Ordered-mesoporous-silica-thin-film-based chemical gas sensors with integrated optical polarimetric interferometry

A technology to make integrated optical chemical gas sensors using sol-gel surfactant-templated mesoporous thin films was demonstrated. The sensor was prepared by dip coating of an ordered mesoporous silica thin film onto a tapered thin TiO2 layer sputtered on top of a tin-diffused glass waveguide. The sensor employs single-beam polarimetric interferometry to detect molecular adsorption in the mesoporous silica film because the adsorption can lead to a change of the phase difference (Delta phi) between the fundamental transverse electric (TE0) and magnetic (TM0) modes propagating in the waveguide. The sensor is sensitive to humidity and NH3 gas at room temperature (RT). The sensitivity of Delta phi > 180 degrees was detected for 40 min exposure of the sensor to 2 ppm of NH3 in dry air. The sensor was recoverable from the HN3 exposure but its recovery was too slow to cause a detectable Delta phi per unit time. The slow response and recovery of the sensor were ascribed to slow diffusion of gaseous molecules in the laterally oriented tube-like pores of the silica film. (c) 2006 American Institute of Physics.