Katy J. McKenzie

Direct electrochemistry of nanoparticulate Fe2O3 in aqueous solution and adsorbed onto tin-doped indium oxide

Nanoparticulate iron oxides occur naturally, for example, in soil, water, and in the cytoplasm of living cells. The redox properties and detection of these nanoparticles are therefore of considerable importance. Understanding and mimicking nanoparticle-based redox reactions may lead to new types of water-based electrochemical processes. In this study, the electrochemical detection of 45 nm diameter Fe2O3 nanoparticles dissolved in aqueous buffer solutions is investigated as a model system. Voltammetric experiments with nanoparticulate Fe2O3 are reported based on two complementary approaches: (i) Fe2O3 nanoparticles adsorbed onto tin-doped indium oxide (ITO) electrodes are shown to give well-defined voltammetric reduction responses and (ii) hydrodynamic voltammety in the presence of fast (24 kHz ultrasound-enhanced) mass transport is shown to allow the direct detection of Fe2O3 nanoparticles in solution. Both the adhesion and the electrochemical reactivity of Fe2O3 nanoparticles at ITO electrode surfaces are strongly affected by the solution composition and the pH.

Nanoporous iron oxide membranes: layer-by-layer deposition and electrochemical characterisation of processes within nanopores

A versatile procedure for the formation of nanoporous metal oxide membranes is reported, based on a layer-by-layer deposition procedure (‘directed assembly’) of metal oxide nanoparticles with appropriate ‘linker’ molecules; here Fe2O3 particles and phytic acid. Two types of nanoporous Fe2O3 membranes have been prepared and characterised: (A) a nanofilm deposit composed of 4–5 nm diameter Fe2O3 particles linked by phytic acid and (B) a nanoporous film formed after calcination of the type A deposit at 500°C in air. The nanofilm deposits are characterised by microscopy (SEM and AFM) and by electrochemical methods. Mechanically stable and homogeneous nanofilm deposits with controlled thickness (ca. 3 nm per layer deposited) were obtained. Transport of small molecules or ions through the nanoporous structure and their electrochemical conversion are shown to be fast in the presence of a sufficiently high concentration of supporting electrolyte. During the electrochemical oxidation of ferrocyanide, Fe(CN)64−, the nanoporous structure of the type A deposit is shown to act as an ‘active’ membrane, which changes the electrode kinetics by ‘double-layer superposition’ effects. For the second type of nanofilm, type B, ferrocyanide is accumulated by adsorption within the porous structure.

Hydrophobic silica sol–gel films for biphasic electrodes and porotrodes

Hydrophobic sol-gel films from methyltrimethoxysilane (MTMOS) are deposited onto glass and tin-doped indium oxide (ITO) coated glass substrates. Uniform and microporous films of ca. 200 nm thickness are obtained and investigated by scanning electron microscopy and by electrochemical techniques. From cyclic voltammograms for the oxidation of ferrocenedimethanol in aqueous 0.1 M KNO3 apparent diffusion coefficients and free volume data for processes within the film are derived and it is demonstrated that the film morphology can be controlled by the deposition timing. Two novel types of biphasic electrodes for observing liquid/liquid ion transfer reactions are introduced: (i) an ITO electrode coated with a hydrophobic sol-gel film and (ii) a hydrophobic sol-gel film on glass sputter-coated with 20 nm porous gold (porotrode). For the t-butylferrocene redox system deposited in the form of an organic liquid, very low and morphology dependent current responses are observed on modified ITO electrodes. However, the porotrode system allows biphasic electrode reactions to be driven with high efficiency and with no significant morphology effect of the hydrophobic sol-gel film. This type of nanofilm-modified electrode system will be of interest for biphasic sensor developments.

