Synthesis scheme of Fe3O4-SiO2-GPS-CDI core-shell NPs. The detailed steps for surface functionalization include (1) Fe3O4-SiO2 NPs were hydroxylated in piranha
solution (H2SO4-H2O2) to create hydroxyl groups on the NP surface for coupling to GPS. (2) Surface modified Fe3O4-SiO2 NPs was redispersed in an aqueous solution of
GPS and toluene in order to bind silane group of GPS to the hydroxyl group on the surface of the modified Fe3O4-SiO2 NPs. The obtained Fe3O4-SiO2-GPS NPs have epoxy
groups on its surface. (3) Prior to functionalization of the Fe3O4-SiO2-GPS NPs with CDI, epoxy groups of the NPs were converted into hydroxyl groups. (4) The hydroxylated
Fe3O4-SiO2-GPS NPs were redispersed into acetonitrile containing CDI. During the sonication, acylimidazole leaving groups of CDI bind to hydroxyl groups to form reactive
carbonyl imidazole groups. Since amino or carboxyl groups are the most common functional groups in biomolecules, either of these two functionalities can be present on
the Fe3O4-SiO2-GPS-CDI NPs surface to allow further derivatization with biological ligands. Therefore, the Fe3O4-SiO2-GPS-CDI NPs with available carbonyl imidazole groups
can couple the amino group on the biomolecules.