Arginine- and serine-functionalized boron-doped graphene quantum dots with dual-emission fluorescence for ultrasensitive detection of Fe3+ in iron supplementation beverages†

The widespread application of graphene quantum dots (GQDs) in bioanalytical sensing is hindered by inherent limitations such as low quantum yield under visible light excitation and interference from biomacromolecule-induced background fluorescence. The paper reports a dual-emissive fluorescent probe based on arginine- and serine-functionalized boron-doped graphene quantum dots (RSB-GQDs), synthesized via a one-step thermal annealing strategy using citric acid, arginine, serine, and boric acid as precursors. The resulting RSB-GQD offers a unique dual fluorescence emission at distinct excitation wavelengths: intense blue fluorescence (λ_em = 460 nm) upon 370 nm ultraviolet excitation, and robust yellow fluorescence (λ_em = 560 nm) under 480 nm visible light excitation. The boron doping effectively modulates the electronic structure by narrowing the bandgap, significantly enhancing visible light absorption and enabling nearly equal emission intensities for both fluorescence bands. Leveraging this dual-emission property, yellow fluorescence undergoes rapid and selective quenching upon Fe³⁺ addition, while the blue emission almost remains unaffected, serving as an internal reference. The quenching efficiency demonstrates a linear correlation with Fe³⁺ concentration (0–50 μM), achieving an ultra-low detection limit of 75 nM (S/N = 3) and outperforming conventional methods. Notably, the system exhibits an exceptional selectivity for Fe³⁺ over Fe²⁺ and common biomacromolecules (e.g., proteins and amino acids), attributed to strong coordination between Fe³⁺ and surface functional groups (–NH₂, –OH, and –COOH). The practical applicability was validated through accurate quantification of free Fe³⁺ in commercial iron-fortified beverages, with recoveries of 99.4–100.8% and relative standard deviations <3.5%. This study also provides a versatile platform for designing multi-color GQD-based probes with minimized background interference, offering broad prospects in food safety monitoring and biomedical diagnostics.