Surfactant-Mediated Surface Engineering Redirects Cr2O3 Nanoparticles Activity in Pea Seeds from Toxicity to Hormesis and Organ-Specific Growth Stimulation

Abstract

Rational design of effective nano-agrochemicals requires a profound understanding of how surface chemistry determines their interaction with plants. This study employed an integrated approach to investigate surfactant-stabilized chromium (III) oxide nanoparticles (Cr2O3 NPs). Quantum-chemical modeling predicted two distinct stabilization mechanisms: covalent chelation for cocamidopropyl betaine (CB, η = 0.033 eV) and electrostatic interactions for alkyldimethylbenzylammonium chloride (ADBAC, η = 0.057 eV). Experimental characterization via FTIR spectroscopy confirmed these predictions, revealing a chelating carboxylate layer on CB-Cr2O3 NPs and an ammonium-rich cationic surface on ADBAC-Cr2O3 NPs. X-ray diffraction confirmed the phase purity of the Cr2O3 NPs (rhombohedral, space group R3̅c), and scanning electron microscopy showed that the samples consisted of primary NPs of 25–100 nm. Biological assays on pea seeds revealed a clear hormetic response. ADBAC-Cr2O3 NPs at 0.1 mg/L acted as a potent roots-specific stimulant, increasing average root length from 27.0 mm to 49.8 mm. In contrast, CB-Cr2O3 NPs promoted more balanced growth, significantly enhancing sprout length by 34% to 12.7 mm. However, elevated concentrations (> 1.0 mg/L) induced pronounced toxicity, drastically reducing germination, which highlights the critical importance of precise dosage control. This work demonstrates that surface engineering of Cr2O3 NPs with surfactants serves as a functional switch, enabling targeted manipulation of plant physiology and providing a framework for designing precision nano-biosimulants.