A novel colorimetric tuning fork sensor for ammonia monitoring

In this work, we reported the design and performance of a novel colorimetric tuning fork (CTF)-based gas sensor that could simultaneously capture both chemical and mechanical information of the same sensing event. The CTF sensor could transduce optical signals from a colorimetric reaction and frequency signals from the mass change. By transducing 2-dimensional (2D) sensing signals on the same CTF sensing unit, different aspects of physicochemical properties could provide a rich spectrum of binding events, leading to more accurate, sensitive, and reproducible gas detection. We demonstrated the CTF-based 2D sensing concept by coating bromophenol blue on the prongs of a micro quartz tuning fork (MQTF) as the receptor for ammonia detection. Our test results exhibit CTF ammonia sensor could reversibly detect ammonia in the air over a wide range (500 ppb - 1000 ppm) with a detection limit of 500 ppb and excellent selectivity. Due to its small size (a few millimeters), low cost (less than $1), simple fabrication process, and high performance, the CTF sensor could serve as an excellent chemical sensing unit for wearables and point-of-care screening tools.