A practical difficulty in assessing the properties of ensembles of NDs is the large variability of the optical response in heterogeneous samples . This has been ascribed to inter-aggregate interactions, size distribution and different efficiency in NV center initialization in ND aggregates . To circumvent this problem, we resorted to use a wide-field fluorescence microscope to spatially resolve ODMR spectra in different parts of the sample. The heterogeneity in ND deposition is apparent in Fig. , where FL images clearly inhomogeneous aggregation and concentration of NDs. ODMR contrast in these samples depends on the selected ROIs, characterized by different levels of FL [Inset Fig. ], with lower variability observed in the 156 nm NDs compared to the 48 nm NDs. Conversely, the ODMR contrast from the bulk diamond is uniform throughout the image. Despite region and sample dependent contrast, our data show consistent trends for NDs in different samples and ROIs, thus suggesting that the effect is robust and reproducible. Charge dynamics and spin properties of NVs also depend on the ND microenvironment. To explore the effects described above in a typical bioassay, we incubated the NDs in cell cultures of macrophages (RAW 264.7). Internalization in macrophages is described in the Methods and illustrated in Fig. . The composite figures on the right show the NDs (orange) internalized in the cells, together with the actine filaments (green) and the nuclei (blue), for both 156 nm NDs (top row) and 48 nm NDs (bottom row) The concentration of the 156 nm NDs in liquid is much higher than that of the 48 nm NDs, resulting in a greater internalization of the larger NDs. Thus, only the results of 156 nm NDs are reported in the following section.