Fluorescence "on-off-on" Assay of Copper Ions and EDTA Using Amino-Functionalized Graphene Quantum Dots

Copper is an important trace element involved in several physiological processes. The deficiency or excess of Cu in the human body may cause some serious diseases. EDTA has been widely employed in many industry fields owing to its excellent chelating ability. The poor biodegradability of EDTA makes itself a persistent substance in the natural environment. This work provided a fluorescence "on-off-on" strategy for the sequential determination of trace Cu²⁺ and EDTA. Amino-functionalized graphene quantum dots (afGQDs) were synthetized via the thermal pyrolysis of citric acid. Fluorescence resonance energy transfer (FRET) between afGQDs and 1-(2-pyridylazo)-2-naphthol (PAN) effectively quenched the fluorescence of this carbon-based nanomaterial. The generation of the Cu²⁺-PAN complex caused the increased FRET efficiency and the further fluorescence decline. The change of the fluorescence intensity sensitively responded to copper ions. The linear range and the limit of detection (LOD) were 1 nM-10 μM and 0.87 nM, respectively. EDTA could decompose the Cu²⁺-PAN complex and liberate PAN, which weakened the FRET efficiency and led to the fluorescence recovery. The increasing degree of the fluorescence intensity was closely related to EDTA within a concentration range from 10 nM to 10 μM with a LOD at 4 nM. Copper ions in the water and human serum samples and EDTA in the trypsin-EDTA sample were successfully detected based on the proposed fluorescence method.