A new HRP/catalase biosensor based on microconductometric transduction for nitrite determination

An interdigitated conductimetric electrode system using a combination of peroxidase/catalase has been developed to determine nitrite in water samples. A peroxidase (HRP) was located in the inner layer while the outer contained catalase. Catalase catalyzed the breakdown of H₂O₂ into H₂O and O₂ thus consuming totally H₂O₂, the substrate of HRP. The latter, in presence of H₂O₂, generates a conductometric signal due to the reduction of H₂O₂. Nitrite was selected as an inhibitor of catalase. In the presence of H₂O₂, the nitrite addition blocked the H₂O₂ consumption by catalase. Since nitrite had no effect on HRP activity, its inhibitive effect on catalase leads to an increase in the conductometric signal. The bienzyme sensor exhibits an increase in conductometric response for nitrite concentration, leading to high values of conductivity. In both case, the detection limit of nitrite is 0.3 µM and for bienzyme sensor the dynamic range is from 0.3 µM to 446 µM.     

Photoelectrochemical reactions on semiconductor materials

The cathodic reduction of Fe³⁺ ions and I₂ on ann-GaAs electrode was studied. The variation in the current density with the concentration of the oxidant has been interpreted as being the result of a reduction mechanism involving interface or surface states, an interpretation that is amply substantiated by experimental data. The effect of the surface modification with SiW₁₂O₄₀ ^4– on the reduction process was studied. Prior to this electrode activation, the rate constant for electrons being transferred from the conduction-band to the interface or surface states,k _S, was observed to be independent of electrode potential, whereas in the case of the modifiedn-GaAs,k _S depends on band-bending. On the other hand, the rate constant for electrons being transferred from the interface or surface states to oxidant species,k _ox, does depend on electrode potential in the case of the unmodifiedn-GaAs, and is independent of band-bending in the case of the modifiedn-GaAs. This change may be attributed to the filling of the active surface or interface states or their redistribution after the electrode surface activation.     

Direct Interfacial Excitation from TiO2 to Cu(II) Nanoclusters Enables Cathodic Photoresponse for Hydrogen Evolution under Visible-Light Irradiation

The titanium dioxide (TiO₂) photocatalyst is only active under UV irradiation due to its wide-gap nature. A novel excitation pathway denoted as interfacial charge transfer (IFCT) has been reported to activate copper(II) oxide nanoclusters-loaded TiO₂ powder (Cu(II)/TiO₂) under visible-light irradiation for only organic decomposition (downhill reaction) so far. Here, the photoelectrochemical study shows that the Cu(II)/TiO₂ electrode exhibits a cathodic photoresponse under visible-light and UV irradiation. It originates from H₂ evolution on the Cu(II)/TiO₂ electrode, while O₂ evolution takes place on the anodic side. Based on the concept of IFCT, a direct excitation of electrons from the valence band of TiO₂ to Cu(II) clusters initiates the reaction. This is the first demonstration of a direct interfacial excitation-induced cathodic photoresponse for water splitting without any addition of a sacrificial agent. This study is expected to contribute to the development of abundant visible-light-active photocathode materials for fuel production (uphill reaction).     

On the oxygen reduction reaction catalyzed by Ti -Cu binary films in 0.5 M sulfuric acid solution

Ti-Cu binary films co-sputtered in vacuum are catalytically active for the oxygen reduction in 0.5 M H₂SO₄. The activity for the oxygen reduction reaction (ORR) increased with increasing the Cu-content in the Ti-Cu films and it reached to a maximum with the copper composition up to 90 at.%. The constant Tafel slope of ~ 190 mV/decade which is comparable to that obtained on pure Cu films indicates that the active sites for oxygen reduction is copper sites. Through investigation of Tafel polarization, the Ti-Cu films revealed a constant Tafel slope (i.e., 190 mV/decade) similar to that of ORR on the pure Cu film. This infers that the electrochemical reduction of oxygen is predominated on the Cu-sites in the film. In the cyclic voltammograms, the strong broad peak should have arisen from the oxidation of Cu to Cu⁺ and Cu²⁺ ions. This oxidation indicated that the Ti-Cu films are unstable and the Cu-component is susceptible to dissolution in 0.5 M H₂SO₄. This dissolution caused a loss of catalytic activity in the films. Preparing the Ti-Cu films enriched in Ti will stabilize these films to prevent the Cu-dissolution.     

