Single-Particle Correlated Imaging Reveals Multiple Chromophores in Carbon Dot Fluorescence.

Carbon dots are remarkable nanomaterials with many applications, but the sources of their emission are still uncertain. Carbon dots exhibit complex behaviors such as excitation-dependent emission due to their heterogeneous composition and structure. Most studies have been carried out on the ensemble level, where sample heterogeneity remains hidden. Understanding the complex emission of carbon dots requires single-particle measurements. Here, we determined that for red-emitting carbon dots made from two bottom-up precursors, there is a significant population of dots with more than one emitting moiety. Polarization-resolved, single-dot emission microscopy revealed subpopulations of carbon dots based on their emission intensity and polarization. For the multichromophoric carbon dots, we found an average of about four emitters. Single-particle spectroscopy, acquired in parallel to the emission trajectories, and molecular dynamics simulations furthermore established that the countable chromophores in the carbon dots are chemically similar, considering the rather narrow room-temperature emission line width and the absence of significant spectral diffusion.