Effective Immobilization of Monomeric Methylene Blue on Hydroxyapatite Nanoparticles by Controlling Inorganic–Organic Interfacial Interactions

• Published in: Inorganic chemistry Vol. 61; no. 12; pp. 4865 - 4878
• Main Authors: Yamada, Iori, Kataoka, Takuya, Ikeda, Ryota, Samitsu, Sadaki, Tagaya, Motohiro
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28.03.2022
• Subjects: Durapatite; Methylene Blue; Nanoparticles
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.1c03569

We successfully synthesized methylene blue (MB+)-immobilized hydroxyapatite (HM) nanoparticles by changing the initial P/Ca molar ratio. The immobilized amount of MB+ increased with increasing the initial P/Ca molar ratio from 0.6 to 4.0, and the HM had an elliptical shape (long length, 21–24 nm; short length, 11–13 nm) irrespective of the initial P/Ca molar ratio. Upon increasing the initial P/Ca molar ratio, the number of carbonate ions on the HM surface decreased, which would be owing to the electrostatic repulsion by the surface phosphate ions (i.e., P–O–), the surface P–OH mainly dissociated to form P–O–, and the electrostatic interaction of P–O– with MB+ enhanced. The bonding of MB+ with surface P–OH and Ca2+ sites of hydroxyapatite would be hydrogen-bonding and Lewis acid–base interactions, respectively. The optimum synthesis condition for MB+ immobilization at the monomer state was found to be the initial P/Ca molar ratio of 2.0. Here, the existence percentage of the MB+ monomer and the molecular occupancy of the surface carbonate ion at the initial P/Ca molar ratio of 2.0 were higher than those at 4.0 with no significant difference in the immobilized amount of MB+, indicating that MB+ at the initial P/Ca molar ratio of 4.0 is more aggregated than that at 2.0. These results suggested that a part of carbonate ions has a role as a spacer to suppress MB+ aggregation. Accordingly, the interfacial interactions between the MB+ monomer and the hydroxyapatite surface were clarified, which can effectively be controlled by the initial P/Ca molar ratio. These findings will provide fundamental and useful knowledge for the design of calcium phosphate–organic nanohybrids. We believe that these particles will be the base materials to realize diagnostic and/or therapeutic functions through the molecular state control by optimizing the synthesis conditions.