Significance of Optimization of Phospholipid Poly(Ethylene Glycol) Quantity for Coating Carbon Nanohorns to Achieve Low Cytotoxicity

In biological studies, phospholipid poly(ethylene glycol) (PLPEG) is used to disperse nanocarbons, which effectively reduces the macrophage uptake of nanocarbons. Single-walled carbon nanohorn (CNH), a type of nanocarbon, was dispersed with PLPEG in this study, and the effect of PLPEG quantity on the cytotoxicity of PLPEG-CNH in RAW264.7 mouse macrophages was revealed. The results were compared with the macrophage uptake quantity of PLPEG-CNH or agglomeration size of PLPEG-CNH. It is concluded that the optimum PLPEG:CNH weight ratio was 0.5:1 and 1:1. At the higher or lower ratios, the PLPEG-CNH toxicity and the macrophage uptake quantity increased and large agglomerates appeared. These tendencies at the lower PLPEG:CNH ratio were caused by the CNHs surfaces not being sufficiently well covered by PLPEG. The tendency at the higher PLPEG:CNH ratio was curious, for which we deduced that PLPEG adsorbed on CNHs was detached to form micelles, leading to the reappearance of the CNHs not well covered with PLPEG.