Cellular uptake and phototoxicity of surface-modified fluorescent nanodiamonds

Bioconjugated fluorescent nanodiamonds (FNDs) show potential for effectively targeted imaging and the enhanced photokilling of cancer cells. In this study, we investigated the mechanisms involved in the cellular uptake of surface-modified 140 nm FNDs and evaluated their cytotoxicity and phototoxicity following particle internalization. Through an analysis of flow cytometry, the internalized FND-Tf and FND-NH ₂ particles reached their respective saturation values, with half-life times of approximately 0.8 h and 1.4 h. The determination indicated that the receptor-mediated endocytosis of FND-Tf particles was highly effective and nearly twice as efficient as the endocytic process of FND-NH ₂ particles mainly through surface electrostatic interactions. The cytotoxicity of internalized particles was evaluated using the MTT assay, indicating that at saturation concentrations, the rate of proliferation of FND-treated cells decreases to less than half that of untreated cells. An examination of the phototoxicity of internalized FND-Tf particles under irradiation using a 532 nm laser revealed that the FND-treated cells could be killed using less than half the energy required for untreated cells. The enhanced cellular killing was attributed to FND particles converting light to thermal energy, with the support of Raman spectra of nanodiamond clusters that showed an increase in particle temperature under irradiation. Our results demonstrated that bioconjugated FNDs could be highly effective in the targeting of cancer cells for fluorescence imaging and photokilling with a minimum of collateral cell damage.