In the past decade, a new exciting class of metal-free and carbon-rich fluorescent nanomaterials has been developed. By far, the most example is carbon nanodots (CNDs). CNDs are usually prepared by two main procedures: top-down cutting route from different carbon resources and bottom-up carbonization method from different molecules or polymers. CNDs prepared from the former route possess good carbon lattice with relative low quantum yield (QY), while the latter route is the reverse. However, CNDs prepared from bottom-up carbonization of citric acid based molecules usually contain small fluorophore molecules with high photoluminescence (PL). As a result, researchers should be aware of this significant component in CNDs, and figure out the relationship between chemical structures and PL centers. (C) 2015 Elsevier Ltd. All rights reserved.