*2.5. Characterization*

Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images were acquired using a JEOL EM-2010 microscope (JEOL, Tokyo, Japan) at an accelerating voltage of 200 kV. The powder X-ray diffraction (XRD) pattern was obtained by a Rigaku D/Max-3C diffractometer (Cu K α radiation, λ = 0.15418 nm, Rigaku Co. Ltd., Tokyo, Japan). The UV-Vis absorption spectra were acquired by a Jasco V-570-type spectrophotometer (Jasco SLM-468, Tokyo, Japan). Elemental analysis was performed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using an ICPS-7500 spectrometer (Shimadzu, Kyoto, Japan). The MTT cell proliferation assay was performed by an ELx808TM absorbance microplate reader (Biotek Instruments Inc., Winooski, VT, USA).
