Novel fluorescent sensor membranes for alcohols based on p-N, N-dioctylamino-4′-trifluoroacetylstilbene

We report on a chromogenic ligand which reversibly interacts with alcohols resulting in a change in both absorbance and fluorescence. When embedded in membranes composed of plasticized PVC, p-N,N-dioctylamino-4′-trifluoroacetylstilbene (ETHT 4004) shows a significant signal change on exposure to aqueous ethanol with a decrease in absorbance at 453 nm and an increase in absorbance at 373 nm. At the same time, the fluorescence at 576 nm decreases accordingly when excited at 452 nm. The sensor layer exhibits a dynamic range from 1 vol% to 50 vol% ethanol with highest sensitivity in the 5–40 vol% range. The fluorescence of the sensor membrane is virtually insensitive to changes in pH. However, the magnitude of the relative signal change between plain buffer and buffer-containing ethanol is pH-dependent. A protective PTFE layer was, therefore, applied to the sensor membrane. This prevents interference from ambient light, but also, and this is most important, from pH and ionic interferents. The response time of PTFE-coated membranes is 3–4 min for forward response, and 5–10 min for reverse response. The selectivity of the dye in the sensor membranes is mainly affected by the lipophilicity of the alcohols.