Europium-decorated carbon quantum dots for dual Cu²⁺/Fe³⁺ detection and live-cell bioimaging

The present study demonstrated europium-decorated carbon quantum dots (Eu-CQDs), which were synthesized through a one-pot hydrothermal process using sucrose and melamine as the carbon and nitrogen sources and EuCl₃ as the rare earth metal ion of Eu. The resulting blue-emissive Eu-CQDs were confirmed using spectroscopic and microscopic techniques, including UV-Vis absorption, fluorescence (FL), FT-IR, XRD, TEM, and XPS. The Eu-CQDs showed significant UV-Vis and FL emission at wavelengths of 320 and 424 nm, respectively, with a quantum yield (QY) of 33.6%. TEM analysis revealed well-dispersed spherical Eu-CQDs with an average size of 3.19 ± 0.21 nm. Remarkably, the Eu-CQDs served as an outstanding FL probe for the selective sensing of ferric ions (Fe³⁺) and copper ions (Cu²⁺) through FL dynamic quenching, especially photo-induced electron transfer (PET), while other metal ions produced minimal intensity changes. The detection limits were 65.6 nM for Cu²⁺ and 72.1 nM for Fe³⁺, with a linear relationship observed in the range of 0-20 µM. However, the dynamic quenched FL of Eu-CQDs/Cu²⁺ and Eu-CQDs/Fe³⁺ was swiftly enhanced by EDTA via a turn-on response. The practical use of the Eu-CQDs probe for quantifying the Cu²⁺ and Fe³⁺ ion concentrations was conducted with real water samples, and the findings indicated satisfactory recoveries (101.1% for Cu²⁺ and 104.8% for Fe³⁺) and good reproducibility, which aligned with the results obtained from UV-Vis spectroscopy. Additionally, the WST assay was assess the cytotoxic effects of Eu-CQDs, and their capability to image live HCT-116 (human colon cancer cell line) cells was assessed. These results indicate that the Eu-CQDs used for the nanoscale sensing of Fe³⁺ and Cu²⁺ metal ions are both selective and sensitive, and they can be successfully used for live cell imaging of cancer cell lines when used with this probe.