Time-resolved photoluminescence of pH-sensitive carbon dots

• Published in: Carbon (New York) 2019-04, Vol.144, p.500-508
• Main Authors: Basu, Nabaruna, Mandal, Debabrata
• Format: Article
• Language: English
• Subjects: Carbon dots; Chemical compounds; Current carriers; Emission analysis; Emission spectra; Emissions; Excitation; Excitation spectra; Fluorescence; Luminescence; Photoluminescence; Radiative recombination; Wavelengths
• ISSN: 0008-6223; 1873-3891
• DOI: 10.1016/j.carbon.2018.12.056

Time-resolved photoluminescence emission were performed to study the ≥100 ps time-scale photophysics of water soluble carbon dots (CDs) dispersed in different pH solutions. Excitation of the core and high-energy edge states at excitation wavelengths λex ≤ 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 λex ∼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 λex ∼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. [Display omitted]