It is of great significance to synthesize carbon dots (CDs) with desirable hydrophilicity for the ever-growing application of CDs in different fields. In this study, the hydrophilic and hydrophobic CDs were facilely prepared by solvothermal treatment of o-dihydroxybenzene and urea in N,N-dimethylformamide (DMF). Optimization experiments revealed that the solvothermal temperature has a great impact on the surface states of the CDs. The hydrophobic CDs with a contact angle of 110.7° was obtained at 200 °C. The structural and optical characterizations, along with theoretical calculations elucidated that the lipophilic nature of the CDs was resulting from the formation of polymer chains. The presence of extended conjugated sp²-domains and amino groups contributed to the red emission of the CDs synthesized at low reaction temperatures (160–200 °C). With the further increase of solvothermal temperature, the hydrophobic CDs were gradually transformed to the hydrophilic state accompanying the blue shift of the fluorescence of the CDs. The highly hydrophilic CDs with a contact angle of 25.9° were obtained at 240 °C due to the increased formation of hydrophilic functional groups on the surface of CDs. The red emissive CDs exhibited a sensitive color and fluorescence response to ethanol content while the fluorescence of the blue emissive CDs remained constant. By combining the two kinds of CDs, a dual-emission sensor was constructed, which was successfully applied for the evaluation of the alcoholic strength in commercial Baijiu commodities in both fluorometric and colorimetric modes.

As a new type of fluorescent nanomaterials, carbon dots (CDs) have exhibited excellent photoluminescence properties with tunable emission and high quantum yields, hence they have attracted an increasing interest in diverse research areas. The photoluminescence performance of CDs is primarily influenced by their precursors, which directly or indirectly determine the structures and specific functions of the resultant CDs. In this review, we aim to summarize the recent progress on synthesis of CDs using small aliphatic molecules, anilines, polyphenol, polycyclic aromatic hydrocarbons, organic dyes, or biomass as precursors. The associations of the physical and chemical properties of the CDs with their respective precursors are comprehensively investigated, and the potential applications and future development of CDs are discussed in detail. It is hoped that this review will open new horizons for CDs preparation by rational selection of the precursors from the vastly available carbon sources and the critical comments presented, here could inspire and guide future research in the design of multifunctional CDs.