Nitrate-selective optical sensor applying a lipophilic fluorescent potential-sensitive dye Original Research Article

An optical sensor has been developed for continuous determination of nitrate that is based on a polymer-stabilized emulsion system consisting of a hydrogel with entrapped plasticizer droplets. The droplets contain a cationic potential-sensitive fluorescent dye (PSD) located near its surface. The cationic PSD also acts as an anion-exchange catalyst that extracts nitrate out of the aqueous solution to form a complex (PSD+/NO3−) that moves into the plasticizer droplet due to its lipophilicity. This process causes a decrease of micro-polarity near the dye, resulting in an increase in the fluorescence intensity of the PSD. In a second detection scheme, the fluorescence intensity information of the PSD is converted into a fluorescence phase shift by mixing into the membrane particles that contain an inert long-lifetime reference luminophore. The second sensing scheme is referred to as “dual lifetime referencing”. Both schemes can be used for reversible sensing of nitrate in the 0.1–50 mM range, with response times in the order of 3 min.