Bioimaging in the near-infrared window is of great importance to study the dynamic processes in vivo with deep penetration, high spatiotemporal resolution, and minimal tissue absorption, scattering, and autofluorescence. In spite of the huge progress on the synthesis of small organic fluorophores and inorganic nanomaterials with emissions beyond 900 nm, it remains a tough challenge to synthesize semiconducting polymers with fluorescence over this region. Here, we synthesized a series of heptamethine cyanine-based polymers with both absorption and emission in the near-infrared region. We prepared these polymers as semiconducting polymer dots (Pdots) in pure water with great biocompatibility. The fluorescence quantum yield of the Pdots can be as high as 14% with a full width at half-maximum of 53 nm, and their single-particle brightness is more than 20 times higher than commercial quantum dots or ∼300 times brighter than Food and Drug Administration (FDA)-approved indocyanine green (ICG) dyes. We further demonstrated the use of cyanine-based Pdots for specific cellular labeling and long-term tumor targeting in mice. We anticipate that these cyanine-based ultrabright Pdots could open up an avenue for next generations of near-infrared fluorescent agents.
Keywords: NIR; Pdots; cyanine dye; fluorescence; polymer.