Oxygen-Rich Graphene Quantum Dots Enable Detection of Pr³⁺, Ho³⁺, and Er³⁺ Via Spectral Overlap Engineering

Rare-earth impurities in high-purity rare-earth materials can significantly affect their performance. Therefore, developing accurate and rapid methods for detecting rare-earth has become an urgent necessity. Graphene quantum dots (GQDs) emerge as promising candidates for fluorescent probes. This study presents a novel approach to determining the concentrations of energy-resonant rare-earth ions (Pr³⁺, Ho³⁺, Er³⁺) using oxygen-rich GQDs. These GQDs were synthesized through an oxidation-cutting method involving H₂O₂ and KOH, followed by separation using different dialysis bags. The fluorescence intensity of the GQDs displayed strong linear correlations with the concentrations of Pr³⁺, Ho³⁺, and Er³⁺ within the ranges of 5–200 µM, 100–350 µM, and 20–150 µM, respectively. The case of industrial separation of high-purity Pr³⁺ was simulated. For Pr³⁺, the fluorescence intensity still shows a linear correlation in the range of 10–130 µM under the interference of Nd³⁺. The sensing mechanisms were systematically investigated using comprehensive multidimensional characterization techniques. UV-Vis and Raman spectroscopy reveal that Pr³⁺ ions form ground-state complexes with GQDs through synergistic coordination involving the π-conjugated system and oxygen atoms’ lone-pair electrons. TRPL demonstrated spectra resonance-based energy transfer from GQDs to Pr³⁺, accompanied by quantum energy loss during relaxation. Furthermore, the FL spectrum of Pr³⁺ indicates that the formation of the ground-state complex induces ground-state energy-level splitting, which facilitates the fluorescence resonance energy transfer. The combined effects of static and dynamic quenching lead to the fluorescence decay of GQDs. This work demonstrates the promising potential of GQDs for the rapid and accurate detection of rare earth ions.