Quantitative Evaluation of the Cellular Uptake of Nanodiamonds by Monocytes and Macrophages

Fluorescent nanodiamonds (FNDs) with negative nitrogen-vacancy (NV⁻) defect centers are great probes for biosensing applications, with potential to act as biomarkers for cell differentiation. To explore this concept, uptake of FNDs (≈120 nm) by THP-1 monocytes and monocyte-derived M0-macrophages is studied. The time course analysis of FND uptake by monocytes confirms differing FND-cell interactions and a positive time-dependence. No effect on cell viability, proliferation, and differentiation potential into macrophages is observed, while cells saturated with FNDs, unload the FNDs completely by 25 cell divisions and subsequently take up a second dose effectively. FND uptake variations by THP-1 cells at early exposure-times indicate differing phagocytic capability. The cell fraction that exhibits relatively enhanced FND uptake is associated to a macrophage phenotype which derives from spontaneous monocyte differentiation. In accordance, chemical-differentiation of the THP-1 cells into M0-macrophages triggers increased and homogeneous FND uptake, depleting the fraction of cells that were non-responsive to FNDs. These observations imply that FND uptake allows for distinction between the two cell subtypes based on phagocytic capacity. Overall, FNDs demonstrate effective cell labeling of monocytes and macrophages, and are promising candidates for sensing biological processes that involve cell differentiation.