Fluorescence images were taken to evaluate the different amount of internalization of NDs in different cells. To this end, we extracted the ODMR contrast from ROIs of different size and location [Fig. . Representative examples of ODMRs curves extracted from single-cell ROIs (~6x6 µm) or a cell-aggregates ROI (from ~40x40 µm to ~100x100 µm) are shown in Fig. and Fig. , respectively. Despite some line broadening, compared to the bare NDs, the NV -central bands and 13 C sidebands can still be resolved. As for the previous samples, when increasing the laser power, the linewidth of the resonances decreases with the laser power, while the SNR increases, therefore improving the resolution of the NV -strain split doublet. Also in this case, we do not observe any variation depending on the temperature (i.e., no shift of central resonances), despite a possible MW absorption from the water in the cells or in the biological environment. Fig. shows the evolution of ODMR contrast with laser power for different ROIs. While qualitatively similar, the ODMR contrast dependence on laser power is more variable than the one observed in the bare NDs of Fig. , ranging from a small reduction at the highest laser power, to a plateau-like behavior or even a slight increase. We speculate that this wider heterogeneity may be due to differences in the microenvironment, and particularly to the different pH of various cellular compartments (e.g., pH≈5 in lysosomes compared to pH≈7.2 in the cytoplasm).