N-Doped Carbon Dots as Fluorescent “Turn-Off” Nanosensors for Ascorbic Acid and Fe³⁺ Detection

Carbon-based sensors for the detection of ascorbic acid (AA) and ferric ions have drawn a great deal of attention recently, owing to their excellent optical properties and good biocompatibility for biological applications. In this work, a series of nitrogen-doped carbon dots (NCDs) were fabricated by a microwave-assistant approach with a combination of dl-malic acid and urea as precursors. Under optimal experimental conditions, NCDs with an average size of 4.52 ± 0.05 nm were prepared, containing amino N and C–N functional groups on the surface of carbon cores. Optical analysis showed that the NCDs exhibited excitation-dependent and concentration-dependent emission properties. A single emission at 450 nm was observed with two luminescent centers at 280 and 370 nm for concentrations ranging from 0.02 to 0.08 mg/mL. Moreover, the NCDs were further used as a fluorescence sensor to detect AA and Fe3+ in solution. From the metal ion sensing research, Fe3+ demonstrated significant quenching abilities on NCDs with a detection limit of 1.9 μM. More importantly, the NCDs also showed an excellent quenching response by AA through the static quenching mechanism and inner filter effect with a detection limit of 2.6 μM. Additionally, the low cell toxicity against MA104 and 293T cells from both monkeys and humans were affirmed, respectively. Therefore, the NCDs developed in the present work provide a “turn-off” strategy for the highly sensitive detection of AA and Fe3+ ions and can be potentially applied in both environmental and biological systems.