(A) Schematic illustration of the synthesis and functionalization of Fe3O4 NPs. GAPT-Zn chelate groups have strong binding to phosphate groups. Molecules and NPs are not drawn to scale. (B) TEM image of as-synthesized Fe3O4-OA/OE NPs, (C) PXRD pattern of Fe3O4-OA/OE NPs. Inset shows the size distributions of the Fe3O4-OA/OE (red) and Fe3O4-GAPT-Zn (blue) (D) FTIR of Fe3O4-OA/OE (blue), Fe3O4-NH3+/PEG (red), and Fe3O4-GAPT (green) NPs. (E) TEM image of the functionalized Fe3O4-GAPT-Zn NPs. (F) TGA analysis of Fe3O4-OA/OE(blue), APTMS and Si-PEG (red), and GAPT coupled to APTMS (green). (G) Functionalized Fe3O4-GAPT-Zn NPs well-dispersed (left) and collected by a magnet (right) in aqueous solutions.