Fish scales weigh heavy on the fish processing industry. Getting rid of the waste without irking environmental agencies is an issue. Muhamed Ashraf, at ICAR-Central Institute of Fisheries Technology, was concerned.
He knew that fish scales are made up of nanocrystals of hydroxyapatite, aligned in a regular manner, sandwiching collagen proteins arranged, in a parallel manner, in two or three layers (like plywood) and bound together with some needle-shaped calcium analogues.
Our bones and teeth enamel have hydroxyapatite and the mineral finds use in dental and orthopaedic applications. In labs, it is useful for separating biomolecules. And some toothpastes use the material too. It costs 300 rupees a kilo now – as costly as fish. Meanwhile, the world throws away more than 2 million tonnes of fish scales as waste.
What is the best way to extract hydroxyapatite from fish scales? Without causing environmental problems? Muhamed Ashraf conferred with his colleagues, Stenil Stephen and P. K. Binsi.
One method was to remove collagen chemically and then sinter it by heating it to 1000 degrees centigrade to recover crystalline hydroxyapatite. But that is costly: you have to use acids or enzymes to remove collagen and too much energy is consumed to remove the remaining organic material.
Why not try heating in water? The hydrothermal treatment will destabilise the collagen matrix. The collagenous carbon can then be easily removed as carbon dots – another useful material.
The team set about testing their idea.
They went and collected the scales of rohu, Labeo rohita from the local fish market.
Put one gram of fish scales in dilute acetic acid. Heat it to 180 degrees centigrade for a few hours. Cool it and use ultrasound vibrations to loosen links between macromolecules. Separate carbon dot and hydroxyapatite residues by filtering. Sinter the solid residue by heating. And you have pure hydroxyapatite.
The dark brown filtrate contained carbon dots and dicalciumdiphosphate residue from the needles that once stitched together collagen layers in the fish scale. The researchers could easily separate the two by evaporating to dryness and the re-extracting carbon dots using water. The solid residue that remained was sintered to prepare dicalcium diphosphate.
“The average yield of hydroxyapatite is about six percent and the yield of carbon dots was more than thirty percent”, says Binsi, ICAR-CIFT.
“The carbon dots were around four to five nanometres. They absorb light and becomes fluorescent. The fluorescent spectra indicates that they are made up of graphene-like carbon structures with some oxygen and nitrogen atoms”, says Stenil Stephen.
The hydroxyapatite particles were about six to seven nanometres when measured with X-ray diffraction, but about 25 nanometers when measured with atomic force microscopy. This difference is due to the polycrystalline nature of the hydroxyapatite produced, say the researchers.
The remaining material was dicalcium diphosphate with traces of calcium carbonate – a material usually mixed with hydroxyapatite in dental applications.
“From fish scales, we can now produce three useful materials. We look forward to developing the technology for applications in fish processing industries,” says Muhamed Ashraf, ICAR-CIFT.
Applied Nanoscience, 11:1929–1947 (2021);
Udham P K
Freelance science writer, Pune
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