Fig. 4: Structural evolution of functional groups during protonation treatment.

a, b FTIR and 1H-NMR spectra of OPD with or without protonation treatment. c, d FTIR and 1H-NMR spectra of CDs with or without protonation treatment (black line: spectra of samples without protonation treatment; red line: spectra of samples with protonation treatment).