Magnetic guanidyl-functionalized covalent organic framework composite: a platform for specific capture and isolation of phosphopeptides and exosomes.

A guanidine-functionalized (GF) covalent organic framework (COF) nanocomposite has been developed by a post-synthetic approach for specific capture and separation of phosphopeptides and exosomes. The abundant binding sites on COF can immobilize a large number of gold nanoparticles (AuNPs), which can be used to react with amino groups to graft polyethyleneimine (PEI). Finally, Fe3O4@COF@Au@PEI-GF is obtained through the reaction of PEI and guanidyl group for phosphopeptides and exosomes detection. This composite shows a low detection limit (0.02 fmol), size exclusion effect (β-casein digests:Albumin from bovine serum protein = 1:10,000), good reusability (10 cycles), and high selectivity (β-casein digests:Albumin from bovine serum digests = 1:10,000). For complex biological sample, 4 phosphopeptides can be successfully identified from human serum. Furthermore, for the first time, we used guanidyl-functionalized probe to capture exosomes in human serum, providing a new method for enriching exosomes. The above experiments showed that Fe3O4@COF@Au@PEI-GF not only effectively enrich phosphopeptides and remove macromolecular proteins, but also successfully separate and capture exosomes. This demonstrates the great potential of this composite for the specific enrichment of phosphopeptides and isolation of exosomes.