Toward unified structure-function relationships in carbon dots

Carbon dots (CDs) are an emerging class of carbon-based nanomaterials with broad applications in sensing, bioimaging, optoelectronics, and catalysis. However, a unified understanding of the relationship between their structural composition and functional evolution remains lacking due to the coexistence of various structural units generated during their synthesis via polymerization, crosslinking, carbonization, and surface reconstruction processes. In this review, CDs are revisited from the perspective of structural units, including rigid domains, flexible domains, and surface chemical structures. Their structural characteristics, regulation strategies, and functional roles are systematically summarized. Particular emphasis is placed on how these structural units regulate key electronic-state processes, including exciton localization, exciton recombination, and electron transfer. Furthermore, a structural-unit-driven functional evolution model is proposed, revealing the continuous transition of CDs from exciton-localization-dominated systems to electron-transfer-dominated systems. This framework provides new insights into the structure-property relationships of CDs and guides the rational design of carbon-based functional materials.