Reflectometric Optosensor for Visual Detection of Ammonia Based on Silica Pellet Sensing Material

Abstract

Ammonia (NH 3 ) has been widely used in the manufacture of fertilizers that applied to soil, but the high consumption of fertilizers will end up with water pollution. Owing to the deleterious effects of NH 3 to human and environment, a new optosensor for NH 3 has been fabricated based on silica pellet sensing material. Microsilica was synthesized by sol-gel method in the presence of cobalt(II) chloride hexahydrate (CoCl 2 ·6H 2 O), followed by manual grinding process to obtain microsized silica particles. Due to the non-transparent pellet material used for NH 3 sensing, a fiber optic reflectance spectrophotometer was employed for monitoring of reflectance signal transduction event as the pellet colour changed from pink to blue hue upon reaction with NH 3 at optimum pH 13. Due to the high porosity and surface area of silica microparticles were used as immobilization matrix, the immobilized Co 2+ ion demonstrated broad dynamic linear range from 18 to 100 ppm NH 3 with a fast response time of 3 min. The reflectometric sensing protocol involves a single-step NH 3 assay which merely requires dispensing small aliquots of NH 3 onto the reaction surface of the pellet sensor. This makes on-site NH 3 detection more user-friendly and convenient when compared to traditional electrochemical-, infrared-and gas chromatography-based methods.