This study explores the fluorescence properties of the hydrothermal reaction of citric acid and urea under different conditions, as they have attracted considerable attention in recent years. However, so far, the detailed formation process and the precise chemical structure of the resulting fluorescent products remain inadequately understood. The results reveal that under mild conditions, fluorescence is mainly due to organic molecules, identified as citrazinic acid through acid treatment with spectroscopic techniques. Moreover, the fluorescence shifts towards carbonized quantum dots (CDs) is both temperature and reaction duration-dependent. Comparative analysis versus graphene quantum dots and citrazinic acid elucidated differences in solution properties, including excitation-dependency, photobleaching and fluorescence lifetime. With reference to previous findings in literature, this study provides for an innovative, detailed understanding on the evolution of fluorescent species, by tuning the reaction conditions. Hence, they propose for excellent future strategic application in the nanotechnology and theranostic fields.