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A novel gas sensor composed of electrospun nanofibrous membranes (FM) and quartz crystal microbalance (QCM) was successfully fabricated. The electrospun nanofibers can be deposited on the QCM electrode by electrospinning the homogenous blend solutions of cross-linkable poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA). Moreover, the PAA fibrous membranes with different morphology can be deposited on the QCM electrode by electrospinning the PAA solutions with various solvent compositions of H2O and ethanol. Sensing experiments were examined by measuring the resonance frequency shifts of QCM due to the additional mass loading. The results showed that the sensing properties were mainly affected by the content of the PAA component in nanofibrous membranes, the morphology of the fibrous membranes, the concentration of NH3, and the relative humidity. Additionally, the sensitivity of the FM coated QCM (FM-QCM) sensor was much higher than that of a continuous film coated QCM (CF-QCM) sensor. Furthermore, the PAA FM-QCM sensors exhibited high sensitivity towards low concentrations of ammonia, as low as 130 ppb at the relative humidity of 40 %. The pre-sorbed water in the fibrous membranes was proved to be the key factor affecting the sensitivity of FM-QCM sensors for ammonia.