This study presents a novel approach to nanoscale thermometry using nitrogen-vacancy (NV) color centers in diamond. The method enables temperature detection with high sensitivity and spatial resolution, down to 1.8 mK and 200 nm, respectively. By integrating nanodiamonds and gold nanoparticles into a single human embryonic fibroblast, the researchers demonstrate temperature-gradient control and mapping at the sub-cellular level, opening new possibilities for life sciences applications. The NV-based thermometry relies on the temperature-dependent lattice strain of diamond, which is reflected in the spin properties of the NV centers. The zero-field splitting of the NV spin states is strongly temperature-dependent, enabling fluorescence-based thermometry via precision electron spin resonance spectroscopy. The sensitivity of the sensor is determined by the NV spin coherence time, integration time, and a correction factor for imperfect readout and initialization. The researchers demonstrate the high spatial resolution of NV-based thermometry by using diamond nanocrystals (nanodiamonds). This allows for the measurement of temperature at nanometer scales, with the ability to detect temperature changes as small as 44 mK. The method is also compatible with living cells, as shown by the successful introduction of nanodiamonds and gold nanoparticles into human embryonic fibroblast cells. The study also demonstrates the ability to control and monitor temperature at the nanoscale using a laser-heated gold nanoparticle. The temperature change is measured with high accuracy, and the results are in excellent agreement with theoretical predictions based on the heat equation. The method is further validated by showing that the temperature change is localized to the gold nanoparticle, confirming the method's ability to detect local heating. The experiments show that the quantum spin of NV centers in diamond can be used as a robust temperature sensor with sub-micron spatial resolution, sub-degree thermal sensitivity, and bio-compatibility. The sensitivity of the current measurement can be enhanced by improving the NV coherence time and increasing the number of NV centers within the nanocrystal. The ultimate sensitivity is estimated to be 80 μK/√Hz, allowing for the sensing of sub-kelvin temperature variations with millisecond time resolution. The method is also applicable in solution, where the ultimate accuracy is limited by residual heating during the measurement process. The spatial resolution can be improved by using far-field sub-diffraction techniques. The potential applications of this method include the real-time measurement and control of sub-cellular thermal gradients, the monitoring and control of chemical reactions, and the identification and killing of malignant cells without damaging surrounding tissue. The study highlights the potential of NV-based thermometry as a powerful tool for biological and chemical temperature sensing.This study presents a novel approach to nanoscale thermometry using nitrogen-vacancy (NV) color centers in diamond. The method enables temperature detection with high sensitivity and spatial resolution, down to 1.8 mK and 200 nm, respectively. By integrating nanodiamonds and gold nanoparticles into a single human embryonic fibroblast, the researchers demonstrate temperature-gradient control and mapping at the sub-cellular level, opening new possibilities for life sciences applications. The NV-based thermometry relies on the temperature-dependent lattice strain of diamond, which is reflected in the spin properties of the NV centers. The zero-field splitting of the NV spin states is strongly temperature-dependent, enabling fluorescence-based thermometry via precision electron spin resonance spectroscopy. The sensitivity of the sensor is determined by the NV spin coherence time, integration time, and a correction factor for imperfect readout and initialization. The researchers demonstrate the high spatial resolution of NV-based thermometry by using diamond nanocrystals (nanodiamonds). This allows for the measurement of temperature at nanometer scales, with the ability to detect temperature changes as small as 44 mK. The method is also compatible with living cells, as shown by the successful introduction of nanodiamonds and gold nanoparticles into human embryonic fibroblast cells. The study also demonstrates the ability to control and monitor temperature at the nanoscale using a laser-heated gold nanoparticle. The temperature change is measured with high accuracy, and the results are in excellent agreement with theoretical predictions based on the heat equation. The method is further validated by showing that the temperature change is localized to the gold nanoparticle, confirming the method's ability to detect local heating. The experiments show that the quantum spin of NV centers in diamond can be used as a robust temperature sensor with sub-micron spatial resolution, sub-degree thermal sensitivity, and bio-compatibility. The sensitivity of the current measurement can be enhanced by improving the NV coherence time and increasing the number of NV centers within the nanocrystal. The ultimate sensitivity is estimated to be 80 μK/√Hz, allowing for the sensing of sub-kelvin temperature variations with millisecond time resolution. The method is also applicable in solution, where the ultimate accuracy is limited by residual heating during the measurement process. The spatial resolution can be improved by using far-field sub-diffraction techniques. The potential applications of this method include the real-time measurement and control of sub-cellular thermal gradients, the monitoring and control of chemical reactions, and the identification and killing of malignant cells without damaging surrounding tissue. The study highlights the potential of NV-based thermometry as a powerful tool for biological and chemical temperature sensing.