Hydrophilic polydopamine-coated magnetic graphene nanocomposites for highly efficient tryptic immobilization

In this work, polydopamine-coated magnetic graphene (MG@PDA) nanocomposites were synthesized by a facile method. Trypsin was then directly immobilized on the surface of the nanocomposites through simple PDA chemistry with no need for introducing any other coupling groups. The as-made MG@PDA nanocomposites inherit not only the large surface area of graphene which makes them capable of immobilizing high amount of trypsin (up to 0.175 mg/mg), but also the good hydrophilicity of PDA which greatly improves their biocompatibility. Moreover, the strong magnetic responsibility makes them easy to be separated from the digested peptide solution when applying a magnetic field. The feasibility of the trypsin-immobilized MG@PDA (MG@PDA-trypsin) nanocomposites for protein digestion was investigated and the results indicated their high digestion efficiency in a short digestion time (10 min). In addition, the reusability and stability of the MG@PDA-trypsin nanocomposites were also tested in our work. To further confirm the efficiency of MG@PDA-trypsin nanocomposites for proteome analysis, they were applied to digest proteins extracted from skimmed milk, followed by nano RPLC-ESI-MS/MS analysis, and a total of 321 proteins were identified, much more than those obtained by 16-h in-solution digestion (264 proteins), indicating the great potential of MG@PDA-trypsin nanocomposites as the supports for high-throughput proteome study.