Fluorosensors for ammonia using rhodamines immobilized in plasticized poly(vinyl chloride) and in sol-gel; a comparative study

We report on fluorosensors for ammonia using rhodamines incorporated in plasticized poly(vinyl chloride) (PVC) and in sol-gel matrix. The response of rhodamine B is due to a change in its molecular structure. When exposed to ammonia, the rhodamine is converted into a colorless, non-fluorescent lactone. As a result, the decrease in fluorescence intensity is proportional to the concentration of ammonia. To improve sensitivity we make use of fluorescence energy transfer using donor-acceptor complexes. Ion pairs consisting of a fluorophore (rhodamine B or tetramethylrhodamine ethyl ester) and an absorber (bromophenol blue) were immobilized in PVC and in sol-gel. Response times of the optode layers are in the order of 2 to 8 min depending on the ammonia concentration and the matrix used. Limits of detection are as low as 0.1 μg ml−1 ammonia. The sol-gel is appropriate for use in sensor technology and comparable to PVC layers with respect to response mechanism and response time. Sol-gel based ammonia sensors are advantageous over PVC layers in their storage stability (≥6 months) and the relative insensitivity to protons in response to ammonia.