In-situ scanning electrochemical microscopy interrogation on open-circuit release of toxic Ni2+ ion from Ni-containing carbon nanomaterials and nickel-hexacyanoferrate formation in physiological pH and its thiol-electrocatalysis relevance

•Ni-impregnated carbon nanomaterials are toxic to physiological systems.•Ferricyanide ion is found to trigger and complexation with Ni2+ ion at the open-circuit condition.•RC-SECM has been adopted for the mechanism and imaging of Ni2+ release as Ni-hexacyanoferrate.•The OCP assisted NiHCF formation is much better electrocatalyst than potential cycled prepared NiHCF.•High-performance electrocatalytic oxidation of thiol was demonstrated as a model. The release of toxic/carcinogenic Ni2+-ion from the Nickel-embedded matrix is believed to be strongly influenced by acid solution-assisted corrosion reaction on the interface. In neutral pH, the passive nickel oxide formed on the surface is expected to protect the underlying nickel from any corrosion attack. But in reality, several biological factors are found to influence nickel corrosion and metal ion release. The present investigation is an in-situ scanning electrochemical microscopy (SECM) based mechanistic-interrogation of a commercial Ni-loaded MWCNT sample (MWCNT-Ni) that has been modified on a glassy carbon electrode surface, in presence of ferricyanide ion at a neutral pH condition. The facile chemical reaction between Ni2+ and ferricyanide ion to form Nickel-hexacyanoferrate (NiHCFOCP) at an open-circuit potential (OCP) condition in pH 7 phosphate buffer solution has been utilized as a SECM imaging tool. The surface-confined formation of NiHCFOCP on MWCNT-Ni showed a well-defined redox peak with a surface-excess value, 8.6 × 10−9 mol.cm−2. Initial scanning electron microscopic (SEM) characterization of MWCNT-Ni and MWCNT-NiHCFOCP didn't show any differentiation. On the other hand, constructed redox-competition SECM (RC-SECM) experiment yielded an obvious visualization of Ni2+ release as a specific crystalline unit due to the formation of NiHCFOCP on the MWCNT-Ni surface. Extended experiments with substrate generation-tip collection and tip generation-substrate collection modes SECM studies support the observation. This result correlates the toxic Ni2+ release from Ni-loaded carbon contaminants in the biological system at various electroactive species conditions. As an independent work, MWCNT-NiHCFOCP modified electrode has been demonstrated as an efficient and selective electrocatalyst and a home-made electrochemical detector for flow-injection analysis of thiol (cysteine as a model) in neutral pH with a detection limit of 2.08×10−12 g/20µL without any surface fouling problem. [Display omitted]