A Flexible Chemiresistive Sensor Combining Halogen Bonding and Swelling-Mediated Permeation for Field Detection of Plant Stress VOCs

Volatile organic compounds (VOCs) act as vital early indicators of biotic or abiotic stress in plants, and their accurate, timely detection is significant for early crop health warning. To address the need for field-deployable sensors with high selectivity, operational stability, and practical usability, we developed a resistive VOC sensor with a sensing layer of 4-fluorothiophenol (FTP)-functionalized silver nanowires embedded in a poly(vinyl alcohol)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) matrix. Reversible halogen bonding between FTP ligands and aldehyde/ketone groups enables analytes to induce swelling of the PVA matrix, producing significant signal amplification by altering resistance, while aldehydes and ketones are differentiated by polarity and steric hindrance, with optimal responsiveness to aldehydes. The sensor achieves a linear response to aldehydes over 25–200 ppm (R2 = 0.99), an estimated detection limit of 11.6 ppb, and response and recovery times of ~30 s, with reliable resistance to humidity and temperature variations. Combining stomatal observation with real-time sensor recordings of mechanical damage and pre-symptomatic early blight, we monitored plant stress multidimensionally, detecting disease signals three days before visible symptoms. Principal component analysis establishes stress-type fingerprints, and long short-term memory forecasts VOC emission trends, providing diagnostic classification and predictive intelligence for early intervention and a low-cost, non-destructive pathway for plant health monitoring.