Development of a selective molecularly imprinted polymer composite electrospun nanofiber sensor for a multifunctional platform for monitoring fruit tree health

Studying volatile compounds emitted by plants is crucial in modern agriculture, providing insights into plant health, environmental interactions, and crop management. Plant volatile organic compounds (PVOCs) act as chemical signals, facilitating communication with pollinators, herbivores, and beneficial microorganisms. Understanding PVOC dynamics helps decode plant phenology events (e.g., flowering, fruit ripening), nutritional deficiencies, stress responses, and defence mechanisms. Terpenes are a class of PVOCs emitted during distinct growth stages as well as abiotic and biotic stresses.

Monitoring PVOCs (terpenes) allows for early detection of nutrient shortages, pest infestations, and disease outbreaks, enabling targeted interventions that reduce fertiliser and pesticide use, ultimately minimising crop losses. By leveraging PVOC monitoring, farmers can optimise resource allocation, enhance crop yield and quality, and reduce environmental impact, thus promoting sustainable agroecosystem management.

The MOSSA project integrated sensor technologies into IoT-based digital platforms for plant health monitoring. This project developed distinct interconnected units for each platform:

- TREE Unit – Tracks plant physiological parameters, including water consumption, biomass growth, and leaf stability.

- VOC Unit – Detects PVOC (terpene) emissions from lemon trees to monitor stress-related emission patterns.

- Power Unit – Powers the multi-sensing platform through energy harvesting.

Two different nanotechnological approaches were hired to achieve the VOC Unit goal. Electrospinning (ES) is a key nanotechnology for developing ultra-sensitive sensors, offering advantages in production efficiency and costs. The potential of ES technology to generate nanofibrous networks with various architectures featuring excellent specific surface area and remarkable porosity was combined with the exceptional selectivity of molecular imprinting technology (MIT) characterised by typical biological recognition mechanisms (e.g. enzyme-substrate, antibody-antigene, biological receptors) to developing highly sensitive and selective VOC (terpene) sensors, specifically for limonene, a key biomarker of plant biotic and abiotic stress. These sensors demonstrated extraordinary specificity, even distinguishing between stereoselective compounds. The VOC Unit, which incorporated MIT/ES sensors for limonene detection, allowed real-time monitoring of emission dynamics from lemon trees under simulated stress conditions, such as drought and pest injuries. 

The Tree Unit monitored plant health by recording sap flow, tree growth, trunk temperature, air conditions, and incoming radiation under the canopy. Sap flow, a key indicator of transpiration and water status, was measured using heat transport as a tracer within xylem tissue. After laboratory evaluation, the HPV method was selected, using a 6-second heat pulse at ~4W power.

A 4-chip ASM Osram sensor spectrometer measured incoming radiation across 28 spectral bands. An infrared dendrometer tracked tree growth, while an improved radial increment sensor achieved 0.46 m resolution with an absolute error <10 µm. A hygrometer recorded air temperature and humidity.

The Power Unit utilised a solar energy module based on a 450 nm 3D perovskite light harvester (1.65 eV band gap)between ETL and HTL layers. The ETL, composed of compact and mesoporous TiO₂, supported crystal growth and enhanced charge extraction. This solar cell module efficiently harvested solar energy, ensuring a continuous power supply for the sensing platform.

These innovations open new possibilities for plant health monitoring, contributing to precision agriculture and enabling more sustainable and efficient agrosystem management.