abstract

Fluorescent nanothermometry has revealed pronounced inhomogeneous temperature distributions within cells, establishing the field of single-cell thermal biology. However, this finding has sparked a controversial discussion known as the "10^5 gap issue", stemming from a simple heat conduction calculation suggesting such large temperature distributions should not exist within cells. Here, we address this issue using label-free mid-infrared photothermal microscopy, which enables measurement of heat-induced temperature variations under local thermal equilibrium via micrometer-scale refractive index changes. First, we measured intracellular thermal diffusivity via transient thermal decay and determined that thermal diffusivity within living cells is 93-94% of water in the cytoplasm and nucleus. This result contradicts the hypothesis that intracellular thermal conduction is considerably slower than in water. Next, we compared fluorescent nanothermometry with our label-free microthermometry by measuring heat-induced temperature variations in living cells and found that the fluorescent nanothermometry exhibited a slowly varying signal in addition to a rapidly responding temperature change. This result clearly verifies that fluorescent nanothermometers are sensitive to an additional nonconductive factor beyond the temperature defined under local thermal equilibrium, exhibiting behavior inconsistent with thermal conduction. Our comparative measurements using fluorescent nanothermometry and label-free microthermometry suggest that this non-conductive factor may originate from slowly varying nanoscale dynamics within cells. Consequently, this work clarifies that the "10^5 gap issue" arises from comparing two fundamentally different physical quantities: the temperature defined under local thermal equilibrium and the non-conductive factor detected by fluorescent nanothermometers.