Electronic synergy between ligands of luminol and isophthalic acid for fluorescence ratiometric detection of Hg2

Abstract

Stimulus-responsive double-ligand luminol-Eu-IPA infinite coordination polymer nanoparticles (luminol-Eu-IPA CPNPs) were prepared as a ratiometric fluorescence probe for highly selective detecting Hg²⁺. The CPNPs were constituted of Eu³⁺ as the nuclear metal coordinated by isophthalic acid (IPA) together with luminol as an auxiliary ligand. The photoinduced electron transfer (PET) occurring from IPA to luminol prevented the antenna effect between IPA and Eu³⁺, leading to the quench fluorescence of Eu³⁺ under light excitation. As Hg²⁺ has a high affinity to N atom of luminol and the spin-orbit coupling effect, spectroscopically and magnetically silent properties, the fluorescence intensity of luminol was quenched. Meanwhile, the PET effect between luminol and IPA was interrupted under the presence of Hg²⁺. This process resulted in a significant decrease in the fluorescence intensity of luminol and a significant increase in the fluorescence intensity of Eu³⁺. Therefore, the fluorescence ratiometric detection of Hg²⁺ was performed by monitoring the ratio of the fluorescence at 617 nm of Eu³⁺ to that at 430 nm of luminol. The linear range was from 0.05 to 20 μM with a detection limit as low as 13.2 nM Hg²⁺ (S/N = 3). Due to the fluorescence of luminol be quenched and the effect of PET be disrupted simultaneously, the probe exhibiting excellent detection selectively can avoid false positive signals, which was demonstrated for monitoring mercury ions in real water samples. Precision in positioning ligands in CPNPs is an advantage to achieve high specificity in comparison to traditional organic dendrimers or precious metal nanomaterials.

Keywords: Heavy metal ion; Lanthanide; Photoinduced electron transfer; Ratiometric fluorescence; Visual quantification.