Abstract
Carbon dots have attracted much attention due to their unique optical, chemical and electronic properties enabling a wide range of applications. The properties of carbon dots can be effectively adjusted through modifying their chemical composition. However, a major challenge remains in understanding the core and surface contributions to optical and electronic transitions. Here, three blue luminescent carbon dots with carboxyl, amino and hydroxyl groups were comprehensively characterized by UV-vis absorption and emission spectroscopy, synchrotron-based X-ray spectroscopy, and infrared spectroscopy. The influence of the surface functionality on their fluorescence was probed by pH-dependent photoluminescence measurements. Moreover, the hydrogen bonding interactions between water and the surface groups of carbon dots were characterized by infrared spectroscopy. Our results show that both core and surface electronic states of blue luminescent carbon dots contribute to electronic acceptor levels while the chemical nature of the surface groups determines the hydrogen bonding behavior of the carbon dots. This comprehensive spectroscopic study demonstrates that the surface chemistry has a profound influence on the electronic configuration and surface-water interaction of carbon dots, thus affecting their photoluminescence properties.
Highlights
Carbon dots (CDs) commonly consist of a carbogenic core stabilized with ligands and/or surface groups.[1,2] The graphitic, amorphous or polymeric core determines the type of CD, and Surface functionalization of CDs is of vital importance for many applications
We previously demonstrated that monitoring the OH vibrations of water molecules during exposure to humid air using attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) was a sensitive method for probing the hydrogen bond (HB) environment around carbon nanomaterials.[27]
A more profound assignment of the origin of the optical properties of CDs is hampered by the different synthetic routes used to prepare the commercial CDs, which complicates the evaluation of the exact role played by the different surface groups
Summary
Carbon dots (CDs) commonly consist of a carbogenic core stabilized with ligands and/or surface groups.[1,2] The graphitic, amorphous or polymeric core determines the type of CD, and Surface functionalization of CDs is of vital importance for many applications. Which appears at a higher energy region (539–550 eV) consists of C–O, O–H, CvO and epoxide groups.[37,38] The distinguishable feature at 545.3 eV for 1s → σ*CvO in carboxyl groups is only observed in CDs-COOH (Fig. S3, ESI†).[35] In the N K-edge XA spectrum of CDs-NH2, the contribution of nitrogen to the 2058 | Nanoscale, 2019, 11, 2056–2064
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