In Situ Cyanide-to-Bipyridine Coordination-Sphere Reconstruction of [Fe(CN)6]3– into Surface-Confined Fe(bpy)32+ on Nafion/Carbon Soot for Selective Electrocatalytic Nitrite Oxidation and Sensing

Abstract A simple electrochemical strategy is introduced for the in situ synthesis of a surface-confined tris(2,2′-bipyridine)iron(II), [Fe(bpy)3]2+, complex through coordination-sphere reconstruction (CSR) of hexacyanoferrate(III), [Fe(CN)6]3–. The process involves an electrochemically driven cyanide-to-bipyridine ligand exchange directly on a Nafion/carbon soot (Nf-CS)-modified electrode, thereby avoiding conventional multistep solution-phase synthesis. Cyclic voltammetry, Raman spectroscopy, Fourier transform infrared spectroscopy, UV–vis spectroscopy, high-resolution mass spectrometry, and electrochemical quartz crystal microbalance measurements collectively confirm the formation of surface-confined [Fe(bpy)3]2+ and provide evidence for transient Fe2+/Fe3+ intermediates during CSR. A selective Prussian Blue test further confirms CN– release, supporting the proposed ligand-exchange mechanism. The resulting GCE/Nf-CS@[Fe(bpy)3]2+ electrode exhibits remarkable electrocatalytic activity toward nitrite oxidation, with a sensitivity of 150 μA mM–1, approximately 10-fold higher than that of chemically synthesized [Fe(bpy)3]2+-based electrodes. The electrode also demonstrates a low overpotential, excellent operational stability, and negligible fouling. Coupling with batch injection analysis enables rapid, selective, and reproducible nitrite determination in serum, urine, saliva, spinach, and wastewater samples, with recoveries of 98–101%. This new approach provides a versatile platform for developing a variety of surface-confined molecular electrocatalysts and functional inorganic materials for real-time applications.