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. (C) 2001 Elsevier Science B.V. All rights reserved.