Scientists have discovered that by microwaving fish waste, they can quickly and efficiently create carbon nano-onions (CNOs)—a unique nanoform of carbon that has applications in energy storage and medicine. This method could be used to make cheaper and more sustainable LEDs in the future. The researchers from Nagoya Institute of Technology in Japan published their findings in Green Chemistry.
CNOs are nanostructures with spherical carbon shells in a concentric layered structure similar to an onion. They have “drawn extensive attention worldwide in terms of energy storage and conversion” because of their “exceptionally high electrical and thermal conductivity, as well as large external surface area,” per the paper. They’ve been used in electronics and for biomedical applications, such as bio-imaging and sensing and drug delivery, write the authors in the study.
Though CNOs were first reported in the 1980s, conventional methods of manufacturing them have required high temperatures, a vacuum and a lot of time and energy. Other techniques are expensive and call for complex catalysts or dangerous acidic or basic conditions. This “greatly limits the potential of CNOs,” per a statement from Nagoya Institute of Technology.
The newly discovered method requires only one step—microwave pyrolysis of fish scales extracted from fish waste—and can be done within ten seconds, per the authors.
How exactly the fish scales are converted into CNOs is unclear, though the team thinks it has to do with how collagen in the fish scales can absorb enough microwave radiation to quickly increase in temperature. This leads to pyrolysis, or thermal decomposition, which causes the collagen to break down into gasses. These gasses then support the creation of CNOs.
This method is a “straightforward way to convert fish waste into infinitely more useful materials,” and the resulting CNOs have a high crystallinity, which gives them “exceptional optical properties,” per the statement. They also have high functionalisation, which means they're "bonded to other small molecules on their surface," writes Ellen Phiddian for Cosmos. This combination of attributes means the CNOs can glow bright blue, per Cosmos.
“The CNOs exhibit ultra-bright visible-light emission with an efficiency (or quantum yield) of 40 percent,” says Takashi Shirai, a coauthor and professor in the Nagoya Institute of Technology’s Department of Life Science and Applied Chemistry, in a statement. “This value, which has never been achieved before, is about 10 times higher than that of previously reported CNOs synthesized via conventional methods.”
Because of these excellent optical properties, the CNOs could be used to create “large-area emissive flexible films and LED devices,” Shirai says in the statement. “These findings will open up new avenues for the development of next-generation displays and solid-state lighting.”