Metabolizable Small Gold Nanorods: Size-dependent Cytotoxicity, Cell Uptake and <i>In Vivo</i> Biodistribution

Gold nanorods (AuNRs) with unique plasmonic properties in the near-infrared region have promising biomedical applications but suffer from poor <i>in vivo</i> clearance because of the large size. In this study, small AuNRs with a diameter of 7 nm (designated as sAuNRs) are found to have low toxicity and high clearance rates <i>in vivo</i>. Compared to common AuNRs with a diameter of 14 nm (designated as bAuNRs), sAuNRs exhibit similar surface plasmon resonance bands and photothermal efficiency as bAuNRs but have lower cytotoxicity as well as higher cell uptake. The <i>in vivo</i> biodistribution study indicates that only 0.68% of the intravenously injected sAuNRs remain in the body after 30 days, but the residual amount in the body after injection of bAuNRs is as high as 12.3%. The results demonstrate that the smaller AuNRs having lower toxicity and increased clearance <i>in vivo</i> have large clinical potential.