Sustainable process to co-synthesize nano carbon dots, nano hydroxyapatite and nano β-dicalcium diphosphate from the fish scale

The conventional protocols for the extraction of biological hydroxyapatite employ the prior removal of organic material using severe chemical treatment, which quite often limit the purity and crystalline properties of synthesized hydroxyapatite. The present study describes a process to co-synthesize four high value nano products from scale of Labeo rohita, viz. nano carbon dots, nano hydroxyapatite, nano beta-dicalcium diphosphate and nano carbon dot-hydroxyapatite composite. In the process, the organic matter in fish scale is converted to nano carbon dots by a hydrothermal process using acetic acid (1:1) and the mineral residue precipitated was converted to hydroxyapatite, following a sintering protocol. The acetic acid not only functioned as a medium to release the mineral-organic linkage but also acted as a base to hydrolyze the collagenous proteins linked with calcium phosphate in the fish scales. The hydrolyzed collagenous protein was in turn transformed into fluorescent nano carbon dots. beta-Dicalcium diphosphate was seen attached over carbon dots, which was separated by crystallization followed by high temperature sintering. Hydroxyapatite: carbon dot composite was synthesized through a hydrothermal method. The spectral and morphological studies of the composite showed strong interaction between carbon dot and hydroxyapatite. All the synthesized materials were characterized using ultraviolet visible spectroscopy, Fourier transform infrared spectroscopy, powder X-ray diffraction, atomic force microscope, transmission electron microscope and electrochemical impedance techniques. The study proposes a sustainable method for the complete utilization of fish scale by converting into high value nano products.