Surface modification by the attachment of organic groups is a subject of current interest in material science and sensor design. We developed facile procedure for covalent functionalization of optical-active gold thin films using water-soluble arenediazonium tosylates (ADTs). These strongly bonded organic layers were characterized by IR, XPS, water contact angles, AFM, SEM-EDX, UV-Vis, CVA and SERS. According to structural characteristics, spontaneous interaction of ADTs water solution with metal-coated substrates gave close to monolayer organic film. Whereas electrochemical reaction in aqueous media resulted in multilayer coating. Observed shift of plasmon resonance and ehnhanced Raman intensity after modification by ADTs enables to consider prepared materials as a platform for sensing system. Plasmon-active metal surfaces decorated with immobilized organic functional groups can be applied as selective SPR or SERS sensory elements for the detection of trace amounts of bioorganic compounds, toxins and pollutants via further functionalization by molecular recognition elements or as tunable plasmonic devices.