Optode for the specific determination of anionic surfactants

A novel and simple kind of optode for monitoring anionic surfactants was developed by using 1,2-benzo-7-(diethylamino)-3-(octadecanoylimino)phenoxazine (ETH5294) immobilized in a thin plasticized poly(vinyl chloride) membrane. The response to anionic surfactants was the result of an ion-pair coextraction of the surfactant anion and a proton. On exposure to anionic surfactants, the red ETH5294, in its neutral form, was converted to a blue ETH5294 in its protonated form, inducing an absorbance spectral change of the membrane. When the fixed exposure time (4 min) and non-stirring detection modes are used in a buffer solution of pH 7.5, absorbance or absorbance ratio (at 535 and 660 nm) of a membrane of 4 μm thickness changes as a function of the anionic surfactants concentration, exemplified by sodium dodecyl sulfate and sodium dodecylbenzene sulfonate. The linear ranges of the method were 5.0×10−5–1.0×10−2 and 2.0×10−5–1.0×10−3 M, respectively. The sensing membrane showed a good selectivity over cationic surfactants and some common anions including acetate, iodide and nitrate.