Reversible redox-based optical sensing of parts per million levels of nitrosyl cation in organic solvents by osmium chromophore-based monolayers

Exposure of an osmium(II)-based monolayer on glass to organic solvents containing 0.36-116 ppm of NOBF₄ results in one-electron transfer from the covalently immobilized complexes to the inorganic analyte with concurrent optical changes. The NO⁺ induced oxidation of the monolayer can be detected optically with an of-the-self-UV/vis spectrophotometer (260-800 nm). The redox-based NO⁺ detection and quantification system can be reset with water within ∼20 s for at least 40 times. The reaction of the monolayer with a THF solution containing 5 ppm of NOBF ₄ follows pseudo first-order kinetics in the monolayer with ΔG‡_298K = 21.5 ±0.7 kcal/mol, ΔH‡ = 9.5 ± 0.3 kcal/mol, and ΔS‡ = -40.6 ±1.1 eu. The monolayer structure and properties have been resolved by electrochemical measurements and synchrotron X-ray reflectivity measurements in combination with density functional theory calculations (B3LYP/SDD level of theory).