3.1. Preparation and characterization of OHH NPs
The synthesis method of OHH NPs was schematically illustrated in Scheme 1. The morphology of the obtained nanomaterials was characterized through transmission electron microscopy (TEM). The image of Fig. 1A indicated that PB NPs was successfully prepared with a size of ∼100 nm32, 34-36. HMPB NPs were synthesized on basis of PB NPs by the controllable acid etching strategy, accompanying with morphological change from square to spheric. Fig. 1B clearly showed hollow mesoporous Prussian blue (HMPB) were successfully prepared with a diameter of ∼75 nm and the hollow mesoporous structure. After loading ofloxacin and coating with HA, the uniform size of OHH NPs slightly increased to ∼78 nm (Fig. 1C), which was consistent with the result of the dynamic light scattering (Fig. 1D). In Fig. 1E, the zeta potential of HMPB increased from -19 mV to 8 mV after loading ofloxacin owing to the positive charge of ofloxacin, while HA coating layer with a negative charge caused the decrease of HMPB loaded ofloxacin from 8 mV to -15 mV (Fig. 1E). The change of charge indirectly reflected the successful synthesis of OHH NPs. The UV-vis absorption spectra of HMPB NPs, ofloxacin and OHH NPs are shown in Fig. 1F. The as-obtained HMPB NPs possessed a broad absorption in the NIR region with an absorption peak at approximately 710 nm. After loading ofloxacin, a strong absorption peak of OHH NPs appeared at 288 nm which was a characteristic absorption of ofloxacin, indicating that ofloxacin was successfully loaded into HMPB NPs. After coating with HA, the strong absorption peak of OHH NPs red-shifted to 800 nm because of the absorbance of HA. Fig. 1G displayed the FT-IR spectra of HMPB NPs, ofloxacin and OHH NPs. The characteristic peak of HMPB NPs at 2086 cm-1 originated from the –CN stretching vibration. Compared with HMPB NPs, new peaks of OHH NPs appeared at 1414 cm-1, 1708 cm-1 and 1630 cm-1, which was assigned to the C=O, C=C and O-F vibration of ofloxacin, respectively, further demonstrating the successful loading of ofloxacin. Besides, X-ray photoelectron spectroscopy (XPS) was also performed to investigate the chemical composition of OHH NPs. As shown in Fig. 1H, the binding energy peak of HMPB at 708.8 eV was attributed to Fe 2p. However, Fe 2p peak of OHH NPs disappeared due to the presence of HA layer. In addition, obvious binding energy peak located at 689.1 eV corresponding to F 1s was observed from OHH NPs (Fig. S1), indicating successful loading of ofloxacin. The X-ray diffraction (XRD) of OHH NPs showed the same diffraction peak as that of HMPB (Fig. S2), indicating that the crystal structure of HMPB loaded with ofloxacin has no effect. The as-above characterization results solidly demonstrated that OHH NPs was successfully prepared.