English

Toward the Neutralization of Carbon Dots Prepared by Mixed Acid Reflux

• Ning Feng ^{1, 2,} ,
• Hongguang Li ^{1, ,} ,
• Jingcheng Hao ^{1, ,}

• 1.

  Key Laboratory of Colloid and Interface Chemistry, School of Chemistry and Engineering, Shandong University, Jinan 250100, China

• 2.

  China Research Institute of Daily Chemical Industry, Taiyuan 030000, China

Corresponding author: Email: hgli@sdu.edu.cn (H.L.); Email: jhao@sdu.edu.cn (J.H.), Tel.: +86-531-88363963 (J.H.)

• Received Date: 05 May 2020
  Accepted Date: 05 June 2020
  Revised Date: 05 June 2020
  Available Online: 15 October 2021
• Fund Project:

  the National Natural Science Foundation of China 

Abstract: Carbon dots (C dots) are relatively novel carbon nanomaterials that have attracted significant interest due to their unique photoluminescence, good biocompatibility, and stability. The preparation methods of C dots was usually summarized into "top-down" and "bottom-up", and mixed acid reflux is a top-down strategy that can be used to synthesize C dots, during which neutralization is a necessary step that can significantly influence the properties and potential applications of the final product. Previously, this research area mainly focused on tuning the properties of C dots by changing the starting materials and/or varying the reaction conditions; the influence of the reagents used during neutralization has been largely ignored. As the previously reported C dots prepared by mixed acid reflux were obtained from different starting materials under varied conditions, a meaningful comparison is difficult. Herein, yellow-emitting C dots were prepared by mixed acid-refluxing a carbon-rich material derived from fullerene carbon soot. For the same batch of as-prepared C dots, the influences of four reagents, i.e., NaOH, Na₂CO₃, K₂CO₃, and NH₃·H₂O, during neutralization on the structures and photoluminescence of the resulting C dots were investigated in detail. The results of thermogravimetric analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy clearly showed that the reagent used during neutralization can affect the degree of dissociation of the acidic functional groups on the C dots. This is further supported by examination of the C dot/surfactant mixtures where subtle changes in the phase behavior were observed. Structural changes of the C dots cause variations in their surface states, ultimately altering the optical characteristics, including UV-vis absorption and fluorescence. Among the treated C dots, the sample prepared with Na₂CO₃ showed the strongest emission under the same excitation wavelength, while that prepared with NH₃·H₂O exhibited a distinct red shift (~8 nm) in the emission curve. The results presented herein provide clear evidence that neutralization reagent selection is important for optimizing the properties of the resulting C dots obtained by mixed acid reflux. In addition, the photoluminescence of the C dots can be influenced by their counterions, providing a novel method for tuning the properties of C dots while explaining their behavior in saline solutions. In short, the basicity of the neutralizing reagent and the type of counterions affect the structure of the C dots surface, which brings different performances. This work reminds researchers that it is necessary to use the type of neutralizing reagent as an experimental condition when preparing C dots in the future.

Key words:
• Carbon nanomaterial
•  /  Carbon dot
•  /  Mixed Acid reflux
•  /  Photoluminescence
•  /  Top-down
•  /  Neutralization
•  /  Phase behavior

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