Antibacterial efficacy of bone mimicking-hydroxyapatite nanoplates with varying morphology

The living bone comprises hydroxyapatite (HA) nanocrystals as a 2-dimensional (D) nanoplate. However, detailed investigation on the antibacterial activity of HA nanoplates is limited in available reports. Therefore, this study presents the antibacterial efficacy of different morphologies of HA nanocrystals; HA nanorods, nanoplates and thin nanoplates against E. coli and S. aureus bacteria. The highly biocompatible, polyvinyl alcohol (PVA) was evolved as a capping agent to control the morphology of HA nanocrystals, such as nanorods; (70 - 500 x 22 - 45 nm), nanoplates; (150 - 500 x 60 - 250 x 7 - 40 nm) and thin nanoplates (thickness: 7 - 23 nm, length: nm to mu m), via hydrothermal route. The thickness of HA thin nanoplates closely mimics the natural HA nanoplates, present in living bone. The possible molecular mechanism controlling the growth of different morphological HA nanocrystals is also discussed in detail. The antibacterial activity of various concentrations (0.2, 2 and 20 mg/ml) and morphologies of HA nanocrystals was evaluated using quantitative, qualitative, and reactive oxygen species (ROS) assays. The results demonstrated that HA nanorods and thin nanoplates exhibited significantly higher antibacterial response at concentrations of 0.2 and 20 mg/ml, respectively, as compared to other HA nanocrystals. Specifically, the antibacterial response is observed to be the highest on 20 mg/ml concentrated HA thin nanoplates. The bactericidal effect of 0.2 mg/ml concentrated HA nanorods was due to over production of ROS within bacterial cells. In contrast, 20 mg/ml concentrated thin HA nanoplates showed antibacterial potential due to the synergistic effect of ROS generation and their electrostatic repulsion with bacteria.