Carbon Dot with pH Independent Near-Unity Photoluminescence Quantum Yield in an Aqueous Medium: Electrostatics-Induced Förster Resonance Energy Transfer at Submicromolar Concentration.

We report the synthesis and dynamical behavior of a carbon dot (CD) with near 100% photoluminescence quantum yield in water for a very large pH range (1-12). This CD exhibits a rotational correlational time of only ∼130 ps, signifying the whole CD is not exhibiting photoluminescence. Unlike most carbon-based nanoparticles (which act as a quencher of fluorescence), this CD could act as a donor, and the Förster model could account for the experimental observables for the resonance energy transfer (RET) experiment quite well. Based on two dynamical measurements, it could be shown that the fluorescing moiety is located inside the core of the CD. Importantly, for this CD, RET experiments could be performed with a very low concentration (500 nM) of the acceptor. This kind of electrostatics-driven RET at very low concentration is quite important in bioimaging. This ultrabright CD is nontoxic and useful for bioimaging in mesenchymal stem cells.