Efficient dual-mode colorimetric/fluorometric sensor for the detection of copper ions and vitamin C based on pH-sensitive amino-terminated nitrogen-doped carbon quantum dots: effect of reactive oxygen species and antioxidants.

We present a facile strategy for highly sensitive and selective determination of copper(II) ions and vitamin C (ascorbic acid, AA) using new amino-terminated nitrogen-doped carbon quantum dots (CQDs) synthesized from melamine as the carbon and nitrogen source by the hydrothermal method. The CQDs have superior optical features, including a pH-sensitive photoinduced electron transfer process. The CQDs form a complex with Cu2+ ions, leading to the development of naked-eye, colorimetric, and fluorometric determination. AA reduces the Cu2+ ions to Cu+ ions, which cannot form the complex. Thus the absorbance and fluorescence of the CQDs are recovered by addition of AA because of dissociation of the complex into Cu+ and CQD. The in situ generation of reactive oxygen species when AA is added to Cu-CQD complexes in the presence of dissolved oxygen leads to the sensitive determination of AA, proposed on CQDs for the first time. The in situ generation of reactive oxygen species was confirmed by a fluorescence method using a hydroxyl radical indicator (i.e., coumarin). This novel "turn-off/turn-on" sensing approach using amine-functionalized CQDs is potentially applicable to determining the concentration of Cu2+ ions and AA in the areas of materials chemistry, nanobiomedicine, nanobiotechnology, and bioengineering because of its high sensitivity, high selectivity, low cost, simple naked-eye readout, and good linearity. Graphical abstract.