Extraction of nano carbon dots, hydroxyapatite and dicalcium diphosphate from fish scale: A green synthesis protocol

Globally, 8 to 10 percent of process discards generated from seafood industry is accounted by fish scales alone. Fish scale is primarily composed of hydroxyapatite (HAP), collagen and other minerals. Nano carbon dots (CD are rich source of electrons, nitrogen, hydroxyl and carboxyl groups, and exhibits excellent photoluminescence characteristics. Fluorescent CDs synthesized from collagen were found to have potential applications in biomedical field. Likewise, HAP and dicalcium phosphate (DCP) synthesized from fish scale were reported to have superior tissue regeneration properties over their synthetic counterparts. In the present study, HAP, CD and dicalcium diphosphate were co-extracted from fish scale following a combination protocol of hydrothermal extraction and high temperature sintering. During hydrothermal process the collagen fibres in fish scale underwent a relaxation sweep, thereby permitting the acetate ions to interact with collagen matrix and interlink calcium phosphate / carbonate layers. In turn, the collagen molecules underwent acetylation and reoriented the bonds to form soluble nano sized fluorescent carbon dots. The acetate selectively reacted with carbonate and released calcium phosphate, which could be recovered by filtration and subsequent sintering process. Nano sized fluorescent CD, HAP and dicalcium phosphates synthesized were characterized using FTIR, UV-Vis spectrophotometer, Atomic force microscope, TEM, XRD and electrochemical impedance spectroscopy. Further, a nano composite of CD and HAP was synthesized and the proportions of individual components were optimized based on electrochemical impedance spectra. The extracted HAP and carbon dots were found to be non-toxic on MTT assay. The methodology proposed in the present study can be economically scaled up and hence can be easily adopted by the industry. Moreover, due consideration was given to ensure that the proposed methodology is well in line with the UNs SDG goal.