Fluorescent response of sol–gel derived ormosils for optical ammonia sensing film

Optical sensing films for ammonia have been investigated based on a fluorescent indicator aminofluorescein (AF). AF was immobilized in diverse organically modified silicates (ORMOSILSs) obtained by copolymerizing various proportions of methyltrimethoxysilane (Me–TriMOS), phenyltrimethoxysilane (Ph–TriMOS), dimethyldimethoxysilane (DiMe–DiMOS), and diphenyldimethoxysilane (DiPh–DiMOS) with tetramethoxysilane (TMOS). The effective polarities of ormosils were probed by using the solvatochromic dye ET (30) [2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino)]phenolate. Compared with the fluorescent responses of fluorescein and fluorescein isothiocyanate, the fluorescent intensity enhancement of AF for ammonia was caused by the reaction between ammonia and the NH2 group on AF. The reaction may cause the reduction of the intermolecular self-quenching of AF to make a fluorescence enhancement of sensing film in NH3 solution. The ammonia sensing range and rate of response were found to highly depend on the type and content of organosilicon precursors employed. Films prepared with a 1:3, 1:1.5, 1:1.2, or 1:1 mole ratio (TMOS:Me–TriMOS, TMOS:Ph–TriMOS, TMOS:DiMe–DiMOS, or TMOS:DiPh–DiMOS) was found to be the best in terms of stability and response. Their detection limits for ammonia in water was 0.01, 0.5, 0.2, and 0.5 (mu)g ml^-1, respectively.