Noble metal nanoparticles have been used as sensors for biological molecules because of their unique interactions with light due to the resonant collective oscillations of the conduction electrons known as surface plasmon resonance (SPR). The frequency at which this resonance occurs is strongly dependant upon the dielectric constant of the medium surrounding the particles. We have previously shown that gold nanoparticles chemisorbed on to glass and subsequently functionalized with a biological receptor can optically transduce analyte binding at the surface of the nanoparticles. This is because when the target analyte binds to the receptor functionalized nanoparticles, the dielectric constant of the surrounding media increases, resulting in a measurable shift of the SPR frequency that can be measured as a color change. In this study, we report the extension of this label free optical biosensor to single nanoparticles that are chemisorbed onto glass. Gold nanoparticles in the size range of 13-40 nm and gold nanorods (30 nm diameter, aspect ratio 2.7) are synthesized in solution. The nanoparticles are chemisorbed on to the surface of an amino-terminated silane monolayer under conditions that result in sparse coverage of the nanoparticles on the substrate. Darkfield microscopy is used to detect the SPR shift of a single nanoparticle by analyzing its scattering spectrum as a function of the change in the dielectric constant in the vicinity of a single nanoparticle. Applying this technique to a single nanoparticle offers the advantage of effective detection of a target analyte with detection limits on the order of a few hundred molecules.