Surface Self-Transforming FeTi-LDH Overlayer in Fe2O3/Fe2TiO5 Photoanode for Improved Water Oxidation

Integrating hematite nanostructures with efficient layer double hydroxides (LDHs) is highly desirable to improve the photoelectrochemical (PEC) water oxidation performance. Here, an innovative and facile strategy is developed to fabricate the FeTi-LDH overlayer decorated Fe₂O₃/Fe₂TiO₅ photoanode via a surface self-transformation induced by the co-treatment of hydrazine and NaOH at room temperature. Electrochemical measurements find that this favorable structure can not only facilitate the charge transfer/separation at the electrode/electrolyte interface but also accelerate the surface water oxidation kinetics. Consequently, the as-obtained Fe₂O₃/Fe₂TiO₅/LDH photoanode exhibits a remarkably increased photocurrent density of 3.54 mA cm⁻² at 1.23 V versus reversible hydrogen electrode (RHE) accompanied by an obvious cathodic shift (≈140 mV) in the onset potential. This work opens up a new and effective pathway for the design of high-performance hematite photoanodes toward efficient PEC water oxidation.     

Preparation of Ce1?xFexO2 solid solution and its catalytic performance for oxidation of CH4 and CO

A series of Ce_1−x Fe _x O₂ (0 < x ≤ 0.5) catalysts were prepared by the co-precipitation method, and their catalytic performances were investigated for the total oxidation of CO and CH₄ as model reactions. X-ray diffraction (XRD) and Raman spectroscopy results show that Ce_1−x Fe _x O_2−δ solid solutions are formed with x ≤ 0.2. Ce_0.9Fe_0.1O₂ solid solution presents superior catalytic performance for CH₄ and CO oxidation, while Ce_1−x Fe _x O₂ with x > 0.2 shows less active for CO and CH₄ oxidation. The results of H₂-temperature programmed reduction (H₂-TPR), CH₄-temperature programmed surface reaction (CH₄-TPSR) and CO-TPSR reveal that, the surface oxygen of catalyst is relevant to CO oxidation, which was promoted by the oxygen vacancies formed in Ce–Fe–O solid solution, while the easier lattice oxygen migration property and the favorable reducibility of the catalysts is responsible for the promoted catalytic performance for CH₄ oxidation.     

The electrocatalytic application of RuO2 in direct borohydride fuel cells

A high electrocatalytic activity of RuO₂ has been found for oxygen reduction reaction (ORR) in the cathode of direct borohydride fuel cells (DBFCs). The electron transfer number n during the ORR changes from 3.58 to 3.86 and the percentage of the intermediate product H₂O₂ decreases from 20.8% to 7.2% correspondingly when the disk potential scans negatively from −0.39 V to −0.8 V versus Hg/HgO. Peak power densities of 425 mW cm⁻² has been obtained at 60 °C, when RuO₂ has been used as a cathodic catalyst in DBFCs. RuO₂ displays low sensitivity to the BH₄⁻ oxidation in DBFCs. Moreover, RuO₂, as a cathodic catalyst, demonstrates a superb stability during a 200-h durability test. The identical X-ray diffraction (XRD) patterns of the RuO₂ before and after the durability test also prove its stability.     

(Fe3O4)-graphene oxide as a novel magnetic nanomaterial for non-enzymatic determination of phenylalanine

Fe₃O₄-graphene oxide (GO) modified glassy carbon (GC) electrode was used as a new magnetic nanosensor for determination of phenylalanine (Phe). It was found that Fe₃O₄-GO has been stably absorbed on GC electrode modified by simple technique. The cyclic voltammograms of the modified electrode in an aqueous solution displayed a pair of well-defined, stable and irreversible reductive/oxidation redox systems. The apparent electron transfer rate constant (k_s) and transfer coefficient (α) were determined by cyclic voltammetry and were approximately 9.3 s^? 1 and 0.67, respectively. The modified electrode showed excellent catalytic activity towards the oxidation of Phe at an unusually positive potential in buffer solution. This nanosensor also displayed fast response time, high sensitivity, low detection limit and had a remarkably positive potential oxidation of Phe that decreased the effect of interferences in analysis.     

