Biogenic synthesis of hydroxyapatite: A sustainable approach using Hylocereus undatus

Hydroxyapatite (HAp), a bioceramic and calcium phosphate-based material, has attracted significant attention, mainly because it serves as a biocompatible material with promising dielectric properties, suitable for enhancing osseointegration. In this work, HAp was synthesized via sol-gel method using a novel green template approach, incorporating the extract of Hylocereus undatus cladodes, thereby achieving an environmentally friendly synthesis. Thermogravimetric and Differential Scanning Calorimetry (TGA/DSC) analysis revealed a clear pattern involving adsorbed water evaporation, lattice water loss, formation and dehydroxylation of HAp. XRD and Raman spectroscopy verified phase composition and the presence of secondary NaNO 3 , which was substantially reduced by a post-synthesis washing step. Scanning Electron Microscopy (SEM) analysis revealed that particle morphology and size were influenced by extract concentration and heat treatment temperature, with higher extract concentrations promoting equiaxed particles, while lower extract concentrations favoured the formation of rod-like shapes, both relevant for biomedical applications. Impedance Spectroscopy demonstrated that, regardless of the extract concentration and heat treatment temperature, the dielectric properties were strongly influenced by the excitation frequency, with all samples exhibiting a behavior typical of dielectric materials. Sample 12wHT400 meets the desired criteria across four frequencies (100, 1,000, 10,000, and 100,000 Hz), exhibiting a dielectric constant between 18.5 and 22.0, higher than the dielectric losses, and the lowest conductivity values (3×10 -9 to 1×10 -5 Ω -1 ·m -1 ), indicating suitability for prolonged polarization retention.