Covalent organic framework-MnO2 nanoparticle composites for shape-selective sensing of bithiols

Covalent organic frameworks (COFs) for detecting biological macromolecules in water or biological environments are generally challenging. In this work, a composite material IEP-MnO2 is obtained by combining manganese dioxide (MnO2) nanocrystals and a fluorescent COF (IEP), which is synthesized by using 2,4,6-tris(4-aminophenyl)-s-triazine and 2,5-dimethoxyterephthalaldehyde. By the addition of biothiols, such as glutathione, cysteine or homocysteine with different sizes, the fluorescence emission spectra of IEP-MnO2 changed (“turn-on” or “turn-off”) via different mechanisms. The fluorescence emission of IEP-MnO2 increased in the presence of GSH by the elimination of the FRET (Förster resonance energy transfer) effect between MnO2 and IEP. Surprisingly, due to the formation of a hydrogen bond between Cys/Hcy and IEP, the fluorescence quenching for IEP-MnO2 + Cys/Hcy may be explained via the photoelectron transfer (PET) process, which endows IEP-MnO2 with specificity in distinguishing the detection of GSH and Cys/Hcy compared to other MnO2 complex materials. Therefore, IEP-MnO2 was used to detect GSH and Cys in human whole blood and serum, respectively. The limit of detection for GSH in whole blood and Cys in human serum was calculated to be 25.58 μM and 4.43 μM, which indicates that IEP-MnO2 can be used to investigate some diseases related to GSH and Cys concentration. Moreover, the research expands the application of covalent organic frameworks in the fluorescence sensing field.