The contamination of some metal ions in soil and water can cause oxidative stress to plants through several mechanisms such as generation of reactive oxygen species, which impair growth, development and productivity of crops. The oxidative stress in plants results in elevation in NO levels which can be adopted as a marker for tracking the stress. Herein we report a new approach for tracking oxidative stress induced by metal ions in plants by using a NIR-II fluorescence probe (Cy-TPE-NH). As for this NIR-II probe, a heptamethine cyanine serves as the fluorophore backbone, and two triphenylethenes (TPEs) are attached on its each terminal respectively; moreover, an electron-donating benzylamine is linked onto its meso-position as the responsive element for NO detection, and three tri(ethylene glycol) chains are introduced to enhance hydrophilicity of the cyanine to ensure its applicability in aqueous environment of the plants. The N-nitrosation of the benzylamine caused by the presence of NO produces an electron-withdrawing benzyl-N-nitro group, thereby changing the electron state of the probe and resulting in strong fluorescence emission at the near infrared II region. When absorbed by plants suffering oxidative stress induced by cadmium and aluminum ions, the probe generates prominent fluorescence signals, achieving real-time and sensitive imaging of oxidative stress of plants.

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