荧光碳点具有激发波长依赖的独特性质, 有望基于此制备检测溶液pH值的荧光探针。以柠檬酸和尿素为原料、N,N-二甲基甲酰胺为溶剂, 采用一步溶剂热法在200 ℃下保温12 h制备了一种新型的具有橙-绿双波段荧光发射性能的水溶性碳点。采用透射电子显微镜、X射线衍射、拉曼光谱、傅里叶变换红外光谱和X射线光电子能谱等方法对荧光碳点的组成和形貌进行了表征, 还通过荧光发射光谱和紫外-可见吸收光谱对其光学性能进行了研究。结果表明, 制备的碳点粒径为2.7～4.3 nm, 表面带有大量含氧官能团, 具有良好的水分散性。在440 nm和540 nm波长光激发下分别呈现绿色(500 nm)和橙色 (590 nm)双波段荧光发射。合成的荧光碳点发光性能对pH值具有敏感性: 在强碱性溶液中, 590 nm的荧光强度比水溶液中提高了6.71倍, 同时吸收峰的蓝移使得自然光下其溶液颜色发生了明显改变, 具有强碱性指示剂的作用; 在pH值为2～6的酸性溶液中, 500 nm与590 nm发光峰强度比与pH值之间呈现良好的线性关系, 展现了作为pH值比率荧光探针的应用潜力。

Carbon dots (CDs) are excitation wavelength-dependent and are expected to be a new type pH fluorescent probe. CDs that exhibited bright orange-green dual fluorescence emission bands were solvothermally synthesized in one pot by heating of citric acid (CA) and urea in N, N-dimethylformamide (DMF) at 200 ℃ for 12 hours. The CDs were characterized through transmission electron microscope, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy for composition, phase and morphology. In addition, ultraviolet - visible absorption spectra and fluorescence spectroscopy were used to measure the optical performance for the as-prepared CDs. It was found that the diameter of the CDs ranged from 2.7 nm to 4.3 nm with oxygenous groups on their surface and the CDs had good dispersion in aqueous solution. They showed PL peaks at green (500 nm) and orange (590 nm) under excitation of 440 nm and 540 nm respectively. These kinds of CDs had sensitivity to pH since their PL intensity of 590 nm wavelength in alkaline condition was 6.71 times higher than in neutral water and the UV-Vis absorbance peak showed blue-shifted leading to the color change of the solution. Besides, PL intensity ratio of 500 nm to 590 nm was linear against pH values from 2 to 6 that laid the foundation for the use of CDs in pH sensing applications.