| Summary: | Nanodiamonds have been studied for several biomedical applications due to their inherent biocompatibility and low cytotoxicity. Recent investigations have shown perspectives in using fluorescent nanodiamonds as nanothermometers because of their optical properties’ dependence on temperature. Easy and accurate localized temperature sensing is essential in a wide variety of scientific fields. Our work demonstrated how the fluorescence spectrum of high-pressure high-temperature fluorescent nanodiamonds of three different sizes: 35 nm, 70 nm and 100 nm, changes with temperature within an important biological temperature range (25 °C to 60 °C). Taking advantage of this phenomenon, we obtained nanothermic scales (NS) from the zero phonon lines (ZPL) of the NV<sup>0</sup> and NV<sup>−</sup> colour centres. In particular, the 100 nm-sized features the more intense fluorescence spectra whose linear dependence with temperature achieved 0.98 R<sup>2</sup> data representation values for both NV<sup>0</sup> and NV<sup>−</sup>. This model predicts temperature for all used nanodiamonds with sensitivities ranging from 5.73% °C<sup>−1</sup> to 6.994% °C<sup>−1</sup> (NV<sup>0</sup>) and from 4.14% °C<sup>−1</sup> to 6.475% °C<sup>−1</sup> (NV<sup>−</sup>). Furthermore, the non-cytotoxic interaction with HeLa cells tested in our study enables the potential use of fluorescence nanodiamonds to measure temperatures in similar nano and microcellular aqueous environments with a simple spectroscopic setup.
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