Development Of Water-Sensitive Polymer Nano-Composite Films And Their Application In Rapid Detection Of Agricultural Spraying

Plant protection unmanned aerial vehicle(UAV)-based spraying technology in precision agriculture offers precise delivery of pesticides and fertilizers,potentially reducing the nonpoint source(NPS)pollution associated with the excessive use of agrochemicals.However,its application is currently limited due to the lack of detection technologies that can evaluate rapidly and precisely the deposition of spraying droplets.In specific,the traditional test paper-based droplet detection technology suffers from defects that the test paper cannot be reused,and the results cannot give feedback in real-time.Here,we developed an optically water-responsive polyvinyl alcohol/polydimethylsiloxane(PVA/PDMS)bilayer film as a reusable droplet test medium.The optical response signal could be captured on-site using an outdoor imaging device,and deposited droplets were further extracted using image-recognition algorithms,thus enabling rapid on-site evaluation of UAV agricultural spraying quality.While the response signal of PVA/PDMS bilayer film could be easily interfered with in the field environment,cesium lead bromide perovskite nanocrystals(CsPbBr3 NCs)were therefore introduced into the PVA layer to achieve fluorescence response,thus emancipating the response signal from various background interferences to improve the environmental adaptability of the film,allowing for the "wearable" detection of deposited droplets on plant leaves.The main contents and conclusions of this work are as follows:(1)Development and optimization of water-responsive crosslinked PVA/PDMS bilayer film:The PVA/PDMS bilayer film was derived from two-step spin-coating and parameters including the thickness and crosslink density of PVA top-layer,as well as the mixing ratio of PMDS bottom-layer were optimized.As swelled by water,the surface of PVA layer could generate reversible winkles and trigger a light scattering response,switching the direct transmittance of the film from 90 to 10%.Such response showed good reversibility for 115 wetting-drying cycles.The response signal(scattered light)could be detected via digital imaging.The grayscale values of pixels reflected the intensity of scattered light,and their distribution in the image reflected the spatial distribution of scattered light.Therefore,the amount and spatial distribution of water absorption of the film were calculated and mapped quantitatively from the grayscale distribution of the digital image.(2)Application of crosslinked PVA/PDMS bilayer film in the rapid droplet detection of UAV agricultural spraying:A patterned PVA array was prepared on the surface of PVA/PDMS bilayer film via direct UV lithography.Compared with the continuous PVA layer,the arrayed layout limited the diffusion of droplets in the PVA layer,thus improving the resolution of droplet detection.The film was integrated into a"box-type" device with a built-in micro-camera to allow for on-site and real-time imaging.The droplet statistics,including size and distribution,could further be extracted from live images accurately after background subtraction.The "box-type"device was applied in the actual UAV spraying operation and realized the rapid on-site detection of droplet deposition.The results were consistent with the traditional "watersensitive paper",showing good prospects for practical application.(3)Preparation of fluorescent water-responsive CsPbBr3 NCs@PVA/PDMS composite film and its application in the detection of agricultural spraying:CsPbBr3 NCs were introduced into the crosslinked PVA top-layer of the bilayer film in situ via a "swelling-deswelling" microencapsulation strategy.Under spatial nano-confinement of the crosslinked PVA networks,the encapsulated CsPbBr3 NCs could undergo reversible nanocrystal-to-ionic transformation upon application/removal of water stimulus,along with an on/off switching response of fluorescence.By increasing the crosslink density of PVA networks,their confinement to precursor ions dissociated from CsPbBr3 NCs could be enhanced,thus improving the reversibility of the fluorescence response of nanocrystals.When the content of crosslinking sensitizer reached 8 wt%,the photoluminescence intensity hardly decreased in 100 wetting-drying cycles.The composite films were attached to plant leaves as "wearable" fluorescent sensors to realize the detection of droplet deposition on leaves in situ.Their fluorescence response showed excellent anti-interference performance and adapted well to high humidity and strong sunlight conditions.Moreover,the composite film had good biocompatibility,with an extremely low Pb ion leakage of only 0.79%after 20 uses in 20 days.