Overcoming the pH dependency of optical sensors: a pH-independent chloride sensor based on co-extraction Original Research Article

A chloride-selective optode is presented which is based on a polymer-stabilized emulsion system which consists of a hydrogel into which plasticizer droplets are incorporated containing a neutral chloride-selective ionophore. A solvatochromic cationic fluorescent dye is located in the hydrogel. When the ionophore extracts chloride out of the aqueous solution into the plasticizer droplets, the positively charged dye is co-extracted for reasons of electroneutrality. This concerted reaction causes a decrease of micro-polarity near the dye, resulting in an increase of its fluorescence intensity along with a bathochromic shift of the emission maximum. This system can be used for pH-independent reversible sensing of chloride in the 10–100 mM concentration range with response times of the order of 3 min.