Smart multicolor fluorescent polymer hydrogel is a kind of polymer soft material with adjustable luminous color. Its three-dimensional cross-linked network contains a large number of unique structural characteristics of water molecules, which makes it often compatible with both classic fluorescent polymer aqueous solutions and solid state. The common performance advantages of both bulk materials have huge application prospects in the fields of sensor detection, information anti-counterfeiting, bionic drive, and soft robots, and are one of the important development directions in the field of advanced light-emitting polymer materials.

In recent years, researcher Chen Tao and researcher Lu Wei of the Intelligent Polymer Materials Research Group of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences have been committed to the construction of intelligent fluorescent polymer hydrogels and their application research on bionic drive, sensor detection, etc., and they have developed successively Developed a series of new polymer hydrogel materials with intelligent adjustable fluorescence color/intensity (Aggregate, 2021, 1, e37; Adv. Intell. Syst. 2021, 2000239; Angew. Chem. Int. Ed., 2021, 60, 8608; Angew. Chem. Int. Ed., 2019, 58, 16243; ACS Macro Lett. 2019, 8, 937; Adv. Funct. Mater., 2019, 29, 1905514; Adv. Funct. Mater., 2018, 28, 1704568; ACS Sensors, 2018, 3, 2394; ACS Appl. Mater. Interfaces 2017, 9, 23884) . However, compared with organisms such as chameleons, which can show colorful skin color changes in nature, the multicolor fluorescent polymer hydrogel prepared by the scientific research team still has a narrow range of luminous color changes under external stimuli, especially it is still difficult to use a single The hydrogel realizes the mutual conversion of red, green and blue fluorescence in response to stimuli.

After careful comparison studies, it is found that the real chameleon skin and the reported multicolor fluorescent polymer hydrogels have different structures. For example, synthetic multicolor fluorescent hydrogels mostly combine two or more fluorophores at the same time. It is prepared by introducing a single isotropic cross-linking network, and a recent biological study (M. C. Milinkovitch, et al. Nat. Commun. 2015, 6, 6368) shows that the skin of a leopard chameleon contains two densely arranged layers The iridescent cell (similar to the core-shell structure, as shown in Figure 1b). This cell not only contains pigments, but also contains many photonic crystals. Chameleons use muscle movement to control the arrangement of crystals in different cell layers to present colorful The structural color changes. This significant difference in material structure prompted the research team to think about whether it is possible to construct a similar multilayer core-shell structure in polymer hydrogels, so that fluorophores with different responsiveness are distributed in different hydrogel layers, so that they can be imaged. The chameleon skin shows the intelligent response of red, green, blue, purple and other luminous colors that change to each other?

In order to verify this idea, the scientific research team proposed a material construction method of ion diffusion-induced interfacial polymerization, and successfully combined the red rare earth europium complex (R) and the aggregation-induced luminescence (AIE) blue naphthalimide (B) And pH-responsive green perylenetetracarboxylic acid (G) and other three fluorophores were introduced into the core layer, first shell layer and second shell layer of the core-shell structure polymer hydrogel respectively. Among them, the AIE-type naphthalimide was co-coated Valence grafted to the temperature-sensitive poly-N-isopropylacrylamide (PNIPAM) polymer chain, and has a temperature-induced blue fluorescence significantly enhanced properties. Thanks to this unique biomimetic multilayer core-shell structure design, the three-color fluorophores have a long spatial distance without complex photophysical interactions (such as energy transfer, complex/association luminescence, etc.). The blue and green fluorescence intensities of the first shell and the second shell of the gel can be independently and continuously adjusted by temperature and pH stimuli, respectively, so as to successfully realize intelligent response of red, green, blue, purple and other luminous colors Mutual transformation. This research work preliminarily proves the feasibility of using the biomimetic multilayer core-shell structure to prepare smart multicolor fluorescent polymer hydrogels, which is expected to play an important role in the application of seafood freshness detection, information security and encryption, and flexible discolored skin. The work was recently published on Cell Reports Physical Science 2021, 2, 100417 under Cell Press under the title "A panther chameleon skin-inspired core@shell supramolecular hydrogel with spatially organized multi-luminogens enables programmable color change". 

Figure 1 The structure of a bionic core-shell fluorescent polymer hydrogel inspired by chameleon skin and its intelligent multicolor fluorescence regulation

 (Polymer Laboratory Lu Wei)

Source: http://nimte.cas.cn/news/progress/202105/t20210508_6010439.html

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