Postsynthetic Functionalization of Zr4+-Immobilized Core–Shell Structured Magnetic Covalent Organic Frameworks for Selective Enrichment of Phosphopeptides

Chemical modification of covalent organic frameworks (COFs) is indispensable for integrating functionalities of greater complexity and accessing advanced COF materials suitable for more potential applications. Reported here is a novel strategy for fabricating controllable core-shell structured Zr⁴⁺-immobilized magnetic COFs (MCNC@COF@Zr⁴⁺) composed of a high-magnetic-response magnetic colloid nanocrystal cluster (MCNC) core, Zr⁴⁺ ion-functionalized two-dimensional COFs as the shell by sequential postsynthetic functionalization and, for the first time, the application of the MCNC@COF@Zr⁴⁺ composites for efficient and selective enrichment of phosphopeptides. The as-prepared MCNC@COF@Zr⁴⁺ composites possess regular porosity with large surface areas, high Zr⁴⁺ loading amount, strong magnetic responsiveness, and good thermal/chemical stability, which can serve as an ideal adsorbent for selective enrichment of phosphopeptides and simultaneous size exclusion of biomacromolecules, such as proteins. The high detection sensitivity (10 fmol) together with the excellent recovery of phosphopeptides is also obtained. These outstanding features suggest that the MCNC@COF@Zr⁴⁺ composites are of great benefit for pretreatment prior to mass spectrometry analysis of phosphopeptides. In addition, the performance of the developed approach in selective enrichment of phosphopeptides from the tryptic digests of defatted milk and directly specific capture of endogenous phosphopeptides from human serum gives powerful proof for its high selectivity and effectiveness in identifying the low-abundance phosphopeptides from complicated biological samples. This study not only provides a strategy for versatile functionalization of magnetic COFs but also opens a new avenue in their use in phosphoproteome analysis.