From Biological Inspiration to Molecular Engineering: Carboxylate-Enhanced NDI Fluorophores for Mercury Sensing

Drawing inspiration from biomimetic design, increasingly recognized in developing organic molecules for sensing applications, this study tackles the challenge of selectively detecting mercury ions. Traditional detection methods often rely on sulfur-containing functional groups, such as thiol and disulfide linkages, which, despite their effectiveness, can compromise specificity due to cross-reactivity with other metal ions. This research explored the potential of incorporating oxygen and nitrogen functionalities, drawing from the natural interactions of mercury with biomolecules in the human body, to improve selectivity in sensing applications. Naphthalene diimide (NDI) derivatives were synthesized, integrating hydroxyl or carboxylate groups at specific locations on the molecules to enhance selectivity. The standout compound, featuring these functional groups, demonstrated exceptional selectivity towards mercury ions compared to fifteen other metal ions. It further showed the applicability in real sample monitoring for mercury ions. This enhanced selectivity was verified through fluorescence sensing techniques and further substantiated by density functional theory (DFT) simulations, which provided deep insights into the binding energies (BE) and the mechanisms of ligand-to-metal charge transfer (LMCT) between the sensing molecule and Hg2+ ions. Additionally, the study observed a limit of detection (LOD) of approximately 42 ppb and a limit of quantification (LOQ) of 141 ppb, with a detection range extending from 42 ppb to 1330 ppb.