3.1. Adsorption of proteins onto calcium salt particles
Fig. 1 shows the protein adsorption capacity of different calcium salts. It suggested that the adsorbed SC concentration (defined as
the protein adsorption per unit mass of calcium salts) increased with the initial SC concentration until a saturated adsorption state
was reached. Although the mean diameters of calcium particles are quite similar for all three calcium particles, there are differences in
their adsorption curves. For CaCO3, the saturation adsorption state could be reached at very low protein concentration, which suggested a much smaller adsorption capacity than HA and TCP. According to the morphology characterization of different calcium
particles in supplementary information (Fig. S4), the particle surface for CaCO3 was relatively smooth while the HA and TCP particles were much rougher and more likely packed by small ones as the scanning electron microscope (SEM) or transmission electron
microscope (TEM) images indicated. This could explain why the specific surface area of CaCO3 particle (13.8 m2/g) was smaller than
those of HA (88.6 m2/g) and TCP (67.2 m2/g) particle. The smoother surface of CaCO3 provides limited binding sites if with the same
quantity of calcium particles. So there are more protein adsorbed on the rough surfaces of HA and TCP which give more space for protein binding.
The electrostatic interactions were illustrated as the main
driving force for protein adsorption onto calcium particles in the previous studies (Feng, Chen, & Zhang, 2002; Zhu et al., 2007). The
protein can interact with calcium particles through both amino groups with the positively charged sites (Ca2) and carboxyl groups
with the negatively charged (CO3 2_ and HCO3
_ for CaCO3; PO4