Gold nanoparticles of 10 nm and 250 nm were intravenously injected in rats. At 24 h after administration, tissues were collected and prepared for transmission electron microscopy (TEM). In the liver and spleen of animals treated with 10 nm gold nanoparticles, groups of nanoparticles were observed that could be positively identified by Energy Dispersive X-ray (EDX) analysis to contain gold, while nanoparticles could not be detected in the heart, kidney and brain. The 10 nm gold nanoparticles were present in the phagocytic cells of the reticulo-endothelial system (RES). The 250 nm gold nanoparticles could not be detected in any of the organs investigated. Considering the number of 250 nm gold nanoparticles administered, calculations showed that it would indeed be almost impossible to detect the 250 nm gold nanoparticles in TEM preparations in view of the very low number of particles that would be theoretically present in one TEM tissue section. This shows that relatively high numbers of nanoparticles need to be administered to enable the detection of nanoparticles in organs by TEM. In a number of samples, several globular structures of approximately the expected size were found in liver cells and the endothelium of blood vessels in the brain. However, elemental analysis with EDX detection showed that these structures did not contain gold. Our studies thus indicate that the in vivo
identification of nanoparticles cannot only depend on the detection of nanosized structures in cells. An additional identification of the composing elements of the nanomaterial is necessary for a positive identification of the nanomaterial.