Adsorption and reactivity of hydrous iron oxide nanoparticles on boron-doped diamond

Abstract Hydrous iron oxide nanoparticles of 8–16 nm diameter are adsorbed from an aqueous sol onto the surface of polished boron-doped diamond electrodes. Boron-doped diamond acts as an inert substrate with low background currents and a wide potential window, allowing electrochemical and electrocatalytic properties of an electrically non-conducting material such as hydrous iron oxide to be studied. The electrochemical reduction of hydrous ferric oxide in aqueous phosphate buffer at pH 5 at a potential of ca. −0.2 V vs. SCE is accompanied by stripping of the deposit, which allows the determination of the approximate coverage of the oxide on the boron-doped diamond electrode, typically 1.4×10 −9 mol cm −2 . Next, hydrous ferric oxide is shown to act as an efficient electrocatalyst for the oxidation of hydroxide to dioxygen. The process is proposed to occur with apparently one electron per hydroxide via a C het , rev E rev C het , irrev -type process, initiated by deprotonation of an active iron oxide surface site. It is also demonstrated that the reaction of hydrogen peroxide on hydrous ferric oxide surfaces (a heterogeneous Fenton-type process) is similarly triggered by the deprotonation of an active surface site.

Mesoporous TiO2 carboxymethyl-γ-cyclodextrate multi-layer host films: effects on adsorption and electrochemistry of 1,1′-ferrocenedimethanol

TiO2 (anatase) nanoparticles are readily deposited layer-by-layer in the form of thin films with a carboxymethyl-γ-cyclodextrate binder. Electron microscopy, voltammetric, and quartz crystal microbalance data demonstrate that the film grows homogeneously and is electrically connected to the ITO electrode surface. 1,1′-Ferrocenedimethanol is employed as an adsorbing redox system to study the voltammetric characteristics of the mesoporous host film. The binding constants for the homogeneous complexation of 1,1′-ferrocenedimethanol with carboxymethyl-γ-cyclodextrin at pH 7, Kred = 1300 ± 200 M−1, and at pH 2, Kred = 1000 ± 200 M−1, are determined assuming 1 ∶ 1 complex formation. In the presence of the TiO2 carboxymethyl-γ-cyclodextrate films, solution phase voltammetric responses are affected due to a lower rate of diffusion of 1,1′-ferrocenedimethanol across the film (possibly due to binding to receptor sites) and due to slow electron transfer at pH 7 but not at pH 2. The TiO2 carboxymethyl-γ-cyclodextrate modified electrode, when dipped into 1,1′-ferrocenedimethanol containing solution, rinsed, and transferred into clean buffer solution, shows characteristic signals for adsorbed 1,1′-ferrocenedimethanol, consistent with weak binding and fast release upon oxidation. There is evidence for two distinct binding sites for 1,1′-ferrocenedimethanol both at pH 7 and at pH 2.

Quartz crystal microbalance monitoring of density changes in mesoporous TiO2 phytate films during redox and ion exchange processes

Nanofilm deposits of TiO2 nanoparticle phytates are formed on gold electrode surfaces by ‘directed assembly’ methods. Alternate exposure of a 3-mercapto-propionic acid modified gold surface to (i) a TiO2 sol and (ii) an aqueous phytic acid solution (pH 3) results in layer-by-layer formation of a mesoporous film. Ru(NH3)63+ is shown to strongly adsorb/accumulate into the mesoporous structure whilst remaining electrochemically active. Scanning the electrode potential into a sufficiently negative potential range allows the Ru(NH3)63+ complex to be reduced to Ru(NH3)62+ which undergoes immediate desorption. When applied to a gold coated quartz crystal microbalance (QCM) sensor, electrochemically driven adsorption and desorption processes in the mesoporous structure become directly detectable as a frequency response, which corresponds directly to a mass or density change in the membrane. The frequency response (at least for thin films) is proportional to the thickness of the mass-responsive film, which suggests good mechanical coupling between electrode and film. Based on this observation, a method for the amplified QCM detection of small mass/density changes is proposed by conducting measurements in rigid mesoporous structures.