Multilayer sol-gel membranes for optical sensing applications: Single layer pH and dual layer CO₂ and NH₃ sensors

Organo-silica sol-gel membranes have been prepared and demonstrated in a single layer format for pH measurement and multiple-layer format for both carbon dioxide and ammonia. The sensors are simple and versatile since the same chemistry and membranes are used for each sensor. The sensors use hydroxypyrenetrisulfonic acid (HPTS) as the indicator immobilized in a base-catalyzed sol-gel containing poly(dimethyl)siloxane, aminopropyltriethoxysilane (APTES) and tetraethylorthosilicate (TEOS). This indicator gel is over coated with a hydrophobic sol-gel to reduce cross reactivity to pH when either CO₂ or NH₃ are examined. The gels are very stable and the sensors retain response up to a 12-month period. Sensors can be stored in buffer or dry without loss of function and have response times to that are comparable to literature values.