Electrochemical deposition of precious metal on carbon nanotube for methanol oxidation

Precious metal nanoparticles were prepared on carbon nanotube (CNT) by sequential and simultaneous deposition methods for the electrocatalytic study of methanol oxidation. All electrochemical measurements were carried out in a three-electrode cell. A Platinum wire and Ag/AgCl were used as auxiliary and reference electrodes, respectively. Suspension of the CNT and Nafion were mixed and dropped on glassy carbon as a working electrode. Cyclic voltammograms in H₂SO₄ electrolyte solution are attributable to hydrogen adsorption and hydrogen desorption resulting in promising electrochemical performance of the prepared precious metal nanoparticles. Cyclic voltamograms of methanol electrooxidation studied in 2 M CH₃OH in 1 M H₂SO₄ show a distinguishing shape with a prominent oxidation wave in the anodic scan contributed to methanol oxidation while the cathodic scan is associated with the accumulation of carbonaceous species.     

Method for effective immobilization of Ag nanoparticles/graphene oxide composites on single-stranded DNA modified gold electrode for enzymeless H2O2 detection

In this paper, we report a new method for effective immobilization of Ag nanoparticles (AgNPs) decorated graphene oxide (AgNP/GO) composites onto thiolated single-stranded DNA decorated Au electrode (AuE) surface. The novel immobilization method is based on the coordination interactions and π–π stacking interactions between DNA bases and AgNP/GO composites. The morphologies of the AgNP/GO nanocomposites are characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It is found that the AgNP/GO-decorated AuE exhibits remarkable catalytic performance for H₂O₂ reduction. This H₂O₂ sensor has a fast amperometric response time of less than 5 s. The linear range is estimated to be from 0.1 mM to 20 mM (r = 0.998) and the detection limit is estimated to be 1.9 μM at a signal-to-noise ratio of 3, respectively.     

Influence of oxygen flow rate on photocatalytic TiO2 films deposited by rf magnetron sputtering

Titanium dioxide (TiO₂) thin films having anatase (1 0 1) crystal structure were prepared on non-alkali glass substrates by rf (13.56 MHz) magnetron sputtering using a TiO₂ ceramic target under various oxygen partial pressures. At a fixed substrate temperature of 400 °C and total gas pressure of 1 Pa after 3 h deposition. Effects of oxygen partial pressure on the structural, surface morphology, and photocatalytic activities of the TiO₂ thin films were investigated. We performed both photoinduced decomposition of methylene blue (MB) and photoinduced hydrophilicity under UV light illumination. The XRD patterns exhibited a broad-hump shape indicating the amorphous structure of TiO₂ thin films. The results showed that when the [O₂/(Ar + O₂)] flow rate increased to 50%, the photoinduced decomposition of MB and photoinduced hydrophilicity were enhanced. The water contact angle after 9 min UV illumination was approximately 4.5°, and the methylene blue (MB) solution decomposition from 12 down to 3.34 μ mol/L for 240 min UV irradiation.     

A novel potentiometric hydrogen peroxide sensor based on pKa changes of vinylphenylboronic acid membranes

A potentiometric hydrogen peroxide (H₂O₂) sensing scheme was developed using arylboronic acid as the electrode modifier. It is well-known that both aliphatic and aryl boronic acid undergo electrophilic displacement reaction with H₂O₂. This reaction involves replacement of boronic acid by the hydroxyl group of peroxide resulting in a change in pK_a value that can be exploited for sensing of H₂O₂. Vinylphenylboronic acid (VPBA) ink was prepared using Nafion as the binder and it was drop cast on an electrode surface. Morphology of the modified electrode was analysed using scanning electron microscopy (SEM). The present modifier exhibited a linear relationship between the difference in electrode potential (?Ep) vs. [H₂O₂] with a Nernstian slope of 26 ± 2 mV in the concentration range of 10^? 1–10^? 5 M. Application of the VPBA modified electrode for hydrogen peroxide sensing was studied in an industrial dye-bleach effluent.     

Improved Oxygen Evolution Kinetics and Surface States Passivation of Ni-Bi Co-Catalyst for a Hematite Photoanode

This paper describes the combinational surface kinetics enhancement and surface states passivation of nickel-borate (Ni-B_i) co-catalyst for a hematite (Fe₂O₃) photoanode. The Ni-B_i-modified Fe₂O₃ photoanode exhibits a cathodic onset potential shift of 230 mV and a 2.3-fold enhancement of the photocurrent at 1.23 V, versus the reversible hydrogen electrode (RHE). The borate (B_i) in the Ni-B_i film promotes the release of protons for the oxygen evolution reaction (OER).     

