On-site detection is essential for the wide-scale deployment of PFAS monitoring. Many existing on-site detection methods operate on an electrochemical basis using molecularly imprinted polymers, metal nanoparticles, and metal-organic frameworks. Unfortunately, many of these sensors involve complex manufacturing processes and uncommon reagents. Here, we will demonstrate an approach that combines widely available phenothiazine-derivative cationic dyes, such as methylene blue, with the nanogram-level sensitivity of the quartz crystal microbalance (QCM). We coated the QCM sensor’s Au electrode surface with nanometer-thin films of dyes in a portable setup, then introduced PFAS solutions over it using a portable syringe pump, completing the on-site process while monitoring changes in the resonance frequency of the QCM crystals. As these perfluorinated anionic surfactants formed ion pairs with the dyes, the mass bound to the sensor increased, creating a measurable difference in resonance frequency. We successfully quantified 370 parts per trillion (ppt) PFAS in solution and demonstrated the reusability of these sensors through plasma surface cleaning.
