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.