Fig. 2: PL evolution of CDs under variable H⁺ ions concentration.

a–f Normalized 3D FL spectra of CDs under different pH condition adjusted by H₂SO₄ and NaOH; g, h one-photon and two-photon fluorescence (FL) spectra of CDs without protonation treatment; i relationship of two-photon FL intensity and femtosecond (fs) laser power.