Carbon dots are shining in the world

Carbon dots (CDs) are an emerging class of materials that has recently attracted considerable attention for biomedical and energy applications due to their strong luminescence, low toxicity, water-solubility, ease of synthesis, high surface area, and the possibility to tune their optical properties. However, such excellent properties and wide applications must be caused by their unique structure. CDs are generally considered to be a core-shell structure, usually without a clear boundary between the carbon core and the polymer shell, with at least one dimension of <10 nm and consisting of a high-density core of sp2/sp3 hybridized carbon atoms and heteroatom. The core can often contain polycrystalline nanodomains containing tiny carbon clusters surrounded by amorphous domains while the polymer surface endows the CDs with specific nature. Therefore, a conventional classification system divides CDs into three main groups based on the carbon core and attached states: graphene quantum dots, carbon quantum dots, and carbonized polymer dots, which can unify the advantages of both polymers and quantum dots. Moreover, with the abundant functional group on CDs, it can be easily modified with many kinds of molecules, metals, or matrices, which gives CDs some customized properties. For example, using different polymers to coat CDs can control their emission wavelength, brightness, lifetime, and so forth. Metals can impart electrical conductivity and partial biological activity to CDs. In that regard, CDs have been converted into tumor-killer, efficient electrocatalysts, batteries, and anti-counterfeiting materials. There are, however, certain unresolved challenges, such as the clarify the structure and photoluminescence mechanism of CDs. Varied research directions focused on expanding the application fields of CDs in addition to the pursuit of material synthesis and better performance. CDs will likely continue to attract researchers from diverse disciplines and contribute to the wider scientific community. So, We want to demonstrate the progress and discuss the future development of this class of nanomaterials in this special issue. This special issue, featuring two reviews, and eight research articles from pioneering and leading scientists in their research fields, covers a broad spectrum of promising applications of cutting-edge research of CDs-based functional nanomaterials, including, crops, electrocatalysis, batteries, room-temperature phosphorescence, biological therapy, and so forth. We would like to take this opportunity to thank the distinguished authors who contributed to this special issue.