The structure and size of ligandsattached to the surfaces of goldnanorods, such as adsorbed surfactants or grafted polymers, are importantconsiderations that facilitate the use of such nanoparticles in thehuman body, in advanced materials for energy harvesting, or in devicesfor single molecule detection. Here, we report small-angle neutronscattering (SANS) measurements from surfactant or poly(ethylene glycol)(PEG) coated gold nanorods in solution, which quantitatively determinethe location, structure, and composition of these surface layers.In addition, by synthesizing gold nanorods using seed crystals whichare coated with deuterated cetyltrimethylammonium bromide (dCTAB),we are able to exploit the isotopic sensitivity of SANS to study,for the first time, the retention of surfactant from the seed crystalsto the final gold nanorod product, finding that very little exchangeof the deuterated with hydrogenated surfactant occurs. Finally, wedemonstrate that, when Au NRs are PEGylated using standard techniques,the surfactant bilayer remains intact, and while mass spectrometrydetects the presence of both surfactant and PEG, the composition asmeasured from SANS is predominantly that of the surfactant. Thesemeasurements not only provide new insight into the synthesis and functionalizationof gold nanorods but provide a quantitative picture of the structureof grafted polymer and surfactant layers on gold nanorod surfaceswhich has implications for the fabrication of plasmonic and biomedicalmaterials. [ABSTRACT FROM AUTHOR]