Temperature-controlled direct detection of volatile fatty acids by a membrane-covered moisture sensor

Volatile fatty acids (VFAs) are important intermediates in the decomposition processes of organic wastes. The detection of VFAs is an indispensable procedure to monitor anaerobic digestion in biogas production. However, the direct, rapid, and sensitive detection of VFAs remains a challenge. Herein, we report the proof-of-concept use of a newly developed membrane-covered moisture sensor (MMS) composed of a moisture sensor chip (MSC) integrated with a hydrophobic polytetrafluoroethylene (PTFE) membrane toward the rapid direct detection of VFAs in aqueous solutions. The detections of propionic acid (PA) and other VFAs were investigated using two methods: Raoult's law-driven evaporation and heating-induced evaporation. The results revealed that the current detected from the transport of gaseous PA through PTFE membrane pores and the subsequent accumulation within MSC electrodes depended on the PA concentration in the gas phase and the controlled temperature. The proposed MMS is speculated to be applicable for detecting VFAs in anaerobic digestion.