基于混酸回流制备碳点的中和过程

Toward the Neutralization of Carbon Dots Prepared by Mixed Acid Reflux

作为碳纳米材料家族的一颗新星,碳点因具有奇特的光致发光特性、良好的生物相容性和稳定性而受到广泛关注。其制备方法通常可以归纳为“自上而下”和“自下而上”两种,而混酸回流是一种自上而下制备碳点的重要方法,中和是其中不可缺少的步骤。本文以富勒烯副产炭灰为原料,利用混酸回流方法制备了黄光发射的碳点,系统研究了NaOH、Na_(2)CO_(3)、K_(2)CO_(3)和NH_(3)∙H_(2)O四种不同中和试剂对碳点结构和发光特性的影响。热重分析、傅里叶变换红外光谱和X射线光电子能谱的研究结果表明:中和试剂的改变,影响碳点表面酸性官能团的解离程度,该结论被表面活性剂复配体系的相行为研究结果所印证。碳点结构的变化,影响了碳点的表面态,导致碳点光谱特性,如紫外-可见吸收和荧光光谱的改变。选用了四种中和试剂中,Na_(2)CO_(3)处理得到的碳点表现出最优的发光性能。我们的研究还表明:中和试剂的选用是调节混酸回流方式,制备碳点性能的重要途径。而且碳点反离子的改变对其发光性能也具有明显影响。总之,中和试剂的碱性和反离子种类共同影响着碳点的表面结构,带来了不同的性能。这项工作提示着研究者在未来制备碳点时有必要将中和试剂的种类作为一项实验条件。

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 acidrefluxing 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,Na_(2)CO_(3),K_(2)CO_(3),and NH_(3)∙H_(2)O,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 Na_(2)CO_(3) 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.