Tecnai f 20

TEM was performed using a Tecnai F-20 and the hydrodynamic diameter and zeta potential were measured using a Nano-ZS 90 (Malvern Instrument, Malvern, UK). Inductively coupled plasma optical emission spectroscopy (ICP-OES) (Spectro Genesis ICP) was used to determine iron concentrations. The UV-VIS spectra were recorded using a UV-VIS spectrometer (Beckman Coulter, DU800). Pierce BCA Protein Assay Kit (Thermo Scientific) was used to determine concentrations of the GOx enzyme. Powder X-ray Diffraction (XRD) was recorded using a Rigaku GiegerFlex D/Max-B X-ray diffractometer operated at 45 kV and 30 mA with a monochromatized Cu Kα radiation wavelength of 1.5406 Å and a scan rate of 2° per minute. Energy-Dispersive X-ray (EDX) was performed using a Quanta 600 field emission gun scanning electron microscope (FEI) operated at 15 kV and equipped with EDX detectors (EDAX, Inc.).