A Multifunctional MOF-on-MOF-Based Dual-Channel Luminescent Signal Readout Strategy for Classifying Phenylglyoxylic Acid and 2,6-Dipicolinic Acid

Abstract

A desirable MOF-on-MOF-based ratiometric fluorescent probe (denoted as HPU-26@ZIF-8@Cit@Eu) was designed through the seed-mediated method for classifying phenylglyoxylic acid (PGA, a real internal exposure level of styrene) and 2,6-dipicolinic acid (DPA, a unique bacterial endospore biomarker) with high selectivity and sensitivity. Characterization techniques were carried out with the PXRD pattern, XPS spectra, UV–vis absorption spectra, and DFT-calculated HOMO and LUMO energies for proven mechanisms. The results indicated that PGA could affect the LMCT-ET process in HPU-26 and be binded to Eu³⁺ in ZIF-8, thus resulting in a significant increase both in the blue fluorescence emission peak at 476 nm and the red fluorescence emission peak at 620 nm. According to the fluorescence intensity ratio (I₆₂₀/I₄₇₆), the limit of detection (LOD) reached as low as 0.63 μM within a wide concentration range from 0 to 100 μM, and a noticeable color change from blue to red could be observed. However, upon exposure to DPA, the energy transfer between Zn²⁺ and the ligand was influenced sharply, without prejudice to the fluorescent intensity of Eu³⁺ in ZIF-8. These phenomena made this probe with a stable reference signal have an ultralow LOD of 0.26 μM for DPA in the range of 0–175 μM, converting the fluorescence color from dark blue to bright blue. Meanwhile, the developed probe was applied to the construction of logic gates and hydrogel-based film for the determination of PGA and DPA in real samples with the help of a smartphone.