From rice husk biowaste to functional sensor: mesoporous silica-immobilized Schiff base for ultra-sensitive Cr(VI) detection and wastewater remediation.

A dual-functional fluorescence sensor and adsorbent, BH@SNP, was engineered by grafting a designed Schiff base ligand — (E)-4-((5-bromo-2-hydroxybenzylidene)amino)-3-hydroxynaphthalene-1-sulfonic acid — onto mesoporous silica nanoparticles synthesized from rice husk biowaste. Detailed SEM/TEM, XRD, BET, and DFT analyses confirm the formation of an amorphous, high‐surface‐area (412 m2/g) network with uniform pores and electronic characteristics optimized to suppress photoinduced electron transfer. Under mildly acidic conditions (pH 4.5), BH@SNP exhibits rapid fluorescence “turn-off” toward Cr(VI) oxyanions, achieving a 9.83 ng/mL detection limit and equilibrium response within 180 s. The sensor maintains ≥ 80% of its initial fluorescence output after one year of storage and retains > 80% activity over seven regeneration cycles using mild EDTA treatment. Concurrent adsorption studies reveal 75% Cr(VI) uptake within 30 min and a maximum capacity of 45.9 mg/g, following Langmuir and pseudo–second‐order kinetics. In both spiked laboratory solutions and real electroplating wastewater, BH@SNP achieves > 94.8% removal efficiency. Combining agricultural‐waste valorization with integrated trace‐level detection and remediation using BH@SNP establishes a scalable, circular-economy platform for sustainable water treatment and heavy‐metal monitoring.