Plasmonic biosensors represent a rapidly expanding interdisciplinary field with numerous transducers which are based principally on the optical measurement of the plasmon shift, that is induced by the change of surrounding water optical index, due to the presence of biomaterials, in a binding event. A functionalization layer is practically used for selective detection of the molecules of interest. In this paper we show how this material can modify the sensitivity of the biosensor and we propose an optimization of the biosensor.