Piezoelectric–Optical Hybrid Sensor for Trace-Level Detection of (E)-2-Hexenal via Tuned Zn–Co/ZIF-8 Frameworks for Plant Health Monitoring

Plants emit volatile organic compounds (VOCs) in response to biotic and abiotic stress, serving as early indicators of health. Among them, (E)-2-Hexenal─a key green leaf volatile (GLV)─signals stress from pathogen infection or herbivory. Real-time detection of (E)-2-hexenal at trace levels is crucial for early diagnosis, crop protection, and environmental sustainability. However, current VOC sensors often suffer from low sensitivity, poor selectivity, and limited detection at low concentrations. Although gas chromatography–mass spectrometry (GC-MS) is the gold standard for VOC analysis, its bulky instrumentation, high cost, and lack of portability make it unsuitable for real-time, in-field applications. To overcome these limitations, we developed a highly sensitive and selective (E)-2-hexenal sensor using a bimetallic Zn–Co/ZIF-8 receptor integrated with a microquartz tuning fork (MQTF) transducer. The sensor combines the high mass sensitivity of the piezoelectric MQTF with the nanoporous structure of Zn–Co/ZIF-8 to enable dual-mode detection: resonant frequency shift and visible color change due to chemosorption. The two-dimensional sensing performance can be finely tuned by adjusting the Zn:Co ratio, which influences both the adsorption behavior and the optical response of the material. By optimizing this ratio, the 1:1 composition demonstrated the best performance across a dynamic range of 125 ppb to 50 ppm, achieving a detection limit as low as 125 ppb. This robust, thermally and chemically stable sensor is low-cost, easy to fabricate, and ideal for real-time and in-field monitoring of plant VOCs. Its implementation could enable early plant stress detection, enhance crop management, and support sustainable agriculture.