Deposition and water repelling of temperature-responsive nanopesticides on leaves

The study addresses the problem that only a small fraction (1–25%) of pesticide active ingredient reaches target organisms due to rain wash-off, photolysis, degradation, and run-off. Weak adhesion of spray droplets to negatively charged, waxy leaf surfaces is a primary cause of loss, exacerbated by climate-related stress (heat waves, heavy rains) that reduces crop yields. The authors hypothesize that nanopesticides formulated with biocompatible, dual-responsive materials can (i) reduce the contact angle and improve deposition during spraying through electrostatic attraction, hydrogen bonding, and van der Waals interactions, and (ii) become more hydrophobic after application as a temperature stimulus is encountered, thereby enhancing water repellency and rainfastness. The purpose is to create a simple, efficient tebuconazole (TEB) nanopesticide via flash nanoprecipitation (FNP) using PDMAEMA-b-PCL, to improve foliar retention, controlled release in plant-relevant pH/temperature, and safety, ultimately increasing pesticide utilization efficiency and resilience to extreme weather.

Prior approaches to improve pesticide droplet deposition and adhesion include nanocomposite supports and additives such as carbon nanomaterials, silica nanoparticles, nanogels, and polymers. Examples: graphene oxide modified with Cu2Se nanocrystals to anchor pesticide on leaves; spiky hollow silica nanoparticles loading spinosad for dual adhesion to hides and pest surfaces; temperature-dependent PNIPAM-based nanogels with tunable structure showing better foliar wettability; nanocarriers like attapulgite aggregates, phosphorylated zein, and polydopamine nanoparticles to enhance adhesion. Despite these advances, challenges remain due to complex formulations, low drug loading, poor degradability, or insufficient interactions. The authors propose using biocompatible, stimuli-responsive materials to enhance wetting via electrostatics and hydrogen bonding during application and to impart hydrophobicity post-application to resist rain wash-off.

• Formulation: Tebuconazole (TEB) nanopesticides (TEB NPs) were prepared via flash nanoprecipitation (FNP) using a confined impinging jet mixer with dilution (CIJ-D). An organic phase (THF) containing TEB and PDMAEMA-b-PCL (varied TEB:polymer mass ratios 0.5:1 to 8:1) was rapidly mixed with water at equal flow rates; the outlet stream was diluted into water, followed by dialysis (MWCO 3.5 kDa) to remove solvent and free TEB.
• Optimization and characterization: Effects of polymer concentration and flow rate on particle size, polydispersity (PDI), encapsulation efficiency (EE), drug loading capacity (DLC), and colloidal stability were assessed (dynamic light scattering over storage at room temperature and at 0, 38, 54 °C). Transmission electron microscopy (TEM) assessed morphology; zeta potential measured by electrophoretic light scattering (pH 6.0); FTIR assessed component interactions. A conventional thermal dynamic assembly method (TM) was also used to prepare comparison particles and evaluate differences in size, PDI, and stability.
• Stimuli-responsive release: In vitro release profiles of TEB from NPs were measured using dialysis in ethanol/water (20:80 v/v) at temperatures 30, 40, 50 °C (pH 6.0) and at pH 4.5, 6.0, 7.4 (30 °C), quantified by UV–Vis (269 nm). Thermal stability was characterized by TGA/DTG from 30–600 °C under N2.
• Photostability: TEB NPs, free TEB, SC (suspension concentrate), and WP (wettable powder), each at 0.3 mg/mL in THF/water, were irradiated under UV (254 nm, 16.4 W/m²); degradation kinetics were analyzed as pseudo-first-order to obtain rate constants and half-lives.
• Foliar wettability and adhesion: SEM visualized deposition on tomato leaves. Contact angles (15 µL droplets) were measured on hydrophilic tomato and hydrophobic wheat leaves for TEB NPs, SC, and WP. Liquid holding capacity (LHC) on leaves was quantified after dipping in water, TEB solution, PDMAEMA-b-PCL solution, TEB NPs, SC, and WP. Rainfastness was modeled by applying FITC-labeled formulations to tomato leaves, drying, then washing with water spray; fluorescence microscopy and ImageJ quantified retained coverage. Temperature effects were evaluated by drying FITC-labeled TEB NPs on tomato leaves at 30, 40, and 50 °C prior to wash-off tests and by measuring contact angles over time (0, 15, 30 s).
• Uptake and bioactivity: FITC-labeled TEB NPs uptake in Botrytis cinerea and translocation in tomato plants were imaged by CLSM. In vitro antifungal activity against B. cinerea and Fusarium graminearum was assessed on PDA with TEB concentrations 0.039–10 µg/mL to determine EC50 (probit regression). In vivo protective and curative efficacies were tested on tomato leaves and wheat coleoptiles inoculated with pathogens at TEB concentrations 5, 20, 100 µg/mL using TEB NPs, SC, WP; lesion sizes were quantified to compute control efficacy.
• Safety assessment: Acute aquatic toxicity was evaluated using zebrafish larvae (5 dpf) exposed to TEB formulations (0.1–10 µg/mL) to determine LC50. In vitro cytotoxicity on BEAS-2B cells was measured by MTT after 24 h exposure to 1–50 µg/mL to assess cell viability.

