A sensing apparatus, system, and use method for selective detection of a target molecule in a gaseous medium with a limit of detection of less than 50 ppm are provided. The sensing apparatus comprises an optical fiber having a core with a tilted grating, and a coating assembly that is both active to surface plasmon resonance (SPR) and reversibly reactive to the target molecule to allow for repeated detection. Upon a compatible light propagating in the optical fiber, surface plasmon waves at an interface between the coating assembly and the medium can be generated to thereby derive information of the target molecule. Signals from core mode optical waves can additionally be used as inherent reference to remove influences of fluctuations from environmental factors and input power level. There is at least one range of concentrations for the target molecule allowing for linear measurement. Multiplexing of a plurality of sensing apparatuses is also disclosed.