In Vitro Immune Activation of Raw264.7 Cells by Nanodiamonds with Different Surface Chemistry

In this study, three commercially available nanodiamonds (NDs) of similar size (about 5 nm in diameter) but with different surface chemistry (-COOH, -NH2 and -poly(ethylene glycol)) were used to study the toxicity and immune activation of Raw264.7 cells. Only ND-PEG is well dispersed in water and cell culture medium (about 10 nm in diameter), while ND-COOH and ND-NH2 only showed some aggregation, about 50 nm in water and about 100 nm in cell culture medium. The three NDs showed different zeta potentials in water but the difference disappeared in cell culture medium due to the surface adsorption of the proteins. The ND-PEG did not show any obvious signs of cytotoxicity or activation on Raw264.7 cells under tested conditions. The ND-COOH and ND-NH2 caused dose-dependent toxicity, with the amino group capped NDs being much more toxic compare to those of the carboxylic acid group at the same exposure conditions. ND-COOH induced a certain immune response in Raw264.7 cells, leading to a higher expression of tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6), especially with a significantly longer incubation time. ND-NH2 also induced significant expression of TNF-α and IL-6 in Raw264.7 cells at first, but this expression was reduced with a longer incubation time due to cell death.