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 NV⁰ and NV⁻ colour centres. In particular, the 100 nm-sized features the more intense fluorescence spectra whose linear dependence with temperature achieved 0.98 R² data representation values for both NV⁰ and NV⁻. This model predicts temperature for all used nanodiamonds with sensitivities ranging from 5.73% °C⁻¹ to 6.994% °C⁻¹ (NV⁰) and from 4.14% °C⁻¹ to 6.475% °C⁻¹ (NV⁻). 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.

Nanothermometry is crucial for understanding physical, chemical, and biological systems, which require precise temperature measurement. Fluorescent nanodiamonds containing nitrogen-vacancy (NV) color centers offer an approach to temperature sensing. In this study, we present the spectrofluorometric characteristics of the NV zero-phonon lines (575 and 637 nm), in 100 nm nanodiamonds in aqueous volume ensembles at a concentration of 0.5 mg/ml, across the temperature range of 30–45 °C. The NV0 and NV− fluorescence intensities achieved high linear correlation values of 0.99 (INV0) and 0.94 (INV−), respectively, demonstrating their efficiency in high precision temperature assessment. Additionally, we explore NV0 width as temperature increases, NV populations intensity ratios, and INV0/INV− ratios to gain insights into thermal quenching phenomena in fluorescent nanodiamonds, where upon heating, an increasing trend for INV−/(INV0+INV−) is observed, while an antisymmetric effect takes place for INV0/(INV0+INV−). These findings indicate the potential of commercial nanodiamonds for precise all-optical fluorescence-based temperature sensing.