[en] Highlights: • Ferrocene-functionalized mesoporous silica films have been prepared on ITO electrodes. • They can be used for indirect amperometric detection of non-electroactive anions. • They can be applied in both flow injection analysis and suppressed ion chromatography. • On-line regeneration is achievable by operating in square wave amperometry - Abstract: Vertically oriented and ferrocene functionalized mesoporous silica thin films have been prepared by combining the electrochemically assisted self-assembly (EASA) method with click chemistry. The resulting films are electroactive, involving an electron hopping mechanism between adjacent ferrocene moieties. The electrochemical oxidation of ferrocene into ferricinium ion generates positive charges that are compensated by the ingress of anions into the film, opening the door to possible indirect amperometric detection of non-electroactive anions by flow injection analysis. Operating in an electrolyte-free flow, each injection of an anion (e.g., NO₃⁻) at an electrode biased at a suitable positive potential (i.e., +0.5 V) gave rise to an amperometric response proportional to the anion concentration. However, to avoid the decrease of the electrochemical signal due to the progressive consumption of ferrocene in multiple successive analyses, it was necessary to regenerate the electrode by reduction of ferricinium moieties, which can be achieved in-situ by square wave amperometry. The feasibility to apply such indirect amperometric detection scheme in suppressed ion chromatography (for detecting anions in mixture) was also demonstrated.

[en] Highlights: • Birnessite is not a simple cationic exchanger toward ammonium ions. • Presence of two adsorption sites of NH₄⁺ on birnessite. • After reaction of birnessite with NH₄⁺, the CEC of the solid decreases. • The Mn average oxidation state increases without any Mn²⁺ release. • The symmetry of the solid changes from triclinic to hexagonal. - Abstract: The ammonium cation interaction with Na-birnessite in aqueous alkaline medium was studied. Solution and solid analysis give evidence that birnessite is not only acting as a cationic exchanger toward NH₄⁺. The surface analysis performed by XPS showed that N1s spectra are characterized by the existence of two different environments: one assignable to an interlayer NH₄⁺ and the second to a chemisorbed N-species. Structural and chemical transformations were observed on birnessite with nitrogen mass balance deficit. The monitoring of NH₄⁺, Na⁺, Mn²⁺, NO₃^- and NO₂^- and solid changes (average oxidation state of Mn, cation exchange capacity, solid nitrogen content and symmetry evolution identified by XRD and FTIR) indicate unambiguously that NH₄⁺ reacts chemically with the birnessite.