Biodistribution and ultrastructural localization of single-walled carbon nanohorns determined in vivo with embedded Gd₂O₃ labels

Single-walled carbon nanohorns (SWNHs) are single-graphene tubules that have shown high potential for drug delivery systems. In drug delivery, it is essential to quantitatively determine biodistribution and ultrastructural localization. However, to date, these determinations have not been successfully achieved. In this report, we describe for the first time a method that can achieve these determinations. We embedded Gd₂O₃ nanoparticles within SWNH aggregates (Gd₂O₃@SWNHag) to facilitate detection and quantification. Gd₂O₃@SWNHag was intravenously injected into mice, and the quantities of Gd in the internal organs were measured by inductively coupled plasma atomic emission spectroscopy: 70-80% of the total injected material accumulated in liver. The high electron scattering ability of Gd allows detection with energy dispersive X-ray spectroscopy and facilitates the ultrastructural localization of individual Gd₂O₃@SWNHag with transmission electron microscopy. In the liver, we found that the Gd₂O₃@SWNHag was localized in Kupffer cells but were not observed in hepatocytes. In the Kupffer cells, most of the Gd₂O₃@SWNHag was detected inside phagosomes, but some were in another cytoplasmic compartment that was most likely the phagolysosome.