Preparation of polyoxometalate-chitosan magnetic composite for the enrichment of phosphopeptides

: A method for the enrichment of phosphopeptides based on magnetic nanoparticles coated with polyoxometalate (POM) was developed. A layer-by-layer assembly technique was used to prepare magnetic nanoparticles (MNPs) coated with polyoxometalate and chitosan. The composites were used for phosphopeptide enrichment prior to analysis by matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). The composites exhibited advantages of rapid magnetic separation, hydrophilicity, positive electricity, and a high selectivity for enrichment of phosphopeptides. β-Casein was selected as a model phosphorylated protein to assess the performance of the enrichment technique. After the enrichment step, the limit of detection was determined to be 0.02 fmol. The prepared POM-based MNPs thus exhibit significant potential in the detection of low-abundance phosphoproteins.