Graphitic Nitrogen Triggers Red Fluorescence in Carbon Dots
Graphitic Nitrogen Triggers Red Fluorescence in Carbon Dots
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DOI：
10.1021/acsnano.7b06399
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发表时间：
2017-12-01
期刊：
影响因子：
17.1
通讯作者：
Zboril, Radek
中科院分区：
文献类型：
--
作者：
Hola, Katerina;Sudolska, Maria;Zboril, Radek
碳点（cd）是一种稳定且具有高度生物相容性的荧光材料，在细胞标记、光学成像、LED二极管和光电子技术等方面具有很大的应用潜力。由于其发射波长提供最佳的组织穿透，红色发射cd在生物医学技术中的应用特别有趣。目前实现红移发射的合成策略包括通过适当的合成策略增加CD的粒径（sp(2)结构域），以及用合适的官能团（例如羧基）调整CD的表面化学性质。在这里，我们提出了一种优雅的方法来制备具有蓝色，绿色，黄色或红色波长的可控荧光的全彩cd。该两步程序包括在甲酰胺中从柠檬酸和尿素合成全彩色CD混合物，然后根据CD电荷的差异通过柱层析分离单个荧光部分。带有最多负电荷的红色发光CDs被作为最后一部分分离出来。XPS, FT-IR，拉曼光谱和DFT计算清楚地证明了CD结构中石墨氮含量的增加是导致分离，表面电荷和光致发光红移的趋势。重要的是，石墨氮在未掺杂体系的HOMO-LUMO间隙内产生中隙态，导致明显的红移光吸收，从而在可见光谱的低能端产生荧光。本研究发现石墨氮是导致CD光致发光红移的另一个关键因素。
Carbon dots (CDs) are a stable and highly biocompatible fluorescent material offering great application potential in cell labeling, optical imaging, LED diodes, and optoelectronic technologies. Because their emission wavelengths provide the best tissue penetration, red emitting CDs are of particular interest for applications in biomedical technologies. Current synthetic strategies enabling red-shifted emission include increasing the CD particle size (sp(2) domain) by a proper synthetic strategy and tuning the surface chemistry of CDs with suitable functional groups (e.g., carboxyl). Here we present an elegant route for preparing full-color CDs with well-controllable fluorescence at blue, green, yellow, or red wavelengths. The two-step procedure involves the synthesis of a full-color-emitting mixture of CDs from citric acid and urea in formamide followed by separation of the individual fluorescent fractions by column chromatography based on differences in CD charge. Red-emitting CDs, which had the most negative charge, were separated as the last fraction. The trend in the separation, surface charge, and red-shift of photoluminescence was caused by increasing amount of graphitic nitrogen in the CD structure, as was clearly proved by XPS, FT-IR, Raman spectroscopy, and DFT calculations. Importantly, graphitic nitrogen generates midgap states within the HOMO-LUMO gap of the undoped systems, resulting in significantly red-shifted light absorption that in turn gives rise to fluorescence at the low energy end of the visible spectrum. The presented findings identify graphitic nitrogen as another crucial factor that can red shift the CD photoluminescence.