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The labeling of cells with fluorescent nanoparticles is promising for various biomedical applications. The objective of this study is to evaluate the biocompatibility and the mechanism of the cellular uptake of fluorescent nanodiamonds (FNDs) in cancer cells (HeLa) and pre-adipocytes (3T3-L1). With flow cytometry and the use of a battery of metabolic and cytoskeletal inhibitors, we found that the mechanism of the FND uptake in both cells is by energy-dependent clathrin-mediated endocytosis. In addition, the surface charge of FND influences its cellular uptake, as the uptake of poly-L-lysine-coated FNDs is better than that of oxidative-acid-purified FNDs at the same concentration in regular medium with or without serum. We also confirm that the proliferative potential of FND-treated and untreated cells does not exhibit any significant differences when measured at bulk cultures, and more stringently at clonal cell density. Further biocompatibility studies indicate that the in vitro differentiation of 3T3-L1 pre-adipocytes and 489-2 osteoprogenitors is not affected by the FND treatment. Our results show that FNDs are biocompatible and ideal candidates for potential applications in human stem cell research.

Fluorescent nanodiamond (FND) has excellent biocompatibility and photostability, making it well suited for long-term labeling and tracking of cancer and stem cells. To prove the concept, the exocytosis of FND particles (size ≈100 nm) from three cell lines--HeLa cervical cancer cells, 3T3-L1 pre-adipocytes, and 489-2.1 multipotent stromal cells--is studied in detail. FND labeling is performed by incubating the cells in a serum-free medium containing 80 μg mL(-1) FND for 4 h. No significant alteration in growth or proliferation of the FND-labeled cells, including the multipotent stromal cells, is observed for up to 8 days. Flow cytometric analysis, in combination with parallel cell doubling-time measurements, indicates that there is little (≈15% or less) excretion of the endocytosed FND particles after 6 days of labeling for both HeLa and 489-2.1 cells, but exocytosis occurs more readily (up to 30%) for 3T3-L1 preadipocytes. A comparative experiment with FND and the widely used dye, carboxyfluorescein diacetate succinimidyl ester, demonstrates that the nanoparticle platform is a promising alternate probe for long-term cell labeling and tracking applications.

In recent years, carbon and carbon-based nanomaterials have received increasing attention for applications in life sciences. Nanodiamond (ND) stands out as a unique new substance in these applications because it holds several momentous properties such as good biocompatibility, excellent photostability and facile surface functionalizability. A number of experiments have shown that ND has the highest biocompatibility of all carbon-based nanomaterials including carbon blacks, multiwalled nanotubes, single-walled nanotubes and fullerenes. Additionally, the surface of ND can be readily derivatized with various functional groups for either covalent or noncovalent conjugation with biomolecules. Furthermore, some radiation-damaged NDs can emit strong and stable photoluminescence (red or green) from nitrogen-vacancy defect centers embedded in the crystal lattice. These properties together make ND a highly promising nanomaterial for both in vitro and in vivo applications.

ND particles possess a unique set of properties attractive for drug delivery applications, including exceptional biocompatibility, large carrier capacity and versatile surface chemistry properties, which enhance drug binding and provide sustainable drug release. Other unique attributes of NDs embrace bright stable fluorescence based on crystallographic defects. A roadmap toward a clinical translation comprises identification of ND-therapeutic compounds that display marked improvements over clinical standards with respects to efficacy, safety and cost.

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