Surface Plasmon Resonance-Based Fiber Optic Sensor for the Detection of Low Concentrations of Ammonia Gas

Fabrication and characterization of a highly sensitive surface plasmon resonance-based fiber optic sensor for the detection of low concentrations of ammonia gas have been reported. The sensor probe is fabricated by coating an unclad core of the optical fiber with successive layers of indium tin oxide (ITO) and bromocresol purple (BCP). Increase in the concentration of ammonia gas around the sensing probe increases the resonance wavelength linearly implying that the absorption of ammonia gas by BCP layer increases its refractive index. In addition, ITO layer also contributes to the increase in the resonance wavelength because it is porous and has grains, which allow the reaction products to enter the pores of the ITO layer causing swelling of the layer resulting in mechanical stress and hence increase in the refractive index. To achieve maximum sensitivity of the sensor, the thickness of the BCP layer is optimized and is found to be 70 nm. The sensitivity of the sensor with optimized thickness of BCP layer is 1.891 nm/ppm and is larger than the sensitivity values obtained in the cases of Ag/BCP and Cu/BCP-coated probes for the concentration range 1-10 ppm of the ammonia gas. The selectivity of the probe is checked by carrying out experiments on the probe with different gases and it is observed that the probe is highly selective for ammonia gas. The sensor has many advantages, such as low cost, online monitoring, and remote sensing, due to the fabrication of the surface plasmon resonance probe on an optical fiber.