Semiconducting Polymer Dots with Dually Enhanced NIR-IIa Fluorescence for Through-Skull Mouse Brain Imaging.

Fluorescence probes in NIR-IIa region (1300-1400 nm) show drastically improved imaging owing to the reduced photon scattering and autofluorescence in biological tissues. However, organic fluorophores in this region are largely unexplored. Here, we develop NIR-IIa polymer dots (Pdots) by a dual fluorescence enhancement mechanism. First, we used the aggregation induced emission of phenothiazine to reduce the nonradiative decay pathways of the polymers in condensed states. Second, we minimized the fluorescence quenching by different levels of steric hindrance to further boost the fluorescence. The resulting Pdots displayed a fluorescence QY of ~1.7% in aqueous solution, suggesting an enhancement of ~21 times in comparison with the original polymer in tetrahydrofuran (THF) solution. Small animal imaging by using the NIR-IIa Pdots exhibited a remarkable improvement in penetration depth and signal to background ratio, as confirmed by through-skull and through-scalp fluorescent imaging of the cerebral vasculature of live mice. This study indicates the promising potential of NIR-IIa Pdots for in vivo fluorescence imaging.

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