Bifunctional PEI@MSN@COP nanohybrid for efficient adsorption and fluorescence sensing of mercury (II) in water

Mercury contamination remains a major environmental concern because of the toxicity, persistence and bioaccumulation of Hg²⁺ ions. This study successfully synthesised a hierarchical bifunctional nanohybrid (PEI@MSN@COP) by integrating polyethyleneimine (PEI) and a nitrogen/phosphorus-rich COF-like polymer (HM-COP) onto mesoporous silica nanoparticles (MSNs) for the concurrent adsorption and fluorescence-based detection of Hg²⁺ in water. The produced nanohybrid was thoroughly characterised using FT-IR, PXRD, TGA, BET, FE-SEM, TEM, zeta potential analysis and ICP-OES studies. The results confirmed successful hierarchical functionalisation while preserving the mesoporous architecture of the material. Owing to the synergistic effects of the mesoporous structure and donor-rich functional groups, PEI@MSN@COP exhibited a high Hg²⁺ adsorption capacity of 1058.41 mg g^-1 under optimal conditions. The adsorption data showed the best overall agreement with the Temkin isotherm and pseudo-second-order kinetic models. These findings indicate heterogeneous adsorption sites and surface-controlled adsorption behaviour. Adsorption data showed the best overall agreement with the Temkin isotherm and pseudo-second-order kinetic models, indicating heterogeneous adsorption sites and surface-controlled adsorption behaviour. Thermodynamic analysis revealed that the adsorption process was spontaneous and endothermic. In addition to adsorption, PEI@MSN@COP exhibited rapid and selective fluorescence quenching toward Hg²⁺, with a detection limit of 0.63 ppm and a response time of approximately 3-5 min. Competitive adsorption studies and industrial wastewater experiments further demonstrated the preferential adsorption of Hg²⁺ under chemically complex conditions together with favourable operational stability of the nanohybrid. The integrated adsorption and fluorescence sensing properties demonstrate the potential of PEI@MSN@COP as a multifunctional platform for Hg²⁺ monitoring and remediation in contaminated water systems.