NanoComposix’s gold nanorods are surface plasmon resonant (SPR) rod-shaped nanoparticles with narrow size distributions and uniform shape. The gold nanorods have absorption peaks that are tunable throughout the visible and short wave near-infrared (SWNIR) regions of the electromagnetic spectrum. Our purification methods remove ammonium surfactants used during synthesis and replace it with citrate, producing a stable, easily displaceable surface chemistry to use as a base for functionalization with a wide variety of molecules (silica, DNA, antibodies, etc.). We also offer PEG-functionalized nanorods, as well as carboxyl-terminated surfaces for bioconjugation.