Novel chloride-selective optode based on polymer-stabilised emulsions doped with a lipophilic fluorescent polarity-sensitive dye

A chloride-selective optode is presented which is based on a polymer-stabilised emulsion system consisting of a hydrogel with entrapped plasticizer droplets. The droplets contain a neutral chloride-selective ionophore and a polarity-sensitive cationic fluorescent dye (PSD) located near the droplet surface. The ionophore extracts chloride out of the aqueous solution through the hydrogel into the plasticizer droplets, followed by a displacement of the positively charged PSD into the plasticizer droplet. This concerted process causes a decrease of micro-polarity near the dye, resulting in a dramatic increase in its fluorescence intensity. The sensing scheme can be used for reversible sensing of chloride in the 1-80 mM concentration range with response times on the order of less than 3 min. The sensor membrane has been investigated in terms of signal change, stability, limits of detection and selectivity for the analyte over interferents.