Ethanol vapor detection in aqueous environments using micro-capacitors and dielectric polymers

A technique for the measurement of ethanol concentrations in aqueous mixtures is reported, in which a permeable membrane is used to transport ethanol vapors to a microelectromechanical (MEMS) chemi-capacitor array. The fixed plate micro-capacitors were filled with a polymeric dielectric material, siloxanefluoro alcohol (SXFA), whose dielectric constant increased upon absorption of ethanol vapor. Measurements of the mixture's liquid-phase concentration were made in the vapor-phase by sampling the saturated vapors through a hydrophobic, vapor permeable nanopore filter. The performance of these sensors was characterized over a range of ethanol/water mixture concentrations and flow cell temperatures. The limit of detection (LOD) for ethanol in water using the capacitive micro-sensors in the present arrangement was found to be 40 ppm.