NIR-II fluorescence imaging of salicylic acid in plants with a metal-free rhodamine ring-opening probe

A NIR-II fluorescent probe Py-RP is constructed via spirolactamization of rhodamine–polymethine and 2-hydrazinobenzothiazole. • Intermolecular hydrogen bond-triggered metal-free spiroring opening enables highly selective and sensitive NIR-II imaging of SA. • The probe achieves real-time monitoring of plant abiotic stress and visual identification of SA residues on fruit surfaces. Fluorescence imaging of salicylic acid (SA) is key to understanding plant protective mechanisms. However, existing SA probes are limited by their visible or NIR-I emission, where tissue scattering and autofluorescence compromise imaging quality. To address this limitation, we developed a rhodamine-based NIR-II fluorescent probe, named Py-RP, by functionalizing a rhodamine–polymethine scaffold with 2-hydrazinobenzothiazole via spirolactamization. Its fluorescence turn-on mechanism originates from a metal-free spirocyclic ring-opening process, initiated by a cooperative hydrogen-bonding network formed between the spirolactam carbonyl group of Py-RP and the ortho-hydroxyl/carboxyl moieties of SA. This structural opening extends the π-conjugated system, enabling distinct NIR-II fluorescence activation upon SA recognition. Py-RP exhibited high selectivity and sensitivity toward SA, with a detection limit of 0.15 μM, allowing for dynamic monitoring of plant responses to abiotic stress and visual analysis of SA residues on fruit surfaces. This rhodamine-based NIR-II probe opens an avenue for advanced research in plant stress physiology. Download: Download high-res image (168KB) Download: Download full-size image