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

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 (I-620/I-476), the limit of detection (LOD) reached as low as 0.63 mu M within a wide concentration range from 0 to 100 mu M, 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 mu M for DPA in the range of 0-175 mu M, 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.