Metal–Organic Frameworks (MOFs)-Based Piezoelectric-Colorimetric Hybrid Sensor for Monitoring Green Leaf Volatiles

Green leaf volatiles (GLVs) are organic compounds emitted by plants in response to insect attacks, offering early detection potential. Current GLV detection methods like gas chromatography-mass spectroscopy (GC-MS) are costly and complex and lack real-time monitoring capability. There is an unmet need for affordable and portable sensors with high sensitivity to monitor GLVs in real time. In this study, we developed a novel sensor capable of capturing piezoelectric and colorimetric signals for the sensitive and selective detection of 1-hexanol, a well-known green leaf volatile. We used a piezoelectric micro quartz tuning fork (MQTF) as the multifunctional transducer. The MQTF's two prongs were coated with a metal-organic framework (MOF)-thymol blue hybrid sensing material, enabling detection through both color change and resonating frequency shift upon 1-hexanol binding. MOFs offer a high surface area and tunable pore size, which enhance sensor sensitivity and selectivity. The sensor's frequency shift indicates mass change due to 1-hexanol binding to MOFs, while the colorimetric sensing signal relies on thymol blue's reaction with 1-hexanol. Our test results demonstrate the sensor's ability to detect 1-hexanol from 62.5 ppb to 250 ppm with high sensitivity and selectivity when the colorimetric and piezoelectric sensing signals are integrated. Due to its compact size, affordability, easy fabrication, wide detection range, and high sensitivity and selectivity, this colorimetric-piezoelectric sensor could serve as an effective tool for early detection of insect herbivore attacks and timely crop protection strategies.