Near-infrared emissive carbon dots for two-photon fluorescence bioimaging

Fluorescent probes with both excitation and emission in the near-infrared (NIR) region are highly attractive in the field of bioimaging. Herein, NIR emissive carbon dots (CDs, λ_max = 683 nm) capable of excitation with a NIR femtosecond pulse laser (850 nm) are reported for the first time. The NIR CDs also hold a variety of superior features including excellent water dispersibility, narrow emission bands, and high quantum yields (QY = 16.8%). Further studies reveal that the emission of the NIR CDs derives predominantly from the surface molecular state mechanism. More interestingly, the NIR CDs are verified to have very low cytotoxicity and show capabilities for two-photon fluorescence bioimaging. Finally, the NIR CDs are shown to be easily modified with fluorescein isothiocyanate (FITC) and moved into living cells. These findings demonstrate that the as-prepared NIR CDs could not only be potentially employed for deep-tissue two-photon bioimaging, but also used as an effective carrier for delivery of drugs that can't (or hardly) enter into living cells directly.