Facile synthesis of core–shell structured magnetic covalent organic framework composite nanospheres for selective enrichment of peptides with simultaneous exclusion of proteins

Selective enrichment of peptides from complex biosamples is essential for mass spectrometry-based proteomics but still remains a challenge. In this work, a facile approach was developed for rapid room-temperature synthesis of core-shell structured magnetic covalent organic framework composite nanospheres (denoted as Fe₃O₄@TbBd) by using monodisperse Fe₃O₄ nanoparticles as the magnetic core and 1,3,5-triformylbenzene (Tb) and benzidine (Bd) as two building blocks in the presence of dimethyl sulfoxide (DMSO). The resultant core-shell structured Fe₃O₄@TbBd nanospheres exhibited high adsorption capacity (28.5 mg g^-1), fast adsorption kinetics (<5 min) and excellent reusability (more than 30 times) for peptides, owing to their specific properties of high surface area (196.21 m² g^-1), large pore volume (0.63 cm³ g^-1), narrow pore size distribution (∼2.8 nm), strong magnetic response (41.5 emu g^-1), as well as good thermal and chemical stability. Moreover, the Fe₃O₄@TbBd nanospheres also showed good selectivity towards hydrophobic peptides and a size-exclusion effect against proteins due to the inherent π-π stacking interaction and interconnected mesoporous channels of covalent organic framework (COF) shells. Taking advantage of these composite nanospheres, selective extraction and efficient enrichment of low abundance hydrophobic peptides from human serum in the presence of high abundance proteins were achieved. Based on the HPLC-Q-TOF/MS results, 29 hydrophobic peptides assigned to 12 proteins were clearly identified in 5 ng μL^-1 human serum digestion upon treatment with Fe₃O₄@TbBd nanospheres, much better than those obtained without treatment, confirming the outstanding performance of the Fe₃O₄@TbBd nanospheres in proteome analysis.