Thin, Flexible Humidity Sensors with Self-Doped Organic Conducting Polymers Waterproofed by Cross-Linked Polyvinyl alcohol for Monitoring Transpiration on Plant Leaves

Recording the transpiration status of plant leaves without burdening biological activity is important for optimizing the growing environment and raising crop productivity. Flexible and light transpiration sensors have attracted great attention because they can attach to leaves with a lower mechanical load than conventional sensors composed of heavy and rigid materials. However, existing flexible sensors require a chemical adhesive or adhesive tape, which would induce deterioration of leaves because sensor attachment causes unstable transpiration. In this study, we develop a thin-film humidity sensor with a thickness of 5 μm and weight of 5 mg that consists of a composite layer including sulfonated polyaniline and poly(vinyl alcohol) (PVA), gold electrodes, and an elastomer layer. The humidity sensor shows sufficient flexibility to conform to uneven surfaces of leaves. The humidity response of the sensor and the effect of annealing treatment on its stability in water are evaluated by X-ray difraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and measurement of resistance changes of the composite layer. Annealing the sensing layers containing PVA at 140 °C renders them insoluble in deionized water. The thin-film sensor can be physically attached to the leaf surface without adhesives, allowing the transpiration activity of plants in response to changes in the light intensity to be measured. The resistance changes in response to the light irradiation pattern for 3 days, suggesting that the thin-film humidity sensor can record the transpiration activity of plant leaves based on photosynthesis without damage to leaves, such as discoloration and disruption.