Reports have suggested that intracellular temperature can vary by ~1 °C depending on location, yet probes inserted inside the cytoplasm inevitably suffer from non‑thermal influences such as pH and ionic strength. To overcome this, we developed an “external” thermometry method using a ~50 nm poly(methyl methacrylate) film that co‑encapsulates a temperature‑dependent fluorophore (EuTTA) and a temperature‑independent reference dye (Rhodamine 101). Cultured cells are grown directly on this fluorescent thermometer nanosheet, and the temperature is self‑calibrated by the ratio of the two emissions, allowing disturbance‑resistant tracking of ~0.01 °C changes at tens‑of‑milliseconds resolution.
Key findings
Non‑excitable cells (HeLa, HEK293)
– The difference between cellular and medium temperatures was < 0.2 °C.
– Even chronic Ca²⁺ leak caused by an RYR1 mutation linked to malignant hyperthermia did not alter whole‑cell temperature.
Rapid Ca²⁺ release with ionomycin
– Local heating > 2 °C occurred near the endoplasmic‑reticulum membrane, rich in Ca²⁺ pumps, yet nanosheet‑detected surface temperature remained within ±0.2 °C, indicating that heat dissipates rapidly at the nanoscale.
Excitable cells (neonatal rat cardiomyocytes, hippocampal neurons)
– During 2 Hz beating or 0.25 Hz synaptic firing, global temperature fluctuations were only ±0.01 °C and ±0.03 °C, respectively.
Mitochondrial uncoupling with CCCP in brown adipocytes and cardiomyocytes generated heat internally, but no global temperature rise was detected by the nanosheet, confirming that metabolic heat remains highly localized at cellular and tissue scales.
These results experimentally support the theoretical prediction that living single cells are essentially isothermal as a whole, with metabolic heat appearing as spatial gradients on the order of 1 µm. Fluorescent thermometer nanosheets provide a platform for non‑invasive temperature mapping of organoids and dense cell sheets, and for analyzing local signals triggered by thermal stimuli—opening broad avenues in cellular thermodynamics.
Article information & citation
Kotaro Oyama, Mizuho Gotoh, Yuji Hosaka et al. “Single‑cell temperature mapping with fluorescent thermometer nanosheets.” Journal of General Physiology, 152 (8): e201912469, 2020.
https://doi.org/10.1085/jgp.201912469