PB@UiO-67-CDC-(CH3)2 as an Ultrasensitive Ratiometric Fluorescence Sensor: Visible “Turn-On” Effect for Detecting Preservatives and Amino Acids

Sensory material which can recognize preservatives and amino acids through different fluorescence changes has significant application value in many fields. Here, a pore-functionalized metal–organic framework (MOF) becomes a versatile platform for fluoro-sensing various preservatives and amino acids with higher emission and multicolorful scaffolds and allows for visible detection. The carbazole groups in a Zr-based MOF UiO-67-CDC (CDC = 9H-carbazole-2,7-dicarboxylic acid) are partially substituted into dimethylation through postsynthetic N-methylation to form a positively charged framework UiO-67-CDC-(CH3)2. Because of electrostatic induction, the postmodification skeleton can efficaciously encapsulate the anionic dye phloxine B (PB) in the cavity of the electron-deficient MOF, and a newfangled fluorescence “turn-on” behavior is ascribed due to the inhibition of photoinduced electron transfer (PET) (from the benzene rings to the quaternary ammonium group on the ligand) and the stunning synergistic effect of the confinement-induced emission (CIE) enhancement. PB@UiO-67-CDC-(CH3)2 shows a unique fluorescence response for preservatives and amino acids with variable colors and visual, quantitative discrimination effects, which could be applied for sensing, switching, and anticounterfeiting identification. Notably, the fluorescence emission peak of dye (PB) and MOF presents a ratiometric change, further improving the detection’s resolution and veracity. The sodium acetate and leucine detection limits are as low as 0.0157 μM and 0.1084 μM, respectively. In addition, a smartphone was used to visualize sodium acetate by identifying the RGB value of the fluorescent changes, and the LOD can be as low as 7.23 μM. These features make the dye-embedded MOF composite a simple, efficient, and instrument-free stratagem for the on-site and convenient detection of preservatives in various samples.