Size-tuned PEGylated NIR-II fluorescent probes for high-contrast plant imaging and TMV detection

The widespread applications of fluorescence imaging in plant science still suffer from challenges including strong auto-fluorescence (chlorophyll) and tissue light scattering, resulting in low signal-to-background ratio (SBR) for in vivo bioimaging. Moreover, the relationship between the transport efficacy of fluorescence probes in plants and their sizes has been rarely investigated. To address these bottlenecks, we developed an ingenious PEG-engineering strategy on the second near-infrared (NIR-II) donor-acceptor-donor (D-A-D) emissive dye (CCNU1020) to adjust the self-assembly nanosizes of NIR-II fluorescence probes, resulting in three variants: SYH1 (170 nm), SYH2 (80 nm), and SYH3 (60 nm). As the polyethylene glycol (PEG) chain length increased, the probes' nanosize decreased from 170 to 60 nm. Among them, SYH3 exhibited the fastest entry velocity into Epipremnum Aureum leaf and spread over the leaf veins evenly than the other two probes, of which SYH1 even could hardly entry into the leaf. Meanwhile, SYH3 demonstrated high-contrast imaging of leaf vein with an exceptional signal to background ratio (SBR, ~ 18.6) superior to that of classical NIR-I indocyanine green (ICG) (~ 3.0) and SYH2. This promising imaging ability of leaf veins achieved by size optimization laid the foundation for the early diagnosis of viral infections. In vivo experiments further confirmed that SYH3 effectively accumulated and monitored in the lesion of Tobacco mosaic virus (TMV)-infected Arabidopsis thaliana, which matched well with the green fluorescent protein (GFP)-labeled results. This work represents a significant step forward in plant bioimaging in the cutting-edge NIR-II region.