In one aspect of the invention, a method or apparatus is described for determining concentration(s) of one or more analytes in a sample using plasmonic excitations. In another aspect, a method relates to designing systems for such concentration determination, wherein metallic nanostructures are used in combination with local electrical detection of such plasmon resonances via a semiconducting photodetector. In certain aspects, the method exploits the coupling of said metallic nanostructure(s) to a semiconducting photodetector, said detector being placed in the “metallic structure's” near field. Surface plasmon excitation can be transduced efficiently into an electrical signal through absorption of light that is evanescently coupled or scattered in a semiconductor volume. This local detection technique allows the construction of sensitive nanoscale bioprobes and arrays thereof.