One-step and large-scale fabrication of flexible and wearable humidity sensor based on laser-induced graphene for real-time tracking of plant transpiration at bio-interface

The rapidly growing demand for humidity sensing in various applications such as noninvasive epidermal sensing, water status tracking of plants, and environmental monitoring has triggered the development of high-performance humidity sensors. In particular, timely communication with plants to understand their physiological status may facilitate preventing negative influence of environmental stress and enhancing agricultural output. In addition, precise humidity sensing at bio-interface requires the sensor to be both flexible and stable. However, challenges still exist for the realization of efficient and large-scale production of flexible humidity sensors for bio-interface applications. Here, a convenient, effective, and robust method for massive production of flexible and wearable humidity sensor is proposed, using laser direct writing technology to produce laser-induced graphene interdigital electrode (LIG-IDE). Compared to previous methods, this strategy abandons the complicated and costly procedures for traditional IDE preparation. Using graphene oxide (GO) as the humidity-sensitive material, a flexible capacitive-type GO-based humidity sensor with low hysteresis, high sensitivity (3215.25 pF/% RH), and long-term stability (variation less than ± 1%) is obtained. These superior properties enable the sensor with multifunctional applications such as noncontact humidity sensing and human breath monitoring. In addition, this flexible humidity sensor can be directly attached onto the plant leaves for real-time and long-term tracking transpiration from the stomata, without causing any damage to plants, making it a promising candidate for next-generation electronics for intelligent agriculture.