MHC synthesis and characterization
The MHC synthesis steps and structure are shown in Figure 2. Briefly, the organic-inorganic covalently bound MHC was synthesized in one step by mixing Fe3+, Fe2+, and silane coupling agents in proportion, slowly titrating alkalinity, and ageing. The TEM image (Figure 3a) shows that MHC has an irregular shape. Comparing to the Fe3O4 TEM image (Figure 3b), it can be seen that the black component in MHC is Fe3O4. The periphery of Fe3O4 is the highly-crosslinked polymer of the organic silane coupling agent. It shows that MHC is a cross-linked integral structure consisting of magnetic components and organic compounds.
The presence of silane coupling agents will form Si-O-Si bonds. Generally, the asymmetric stretching peak of Si-O-Si is at 1046 cm-1.18 Shown in the infrared absorption spectrum of MHC (Figure 3c), there is a clear absorption peak at 1028 cm-1. It may be related to red shift of the Si-O characteristic peak induced by Fe replacing Si in Si-O-Si.19 This indicates that the inorganic Fe components in MHC are coupled to organic components through covalent bonds of Si-O-Fe, which is different from MSC that simply physically mixes magnetic seeds and coagulants. Other absorption wavenumbers are listed in Table S1. In addition, MHC has a high zeta potential (>25 mV) over a wide pH range of 3–11 (Figure S1), which can be attributed to the introduction of positively-charged quaternary ammonium groups.