Thermally Modulated Nanospace Architectures in Apatite Particulate Films for Size-Selective Protein Adsorption

Controlling nanospace architecture in nanoparticle-based films is crucial for regulating protein–material interactions; however, simple strategies to achieve tunable nanospaces remain limited. In this study, citric acid (Cit)-coordinated HA (Cit/HA) nanoparticle dispersions were regulated through temperature-mediated interactions, thereby tailoring the dissociation states and binding configurations of Cit on the nanoparticle surfaces and leading to distinct nanospace structures in the resulting films. By adjusting the synthetic temperature and stirring time, Cit/HA nanoparticle films with nanospace sizes of 20–30 nm, sub-10 nm, and sub-5 nm were obtained. These nanospaces exhibited nanospace-dependent selective adsorption behavior toward representative proteins in fetal bovine serum, including the cell-adhesive vitronectin (VN) and the nonadhesive bovine serum albumin (BSA). The films with sub-5 nm and sub-10 nm nanospaces preferentially adsorbed VN and BSA, respectively, whereas the films with the larger nanospaces did not exhibit significant selectivity. This selective adsorption behavior is associated with a size-matching relationship between protein dimensions and nanospace size. In addition, secondary structure analyses suggested that the adsorption within confined nanospaces was favorable for maintaining protein stability. These results indicate that the nanospace-controlled Cit/HA nanoparticle films exhibit distinct protein adsorption behaviors depending on nanospace architecture, which may be useful for applications such as cell adhesion control and surface functionalization in the biomedical field.