Manganese oxide thin films prepared by pulsed laser deposition for thin film microbatteries

Manganese oxide thin films with various oxidation states (MnO, Mn₃O₄ and Mn₂O₃) have been prepared by pulsed laser deposition using a Mn target at different oxygen partial pressures. The structural and morphological features of the as-deposited thin films are characterized by X-ray diffraction, Raman, field emission scanning electron microscopy (FESEM). The oxidation states of Mn in different thin films are investigated by X-ray photoelectron spectroscopy for both Mn 2p and 3s levels. It is found that the structure, surface morphology, and Mn oxidation state of the thin films can be tuned by oxygen partial pressure during the deposition. As anode for thin film lithium-ion microbatteries, the Mn₃O₄ thin film electrode exhibits the largest reversible capacity up to 800 mAh g⁻¹ with good cycling stability and excellent rate capability. The promising electrochemical performance of the Mn₃O₄ thin film electrode indicates the potential application of Mn₃O₄ thin film anode in all solid-state thin film microbatteries.     

Electrochemical Formation of a Failure Preventing Film on Nickel Surface in Borate Solutions

Cyclic voltammograms (CVs) of the Ni electrode are traced in Na₂B₄O₇ solutions as a function of electrolyte concentration, voltage scanning range, and rate in order to determine the nature of failure protective species formed in the slightly alkaline media. The species formed on the Ni electrode are found to depend on the sweep number due to changes in the activation state of the electrode surface. The voltammograms are characterized by a pronounced anodic peak due to the formation of NiO and a protective passive film corresponding to the formation of β-Ni(OH)₂ before the evolution of oxygen. An additional anodic peak in the vicinity of oxygen evolution potential appeared in the advanced cycles that is attributed to the transformation of β-Ni(OH)₂ to β-NiOOH. The cathodic branch shows only one peak corresponding to the reduction of β-NiOOH to β-Ni(OH)₂. The current density flowing along the anodic oxidation peak varies with the concentration of the electrolyte according to: where a and b are constants. An increase in the scan rate increases markedly the current density flowing along the whole range of the CVs. As the concentration of borate anions increases, the anodic peak potential is shifted toward more positive values, whereas the cathodic peak potential is shifted in the negative direction, indicating the irreversibility of formation of the passive film formed on the electrode surface. A correlation is made between the anodic oxidation processes and their corresponding cathodic one. The failure-protecting film in borate solutions is assumed to be caused by the formation of a sandwich oxide having the form: NiO/β-Ni(OH)₂/β-NiOOH.     

CuO impregnated activated carbon for catalytic wet peroxide oxidation of phenol

This paper presents an original approach to the removal of phenol in synthetic wastewater by catalytic wet peroxide oxidation with copper binding activated carbon (CuAC) catalysts. The characteristics and oxidation performance of CuAC in the wet hydrogen peroxide catalytic oxidation of phenol were studied in a batch reactor at 80 °C. Complete conversion of the oxidant, hydrogen peroxide, was observed with CuAC catalyst in 20 min oxidation, and a highly efficient phenol removal and chemical oxygen demand (COD) abatement were achieved in the first 30 min. The good oxidation performance of CuAC catalyst was contributed to the activity enhancement of copper oxide, which was binding in the carbon matrix. It can be concluded that the efficiency of oxidation dominated by the residual H₂O₂ in this study. An over 90% COD removal was achieved by using the multiple-step addition in this catalytic oxidation.     

Effects of transition metal oxide and Ni addition on the hydrogen-storage properties of Mg

