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Organic thin-film transistors (OTFTs) with chemically-modified dielectric interfaces are proposed as exhaled-alcohol sensors. Although sensors based on OTFTs have high sensitivity to moisture and would therefore be unsuitable for use as exhaled-gas sensors, it was found that a chemically-modified dielectric interfaces play a significant role in the sensitivity and selectivity of ethanol/moisture. OTFTs fabricated on a surface with low surface free energy, such as (tridecafluoro-1,1,2,2,-tetrahydrooctyl) trimethoxysilane (FTS), octadecylmethoxysilane (OTS) and hexamethyldisilazen (HMDS)-treated surfaces, have low sensitivity to moisture. OTFTs fabricated on a surface with a high ratio of dispersive to polar components of the surface free energy, such as HMDS and OTS-treated surfaces, have high selectivity of ethanol to moisture.