Ratiometric fluorescence determination of Gd(III) using plumbagin: method validation and analysis of real samples

In this study, the spectroscopic interaction between plumbagin (PLB) and Gd(III) was studied using UV-vis and fluorescence spectroscopy techniques. When Gd(III) coordinated with PLB, clear spectral changes were found, together with the appearance of two fluorescence emission bands at about 530 and 610 nm. The emission at 530 nm was more sensitive to Gd(III) than the emission at 610 nm. Based on this result, a ratiometric fluorescence index R = I₅₃₀/I₆₁₀ was developed for the quantitative determination of Gd(III) in solution. Results from ¹H-NMR spectroscopy, ESI-MS, FT-IR spectroscopy, XPS, and DFT calculations (HOMO-LUMO analysis) confirmed the predominant formation of the [Gd(PLB)₃]⁺ complex. Spectroscopic evidence supported complex formation and suggested that fluorescence modulation mainly occurs via a static quenching mechanism. The optimal conditions were pH = 8.5, solvent composition of ACN : H₂O = 2 : 3 (v/v), and an excitation wavelength of λ_ex = 320 nm. The ratiometric response increased signal reliability by decreasing susceptibility to instrumental fluctuations and sample matrix interference. The method was successfully used for the determination of gadolinium in fertilizer samples, and results showed good agreement with ICP-MS findings. This study provides new spectroscopic evidence for the Gd-PLB interaction and shows the potential of ratiometric fluorescence strategies for practical elemental analysis.