Synthesis and characterization of novel chromogenic ligands for optical sensing of ethanol

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Abstract
Novel charomogenic ligands are reported which reversibly interact with alcohols resulting in a change of both absorbance and fluorescence. When embedded in plasticized PVC membranes together with tridodecylmethylammonium chloride, N,N-dioctylaminophenyl-4'-trifluoroacetyl-azobenzene (ETHT 4001) shows a significant signal change on exposure to aqueous ethanol solution with a decrease in absorbance at around 490 nm and an increase in absorbance at around 430 nm wavelength. The sensor layer exhibits a dynamic range from 2% to 40% (v/v) ethanol with maximum relative signal changes being as high as 50%. The limit of detection is 1.5% (v/v). The absorbance of the sensor membrane is almost insensitive to changes in pH, however, the magnitude of the relative signal change between plain buffer and buffer containing ethanol is pH dependent. The sensor is selective for primary alcohols. Sensor membranes composed of the fluorescent dye p-N,N-dioctylamino-4'- trifluoroacetylstilbene (ETHT 4004) and tridodecylmethylammonium chloride show a change in fluorescence at 576 nm when excited at 452 nm. In order to prevent cross-sensitivity to pH, a protective teflon layer has been attached to the sensor membranes. The dynamic range is similar to the membranes composed of ETHT 4001, however, the relative signal changes are around 30% for 40% (v/v) ethanol. The response time of teflon-coated membranes is 3 - 4 min for forward, 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.

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