Nanoporous Silica-Alumina Films Fabricated on Silicon Photonic Chips for Selective Ammonia Sensing

Surface-modified nanoporous silica films offer attractive features for analyte-specific gas detection applications. Here we demonstrate the integration of highly porous silica-alumina films on silicon nanophotonic chips and their performance in selective NH3 detection. Prototype sensors with microporous as well as mesoporous silica films were assembled. The incorporation of aluminum in trace amount needed to generate acid sites was achieved during film deposition or using postsynthesis atomic layer deposition. Silicon photonic micro-ring resonators functionalized with both techniques demonstrated a selective response to NH3 relative to CO2. Furthermore, the response was rapid and reversible. The role of preadsorbed water vapor on the reversible nature of the sensor is also investigated. Experimental observations indicate that water vapor preadsorbed on the films leads to fast sensor recovery while maintaining selectivity toward NH3. This could be attributed to the relatively less strong and still selective binding of NH3 on protonated water molecules preadsorbed on the surface acid sites. The potential of modified nanoporous films for portable and low-cost NH3 sensing on optical chips demonstrated here can be exploited in health care as well as industrial applications.