Carbon dots as an "on-off-on" fluorescent probe for detection of Cu(II) ion, ascorbic acid, and acid phosphatase.

Carbon dot (CD)-based fluorescent probes have been widely exploited; however, multi-component detection using CDs without tedious surface modification is always a challenging task. Here, we develop a convenient and simple CD-based "on-off-on" fluorescent probe for detection of copper(II) ion (Cu2+), ascorbic acid (AA), and acid phosphatase (ACP). Cu2+ leads to the fluorescence quenching of CDs. The limit of detection (LOD) for Cu2+ is 2.4 μM. When AA is added into the CDs + Cu2+ solution, Cu2+ is reduced by AA to Cu+, causing the fluorescence recovery of CDs. The fluorescent intensity linearly correlates with the concentration of AA in the range of 100-2800 μM with LOD of 60 μM. Besides, the probe has potential application for detection of AA in real samples such as VC tablets, orange juice, and fresh orange. The probe can also indirectly detect ACP that enzymatically hydrolyzes ascorbic acid-phosphate (AAP) to produce AA. This work expands the application of CDs in the multi-component detection and provides a facile fluorescent probe for detection of AA in real samples. Graphical abstract.