Sustainable synthesis of biocompatible nano-hydroxyapatite from seashell (micro-mollusk) waste

Herein, we report a simple and sustainable wet chemical synthesis of nano-hydroxyapatite (nHAp) from micro-mollusk (seashell) waste collected from Saint Martin's Island, Bangladesh. The collected micro-mollusk contained around 96.7% of CaO measured by Wavelength Dispersive X-ray Fluorescence (WDXRF) spectroscopy. X-ray Diffraction (XRD) pattern of the synthesized nHAp matched with the ICDD standard data and confirmed the hexagonal HAp with nanocrystalline peaks. Crystallite size was estimated by exploiting multiple methods where the size ranged from around 2.6-16.5 nm. The nano-sized nature along with the morphology was confirmed through Field Emission Scanning Electron Microscope (FESEM) and Transmission Electron Microscope (TEM) analysis, where the particle sizes were measured to be 32.11 ± 11.38 nm (N = 105) and 14.95 ± 5.37 nm (N = 33), respectively. Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy were employed to confirm the existing functional groups, primarily the vibrational modes of PO₄ ^3- groups. They also confirmed the presence of B-type carbonate substitution of the synthesized nHAp. X-ray Photoelectron Spectroscopy (XPS) analysis was carried out to confirm the surface elemental composition (Ca, P, O and C), along with Na which was not detected in EDX analysis. Simultaneous Thermal Analysis (STA) indicated a total mass loss of 7.7% up to 597.5 °C and high thermal stability. The specific surface area was measured to be 72.98 m² g^-1 by Brunauer-Emmett-Teller sorptometer (BET) analysis. The in vitro cell viability assay showed >95% cell viability at 100 µg mL^-1. The results demonstrate the successful synthesis of nHAp by utilizing micro-mollusk waste, which is biocompatible and suitable for biomedical and environmental applications.