Highly enhanced photoluminescence and suppressed blinking of N-doped carbon dots by targeted passivation of amine group for mechanistic insights and Vis-NIR excitation bioimaging application

Carbon dots (CDs) have attracted extensive attention owing to their attractive optical properties, small size, low cost, and good biocompatibility. However, there are still challenges in unequivocally unraveling the photoluminescence (PL) origin and the structure-function relationship associated with the surface groups of CDs. Herein, we have successfully enhanced the PL efficiency of GA/N-CDs by targeted passivation of the amine groups of N-doped CDs (N-CDs) with glutaraldehyde (GA), to facilitate their bioimaging applications under one-photon and two-photon excitations. In particular, we have systematically investigated the regulation effects of this surface modification on the optical properties, exciton dynamics, and blinking behavior using steady-state and time-resolved spectroscopies, and single-particle fluorescence imaging microscopes. The effective passivation of the amine surface group of N-CDs not only regulates the energy levels and properties of surface states, but importantly activates an additional emissive center via an aldehyde-amine condensation reaction. Meanwhile, the hydrogen bonding interactions between GA and the related surface groups of N-CDs further restrain the non-radiative exciton relaxations. This synergetic effect greatly enhances the PL efficiency of two separate emission centers and the photostability of GA/N-CDs. Moreover, the photoblinking of N-CDs is significantly suppressed and the two-photon excitation PL property is also markedly increased after the surface modification, endowing GA/N-CDs the outstanding performances for fluorescence bioimaging under Vis-NIR excitations. Our findings contribute to mechanistic insights into the correlation between photophysical properties and surface states/groups, as well as demonstrate an effective post-modification strategy to improve the PL efficiency and stability of carbon-based dots.