Copolymer-mediated synthesis of hydroxyapatite nanoparticles in an organic solvent

The objective of this research is to develop a nanoparticle synthesis scheme that controls nanoparticle shape and surface chemistry concomitantly. Specifically, a method to synthesize hydroxyapatite nanoparticles using a dispersed block copolymer template is explored, which produces spherical and needle-shaped nanoparticles, and at the end of the synthesis, the block copolymer is retained as a surface coating on the nanoparticles. This strategy has been used previously with double-hydrophilic block copolymers (DHBCs) as the dispersed template; however, in this work, an alternative block copolymer chemistry is explored in an effort to extend this method to synthesis in organic solvents, producing nanoparticles that are organophilic instead of hydrophilic. The hydroxyapatite nanoparticles were synthesized using poly(methyl methacrylate)-b-poly(methacrylic acid) (PMMA-b-PMAA) as the dispersed template and tetrahydrofuran as the solvent. The synthesis proceeds following the ionization of the PMAA block of the copolymer and association between this ionized group and the calcium precursor ions. To investigate the degree of shape control available, the concentration of block copolymer solution and the amount of precursor were systematically varied, and the synthesized HAp nanoparticles were characterized. SEM images showed that needle and spherical HAp nanoparticles could be synthesized by changing the block copolymer concentration. TGA, FT-IR, and XRD results indicated that the block copolymer used for synthesis remained on the HAp particle surface. Overall, these results indicate that the shape of the nanoparticles produced by this method was related to the Ca(2+)/COO(-) mole ratio used during synthesis, similar to results obtained with DHBC template synthesis. The qualitative agreement between the shape control mechanisms in the two synthesis schemes suggests that this relationship could be general to the overall synthesis scheme and provide a mechanism for controlling nanoparticle shape with many block copolymer chemistries.