Enhancement of the sensitivity and selectivity of oxidation of H2O2 on platinum wire at low working potential by platinization and covering of heteropolypyrrole film for amperometric micro-biosensor construction

A microelectrode for glucose determination was constructed by immobilization of glucose oxidase (GOx) on a platinized platinum (Pt) by electrochemical polymerization of a solution containing GOx, pyrrole, and a substituted pyrrole, 4-(3-pyrrolyl)-4-oxobutyric acid. Due to platinization and covering with the polymerized heteropolypyrrole (hPPy) film, the electrode prepared showed high sensitivity to H₂O₂ at a low potential and significantly reduced the response to electroactive compounds, such as ascorbate, urate and 4-acetamidophenol. Working at 200 mV (vs. SCE) the electrode showed a linear response to glucose from 1.6 to 10 mM with a high sensitivity of 1 μA/mM, whereas the response to 1 mM ascorbate, urate, and 4-acetamidophenol was 0.53 μA, 18 nA and 4 nA, respectively, which was about 2.5%, 1.0% and 1.0% of that at a bare electrode. The stability of the electrode was tested at intervals of three or five days, and each test lasted about two hours. After 6 months examination, only 30% of its activity was lost. Received: 16 March / Revised: 26 July 1998 / Accepted: 1 August 1998     

Drastic increase of selectivity for H2O2 formation in direct oxidation of H2 to H2O2 over supported Pd catalysts due to their bromination

Incorporation of bromide anions (1.0 wt%) in supported Pd catalysts (viz. Pd supported on Al2O3, ZrO2, SiO2, H-beta or Ga2O3) leads to a drastic increase in their selectivity for H2O2 formation in the direct oxidation of H2 to H2O2 by O2(at room temperature) in an aqueous acidic (0.03 M H3PO4) reaction medium; the selectivity increase is accompanied by a large decrease in the H2O2 decomposition activity of the catalysts.     

Hybrid platinum nanoparticle ensemble for the electrocatalytic oxidation of H2O2: Toward nanostructured biosensor design

This paper reports the electrocatalysis of H₂O₂ oxidation by Langmuir–Blodgett (LB) thin films based on polymer-grafted platinum nanoparticles (PtNPs). 4-mercaptoaniline-functionalized PtNPs have been chemically modified to obtain poly(methacrylic acid)-grafted platinum nanoparticles (PMAA-PtNP) via surface-ATRP. These elementary bricks are used to form mixed LB films consisting of controlled mixtures of PMAA-PtNP and a redox-inactive fatty acid molecule to tune the PtNP surface density. Nanostructures are formed in which the number of layers and the number of particles in each layer may be varied. The nanostructure morphology strongly depends on the amount of PtNP. High Pt content leads to quite large nanoparticle domains whereas low content gives small domains with a finely divided nanostructure. The H₂O₂ oxidation current increases when the PMAA-PtNP surface density decreases. This result is discussed in terms of electrocatalyst accessibility modification related to the nanostructure of the films.     

Preparation of compact Al2O3 film on metal for oxidation resistance by polyvinylpyrrolidone

Oxidation resistance of metal at high temperature can be improved by an environmentally friendly solution deposition approach. Stable precursor solution with high oxide concentration, favorable viscosity and low surface tension was prepared using aluminum sec-butoxide (ASB) and polyvinylpyrrolidone (PVP) as starting raw materials. Alumina sol-gel films were deposited onto metal by spin-coating followed by heat treatment. When PVP was added according to an amount of 50 mg/mL into a sol with an ASB/H₂O molar ratio of 1:35, the as-obtained sol exhibited favorable gelation time and viscosity. The surface tension of the alumina sol with PVP was examined to be lower by 32% than the sol (ASB:H₂O = 1:100) without PVP. TG-DTA analyses show the densification of the alumina gel film with PVP was progressed within a wide temperature range from 200 to 650 °C. Crack-free Al₂O₃ film with a thickness up to 1.5 μm was successfully produced on metallic substrate by three spin-coating cycles. SEM and XRD analyses revealed the gel film transformed into compact α-Al₂O₃ material after calcined at 1,000 °C for 0.5 h. The weight gained by the samples during firing at 1,000 °C indicated that the Al₂O₃ coating film could reduce the rate of oxidation by ∼81%. The hardness of the Al₂O₃ film coated metal was higher by 260% than the uncoated metal that was calcined at 1,000 °C for 0.5 h. It was confirmed by adhesion test that both the alumina/PVP hybrid film and the as-produced α-Al₂O₃ coating film had strong adhesion.     

