Optical alcohol sensor using lipophilic Reichardts dyes in polymer membranes

Two lipophilic derivatives of Reichardtʹs phenolbetaine were dissolved in thin layers of plasticized poly(ethylene vinyl acetate) copolymer. These layers were coated with microporous white PTFE in order to allow reflectance (transflectance) measurements to be performed. The sensor layers respond to aqueous ethanol with a colour change from green to blue when increasing the ethanol content. The highest signal changes are observed at a wavelength of 750nm with a linear calibration function up to 20% v/v ethanol and a detection limit of 0.1% v/v. The layers also exhibit strong sensitivity to acetic acid which affects real measurements of beverages. However, this limitation can be overcome by adjusting the basic pH of the sample solution. The correlation of ethanol content measured by enzymatic tests or gas chromatography correlates well with the data obtained using the ethanol sensor described here.