2.4. Characterization

Transmission electron microscopy (TEM) images were acquired on a JEM-2100UHR microscope (JEOL, JEM-2100UHR, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Inc., ESCALAB 250Xi, Asheville, NC, USA) and energy dispersive spectrometer (EDS) were applied to analyze the chemical constitutions of CaS, CaS@PP and I-CaS@PP. X-ray diffraction (XRD, Bruker, Bruker D8, Karlsruhe, Germany) was performed to analyze the crystal structure of CaS and I-CaS@PP. The hydrodynamic particle sizes and zeta potentials of NPs were monitored by dynamic light scattering (DLS, Brookhaven Instruments Corporation, NanoBrook 90 Plus PALS, New York, NY, USA). The loading amount of ICG in I-CaS@PP was determined based on the UV–Vis absorbance peak at 780 nm, and it was calculated according to the followed Eq. (3):

The loading amount of CaS in I-CaS@PP was determined by detecting Ca and S amounts in I-CaS@PP using ICP-MS (Thermo Fisher Scientific Inc., Thermo ICAP PRO, NC, USA).