Enhancement of O2 dissociation on Au111 by adsorbed oxygen: implications for oxidation catalysis

We show that the dissociation probability of O2 on the reconstructed, Au111-herringbone surface is dramatically increased by the presence of some atomic oxygen on the surface. Specifically, at 400 K the dissociation probability of O2 on oxygen precovered Au111 is on the order of 10(-3), whereas there is no measurable dissociation on clean Au111, establishing an upper bound for the dissociation probability of 10(-6). Atomic oxygen was deposited on the clean reconstructed Au111-herringbone surface using electron bombardment of condensed NO2 at 100 K. The dissociation probability for dioxygen was measured by exposing the surface to 18O2. Temperature programmed desorption (TPD) was used to quantify the amount of oxygen dissociation and to study the stability of the oxygen in all cases. Oxygen desorbs as O2 in a peak centered at 550 K with pseudo-first-order kinetics; i.e., the desorption peak does not shift with coverage. Our interpretation is that the coverage dependence of the activation energy for dissociation (deltaE(dis)) and/or preexponential factor (nu(d)) may be responsible for the unusual desorption kinetics, implying a possible energy barrier for O2 dissociation on Au111. These results are discussed in the context of Au oxidation chemistry and the relationship to supported Au nanoparticles.     

Functionalization of graphene with Prussian blue and its application for amperometric sensing of H2O2

As a two-dimensional carbon material with high surface area and conductivity, graphene shows great promise for designing composite nanomaterials to achieve high-performance electrochemical devices. In this work, we prepared graphene-based nanocomposite material by electrochemically depositing Prussian blue (PB) nanoparticles on the surface of graphene. Fourier transform infrared spectra, SEM, and cyclic voltammetry were used to characterize the successful immobilization of PB. Compared with PB films and graphene sheets, the PB–graphene composite films showed the largest current response to the reduction of H₂O₂, probably due to the synergistic effects between graphene sheets and PB nanoparticles. Therefore, a fast and highly sensitive amperometric sensor for H₂O₂ was obtained with a detection sensitivity of 1.6 μA μM^?1 H₂O₂ per cm² and a linear response range of 50～5,000 μM. The detection limit of H₂O₂ was 20 nM at a signal-to-noise ratio of 3. These obtained results are much better than those reported for carbon nanotubes-based amperometric sensors.     

Template-assisted synthesis of rough Ag nanorods and their application for amperometric sensing of H2O2

Silver nanorods (Ag NRs) with rough sidewalls were successfully obtained via extremely simple template-assisted electrochemical deposition followed by selective dealloying. Ag–Zn NRs with an average diameter of ∼250 nm and the atomic ratio of about 1:1 were synthesized inside the nanochannels of a polycarbonate membrane. A chemical etching of Zn in a sulfuric acid solution led to the formation of NRs with smaller diameters, rough sidewalls, and much lower Zn content. Such kind of a material exhibited quite promising electrocatalytic properties toward reduction of hydrogen peroxide and can be used as an amperometric sensor for the detection and determination of H₂O₂.     

Electro‐oxidation and reduction of H2 on platinum studied by scanning electrochemical microscopy for the purpose of local detection of H2 evolution

Electrochemical detection of H₂ using scanning electrochemical microscopy (SECM) has shown to hold great promise as a sensitive characterization method with high spatial resolution for active surfaces generating H₂. Herein, the factors contributing to the current that is measured by SECM in generation/collection mode for H₂ detection are studied. In particular, the concentration gradient of H₂ at the substrate, the H₂/H⁺ recycling between the SECM tip and substrate and hemispherical profile of H₂ diffusion has been discussed. It was postulated that H₂/H⁺ recycling plays a dominant role in the oxidative current measured in generation/collection mode of SECM when the microelectrode is positioned in close vicinity of substrate. Copyright © 2015 John Wiley & Sons, Ltd.     

Infrared matrix isolation study of the oxidation of H2S by CrCl2O2

The thermal and photochemical reactions between CrO2Cl2 and H2S have been investigated using matrix isolation infrared spectroscopy. Twin-jet co-deposition of the two reagents into argon matrices at 14 K followed by irradiation with light of lambda > 300 nm led to the growth of a number of new bands. These have been assigned to the HSOH-CrCl2O complex, and to SO2. These assignments were supported by deuterium isotopic substitution and high level density functional calculations. Merged-jet deposition with the reaction zone at temperatures above 125 degrees C led to substantial yields of SO2 and HCl, as well as the complex of HCl with excess parent H2S. The evidence suggests that the thermal and photochemical reactions may follow different pathways.     