• Efficient nanoformulation: FNP produced spherical TEB NPs with hydrodynamic size ~122 nm (TEM ~100 nm) and narrow PDI (~0.212), high EE (~up to ~90%), and excellent storage stability at room temperature; particles remained stable at 0 and 38 °C over 3 weeks, with some growth at 54 °C.
• Surface charge: TEB NPs exhibited positive zeta potential (+35.7 mV) versus commercial SC (−30.3 mV) and WP (−14.2 mV), favoring electrostatic attraction to negatively charged foliage.
• FTIR confirmed successful incorporation of TEB into PDMAEMA-b-PCL NPs.
• Stimuli-responsive release: At pH 6.0, cumulative release at 48 h increased with temperature: 73.6% (30 °C), 80.2% (40 °C), 94.5% (50 °C). At 30 °C, cumulative release at 48 h increased with acidity: ~80% (pH 4.5), 73% (pH 6.0), 44% (pH 7.4).
• Thermal stability: TEB NPs were thermally stable up to ~300 °C, with TGA profile resembling PDMAEMA-b-PCL.
• Photostability: Under UV (254 nm), TEB NP photolysis rate constant k = 0.0498 h⁻¹ vs free TEB 0.1382 h⁻¹, WP 0.1065 h⁻¹, SC 0.0857 h⁻¹. Half-lives: free TEB 5.02 h, WP 6.51 h, SC 8.09 h, TEB NPs 13.92 h, demonstrating enhanced photostability by encapsulation.
• Foliar wetting and deposition: On tomato and wheat leaves, TEB NPs had smaller contact angles than SC and WP; e.g., tomato: TEB NPs 57.6±2.3°, SC 75.5±5.6°, WP 82.2±3.5°; wheat: TEB NPs 84.7±4.7°, SC 120.8±4.2°, WP 124.4±3.5°. SEM showed uniform NP deposition (~120–140 nm) vs aggregated commercial formulations (>400 nm to microns).
• Liquid holding capacity: TEB NPs yielded the highest LHC on both tomato and wheat leaves, exceeding water, TEB solution, polymer alone, SC, and WP (attributed to hydrogen bonding and improved wettability).
• Rainfastness: After washing, FITC-labeled TEB NPs retained strong fluorescence with 88.58% coverage retained (vs ~32.47% for free TEB solution). Retention ratios for commercial formulations were lower (e.g., WP and SC ~60.6% and ~68.2%). Overall anti-wash efficiency increased by ~37% vs commercial formulations.
• Temperature-enhanced water repellency and retention: Drying TEB NPs on leaves at higher temperatures increased contact angle and retention: contact angle 80.7° (30 °C) vs 108.6° (50 °C); post-wash retained coverage: 81.6% (30 °C), 93.4% (40 °C), 97.6% (50 °C).
• Uptake and translocation: FITC-labeled NPs were observed in fungal mycelia and on/within tomato leaves adjacent to treated areas; low fluorescence in tomato flesh indicated low residue risk in edible parts at 24 h.
• Antifungal efficacy: EC50 (µg/mL) for TEB NPs vs SC vs WP: B. cinerea 0.339 (0.244–0.470) vs 0.672 vs 0.757; F. graminearum 0.344 (0.224–0.531) vs 0.712 vs 0.873, indicating higher potency of NPs.
• Safety: Zebrafish larvae LC50 (µg/mL): TEB NPs 12.813 (6.342–25.888), WP 4.583, SC 0.449; TEB NPs showed >25-fold lower acute toxicity than SC. BEAS-2B cell viability remained higher with TEB NPs than with SC/WP at comparable concentrations.

The dual-responsive PDMAEMA-b-PCL carrier produced small, positively charged TEB nanoparticles that enhance initial droplet spreading and adhesion via electrostatic attraction to negatively charged leaf surfaces and hydrogen bonding with wax layer components. Following application, the thermoresponsive behavior increases hydrophobicity at elevated temperatures, raising water contact angles and enabling rainwater to bead and roll off, which reduces wash-off and improves retention. Encapsulation protects TEB from photodegradation, extending half-life and potentially reducing required doses. The pH- and temperature-responsive release profile aligns with plant apoplastic conditions and hot-season pathogen pressure, offering controlled delivery when and where needed. Enhanced deposition, retention, and uptake translate into significantly lower EC50 against B. cinerea and F. graminearum compared with commercial SC and WP, while safety assessments indicate reduced acute aquatic toxicity and lower in vitro cytotoxicity. Collectively, these results address the central goal of maximizing pesticide utilization efficiency and resilience to extreme weather by improving foliar adhesion and water repellency.

This work presents a simple, rapid FNP-based method to formulate tebuconazole into small, stable, positively charged, temperature- and pH-responsive nanoparticles using PDMAEMA-b-PCL. The formulation improves foliar wetting and uniform deposition during spraying, imparts water-repellent behavior upon heating with superior rainfastness, enhances photostability, and delivers higher antifungal potency than commercial SC and WP. Safety assessments suggest better biocompatibility, with substantially reduced acute aquatic toxicity and lower cell cytotoxicity. Future research could include field-scale trials under diverse climatic conditions, evaluation across different crops and pathogens, exploration of other active ingredients within this carrier platform, long-term environmental fate and non-target impact studies, and formulation optimization for scalable manufacturing and shelf-life.