Quantitation of Cell-associated Carbon Nanotubes: Selective Binding and Accumulation of Carboxylated Carbon Nanotubes by Macrophages
The accumulation by macrophages of pristine multi-walled carbon nanotubes (P-MWNTs) and carboxylated multi-walled carbon nanotubes (C-MWNTs) coated with Pluronic® F-108 (F108) was measured by quantifying CNTs extracted from cells. Mouse RAW 264.7 macrophages and differentiated human THP-1 (dTHP-1) macrophages accumulated 80-100 times more C-MWNTs than P-MWNTs during a 24-hour exposure at 37 °C. Carboxylated single-walled carbon nanotubes (C-SWNTs) were also accumulated more than pristine single-walled carbon nanotubes (P-SWNTs). The selective accumulation of carboxylated carbon nanotubes (C-CNTs) compared to pristine carbon nanotubes (P-CNTs) suggested that there might be cell surface sites on macrophages that bind C-CNTs. The binding of C-CNTs to cells was measured at 4 °C and in medium lacking fetal bovine serum (FBS), conditions that minimize potential interference by endocytosis or serum proteins. Strong binding of C-MWNTs and C-SWNTs to cells was detected whereas the binding of P-MWNTs and P-SWNTs was low, consistent with the selective accumulation of C-MWNTs and C-SWNTs at 37 °C. Further, the binding of C-CNTs to cells was reduced when the medium contained FBS, suggesting that one or more components of FBS interfere with the binding of C-CNTs to cells. Altogether, the evidence supports the hypothesis that macrophages contain binding sites for C-CNTs that facilitate the high accumulation of C-CNTs compared to P-CNTs.