Gold nanorods are tiny, rod-shaped gold particles. They have two distinctive different absorption bands derived from the transverse and longitudinal surface plasmon resonance of free electrons in the visible and near-infrared (NIR) regions, respectively[1, 2]. Gold nanorods absorb NIR light, which is suitable for in vivo applications such as imaging and for photoradiation therapy because of maximal penetration of light into tissues. Gold nanorods have a photothermal effect, i.e., the adsorbed light energy is converted to heat. Therefore, gold nanorods are expected to be contrast agents for NIR imaging and exothermic nanodevices for photothermal therapy.
Gold nanorods are prepared in the presence of the cationic detergent 1-hexadecyltrimethylammonium chloride (cetyltrimethylammonium chloride; CTAB), which acts as a stabilizer of gold nanorods. To apply gold nanorods as medical nanodevices, biocompatible gold nanorods have been prepared by coating with phosphatidylcholine or by modifying gold nanorods with polyethylene glycol (PEG)[6, 7]. PEG-modified gold nanorods show high dispersion stability, high circulation stability in the blood after intravenous injection into mice, and accumulation in tumors mediated by the enhanced permeability and retention (EPR) effect[8, 9]. The PEG-modified gold nanorods have been applied to NIR imaging and photodynamic/photothermal therapy of tumors[8, 10–13].
The size and aspect ratio of gold nanorods are important factors of their uptake into cells[14–16] and biodistribution[17–20]. Arnida et al. reported that rod-shaped gold nanoparticles accumulated more efficiently in tumors and less efficiently in the liver compared with sphere-shaped gold nanoparticles. However, the compared gold nanorods and gold nanospheres had different volumes and zeta potentials that affected cellular uptake (e.g., phagocytosis). Therefore, if we examine the effect of the geometry of gold nanoparticles on biodistribution, other parameters should be equivalent.
In the present study, we prepared two types of gold nanorods that had distinct aspect ratios but had an identical volume, zeta potential, and surface structure. Biodistributions of these nanorods after intravenous injection into tumor-bearing mice were then compared.