An optical sensor is provided, comprising (a) a silicon nanowire of finite length having an electrical contact pad at each end thereof; and (b) a plurality of self-assembled molecules on a surface of the silicon nanowire, the molecules serving to modulate electrical conductivity of the silicon nanowire by either a reversible change in dipole moment of the molecules or by a reversible molecule-assisted electron/energy transfer from the molecules onto the silicon nanowire. Further, a method of making the optical sensor is provided. The concept of molecular self-assembly is applied in attaching functional molecules onto silicon nanowire surfaces, and the requirement of molecule modification (hydroxy group in molecules) is minimal from the point view of synthetic difficulty and compatibility. Self-assembly will produce well-ordered ultra-thin films with strong chemical bonding on a surface that cannot be easily achieved by other conventional methods.