Time-resolved photoluminescence of pH-sensitive carbon dots

Time-resolved photoluminescence emission were performed to study the >= 100 ps time-scale photo-physics of water soluble carbon dots (CDs) dispersed in different pH solutions. Excitation of the core and high-energy edge states at excitation wavelengths lambda(ex) <= 350 nm produce hole-electron excitonic charge-carriers but do not yield any photoluminescence emission, implying efficient non-radiative recombination mechanisms for the charge-carriers. Using lambda(ex) similar to 375 nm, surface fluorophores as well as low-lying edge-states are excited, producing emission over a broad range. However, the emission characteristics vary strongly depending on the pH condition and the choice of excitation wavelength. In particular, the edge state emission is abruptly extinguished at pH >= 5, which also marks the onset of deprotonation of surface-bound -COOH groups of the CDs, indicating that the -COO- anionic centers promote quenching of edge-state emission. Using lambda(ex) similar to 450 nm, only the surface fluorophores are excited. The deprotonation of phenolic -OH group at pH > 9 causes a sharp red-shift in the emission spectra at all excitation wavelengths. The pH-sensitive emission behavior of the CDs enable them to serve as optical pH sensors. (C) 2018 Elsevier Ltd. All rights reserved.