Thermal transition analysis of fish scale originated hydroxyapatite: AFM and electrochemical impedance spectroscopy as complimentary techniques

Several studies are reported on extraction and characterization of hydroxyapatite (HAp) from biological sources by heat sintering method; however, conversing observations are reported on the optimized sintering conditions and crystalline characteristics of the extracted HAp. The present study systematically depicts the different stages of collagen elimination and crystallization of mineral phase in fish scale occurring during different conditions of heat sintering. The pre-sintering temperature as optimized by thermal analysis by TGA, DTG and DTA curves was at 450 °C. The microcrystalline and structural characteristics of extracted HAp samples were established by SEM, elemental mapping, XRD, FTIR and surface area analysis (BET). Accordingly, complete desorption of residual collagen from scale surface was evidenced at 800 °C, and that from matrix interspace at 900 °C. Stable HAp nanocrystal formation could be achieved at 1000 °C, above which the characteristic features of recrystallization and phase transformation were evident. Hence, 1000 °C was considered as the rate limiting temperature for HAp formation. This could be clearly established by atomic force microscopy (AFM) and electrochemical impedance analysis. Conclusively, the present study established AFM and electrochemical impedance analysis as complimentary techniques to the routine analytical techniques employed in the earlier reports for the confirmation of microstructural alterations and phase transformations occurring in HAp crystals during sintering process.