Tuning the photoluminescence by engineering surface states/size of S, N co-doped carbon dots for cellular imaging applications.

In this research, we have synthesized carbon dots (CDs) co-doped with nitrogen and sulfur by facile hydrothermal method, using citric acid and cysteine as carbon source. The effect of solid-state thermic treatment (STT) at 303-453 K on the size, surface, fluorescence and cellular cytotoxicity of the CDs were systematically investigated. Through a simple STT, it was possible to tune surface states and the average size of the CDs, causing a permanent red shift. Initially, CDs showed a decrease in cell viability with increasing concentration. However, after STT, its viability remained constant with an increase in concentration. Here, we show the possibility to label the cells cytoplasm according to the CDs fluorescence emission before (blue emission) and after STT (red emission). The CDs studied in this paper show selective luminescence properties, which are fundamental for any cell imaging application.

Temperature-dependence on the optical properties of chitosan carbon dots in the solid state.

We report the synthesis of chitosan-derived aminated carbon dots with dual fluorescence bands and their influence on the morphology, absorption and emission spectral profiles as well as on the band gap energy in relation to thermal treatment after synthesis. To unravel these changes, we performed spectroscopic measurements in the solid state on two stages at temperatures ranging from 303 to 453 K. For the first heating stage, the emission spectrum showed a 20 nm red shift and a new absorption band at 350 nm, possibly related to new bonds and/or nitrogenous molecular fractions. For the second heating stage in the same temperature range, no displacements in the emission spectrum were observed and both the energy gap and bandwidths for the two emission bands are practically constant, indicating a change nitrogen moiety exposed on the surface. Furthermore, through atomic force microscopy it was noted that the morphology and size of the CDs were not significantly affected by the increase in temperature. It is noteworthy that the values of the Huang-Rhys factor, respectively, 2.584 × 10-10 and 2.315 × 10-9 for band I and II emission after the second heating indicate a mechanism of weak electron-phonon interactions. This work may open a novel perspective for the development of new surface modulation strategies for carbon dots subjected to thermal treatment in the solid state.