Simple Strategy for Scalable Preparation Carbon Dots: RTP, Time‐Dependent Fluorescence, and NIR Behaviors

Abstract Transforming carbon dots (CDs) fluorescent materials into smart materials with complex functions is a topic of great interest to nanoscience. However, designing CDs with regulating fluorescence/phosphorescence that can be visually monitored with the environment changes in real‐time remains a challenge. Here, a very simple strategy, one‐step solvent‐free catalytic assistant strategy, which is low cost, facile, environment‐friendly, and high throughput, is put forward. Hydrogen bond is used to manipulate nanostructure of CDs, and the obtained carbon dots (M‐CDs) show a series of attractive properties including matrix‐free room‐temperature phosphorescence, time‐dependent fluorescence, and near‐infrared emissive characteristics. Different from the traditional aggregation caused quenching or aggregation‐induced emission fluorescent materials, M‐CDs exhibit unprecedented and unique dispersion induced redshift fluorescence phenomenon, promoting the studies of fluorescence from static to dynamic. The causes of this phenomenon are further analyzed in detail. As a kind of intelligent fluorescent materials, this new designed CDs greatly enrich the basic recognition of CDs by illustrating the relationship between redshift fluorescence behaviors and the dispersion states, and may provide with an opportunity for solid‐state fluorescent materials, anti‐counterfeiting, cellular imaging, and hopefully many others.


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ceasing the UV irradiation. And then powders were washed with deionized water three times to remove impurities, and the remnants were dried in an oven with 60 ℃. In the end, the dried powders were dissolved into DMSO to achieve time-dependence fluorescence CDs for further characterizations. The reaction volume can be flexibly scaled up or down, thus the CDs should hold great potential for large-scale synthesis.
In order to verify the reliability of the Water-Washing method, M-CDs are purified by column chromatography for comparison, and the M-CDs obtained by this method are named Column Chromatography M-CDs. The detailed operation process is as follows: The crude products of M-CDs were purified with a silica column chromatography using methanol as eluent. After removing solvent, the resultant M-CDs further dried at 40 °C for 24 h under vacuum. As shown in Figure S17, UV-Vis and FL spectra of Water-Washing M-CDs and Column Chromatography M-CDs in DMSO solutions show that their compositions are consistent. Hence, the Water-Washing method is considered as reliable as column chromatography. The yield of M-CDs obtained by Water-Washing method is about 71%, while that obtained by Column Chromatography method is about 59%.
Synthesis route of CDs 1-3. First, 0.54 g of OPD was dissolved in 35 mL of ethanol; then, the solution was transferred into 50 mL Teflon-lined stainless-steel autoclave.
After being heated at 200 °C for 12 h and then cooled to room temperature, the obtained solution was purified via silica column chromatography using ethanol as the eluent.

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Synthesis route of CDs-4. 0.020 g PPD was dissolved in 10 mL of deionized water and stirred for about 10 min. The mixed solution was transferred to a 20 mL Teflon-lined stainless-steel autoclave and heated at 200 ℃ for 2 h. After the reaction was complete, a cloudy solution with a small amount of precipitate was obtained, which was purified by silica column chromatography using ethanol as the eluent.
Synthesis route of CDs 5-6. First, 0.54 g PPD and 0.08 g AlCl3ꞏ6H2O were dissolved in 35 mL of ethanol; then, the solution was transferred into 50 mL Teflon-lined stainless-steel autoclave. After being heated at 200 °C for 12 h and then cooled to room temperature, the obtained solution was purified via silica column chromatography using ethanol as the eluent. Afterward, CDs samples with different fluorescence colors (CDs-5: red; CDs-6: green) will be obtained.
Synthesis route of M'-CDs. 0.54 g OPD and 0.056 g CuCl2ꞏ2H2O were ground by pestle milling for ten minutes in agate mortar. Then the mixture was transferred to a 20 mL autoclave for heating 12h at 200℃. After that, the autoclave was taken out and cooled to room temperature naturally. The obtained carbonized powders possess weak blue emission under a UV lamp (365 nm), weak yellow room-temperature phosphorescence after ceasing the UV irradiation. And then powders were washed with deionized water three times to remove impurities, and the remnants were dried in an oven with 60 ℃. S5

The evidence of hydrogen bonds among carbon nanoparticles.
In the solvent-free process of preparing M-CDs, we fully consider how to create hydrogen bonds and other non-covalent interactions among CDs, so as to ensure the realization of solid-state fluorescence and room temperature phosphorescence (RTP) of M-CDs. If the M-CDs are purified by dialysis or column chromatography, the hydrogen bonds among CDs will be destroyed directly. As shown in Figure S6a, the obtained M-CDs powder (by using Column Chromatography method) is no longer emitted blue fluorescence under a UV lamp (365 nm). We compared the FT-IR spectra of M-CDs purified by using Water-Washing and Column Chromatography methods. As shown in