A sensor is provided. The sensor includes a substrate, a waveguide having a first surface and a second surface, wherein the waveguide is disposed on the substrate such that at least a portion of the second surface of the waveguide is in physical contact with the substrate, a holder component disposed on at least a portion of the substrate, or the waveguide, or both, wherein the holder component comprises one or more cavities. The sensor further includes at least one microsphere at least partially disposed in a corresponding cavity of the holder component.