Molecular Dynamics Exploration of the Growth Mechanism of Hydroxyapatite Nanoparticles Regulated by Glutamic Acid

Morphological control can enhance the performance of materials like hydroxyapatite (HAP), a well-known bioceramic with various morphologies, including spheres, rods, whiskers, needles, and plates. To obtain certain HAP morphologies, the crystal growth mechanisms at different planes should be investigated. Here, molecular dynamics was employed to understand the mechanism of HAP nanoparticle growth regulated by glutamic acid (Glu). Long-time dynamics simulations and free energy calculations were performed to explore the effect of Glu on calcium and phosphate ion precipitation on the HAP (100) and (001) faces. Without Glu, PO₄^3- prefers binding to the HAP (100) surface, whereas with Glu, the (001) surface is preferred. This could partially explain why HAP changes from needle-like to plate-like with Glu addition in experiments. Our theoretical results indicate that Glu inhibits calcium and phosphate ion deposition on the crystal surfaces by occupying the calcium sites on the outermost layers. In addition, Glu has a strong concentration gradient effect on HAP deposition. At Glu concentrations of >80 mM, ion deposition was inhibited more on the (100) than on the (001) surface. Our results agree with experimental observations and afford insights into complicated HAP crystal growth mechanisms with foreign additives, which will aid in HAP synthesis with morphological control.