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.
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