mance of CDs, novel methods have been proposed. Element doping is considered one of the effective strategies for improved fluorescence QYs. Sun et al. used a series of N-containing bases to synthesize nitrogen-doped CDs with a QY of 94%. [22] Yang and his colleagues condensed citric acid with ethylenediamine to obtain CDs with a QY of up to 80%. [23] Naumov et al. used glucosamine hydrochloride and thiourea as precursors to synthesize nitrogensulfur co-doped graphene quantum dots with a QY of 60%. [24] Nevertheless, obtaining long-wavelength emission CDs and multicolor emission CDs is still difficult. To this end, researchers have proposed another strategy, that is, to purify CDs or obtain multicolor CDs by adjusting the type and ratio of the solvents through a chromatographic column. Jiang and co-workers used three isomers of phenylenediamine as precursors to obtain blue, green, and red CDs and purified them by silica gel column chromatography. [25] Xiong et al. used silica gel column chromatography to separate the reaction solution and obtained CDs with visible light emissions. [26] Usually, the multicolor CDs obtained by silica column chromatography have a narrow emission wavelength range, and the QY is greatly improved, which provides conditions for the further application of CDs.However, column chromatography requires many organic solvents, which are cumbersome and time-consuming and easily cause CD loss. Therefore, the direct determination of polychromatic CDs by controlling various preparation conditions instead of silica column chromatography has become a popular research topic in recent years. Fan and co-workers reported a method for preparing multicolor CDs. They used two isomers of diaminonaphthalene and citric acid as precursors to obtain five-color CDs by controlling the reaction time or adding sulfuric acid. [27] Sun et al. obtained a series of CDs with different emission colors by adjusting the reaction temperature and the ratio of citric acid to urea. [28] In these reports, two conditions need to be adjusted simultaneously to obtain poly chromatic CDs, which requires more steps and makes the synthesis process time-consuming.In this study, a simple method for preparing multicolor CDs only by controlling the ratio of reactants in the asparagine (Asn) and p-phenylenediamine reaction system was proposed. To the In a reaction system of asparagine and p-phenylenediamine, blue, green, orange, and red carbon dots (CDs) are successfully prepared by adjusting the ratio of the two reactants. The obtained multicolor CDs have excellent stable properties and high fluorescence quantum yields (up to 74%). Clarifying the luminescence mechanism of multicolor CDs is based on the investigation of the composition, structure, and fluorescence properties of the CDs combined with quantum chemistry calculations. A high-quality white light-emitting diode (LED) with CIE color coordinates (0.33, 0.33) is successfully fabricated using a multicolor CD-epoxy resin composite, and its color temperature and color rendering index a...

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The use of nanomaterials such as carbon dots (CDs) as rapid detection fluorescent nanoprobes is a hot topic in the field of analytical chemistry. In this paper, blue, green, and red CDs with high fluorescence quantum yields (24, 54, and 38%, respectively) were obtained from m-phenylenediamine and urea by hydrothermal synthesis and chromatographic separation. After investigating the structures and properties of the obtained CDs, three distinct and sensitive fluorescent nanoprobes with different functions were successfully constructed. The first nanoprobe derived from blue CDs could be used to detect the pH of a strong acid solution (pH 3.0); a rapid detection kit was designed based on it. The second nanoprobe based on Green CDs with ellagic acid (EA) modification allowed the sensitive determination of Fe3+ ions in aqueous solutions. The final probe derived from Red CDs could be used to test for organic acids in liquor. The test results of the commercial liquor were consistent with the standard methods. The detection mechanism of the three fluorescent probes was proposed. Using CDs to construct multifunctional fluorescent nanoprobes is beneficial for the application of CDs to rapid and specific detection and recognition.

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