We previously described a strategy to stabilize gold nanorods with a methoxy (polyethylene glycol)-thiol (mPEGSH) which displaces the bilayer of the original surfactant hexadecyltrimethylammonium bromide (CTAB) to provide biocompatibility of the resultant OA contrast agent.20,37 In a typical procedure,38,39 0.250 ml of an aqueous 0.01 M solution of was added to 7.5 ml of a 0.1 M CTAB solution in a test tube. Then, 0.600 ml of an aqueous 0.01 M ice-cold solution was added all at once. Three hours later, 38 µl of this seed solution was added to 10 ml of growth solution containing 9.44 ml of 0.10 M CTAB, 0.40 ml of 0.01 M , 0.06 ml of 0.01 M solution, and 0.032 ml of 0.10 M ascorbic acid. This procedure resulted in synthesis of GNRs with a narrowband optical absorption around 765 nm. After the GNR solution underwent low-speed centrifugation at 1000 g for 10 min, it was centrifuged again at 14,000 g for 10 min. The supernatant was discarded and the pellet resuspended in deionized water for PEGylation as described in previous studies.20,39 Excess mPEG thiol was removed from solution by two rounds of centrifugation at 14,000 g for 10 min, followed by resuspension of the GNR-PEG in phosphate buffer solution (PBS, pH 7.4). The GNR-PEG conjugate was filtered through a 0.22-µm Millipore Express Plus Membrane (EMD Millipore, Billerica, MA). Measurements of optical absorbance proved that the PEGylation and sterilization of PEG-GNR conjugates did not affect plasmonic properties of gold nanorods (Fig. 2). An increase in concentration of PEGylated GNRs was made by centrifugation at 12,000 g for 10 min with the supernatant being removed and the pellet resuspended in sterile PBS up to a concentration of 12.5 nM or , which corresponded to the optical density around 50, as measured with a spectrophotometer (Evolution 201, Thermo Scientific, Waltham, MA). The GNR molar extinction coefficient was calculated around (37), allowing quantitative evaluation of GNR concentrations used for intravenous injections (IV).