[Image above] Illustration of the one-step microwave pyrolysis process to turn fish scales into carbon nano-onions. Credit: Nagoya Institute of Technology, YouTube
By Laurel Sheppard
This year, CTT has covered the recycling or reuse of waste materials in a variety of industries, including semiconductor manufacturing, solar energy, wind energy, and nuclear energy. Today, we consider another sector that struggles with heavy waste generation—the food industry.
The United Nations reports that, globally, around 14% of food produced is lost between harvest and retail while an estimated 17% of total food production is wasted. Food that is lost and wasted accounts for 38% of total energy usage in the global food system.
Finding ways to reuse food waste is one way to combat this loss. For example, biowaste materials obtained from the fish industry have drawn significant attention as a novel raw material for various purposes.
Currently, about 50–75% of fish and seafood byproducts—including viscera, skin, bones, scales, flesh, fins, and shells—are wasted during fish processing. However, these byproducts are a rich source of carbon, nitrogen, oxygen, hydrogen, and sulfur. They also contain a valuable amount of collagen, crude protein, and amino acids.
Researchers have derived valuable advanced ceramic materials from these byproducts, for example, calcium phosphate bioceramics from salmon fish bone wastes. Now, in a recent study, researchers at Nagoya Institute of Technology in Japan produced highly crystalline carbon nano-onions (CNOs) using a one-step microwave pyrolysis of fish scales obtained from black snapper.
CNOs are a newer carbon nanostructure consisting of fullerenes and multiwalled nanotubes that form a concentric structure of spherical shells. The graphitic layers in the structure are composed of many defects and holes, which can be filled with other atoms or molecules to endow the material with different properties. Because of this versatility, CNOs show potential in a wide range of applications, including electronics, photovoltaics, catalysis, and biomedical diagnostics.
The first large-scale synthesis method for CNOs was developed more than a decade after CNOs were first discovered in 1980 as a byproduct of making carbon black. The method involved vacuum annealing a nanodiamond precursor under high temperatures (1,500°C–2,000°C). Although this method became widely used, the resulting CNO cannot be dispersed in either polar or nonpolar solvents, which is required for many applications.
Other CNO fabrication methods include arc discharge, chemical vapor deposition, ion implantation, laser ablation, and liquid phase-thermal pyrolysis. In addition to high temperatures, many of these methods require high-cost carbon sources, additional catalysts, hazardous acids or bases, and post-treatments to improve properties. Other disadvantages include contamination and low crystallinity.
The Nagoya researchers’ new one-step synthesis process avoids most of these drawbacks by taking advantage of the fish scale composition. After a complex cleaning process to remove unwanted fat, color, and calcium, the fish scales are converted to CNOs within 10 seconds using similar frequencies as conventional home microwaves. This fast conversion rate is attributed to the collagen in the fish scales, which absorbs microwaves quickly and results in an extremely rapid increase in temperature.
In addition to yielding CNOs with very high crystallinity, this process causes the CNO surface to be selectively and thoroughly functionalized with (−COOH) and (−OH) groups. When the CNO surface is not functionalized, the nanostructures tend to stick together, making it difficult to disperse them in solvents. However, because the proposed synthesis process produces functionalized CNOs, it allows for an excellent dispersibility in various solvents.
Another advantage of the functionalization and the high crystallinity is exceptional optical properties. The waste-derived CNO demonstrated a visible photoluminescence with a narrow emission width less than 90 nm and a superior quantum yield of 40% (i.e., the probability that a molecule will fluoresce or phosphoresce). This yield is 10 times higher than existing CNOs conventionally prepared using more complicated approaches.
To showcase some of the many practical applications of their CNOs, the researchers used a simple three-step tape casting process to create a flexible film that emitted blue light. Bright solid-state emission under ultraviolet irradiation occurred even at a low concentration of CNOs (2 mg/ml). They then turned the film into an LED that still emitted blue light, which was attributed to the active CNOs.
In a Cosmos Magazine article, coauthor Takashi Shirai, associate professor at Nagoya Institute of Technology, concludes, “These findings will open up new avenues for the development of next-generation displays and solid-state lighting.”
The paper, published in Green Chemistry, is “Fabrication of ultra-bright carbon nano-onions via a one-step microwave pyrolysis of fish scale waste in seconds” (DOI: 10.1039/D1GC04785J).