Sodium dithionite (Na₂S₂O₄) is a common reducing agent widely used in the food, textile, biological science, and other fields. Excessive intake of Na₂S₂O₄ can cause stomach cramps, upper respiratory tract infections, and other types of cell-damaging diseases, such as laryngospasm and bronchospasm. Therefore, a rapid, real-time, and in situ detection method for Na₂S₂O₄ is extremely crucial to establish in biological technology, food security supervision, and the environment. Magnetic resonance/fluorescence dual-mode imaging can not only provide high-resolution structural and histological information but also achieve high-sensitivity functional imaging. In this study, a fluorescence/magnetic resonance dual-modality sodium dithionite molecular probe H2 based on cobalt complexes is designed and developed, using Co³⁺ as the magnetic resonance unit and 7-diethylaminocoumarin-3-carboxylic acid as the fluorescence unit. The probe combines optical imaging with high sensitivity and selectivity and magnetic resonance imaging with strong tissue penetration ability and high spatial resolution, compensating for the deficiencies of single-modal imaging.

Probe H2 is synthesized and characterized by nuclear magnetic resonance (NMR) and mass spectrometry. The identification performance of H2 towards Na₂S₂O₄ in an aqueous solution is studied using a UV?vis spectrophotometer and a fluorescence spectrophotometer. All NMR relaxivity measurements and NMR imaging in vivo are performed on a MesoMR23-060H-I Analyst Analyzing & Imaging system (0.5 T, Shanghai Niumag Corp.). Fluorescence imaging data are collected and processed using Amiview Living Image 2.0 software (PerkinElmer, USA).

The capability of probe H2 ensemble for the detection of Na₂S₂O₄ is studied by UV?vis, fluorescence emission spectrum, and magnetic resonance in Tris buffer solution [V(DMSO)∶V(Tris)=9∶1, pH=7.4]. To evaluate the selectivity of H2 towards Na₂S₂O₄ against other analytes [Figs. 2(b) and 4(a)], no or little effect on the UV?vis and fluorescence spectrum detection of Na₂S₂O₄ is found in the presence of various competitive analytes. The results demonstrate that H2 can be used as a specific probe for Na₂S₂O₄ sensing in aqueous solution. The detection limit is calculated to be 23 μmol/L based on a 3σ/k under the experimental conditions [Fig. 5(a)]. The relaxivity of H2 is gradually enhanced with the increase in the concentrations of Na₂S₂O₄ (0?280 μmol/L), which indicates the conversion of diamagnetic Co³⁺ into paramagnetic Co²⁺ through the redox reaction of Na₂S₂O₄. Additionally, the corresponding T₂-weighted images of H2 show a continuous decrease in spot brightness as the concentration of Na₂S₂O₄ increased [Fig. 7(a)]. The addition of competitive analytes does not noticeably interfere with the responses of the transverse relaxivity (r₂) [Fig. 7(b)]. The magnetic resonance/fluorescence dual-mode imaging results (Figs.10 and 11) demonstrate that H2 enables the successful detection of Na₂S₂O₄in vivo and may potentially be used in biomedical diagnosis fields in the future.

In this study, we design and synthesize a magnetic resonance/fluorescence dual-mode molecular probe H2 for Co(Ⅲ) complexes using 7-diethylaminocoumarin-3-carboxylic acid (L₂) as a ligand. Its structure is characterized by hydrogen nuclear magnetic resonance and high-resolution mass spectrometry. Probe H2 can specifically identify Na₂S₂O₄. After recognition, the fluorescence signal of the H2 solution is quenched, the transverse relaxation signal is enhanced, and the T₂-weighted image becomes darker. The recognition mechanism is verified by high resolution mass spectrometry and cyclic voltammetry. Probe H2 exhibits good light stability, specific selectivity, a suitable pH under physiological conditions and low cytotoxicity. Using 6?8 weeks of nude mice as a model, probe H2 is successfully applied to the visual detection of exogenous Na₂S₂O₄ by magnetic resonance/fluorescence dual-mode imaging in vivo, which has certain application potential in the biomedical field.