Understanding heat dissipation processes at nanoscale during cellular thermogenesis is essential to clarify the relationships between the heat and biological processes in cells and organisms. A key parameter determining the heat flux inside a cell is the local thermal conductivity, a factor poorly investigated both experimentally and theoretically. Here, using a nanoheater/nanothermometer hybrid made of a polydopamine encapsulating a fluorescent nanodiamond, we measured the intracellular thermal conductivities of HeLa and MCF-7 cells with a spatial resolution of about 200 nm. The mean values determined in these two cell lines are both 0.11 ± 0.04 W m−1 K−1, which is significantly smaller than that of water. Bayesian analysis of the data suggests there is a variation of the thermal conductivity within a cell. These results make the biological impact of transient temperature spikes in a cell much more feasible, and suggest that cells may use heat flux for short-distance thermal signaling.

Measuring temperature at or far from equilibrium at the nanoscale is important in many fields of science and engineering. A variety of luminescent nanothermometers have been developed in the past decade for the measurements. Fluorescent nanodiamonds (FNDs) stand out from the rest in terms of biological use, because the nanomaterials contain negatively charged nitrogen-vacancy (NV À ) centers as photostable fluorophores and possess a number of remarkable properties including chemical inertness, negligible toxicity, versatile surface modification ability and, most importantly, exceptional temperature-measurement precision. However, to enable practical applications of FNDs for temperature sensing in biological systems, conjugation of the particles with polymers, biomolecules, and/or other nanomaterials are often required. Gold/diamond nanohybrids are one of these combinations that enhance the temperature measurement versatility of the NV À centers. Here, we provide a review of the recent advances in the research and development of diamond nanothermometry, with special focus on the synthesis, characterization, and applications of gold/ diamond nanohybrids. Current challenges and future perspectives of the FND-based nanothermometry are also discussed.

Thermometers play an important role to study the biological significance of temperature. Fluorescent nanodiamonds (FNDs) with negatively-charged nitrogen-vacancy centers, a novel type of fluorescence-based temperature sensor, have physicochemical inertness, low cytotoxicity, extremely stable fluorescence, and unique magneto-optical properties that allow us to measure the temperature at the nanoscale level inside single cells. Here, we demonstrate that the thermosensing ability of FNDs is hardly influenced by environmental factors, such as pH, ion concentration, viscosity, molecular interaction, and organic solvent. This robustness renders FNDs reliable thermometers even under complex biological cellular environment. Moreover, the simple protocol developed here for measuring the absolute temperature inside a single cell using a single FND enables successful temperature measurement in a cell with an accuracy better than ±1°C.