Redox processes in mesoporous oxide membranes: layered TiO2 phytate and TiO2 flavin adenine dinucleotide films

Enhanced photocatalytic activity of a self-stabilized synthetic flavin anchored on a TiO2 surface

Synthetic flavin molecules were anchored on Degussa P25 titanium dioxide (TiO2). The effect of their presence on the photocatalytic (PC) activity of TiO2 was studied. Under UV light, an increase in the degradation rate of ethanol was observed. This increase was accompanied by stabilization of the anchored flavin against self-degradation. The unprecedented stabilization effect was found also in the absence of a reducing agent such as ethanol. In contrast, under the less energetic visible light, fast degradation of the anchored flavin was observed. These rather surprising observations were attributed to the propensity for charge transport from excited flavin molecules to the semiconductor and to the role that such charge transfer may play in stabilizing the overall assembly. Anchored flavins excited by UV light to their S2, S3 electronic states were able to transfer the excited electrons to the TiO2 phase whereas anchored flavin molecules that were excited by visible light to the S1 state were less likely to transfer the photo-excited electrons and therefore were destabilized. These findings may be relevant not only to anchored flavins in general but to other functionalized photocatalysts, and may open up new vistas in the implementation of sensitizers in PC systems.

Flavin Derivatives with Tailored Redox Properties: Synthesis, Characterization, and Electrochemical Behavior

This study establishes structure-property relationships for four synthetic flavin molecules as bioinspired redox mediators in electro- and photocatalysis applications. The studied flavin compounds were disubstituted with polar substituents at the N1 and N3 positions (alloxazine) or at the N3 and N10 positions (isoalloxazines). The electrochemical behavior of one such synthetic flavin analogue was examined in detail in aqueous solutions of varying pH in the range from 1 to 10. Cyclic voltammetry, used in conjunction with hydrodynamic (rotating disk electrode) voltammetry, showed quasi-reversible behavior consistent with freely diffusing molecules and an overall global 2e(-) , 2H(+) proton-coupled electron transfer scheme. UV/Vis spectroelectrochemical data was also employed to study the pH-dependent electrochemical behavior of this derivative. Substituent effects on the redox behavior were compared and contrasted for all the four compounds, and visualized within a scatter plot framework to afford comparison with prior knowledge on mostly natural flavins in aqueous media. Finally, a preliminary assessment of one of the synthetic flavins was performed of its electrocatalytic activity toward dioxygen reduction as a prelude to further (quantitative) studies of both freely diffusing and tethered molecules on various electrode surfaces.

Nano-TiO(2)-flavin adenine dinucleotide film redox processes in contact to humidified gas | salt electrolyte

Redox processes in nano-TiO(2)-flavin adenine dinucleotide (TiO(2)-FAD) layer-by-layer assembled films on ITO substrate electrodes are investigated and compared in contact to aqueous electrolyte media (for dilute and saturated electrolyte) and in contact to solid humidified salt electrolyte (for extreme salt levels and different types of salts). Under these unusual conditions an aqueous microphase present at the gas | salt | electrode interface allows voltammograms to be obtained and redox processes to be analysed. It is demonstrated that the 2-electron 2-proton reduction of FAD can be used as reporter redox system to determine local pH at the electrode | gas | salt interface as pH 15, 12, 7 for contacts to K(3)PO(4), K(2)HPO(4), and KH(2)PO(4), respectively. Exposure to gases such as carbon dioxide is shown to lead to unexpected changes in surface pH. In the future, bio-electrochemical microphase processes under halophilic conditions could be useful for air-quality and rapid gas sensing devices.

Controlling the reactivity of chlorinated ethylenes with flavin mononucleotide hydroquinone

Reduction rate constants of the chlorinated ethylenes cis-1,2-dichloroethylene (cis-DCE), trichloroethylene (TCE), and tetrachloroethylene (PCE) reacted with flavin mononucleotide hydroquinone (FMNH2) under anoxic conditions were investigated. FMNH2 was produced in methanol solvent by the photoreduction of FMN. In aqueous solution, FMN was not fully reduced to FMNH2 but instead yielded the semiquinone radical FMNH*. However, when FMN was anchored to nanocrystalline TiO2, band gap irradiation resulted in electron transfer from the TiO2 conduction band to FMN, thus yielding FMNH2. The FMNH2 generated in aqueous solution on the TiO2 surface was a stronger reductant toward chlorinated ethylenes, relative to FMNH2 in solution. Furthermore, by combining the reactivity of the TiO2 conduction band electrons [TiO2(e-(CB)] with FMNH2, reduction rate constants for the chlorinated ethylenes increased by 2 orders of magnitude relative to FMNH2 alone. The results show how biological molecules such as FMNH2 could have significant effects toward the remediation of organic pollutants.

The effects of conductivity and electrochemical doping on the reduction of methemoglobin immobilized in nanoparticulate TiO2 films

Methemoglobin (bovine) is immobilized from aqueous phosphate buffer (pH 5.5) solution into thin porous TiO(2) (anatase) films at ITO electrode surfaces. Films of TiO(2) are produced in a deposition process employing 40 nm diameter TiO(2) nanoparticles suspended in dry methanol followed by calcination. The pore size in these films is sufficient for methemoglobin (ca. 6 nm diameter) to diffuse into the porous structure (over several hours) and to remain immobilized in electrochemically active form. The electrochemical reduction of methemoglobin immobilized in TiO(2) and immersed in aqueous phosphate buffer at pH 5.5 is observed in two steps with (i) a small quasi-reversible voltammetric response at -0.16 V vs. SCE (Process 1) and (ii) an irreversible reduction peak at ca. -0.5 V vs. SCE (Process 2). The irreversible response is recovered only after slow chemical re-oxidation of hemoglobin to methemoglobin. At sufficiently negative applied potential "electrochemical doping" of the TiO(2) host is observed to lead to a considerably enhanced reduction Process 1. TiO(2) can be temporarily switched from a non-conducting (irreversible electron transfer) into a conducting (reversible electron transfer) state.

Electrochemical properties of core-shell TiC–TiO2nanoparticle films immobilized at ITO electrode surfaces

Titanium carbide (TiC) nanoparticles are readily deposited onto tin-doped indium oxide (ITO) electrodes in the form of thin porous films. The nanoparticle deposits are electrically highly conducting and electrochemically active. In aqueous media (at pH 7) and at applied potentials positive of 0.3 V vs. SCE partial anodic surface oxidation and formation (at least in part) of novel core-shell TiC-TiO2 nanoparticles is observed. Significant thermal oxidation of TiC nanoparticles by heating in air occurs at a temperature of 250 degrees C and leads first to core-shell TiC-TiO2 nanoparticles, next at ca. 350 degrees C to TiO2 (anatase), and finally at temperatures higher than 750 degrees C to TiO2 (rutile). Electrochemically and thermally partially oxidized TiC nanoparticles still remain very active and for some redox systems electrocatalytically active. Scanning and transmission electron microscopy (SEM and TEM), temperature dependent XRD, quartz crystal microbalance, and voltammetric measurements are reported. The electrocatalytic properties of the core-shell TiC-TiO2 nanoparticulate films are surveyed for the oxidation of hydroquinone, ascorbic acid, and dopamine in aqueous buffer media. In TiC-TiO2 core-shell nanoparticle films TiO2 surface reactivity can be combined with TiC conductivity.