Remote light-controlled high-intensity rare earth hybrid luminescent sol-gel reversible phase transition
   Luminescent hydrogels have attracted widespread interest due to their important applications in bioimaging, drug delivery, OLED, and fluorescence sensing. Compared with luminescent groups such as quantum dots and organic dyes, rare earth complexes are considered to be an ideal luminescence center due to their strong light absorption ability, long excited state lifetime, high color purity, rich emission lines, and high quantum efficiency. However, the preparation of rare earth hybrid hydrogels, especially the preparation of high-strength rare earth hydrogels, still faces severe challenges: 1) The high-frequency stretching vibration of OH in water molecules couples with the central rare earth ion, resulting in the central rare earth ion produces non-radiative decay and fluorescence quenching. 2) Another challenge facing rare earth luminescent hydrogels is how to prepare smart gels with stimulus responsiveness. Common external stimulus sources include light, electricity, magnetism, temperature, pH, redox, etc. However, the luminescence properties of rare earth ions are extremely sensitive to the environment. Both acidic or strong alkaline environments can cause fluorescence quenching of rare earth ion; redox can also easily cause changes in the valence of rare earth ions. Therefore, the preparation of stimulus-responsive smart materials containing rare earth ions puts forward higher requirements for stimulus sources. Compared with other stimulus sources, light control has the advantages of remote, clean, fast, and efficient, and light irradiation has become an ideal stimulus source for realizing precise and controllable reversible intelligent gel systems. Therefore, the construction of smart rare earth luminescent hydrogels with light stimulus responsiveness has become the research goal of many researchers.
    Recently, the Li Huanrong team of Hebei University of Technology used a multi-stage hybrid self-assembly strategy to successfully construct a high-strength rare-earth hybrid luminescent hydrogel and realize its remote light-control sol-gel reversible phase transition, which is the first report in a gel with such a high elastic modulus.


    The team has recently been committed to the multi-level hybrid self-assembly strategy to construct a rare earth hybrid luminescent hydrogel with high mechanical strength, high water content and self-repairing properties (Adv. Funct. Mater., 2017, 27, 1604379; Macromolecules , 2017, 50, 1141-1146). Recently, they have synthesized a cyclodextrin-modified rare earth complex. After introducing an azobenzene guest molecule, they obtained a light-stimulus-responsive α-cyclodextrin/azobenzene host-guest inclusion compound and used it as the "molecular glue" cross-linked the nano-clay flakes to obtain a high-intensity luminescent hydrogel, and further successfully realized the reversible phase transition of the gel sol through the alternating stimulation of ultraviolet/visible light. In addition, there is no conflict between the photoisomerization wavelength of azobenzene (365-450 nm) and the excitation emission wavelength of rare earth complexes, and there is no charge/electron transfer process between the two, so the remote light control gel The phase transition of the sol does not affect the luminescent properties of the gel. On this basis, they adjusted the luminescence color of the hybrid hydrogel by adjusting the type and ratio of rare earth ions in the gel (Eu3+/Tb3+).


Related results were recently published in Angew. Chem. Int. Ed. 


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