Electrochemical insights into alkaline hexacyanoferrate-mediated oxidation of creatinine: Evidence for cyanide release, Iron–Oxo formation, and Iron nanoparticle growth

Alkaline hexacyanoferrate (AHCF, ferricyanide in strongly alkaline media, pH ≈ 14) has been widely used since 1962 as an oxidant in catalysis, electroanalysis, and more recently in aqueous redox-flow batteries. However, its oxidative mechanism and chemical stability under strongly alkaline conditions remain poorly understood. Herein, AHCF-mediated oxidation of creatinine (Ctn) was systematically investigated using carbon nanomaterial-modified screen-printed electrodes. Voltammetric and batch injection analyses demonstrated efficient mediated oxidation with good selectivity. Complementary SEM, FTIR, UV–Vis, Raman, ICP-OES, and in situ electrochemical quartz crystal microbalance (EQCM) analyses revealed that AHCF undergoes concurrent decomposition during oxidation. Time-dependent UV–Vis studies showed apparent pseudo-first-order decomposition kinetics with rate constants ( k obs ) of 1.13 × 10 −3 min −1 and 1.73 × 10 −3 min −1 in the absence and presence of Ctn, respectively, indicating that Ctn accelerates AHCF decomposition. Cyanide release was verified by the characteristic Prussian blue (PB) formation test, while ICP-OES, in situ EQCM, and electron microscopy confirmed the release and deposition of iron species as nanoscale particles on the carbon surface. In situ mass measurements further suggest the transient formation of high-valent iron–oxo/hydroxo intermediates (Mw ≈ 128 g mol −1 ), which are proposed to be the actual oxidizing species responsible for substrate transformation rather than ferricyanide itself. The deposited iron species subsequently promote the in situ formation of surface-confined PB molecular electrodes with electrocatalytic activity toward hydrogen peroxide reduction. These findings reveal an overlooked decomposition pathway of AHCF, providing new mechanistic insight into its oxidative behaviour while highlighting important stability and safety considerations for alkaline ferricyanide-based electrochemical systems.