Nanoconfinement and Interface Effects on Calcium Phosphate Aggregation within a 2D Nanochannel: Insights from Deep-Learning Molecular Dynamics.

In this study, we investigated the hydration and aggregation dynamics of Ca²⁺ and phosphate species, as well as the structural characteristics of calcium phosphate clusters, within a two-dimensional (2D) nanochannel using molecular dynamics simulations with a deep learning potential. Our findings show that ion dynamics are markedly enhanced under confinement, primarily due to accelerated water dynamics. Ion hydration within the 2D nanochannel is reduced as a result of layered water distribution and frequent water exchange around ions compared to the bulk phase solution, thereby facilitating coordination between Ca²⁺ and phosphate species despite observed polarization effects. However, an increased energy barrier for association between Ca²⁺ and phosphate species can slow their aggregation within the 2D nanochannel. Since protonated phosphate species exhibit a stronger preference for interfacial water layers than PO₄^3-, fewer protons are present in cluster of Ca²⁺ and phosphate species in the bulk-like region, which facilitates the association of Ca²⁺ and phosphate species. The interfacial enrichment of protonated species can also promote the transformation of amorphous calcium phosphate (ACP) to hydroxyapatite. Our results presented here elucidate the influence of nanoconfinement and interfacial interactions on calcium phosphate aggregation within 2D nanochannels, offering valuable insights into biological and biomimetic mineralization processes.