Fig. 6: Comparison of two-photon emission (TPE) between the 2,3-DAPN, OPD, and CDs.

a TPE of 2,3-DAPN and CDs without protonation treatment, b TPE of 2,3-DAPN and CDs with protonation treatment; c TPE of OPD and CDs without protonation treatment, d TPE of OPD and CDs with protonation treatment; (P-CDs: CDs with the protonation treatment, P-OPD, and P-2,3-DAPN: OPD and 2,3-DAPN molecular at pH 1 condition).