Fluorescent "on-off-on" sensor based on N, P co-doped carbon nanoparticles for specific detection of Fe3+ and ascorbic acid.

Ascorbic acid (AA) is used as a food additive for its antibacterial and antioxidant properties. However, excessive intake of AA is harmful to humans. Therefore, the detection of Fe³⁺ and AA is generally recognized to be meaningful. In this work, a one-step hydrothermal tactics is developed for the preparing nitrogen and phosphorus co-doped carbon nanoparticles (N, P-CNPs) by using sucrose and ammonium orthophosphate as raw materials. The N, P-CNPs not only exhibited an enhanced fluorescent efficiency with a relatively high quantum yield up to 28%, but also showed satisfactory stability, water solubility and photostability. The fluorescence of N, P-CNPs can be effectively quenched by Fe³⁺ by a combination of inner filter effect (IFE) and static quenching and recovered upon the addition of AA due to the easy conversion of Fe³⁺ to reduced states Fe²⁺ by AA. Therefore, a turn-off-on fluorescent sensing strategy can be constructed for sequential detection of Fe³⁺ and AA with detection limits of 0.03 μM and 6 nM with the corresponding linear ranges of 0.05-500 μM and 0.01-500 μM respectively. The proposed fluorescent sensor exhibits excellent sensing performance and has been applied to the determination of Fe³⁺ in tap water and the analyses of AA in canned fruit soup with satisfactory results. Therefore, this fluorescent sensor expressed high potential in water quality monitoring, and could be further used to be a tool for practical food additives detection.