Facile synthesis of NS-doped carbon dots as sensitive "ON-OFF-ON" fluorescent sensor for Cu²⁺ and GSH detection

In this paper, a novel nitrogen and sulfur co-doped carbon quantum dots (NS-CQDs) were successfully prepared by a dehydration exothermic carbonization method. The NS-CQDs exhibited uniform size distribution, splendid photostability, and bright fluorescence emission with a fluorescence quantum yield of 24.1 %. It was found that Cu²⁺ could quench the fluorescence at 467 nm based on the static quenching effect when Cu²⁺ was added to the NS-CQDs. At this time, the fluorescence sensor changed from the "ON" state to the "OFF" state. When glutathione (GSH) was further introduced into the NS-CQDs/Cu²⁺ system, the fluorescence intensity of NS-CQDs was amazingly restored through the coordination reaction between GSH and Cu²⁺. The fluorescence sensor changed from the "OFF" state to the "ON" state. Therefore, NS-CQDs as an "ON-OFF-ON" fluorescence sensor was designed for sequential detection of Cu²⁺ and GSH. Furthermore, this study successfully demonstrated the sensor's ability to selectively detect Cu²⁺ and GSH within a wide concentration range. Specifically, the detection range for Cu²⁺ was 0.1 μM-200.0 μM with a detection limit of 0.07 μM, while the range for GSH was 0.6 μM-180.0 μM with a detection limit of 0.1 μM. Most importantly, the NS-CQDs nanosensor could reliably monitor Cu²⁺ and GSH levels in human serum samples, with significant potential for practical applications.