Influence of cell phenotype and surface modifications on the cytotoxicity of fluorescent nanodiamonds

Fluorescent nanodiamonds (FNDs) have garnered considerable attention in biomedical fields owing to their unique physicochemical properties, including high photostability and biocompatibility. Nevertheless, concerns persist regarding their potential cytotoxicity both in pristine form and upon surface modification. In this study, we systematically investigated the cytotoxicity of FNDs regarding cell phenotype and surface modification. Two surface-modified FNDs were synthesized: mesoporous polydopamine encapsulated FNDs (FND@mPDA) and polyethylene glycol functionalized FND@mPDA (FND@mPDA-PEG). These types along with bare FNDs were applied to two cell phenotype groups, namely non-epithelial cells (A10 and HDFn) and epithelial cells (A549, HeLa, and HaCaT). All three types of FNDs exhibited substantial cytotoxicity in the non-epithelial cells, accompanied by elevated levels of reactive oxygen species. Moreover, precise 3D localization of red fluorescence allowed for quantitative estimations of the FND uptake levels in A549 and A10 cells. We observed that A10 cells internalized bare FNDs more effectively, while the surface modifications (FND@mPDA and FND@mPDA-PEG) further enhanced cellular uptake levels in A10 cells. Interestingly, a statistically significant linear correlation was observed between cell viability and uptake level in A10. Finally, we confirmed that all FND types were internalized via two pathways: clathrin-mediated endocytosis and macropinocytosis. These findings emphasize that the biocompatibility of FNDs crucially depends on cellular uptake, which is strongly affected by both cell phenotype and surface modification. This study provides valuable insights for exploring potential biomedical applications of surface-modified nanodiamonds. • Fluorescent nanodiamonds (FNDs) functionalized with mPDA/PEG for efficient cell delivery. • The cytotoxicity of FNDs in non-epithelial cells increased by reactive oxygen species. • Surface-modified FNDs exhibited increased uptake in non-epithelial cells. • Correlation of uptake/cytotoxicity confirmed by visualizing intrinsic FND fluorescence. • Internalization in non-epithelial cells via clathrin-mediated endocytosis and macropinocytosis.