Wavelength-shift-based visual fluorescence sensing of aspartic acids using Eu/Gd-MOF through pH triggering

With the increasing demand for on-site detection, the current approach of building dual-emission or multi-emission luminescence sensors based on metal-organic frameworks (MOFs) which possess the capacity of self-reference for numerous non-analyte factors falls short of meeting sensing requirements. Therefore, we have designed a novel strategy for constructing wavelength shift-based luminescence sensor named Eu/Gd(TCPP), which exhibits dual-emitting from metal ions Eu3+ and flexible rotating aggregation-induced emission (AIE) ligands H4TCPP (2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine). This sensor was prepared by a simple, green and fast plasma synthesis method. It's worth noting that the fluorescence emission of Eu/Gd(TCPP) shows a specific wavelength shift from ligand peak, and a visual color change from red to blue within a pH range of 4 to 3. Moreover, various characterization data verified that the luminescence switching mechanism of Eu/Gd(TCPP) was attributed to the H+-induced collapse of the Eu/Gd(TCPP) crystal structure, followed by untwisting of free ligands that lose rigid MOFs confinement. This hindered the antenna effect from H4TCPP to Ln3+ ions and restricted the rotation emission of ligand, resulting in the red-shifting of the ligand emission and corresponding luminescence switching. By tactfully utilizing the short-range pH response property of Eu/Gd(TCPP), highly sensitive and selective on-site visual detection of acidic aspartic acid can be achieved.