Unraveling excitation-dependent fluorescence of nitrogen and sodium co-doped carbon dots for dual detection of Fe3+ and Ag+

Carbon dot-based fluorescence sensors show great potential for developing metal ion detectors. This study focuses on developing a nanoprobe for the dual detection of different metal ions by harnessing the excitation-dependent fluorescence (EDFL) behavior of nitrogen and sodium co-doped carbon dots (N:Na-CDs). N:Na-CDs were synthesized using a microwave-assisted hydrothermal technique. The synthesized N:Na-CDs exhibited desirable functional groups containing nitrogen and sodium. Both experimental and computational results demonstrate that the presence of nitrogen and sodium-containing groups can modify N:Na-CDs' electronic and chemical structures, thereby inducing EDFL behavior. Interestingly, the different fluorescence centers of N:Na-CDs as a single nanoprobe can be effectively used for selective and sensitive Fe³⁺ and Ag⁺ detection under 360 and 440 nm excitation wavelengths, respectively. The limit of detection (LOD) of Fe³⁺ and Ag⁺ separately was found as 0.25 and 0.14 $\mu M$ within concentration ranges spanning 1–100 $\mu M$ for Fe³⁺ and 1–200 $\mu M$ for Ag⁺. Finally, the quenching mechanisms of N:Na-CDs due to metal quenchers were elucidated. This study provides insight into understanding the EDFL behavior of N:Na-CDs for effective multimodal sensors in environmental water analysis.