Fiber-optic ammonia sensor using Ag/SnO(2) thin films: optimization of thickness of SnO(2) film using electric field distribution and reaction factor.

A highly sensitive ammonia gas sensor exploiting the gas sensing characteristics of tin oxide (SnO₂) has been reported. The methodology of the sensor is based on the phenomenon of surface plasmon resonance (SPR) with a fiber-optic probe consisting of coatings of silver as a plasmonic material and SnO₂ as the sensing layer. The sensing principle relies on the change in refractive index of SnO₂ upon its reaction with ammonia gas. The capability of the sensor has been tested for a 10 to 100 ppm concentration range of ammonia gas. To enhance the sensitivity, probes with different thicknesses of SnO₂ have been fabricated and characterized for ammonia sensing. It has been found that at a particular thickness the sensitivity is highest. The reason for the highest sensitivity at a particular thickness has been evinced theoretically. The electromagnetic field distribution for the multilayer structure of the probe reveals the enhancement of the evanescent field at the tin oxide-ammonia gas interface, which in turn manifests the highest shift in resonance wavelength at a particular thickness. The selectivity of the probe has been tested for various gases, and it has been found to be most accurate for the sensing of ammonia. A sensor utilizing optical fiber, the SPR technique, and metal oxide as sensing element combines the advantages of a miniaturized probe, online monitoring, and remote sensing on one hand and stability, high sensitivity and selectivity, ruggedness, and low cost on the other.