Scientists have developed a novel strategy to produce near-oxygenless carbon nanodots, helping shed light on the role of oxygen in their optical properties.
Carbon nanodots (CNDs) are a promising type of fluorescent material with potential applications in many fields, including optoelectronics, photocatalysis, and medicine. However, the role of oxygen in CND fluorescence is unclear because conventional synthesis methods cannot produce CNDs without oxygen for comparative analyses. In a recent study, scientists from Sookmyung Women’s University, Korea, have developed a new synthesis pathway to easily obtain near-oxygenless CNDs, paving the way to a deeper understanding of their attractive properties.
In 2004, scientists accidentally discovered carbon nanodots (CNDs) while purifying carbon nanotubes, triggering extensive studies to understand and exploit the remarkable fluorescent properties of these carbon-based nanoparticles. Thanks to their high stability, low toxicity, and attractive fluorescent profile, CNDs are promising candidates for a wide variety of applications in the fields of optoelectronics, photocatalysis, photovoltaics, and even medicine and the life sciences.
However, well over a decade since their discovery, the exact mechanisms behind the photoluminescence of CNDs are not completely understood. So far, researchers have found that oxygen-containing functional groups on CNDs are responsible for some of their attractive optical properties, but the exact role of oxygen has proven difficult to pinpoint. The problem lies in the fact that conventional synthesis procedures for CNDs involve oxidation reactions or oxygen-containing precursors, yielding nanoparticles with a markedly high oxygen content. Without a way to produce oxygenless CNDs, it is impossible to carry out comparative analyses to shed light on the mysterious influence of oxygen in CND fluorescence.
Fortunately, in a recent study published in ACS Applied Nano Materials, a team of scientists from Sookmyung Women’s University, Korea, set out to address this issue that has been plaguing CND research. Led by Associate Professor Woosung Kwon, the team developed a radically different synthetic pathway that yields CNDs with extremely low amounts of oxygen. Unlike conventional methods that involve the carbonization of carbohydrates and organic acids, the new approach is based around the ‘pyrolytic decarboxylation of aromatic carboxylic acids.’ The term ‘carboxylic’ refers to any substance that contains a carboxyl group (–COOH), and ‘pyrolytic decarboxylation’ means the removal of these groups using elevated temperatures (between 200 and 500 °C).
Using this new synthesis pathway, the scientists produced multiple batches of CNDs from different precursors while varying specific parameters. This allowed them to gain insight into the effects of, for example, pyrolysis time and temperature. They characterized the composition of CNDs and their photoluminescent properties through multiple experimental techniques, including, but not limited to, transmission electron microscopy, infrared and X-ray photoelectron spectroscopy, X-ray diffraction, and fluorometry.
The experimental measurements on the CNDs revealed remarkably low oxygen-to-carbon ratios, which ranged from 10% to as little as 0.6%. Most notably, the near-oxygenless CNDs showed an extremely high absolute quantum yield—a measure of fluorescence defined by the ratio of absorbed to emitted photons. “To the best of our knowledge, the quantum yield of 80.4% of our CNDs is the highest reported for long-wavelength fluorescent CNDs, beating the previous record of 65.93%,” highlights Kwon. “The results demonstrate the feasibility of our new synthesis pathway for preparing highly luminescent CNDs.”
The team went one step further and demonstrated one of the potential uses of their CNDs. By combining them with poly(methylmethacrylate), they fabricated thin luminescent films that absorb white light from an LED and stably re-emit it in the yellow to orange range in the visible spectrum, acting as color filter. This showcases how CNDs, which are intrinsically biodegradable and ecofriendly, could find a home in future display applications.
Most importantly, this study will help future researchers deepen their understanding of what goes on under the hood in CNDs. With his eyes on the future, Kwon comments on the implications of their findings: “Our study provides new insights into the synthesis of highly fluorescent CNDs and highlights the serious need for further consideration of the effects of oxygen on the chemical, optical, and electronic properties of CNDs.” Hopefully, progress in this relatively young field will help unlock the potential of CNDs as highly effective fluorophores for a variety of applications.