Surface plasmon resonance based fiber optic ammonia gas sensor using ITO and polyaniline

We present the fabrication and characterization of a surface plasmon resonance based fiber optic ammonia gas sensor. The fiber optic probe is prepared by removing a small section of the fiber cladding and symmetrically depositing a thin layer of ITO (In2O3 + SnO2 in the ratio 90:10) and a thin film of polyaniline over it using dip coating method. The sensor works on intensity modulation as well as wavelength interrogation method. The surface plasmon resonance spectra of the ammonia gas for different concentrations are obtained. It is observed that the resonance wavelength increases as the concentration of ammonia gas increases up to a certain concentration. The shift in resonance wavelength occurs due to the reactivity of the ammonia gas with the polyaniline layer resulting in the decrease in its refractive index. The sensitivity and the operating range of the sensor depend on the thickness of the ITO layer. For the best performance, the optimum thickness of the ITO layer is found to be around 60 nm. In addition, the ITO gives better sensitivity in comparison to gold for ammonia gas sensor.