Silk fibroin particles prepared by phase separation with polyethylene oxide were coated with calcium-deficient hydroxyapatite (CDHA) crystals under various pH conditions. For different pH values, the growth and the morphology of CDHA crystals on the surface of silk fibroin particles were investigated in detail by zeta potential analysis, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction techniques. Negative charges formed by deprotonation of the functional groups on the surface of silk fibroin particles at high pH lead to an increase of binding affinity between the calcium ions of the CDHA crystals and the functional groups of the silk fibroin particles. Consequently, the generation of many CDHA crystals was promoted to deposit on the surface of silk fibroin particles at a high pH value.
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