Fluorescent nanodiamonds as a robust temperature sensor inside a single cell

Abstract

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.

Keywords: cellular imaging; fluorescent nanodiamond; magnetic resonance; nitrogen-vacancy center; temperature sensing.

Figure 1

(a) Energy diagram of the NV⁻ spin sublevels in nanodiamonds. (b) Schematic representation of the experimental setup. A typical fluorescence image is shown (24 μm × 24 μm). (c) ODMR spectra of the FND marked by a yellow circle in (b), measured at 27.0, 32.0, and 37.0°C. Close-up spectra inside the black square are shown on the right.

Figure 2

(a) Illustration of our experimental conditions: FNDs and environmental factors. (b) ΔD/ΔT for the samples from at least five FNDs in each condition. Error bars indicate the standard deviations.

Figure 3

(a) Schematic illustration of temperature sensing inside a cell. (b) Merged bright field and fluorescence image of a HeLa cell before and after fixation. (c) ODMR spectrum of the FND in the live cell that are indicated by the circle in (b). The whole spectrum was obtained in 5 min and subsequently fitted to a sum of two Lorentzian functions. (d) D values from the FND sample in the fixed cell at 27.0, 32.0, and 37.0°C, with a calibration line obtained by linear fitting. From the calibration line, the temperature of the FND in the live cell was determined (dashed line), while the temperature of the medium was maintained at 32.0°C.