Molecularly Engineered Biopolymer Nanoprobes for Fluorescence-Guided Monitoring of Pesticide Residues in Forest Restoration Systems

Chitosan/thiourea nanoprobes enabled selective chlorpyrifos sensing. • Heteroatom-rich carbon surfaces supported molecular recognition. • Static quenching dominated with partial inner-filter contribution. • Detection was linear from 0.06 to 50 μg L⁻¹. • Real samples showed 91.3-97.1% recovery and HPLC-UV agreement. Pesticide residues in soil-water-plant interfaces pose persistent risks to ecosystem recovery, yet rapid sensing platforms that combine sustainable material design, structural interpretability, and matrix-tolerant analytical performance remain limited. Herein, biopolymer-derived fluorescent nanoprobes were engineered from chitosan and thiourea through controlled hydrothermal conversion for selective chlorpyrifos monitoring in restoration-relevant environmental matrices. The nanoprobes exhibited quasi-spherical morphology with an average diameter of 8.5 ± 2.1 nm, amorphous/turbostratic carbon organization, defect-rich aromatic domains, and oxygen-, nitrogen-, and sulfur-containing surface functionalities. These structural features generated blue fluorescence at 445 nm under 340 nm excitation, a quantum yield of 16.8%, nanosecond lifetime behavior, and excellent stability over a wide pH range, ionic strength, storage, and irradiation conditions. Chlorpyrifos recognition was mediated by cooperative hydrogen bonding, donor-acceptor interaction, electrostatic/dipole-assisted association, and π-associated contact between the pesticide molecule and the heteroatom-rich nanoprobe surface. The sensing platform showed concentration-dependent fluorescence quenching over 0.06-50 μg L⁻¹, with a detection limit of 0.018 μg L⁻¹, a quantification limit of 0.060 μg L⁻¹, and 95% signal stabilization within 108 s. Mechanistic analysis indicated dominant static quenching with a partial inner-filter contribution. In forest topsoil extract, runoff water, and plant-surface washings, recoveries ranged from 91.3% to 97.1%, and fluorescence-derived concentrations agreed closely with HPLC-UV validation. These results establish chitosan-derived fluorescent nanoprobes as structurally defined, photophysically stable, and environmentally applicable sensing materials for rapid chlorpyrifos residue monitoring.