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.
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