Molecular Engineering of Bracken Fern‐Derived Carbon via In Situ Benzimidazole Functionalization for Fluorescent Lead(II) Ion Sensing

Bracken fern‐derived activated carbon is molecularly engineered for the selective detection of lead(II) ions in contaminated water. The carbon surface is functionalized with carboxylic acid groups and reacted with o ‐phenylenediamine to enable the in situ formation of benzimidazole moieties, yielding the material BI@AC. X‐ray photoelectron spectroscopy (XPS) and Fourier‐transform infrared (FTIR) analyses confirm the presence of benzimidazole, sulfonic and carboxylic groups, while X‐ray diffraction (XRD) reveals its highly amorphous nature. Field emission scanning electron microscope (FESEM) images show over‐layered porous graphitic sheets with some aggregated graphitic flakes. Moreover, the high‐resolution electron transmission electron microscopy (HRTEM) displays multistacked porous graphitic sheets with a corresponding selected area electron diffraction (SAED) pattern indicating amorphous nature. 1 H‐NMR verifies the consumption of o ‐phenylenediamine during benzimidazole formation. The fluorescence properties of BI@AC enable lead(II) ion sensing through a quenching mechanism, where increasing Pb(II) concentrations cause a significant decrease in fluorescence intensity. The material exhibits high selectivity towards Pb(II) over other metal ions and achieves a detection limit of 2.8 µM within a linear range of 0.5–16.6 µM. Adsorption confirms the binding of Pb(II) to the BI@AC through sulfonic acid and nitrogen atom of benzimidazole as the primary active sites, confirmed using FTIR analysis. This work highlights BI@AC as a sustainable, biomass‐derived sensor for efficient detection of toxic lead(II) ions in aqueous environments.