Covalent Organic Framework Nanohydrogels

Abstract

Nanohydrogelation of covalent organic frameworks (COFs) will undoubtedly open up new applications for them in water, such as aqueous catalysis and biomedicine. It is currently a great challenge to achieve water dispersion of COFs through either bottom-up construction strategies or top-down exfoliating technologies. Herein, poly(N-isopropylacrylamide) (PNIPAM)-postmodified COF nanohydrogels (COF-NHGs) are successfully designed and synthesized via in situ atom-transfer radical polymerization (ATRP) on a scaffold of COFs. During the polymer growth process, the bulk COFs are exfoliated into nanosheets with a lateral size of ∼500 nm and a thickness of ∼6.5 nm. Moreover, their size can be precisely controlled by the degree of polymerization of PNIPAMs. In aqueous solution, the obtained COF-NHGs are assembled into nanohydrogels retaining intra-plane crystallinity and exhibit a temperature-sensitive sol–gel phase transition. With excellent solubility in organic solvents, the COF-NHGs’ intrinsic physical properties in the solution state can be characterized through their solution nuclear magnetic resonance and ultraviolet absorption spectra. These results put forward new opportunities for regulating the solution processability of COFs and building an intelligent, stimuli-response platform of COF–polymer composite nanohydrogels for device applications.