Rational Design of a Deep-Red Aggregation-Induced Emission-Active Iridium Complex for Plant Cell Wall Imaging

Plant cell wall imaging has received consistent attention in characterizing and monitoring the structure and health of plant tissues. To overcome current drawbacks of plant cell wall staining, such as source limitations, low specificity, and auto-fluorescence interference, in this work, we present the rational design of an aggregation-induced emission (AIE)-active, deep-red emissive iridium complex as a specific staining agent of plant cell walls. Cell wall probe [Ir(dpyb)₂(iqbtz)]⁺ (Ir-CW), bearing two rotatable pyridyl moieties, is designed for selective binding to pectin, one of the major components of the cell wall that carries rich carboxylic residues on its polymer chain. The AIE properties, pectin sensing behaviors, pectin binding mechanism, and practical application of cell wall staining of Ir-CW are performed. Results demonstrate that Ir-CW exhibits pectin-responsive AIE properties and selective pectin binding against common interfering species, including metal ions, anions, amino acids, proteins, and saccharides. Notably, Ir-CW can specifically stain plant cell walls with better performance than Congo red, a commercial cell wall staining agent. This work highlights the rationales of developing novel iridium complexes for cell wall staining and widely broadening their applications as powerful tools for botanical studies.