Cap-shaped gold nanoparticles for an optical biosensor

Cap-shaped Au nanoparticles have been formed by evaporating Au on a dense monolayer of randomly adsorbed monodisperse polystyrene (PS) spheres with a diameter of 110 nm. The resulting sample exhibits a large extinction coefficient of optical density of 2.4 with a bandwidth of 100 nm in the visible region. The extinction peak position is found to depend linearly on the refractive index of the environment. As demonstrated by successful detection of octadecanethiolate monolayer formation as well as biotin/avidin binding, the sub-monolayer sensitivity of the simple novel device reported here promises development of inexpensive optical biosensors for massively parallel detection.