Characterization of Carbon Dots

Characterization of Carbon Dots

Within SFB953 ‘Synthetic carbon allotropes’, we focus on the synthesis and characterization of carbon dots (CDs), a new class of highly fluorescent carbogenic nanoparticles < 10 nm in size, which possess unique optical properties and low toxicity.

In our project, hydrothermal bottom-up synthesis is used to produce highly-fluorescent aqueous CDs from citric acid and an amino-precursor. We found that the main fluorescence of these CDs originates from effective molecular fluorophores, which were identified to be a typical pyridone structure.1 Furthermore, we found that oxygen radicals, which irreversibly damage the molecular fluorophores, are responsible for the photobleaching effect, by exposing CDs from citric acid and urea to UV light.2 By replacing urea with cysteine, it was possible to enormously enhance the photostability of the CD solution and to increase the QY up 70%.3 Hydrothermal synthesis yields a mixture of several CD-subspecies, including carbon particles (carbon cores) and molecular fluorophore species.2-3 Therefore, it is extremely important to find an appropriate purification method in order to characterize the structure and fluorescence mechanism of CDs and produce a pure product for further applications.

Figure 1: Isocratic chromatographic separation of the CD solution with the corresponding 3D photoluminescence spectra
Figure 1: Isocratic chromatographic separation of the CD solution with the corresponding 3D photoluminescence spectra

Chromatographic isocratic separation with a simple chromatography column and a H2O:Acetonitrile volumetric ratio 1:1 was used and the CD solution could be separated into three fractions: 1. the carbon core, 2. the carbon core with fluorophores and 3. pure fluorophores (Fig. 1). We also found out, that the exact composition depends enormously on the synthesis conditions and that carbon cores can only be produced for temperatures over 200°C.3 By using HPLC, the expected fractions could be found and identified. At the moment, we are working on a gradient separation process which enables improved separation efficiency and the isolation of more sub-species. Future work will involve fractionation of the CD solution with a new preparative HPLC process followed by the characterization of selected species with Electrospray-Differential Mobility Analysis-Mass Spectrometry and Multi Wavelength Analytical Ultracentrifugation (MWAUC).

Publications:

  1. W.Wang, B. Wang, H. Embrechts, C. Damm, A. Cadranel, V. Strauss, M. Distaso, V. Hinterberger, D. M. Guldi W. Peukert et al., RSC Adv., 2017, 7, 24771-24780.
  2. W. Wang, C. Damm, W. Walter, T. J. Nacken, W. Peukert et al., Phys. Chem. Chem. Phys., 2016, 18, 466-475.
  3.  V. Hinterberger, C. Damm, P. Haines, M. Sattes, D. Guldi, W. Peukert, Nanoscale, 2019, 11, 8464.