Synthesis of Polydopamine-Coated Magnetic Graphene for Cu2+ Immobilization and Application to the Enrichment of Low-Concentration Peptides for Mass Spectrometry Analysis

In this work, Cu2+-immobilized magnetic graphene@polydopamine (magG@PDA@Cu2+) composites were synthesized for the first time. Magnetic graphene prepared via a hydrothermal reaction were easily encapsulated by a layer of polydopamine through the oxidative polymerization of dopamine in alkaline buffer, and it was conveniently modified with Cu2+ ions afterward. The as-prepared magG@PDA@Cu2+ composites were endowed with strong magnetic responsivness, excellent dispersibility and biological compatibility. We applied the novel nanocomposites to the enrichment and identification of low-concentration standard peptides, peptides in standard protein digestions, endogenous peptides in human urine and serum. The enriched peptides were eluted and analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The magG@PDA@Cu2+ composites were proved to exhibit great affinity to both hydrophobic and hydrophilic peptides, thus providing a rapid and facile approach to the extraction of low-concentration peptides. Notably, peptides at an extremely low concentration of 10 pM could be detected by MALDI-TOF MS after enrichment with magG@PDA@Cu2+ composites. The results demonstrated that the magG@PDA@Cu2+ composite is a promising candidate for the enrichment of low-abundance peptides for mass spectrometry analysis.