Ultraviolet (UV) emission plays a vital role in applications such as sterilization, anti-counterfeiting, phototherapy, forensic analysis, and communication. To meet these demands, researchers have developed various UV-emitting materials, including colloidal quantum dots, small molecules, and polymers. However, many of these materials rely on toxic metals like cadmium (Cd) or lead (Pb), critical raw materials, or non-renewable petroleum-based chemicals. Additionally, their synthesis often involves complex and expensive processes, raising significant environmental concerns.
A team of materials scientists, led by Associate Professor Jia Wang from Umeå University in Sweden, in collaboration with Professor Ludvig Edman and colleagues, recently introduced a sustainable alternative. They synthesized carbon dots (CDs) with efficient UV emission, derived from a renewable and abundant biomass feedstock—green tea extract.
This peer-reviewed article was published in Nano Research in April 2025.
"In this study, we convert a common green tea extract into UV-emitting CDs through a simple hydrothermal process," explained Jia Wang, the corresponding authors. Wang is also a research PI with the Wallenberg Initiative Materials Science for Sustainability. "These CDs exhibit a photoluminescence peak at 384 nm with a quantum yield (PLQY) of 17% in water. Interestingly, when dissolved in 'poor' solvents, their emission becomes blue-shifted and nearly five times stronger."
CDs are metal-free, quasi-zero-dimensional nanostructures typically smaller than 10 nm. These nanoparticles can achieve optical performances comparable to inorganic quantum dots while being environmentally friendly. Because CDs can be synthesized from various carbon-rich sources, they hold great potential as a sustainable alternative.
The team highlighted the progress being made in biomass-derived CDs, which have been gaining attention for their environmental benefits. "Biomass sources like food waste, forest and agricultural residues, and even household waste have been used to produce CDs, demonstrating efficient emission in both the visible and near-infrared regions. However, successful UV emission from CDs has been rare," Jia Wang noted.
In this study, the UV emission of CDs exhibited clear solvent dependence. As the solvent polarity index increased, the PLQY decreased, with the highest PLQY of 81% achieved using the highly hydrophobic solvent 3-phenoxyanisole. Wang's team attributed this enhancement to aggregation-induced emission. "These CDs are about 3.5 nm on average," Wang explained. "In poor solvents, they form larger aggregates (10–30 nm) due to reduced dispersibility. This aggregation creates more compact molecular structures and rigid scaffolds, which limits non-radiative recombination and significantly enhances PLQY."
The team also explored the synthesis of CDs from green tea bag powder instead of green tea extract. However, the resulting CDs only emitted blue light, not UV. "Even simple pretreatment steps like extraction or isolation can decisively influence the structure and properties of the final CDs," Wang said. "Rational design of these pretreatments is an important action for advancing biomass-derived functional materials."
Beyond their optical properties, the team is also eager to explore additional applications. "Our preliminary results suggest that these CDs can strongly inhibit the proliferation of SH-SY5Y neuroblastoma cells," said Wang. "Our collaborators are conducting more detailed studies on their antioxidative properties.
The first author of the study is Dr. Henry Opoku. Other contributors include Dr. Junkai Ren, Dr. Xin Zhou, and Dr. Shi Tang from Umeå University, Sweden; Peijuan Zhang and Prof. Dongfeng Dang from the School of Chemistry at Xi'an Jiaotong University, China.
This work was supported by generous funding from J. C. Kempes Minnes Stipendiefond (SMK-21-0015 and SMK-1956), the Swedish Research Council (2019-02345, 2020-04437, and 2021-04778), Bertil & Britt Svenssons stiftelse för belysningsteknik (2021 höst-14 and 2022 höst-31), the Knut and Alice Wallenberg Foundation for a Proof of Concept grant (KAW 2022.0381), the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation (WISE-AP01-D02), and the European Union through an ERC Advanced Grant (ERC, InnovaLEC, 101096650).
About Nano Research
Nano Research is a peer-reviewed, open access, international and interdisciplinary research journal, sponsored by Tsinghua University and the Chinese Chemical Society, published by Tsinghua University Press on the platform SciOpen. It publishes original high-quality research and significant review articles on all aspects of nanoscience and nanotechnology, ranging from basic aspects of the science of nanoscale materials to practical applications of such materials. After 17 years of development, it has become one of the most influential academic journals in the nano field. Nano Research has published more than 1,000 papers every year from 2022, with its cumulative count surpassing 7,000 articles. In 2023 InCites Journal Citation Reports, its 2023 IF is 9.6 (9.0, 5 years), and it continues to be the Q1 area among the four subject classifications. Nano Research Award, established by Nano Research together with TUP and Springer Nature in 2013, and Nano Research Young Innovators (NR45) Awards, established by Nano Research in 2018, have become international academic awards with global influence.
