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

Abstract

In this research, the spectroscopic interaction between plumbagin (PLB) and Gd(III) was studied using UV-Vis, fluorescence 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 is 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 the 1H-NMR, ESI-MS spectrum, FT-IR, XPS and the calculated DFT, HUMO LUMO also confirmed the predominant formation of the [Gd(PLB)3]+ complex. Spectroscopic evidence supported complex formation and suggested that the fluorescence modulation mainly happened from a static quenching mechanism. The optimal conditions were pH = 8.5, solvent composition ACN:H2O = 2:3 (v/v), and excitation wavelength λex=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 the results showed good agreement with ICP-MS. This study provides new spectroscopic evidence for the Gd-PLB interaction and showed the potential of ratiometric fluorescence strategies for practical elemental analysis.