Size-Exclusive Magnetic Graphene/Mesoporous Silica Composites with Titanium(IV)-Immobilized Pore Walls for Selective Enrichment of Endogenous Phosphorylated Peptides

Developing an effective separation method is necessary for identifying low-abundant endogenous phosphorylated peptides with the removal of proteins. In this work, we prepared size-exclusive magnetic graphene/mesoporous silica composites with titanium(IV)-immobilized pore walls (denoted as Ti(4+)-MGMSs) for capturing endogenous phosphorylated peptides for mass spectrometry analysis. The introduction of hydrophilic polydopamine simplified the synthetic process of Ti(4+)-MGMSs, and the ordered mesoporous channels are beneficial to the trapping of endogenous phosphopeptides while large-size proteins are excluded. Furthermore, the magnetic performance greatly simplifies the entire process of enrichment. With all of the advances, the novel Ti(4+)-MGMSs present high enrichment efficiency either from the low concentration of β-casein tryptic digest (0.5 fmol/μL) or the mixture of β-casein tryptic digest and α-casein (or plus bovine serum albumin, with a mass ratio up to 1:500). Besides, Ti(4+)-MGMSs have also been successfully applied to enrich endogenous phosphorylated peptides from human serum and human saliva.