Mg-based hydrogen-storage materials with the compositions of Mg–10 wt%oxide (oxide = Cr₂O₃, Fe₂O₃, MnO, and SiO₂) and Mg–xFe₂O₃–yNi were prepared by reactive mechanical grinding (RMG). Taking into consideration the hydriding and dehydriding rates and the cost of materials, Fe₂O₃ prepared by spray conversion is an appropriate oxide additive to Mg. Mg–5 wt%Fe₂O₃–15 wt%Ni exhibited the best hydrogen-storage performance among the Mg–xFe₂O₃–yNi hydrogen materials. It stored 5.47 wt%H under 1.2 MPa H₂ for 60 min and released 5.42 wt%H under 0.1 MPa H₂ for 15 min at 593 K. The addition of Fe₂O₃ and Ni to Mg by the RMG shortens the diffusion distances through the reduction of the particle size of Mg. These additives are also considered to facilitate nucleation by creating many defects on the surface and in the interior of Mg. The added Fe₂O₃ and Ni themselves may also act as active sites for the nucleation. Ni forms the Mg₂Ni phase by a reaction with Mg, and Fe appears from the reduction of Fe₂O₃ by hydrogen after hydriding–dehydriding cycling.     

Highly Selective Metal-Free Electrochemical Production of Hydrogen Peroxide on Functionalized Vertical Graphene Edges

Electrochemical generation of hydrogen peroxide (H₂O₂) is an attractive alternative to the energy-intensive anthraquinone oxidation process. Metal-free carbon-based materials such as graphene show great promise as efficient electrocatalysts in alkaline media. In particular, the graphene edges possess superior electrochemical properties than the basal plane. However, identification and enhancement of the catalytically active sites at the edges remain challenging. Furthermore, control of surface wettability to enhance gas diffusion and promote the performance in bulk electrolysis is largely unexplored. Here, a metal-free edge-rich vertical graphene catalyst is synthesized and exhibits a superior performance for H₂O₂ production, with a high onset potential (0.8 V versus reversible hydrogen electrode (RHE) at 0.1 mA cm^?2) and 100% Faradaic efficiency at various potentials. By tailoring the oxygen-containing functional groups using various techniques of electrochemical oxidation, thermal annealing and oxygen plasma post-treatment, the edge-bound in-plane ether-type (C? O? C) groups are revealed to account for the superior catalytic performance. To manipulate the surface wettability, a simple vacuum-based method is developed to effectively induce material hydrophobicity by accelerating hydrocarbon adsorption. The increased hydrophobicity greatly enhances gas transfer without compromising the Faradaic efficiency, enabling a H₂O₂ productivity of 1767 mmol g_catalyst^?1 h^?1 at 0.4 V versus RHE.     

Ultraweak emission of the Eu(III) ions in cathodic generated electrochemiluminescence

High voltage cathodic generated electrochemiluminescence (ECL) of selected systems generating weak photon emission was studied. The investigated systems contained europium salts, and co-reactants: H₂O₂ and K₂S₂O₈.During the cathodic pulse polarization of an oxide-covered working aluminum electrode, hot electrons in metal may enter the conduction band of water through the oxide layer/solution interface and turn into hydrated electron after salvation. The ECL intensity and spectra was measured though a suitable cut-off filters, with a photon counting unit attached to a PC computer.On the basis of the analysis of the spectra in particular systems, emitters were identified and simplified scheme of the processes was proposed. The Eu(III)-specific electro generated luminescence is produced via several parallel mechanisms, which usually involve a type of the co-reactant, oxidation state changes of europium ions in aqueous solution and the influence of working electrode surface emission.     

Characterizing the discoloration of methylene blue in Fe/H2O systems

Methylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe⁰/H₂O systems. Likely discoloration mechanisms under used experimental conditions are: (i) adsorption onto Fe⁰ and Fe⁰ corrosion products (CP), (ii) co-precipitation with in situ generated iron CP, (iii) reduction to colorless leukomethylene blue (LMB). MB mineralization (oxidation to CO₂) is not expected. The kinetics of MB discoloration by Fe⁰, Fe₂O₃, Fe₃O₄, MnO₂, and granular activated carbon were investigated in assay tubes under mechanically non-disturbed conditions. The evolution of MB discoloration was monitored spectrophotometrically. The effect of availability of CP, Fe⁰ source, shaking rate, initial pH value, and chemical properties of the solution were studied. The results present evidence supporting co-precipitation of MB with in situ generated iron CP as main discoloration mechanism. Under high shaking intensities (>150 min⁻¹), increased CP generation yields a brownish solution which disturbed MB determination, showing that a too high shear stress induced the suspension of in situ generated corrosion products. The present study clearly demonstrates that comparing results from various sources is difficult even when the results are achieved under seemingly similar conditions. The appeal for an unified experimental procedure for the investigation of processes in Fe⁰/H₂O systems is reiterated.