The paper presents a novel approach to nanoscale thermometry using nitrogen-vacancy (NV) color centers in diamond. The authors demonstrate the ability to detect temperature variations down to 1.8 mK with a sensitivity of 9 mK/√Hz in an ultra-pure bulk diamond sample. By using NV centers in diamond nanocrystals (nano-diamonds), they achieve direct measurement of local thermal environments at lengthscales down to 200 nm. The method is further validated by introducing both nano-diamonds and gold nanoparticles into a human embryonic fibroblast, enabling temperature-gradient control and mapping at the sub-cellular level. This technique offers high sensitivity, bio-compatibility, and a wide temperature range (200 – 600 K), making it a powerful tool for biological research, including temperature-induced control of gene expression and cell-selective treatment of diseases. The authors also discuss the potential applications of this method in optimizing NP-based photothermal therapies and monitoring chemical reactions at the microscopic level.The paper presents a novel approach to nanoscale thermometry using nitrogen-vacancy (NV) color centers in diamond. The authors demonstrate the ability to detect temperature variations down to 1.8 mK with a sensitivity of 9 mK/√Hz in an ultra-pure bulk diamond sample. By using NV centers in diamond nanocrystals (nano-diamonds), they achieve direct measurement of local thermal environments at lengthscales down to 200 nm. The method is further validated by introducing both nano-diamonds and gold nanoparticles into a human embryonic fibroblast, enabling temperature-gradient control and mapping at the sub-cellular level. This technique offers high sensitivity, bio-compatibility, and a wide temperature range (200 – 600 K), making it a powerful tool for biological research, including temperature-induced control of gene expression and cell-selective treatment of diseases. The authors also discuss the potential applications of this method in optimizing NP-based photothermal therapies and monitoring chemical reactions at the microscopic level.