Reuse of waste Myrica rubra for green synthesis of nitrogen-doped carbon dots as an "on-off-on" fluorescent probe for Fe and ascorbic acid detection.

As one kind of high nutrition fruits, abandoned Myrica rubra causes great waste due to short storage period. For resource utilization, we herein fabricated the Myrica rubra-based N-doped carbon dots (MN-CDs) by a facile/green hydrothermal method. MN-CDs, fabricated from four regions of China, displayed significant differences in their corresponding fluorescence intensities (FIs). Interestingly, different batches of waxberry samples from the same region (Wenzhou, China) exhibited slight differences in their FIs, and also an excellent anti-photobleaching and anti-salt capacity. Based on Fe-triggered quenching effect and fluorescent recovery by redox reaction of AA and Fe, MN-CDs were employed to construct an "on-off-on" switch probe for sequential detection of Fe and ascorbic acid (AA). Through Zeta potential, UV spectrum, Stern-Volmer equation, and valence-conduction band theory, the Fe-triggered quenching belonged to a static quenching process, which resulted from the synergistic contribution of inner filtering effect and photo-induced electron transfer mechanisms. The linear ranges for Fe and AA detections were 1-1000 and 0.1-1000 mM. The limits of detection were 0.3 μM for Fe in environmental waters, and 0.03 μM for AA in pharmaceutical tablets and fruit juice samples. Under 365-nm UV lamp, the color changes of test papers were easily observed from dark blue and bright blue in the presence of Fe and AA, and thus the MN-CDs-based switch probe could be satisfactorily used for visually qualitative detection of Fe and AA outdoor with our naked eyes. To sum up, MN-CDs not only realize resource reutilization of abandoned Myrica rubra, but also offer an convenient outdoor approach for qualitative detection of Fe and AA in complex matrices.