Humic acid-based nitrogen-doped graphene quantum dots as “on-off-on” fluorescent sensors for Fe3+ and l-lysine detection

A novel type of humic acid-based nitrogen-doped graphene quantum dots (N-GQDs) exhibiting strong blue fluorescence emission was synthesized using a straightforward hydrothermal method, with coal-derived humic acid and ethanolamine serving as precursors. It was observed that the incorporation of Fe 3+ ions quenches the fluorescence emission of the N-GQDs, the classification of quenching is recognized as static quenching; however, the addition of l -lysine to the N-GQDs/Fe 3+ system effectively restores this fluorescence. Building on these findings, this study further investigates the potential application of N-GQDs as an “on-off-on” fluorescent sensor for detecting both Fe 3+ and l -lysine. It has been established that N-GQDs exhibit commendable selectivity and sensitivity towards Fe 3+ , achieving a detection limit (LoD) of 0.41 μM. Furthermore, the stable N-GQDs/Fe 3+ complex demonstrates significant detection sensitivity to l -lysine in aqueous environments, with a LoD of 1.06 μM. The experimental results indicate that the synthesized N-GQDs possess favorable properties including excellent fluorescence intensity, dispersibility, and water solubility. The developed sensor exhibits considerable potential for improvement and holds promising applications in the domain of water quality monitoring. Additionally, the fluorescence selectivity of N-GQDs towards Fe 3+ was elucidated through calculations involving frontier molecular orbitals.