Abstract:

Carbon quantum dots (CQDs) have attracted extensive interest due to their strong fluorescence as well as inexpensive and plentiful resources for manufacture. There are numerous published reports on the preparation of CQDs and direct applications based on their photoluminescence. Successive chemical modification of CQDs in an appropriate manner might expand the application scope of CQDs and transform them into practical fine chemicals. The various functional groups on the surface of CQDs allow for efficient chemical modification while imparting them with hydrophilicity. Covalent linking of hydrophobic hydrocarbon chains to CQDs would lead to the formation of novel surfactants. Here, a technique for preparing CQD-based cationic surfactants is depicted in detail. This was rare to be reported according to recent publishes. First, a mixture of ethylenediamine tetraacetic acid and ethylenediamine in the presence of hydrogen peroxide in an aqueous medium was pyrolyzed at 180 ℃ for 60 min. The resulting CQDs are represented as OX-CQDs. Then, the OX-CQDs were subjected to quaternization with 1-chlorododecane for obtaining the cationic surfactant (OX-CQDs-C₁₂H₂₅). The OX-CQDs-C₁₂H₂₅ surfactant effectively decreased the surface tension of water from 72.0 to 26.7 mN∙m⁻¹ at the critical micelle concentration of 5.0 mg∙mL⁻¹, thus demonstrating superior performance over several new Gemini cationic surfactants. The OX-CQDs-C₁₂H₂₅ surfactant also decreased the contact angles of water considerably. However, when longer alkyl chains such as －C₁₄H₂₉ or －C₁₆H₃₃ were attached to the CQDs, the corresponding surfactant was less effective in decreasing the surface tension of water. Calculations based on the Gibbs absorption isothermal equation revealed that two more －C₁₂H₂₅ chains were bonded with a carbon quantum dot averagely, implying that the as-prepared CQD-cationic surfactant belonged to the category of Gemini surfactants. Quaternization with 1-chlorododecane also led to a notable enhancement in the antibacterial activity for Escherichia coli as compared with that of unmodified CQDs. The antibacterial percentage approached 100% even the solution was diluted to 0.41 mg∙mL⁻¹, which was much lower than the critical micelle concentration. The fluorescence quantum yield of OX-CQDs-C₁₂H₂₅ reached 6.44%. Experimental results revealed that hydrogen peroxide played a positive role in improving the surface activity and fluorescence quantum yield of OX-CQDs-C₁₂H₂₅. The surface activity, antibiosis, and fluorescence endowed the versatilities of OX-CQDs-C₁₂H₂₅. This novel, economical technique for synthesizing cationic surfactants eliminates the need for introducing hydrophilic groups. The hydrothermal approach for preparing CQDs satisfies the demand for green chemical synthesis. From this aspect, our technique provides efficient access to synthesizing cationic surfactants.

Key words: Carbon quantum dot, Quaternization, Cationic surfactant, Surface activity, Fluorescence, Antibiosis