Suspension balance apparatus for the investigation of the oxidation of refractory and platinum metals at high temperatures and low pressures

A magnetic suspension balance apparatus for oxidation studies at high temperatures and low pressures is described. The balance is mounted on a high—vacuum device with induction heating of the samples. Specific experimental problems of the investigation of gas—metal reactions at higher temperatures are discussed. Results are presented for the oxidation of Ir, Re, Mo, and W in oxygen, air, and water vapor at 1200–2300°C and 10⁻⁴ to 1 mbar. These metals react with oxygen at high temperatures forming volatile oxides, the evaporation of which considerably increases the metal loss in oxygen—containing atmospheres, when compared with high vacuum annealing conditions.     

The comparative study of CO2 + Hads reaction on platinum electrode in H2O and D2O

The observed isotopic effect in CO₂ + H_ads reaction showed that at 0.05 V the rate determining step is

formation whereas at 0.2 V the reorientation of adsorbed intermediate:

is probably the slowest step of reaction. The oxidation of adsorbed product is slower in D₂SO₄ than H₂SO₄ solution like the surface oxidation of platinum. The rate determining step of COOH_ads oxidation is a reaction with OH_ads.     

Separate evolution of H2 and O2 from H2O on visible light-responsive TiO2 thin film photocatalysts prepared by an RF magnetron sputtering method

Visible light-responsive TiO₂ thin film photocatalysts (Vis-TiO₂) have been prepared on Ti metal foil (Vis-TiO₂/Ti) or ITO glass (Vis-TiO₂/ITO) substrates by a radio-frequency magnetron sputtering (RF-MS) method. The UV–Vis spectra as well as photoelectrochemical performance of Vis-TiO₂ were affected by various calcination treatments such as calcination in air or NH₃. Calcination treatment in NH₃ (1.0 × 10⁴ Pa, 673 K) was particularly effective in increasing the visible light absorption of Vis-TiO₂ as well as in enhancing its photoelectrochemical performance and photocatalytic activity. A novel Vis-TiO₂ thin film photocatalyst (Vis-TiO₂/Ti/Pt) was prepared by an RF-MS method where Vis-TiO₂ was deposited on one side of a Ti metal foil substrate and nanoparticles of Pt were deposited on the other side. The separate evolution of H₂ and O₂ from H₂O could be successfully achieved by using an H-type glass cell consisting of two aqueous phases separated by Vis-TiO₂/Ti/Pt and a proton-exchange membrane. It was found that the rate of the separate evolution of H₂ and O₂ was also dramatically enhanced by calcination treatment of Vis-TiO₂ in NH₃.     

Inhibition of CO oxidation on RuO2(110) by adsorbed H2O molecules

Catalytic CO oxidation on the RuO(2)(110) surface was studied at 300 K by scanning tunneling microscopy (STM), high-resolution electron-energy-loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). Upon repeatedly exposing the surface to several 10 L of CO and O(2) at 300 K, STM shows that unreactive features accumulate with each CO and O(2) titration run. HREELS and TDS show formation of increasing amounts of H(2)O, retarded formation of O-cus atoms and incomplete removal of CO-bridge molecules during O(2) dosing, and a changing ratio of single- and double-bonded CO-bridge molecules. It is concluded that H(2)O (presumably from the residual gas) is accumulating at the Ru-cus sites thus blocking them, so that the dissociative adsorption of oxygen is prevented and the CO oxidation reaction is suppressed. Some 10% CO- bridge remains on the surface even during oxygen exposure. Consistent with this interpretation, deactivation of the surface is suppressed at 350 K, at the onset of H(2)O desorption.     

Electrocatalysis by ad-atoms: Part III. Enhancement of the oxidation of carbon monoxide on platinum by ruthenium ad-atoms

The known vibrating interphase method when a.c. is measured fails while investigating very dilute solutions because of the ohmic drop of the potential in the solution. The proposed version of the method is free from this disadvantage. When a mercury electrode is vibrated an alternating voltage U is set up which depends upon the electrode potential E. The alternating voltage-potential curves U-E have a sharp minimum corresponding to the potential of uncharged surface (p.u.s.). Using the method U-E curves in HCl up to 1×10^?5M and in KCl in 5×10^?3M solutions were taken. Obtained p.u.s. data were extrapolated to the zero electrolyte concentration and the null point (n.p.) of mercury in water was determined. Calculation of the U-E curves in 0.01 M HCl at different vibration amplitudes was made. Calculated and measured data are in a good agreement, thus substantiating the principle of the method.     

Optical constants of HNO3/H2O and H2SO4/HNO3/H2O at low temperatures in the infrared region

The complex index of refraction of liquid HNO3/H2O and H2SO4/HNO3/H2O has been obtained at different temperatures and acid concentrations. FT-IR specular reflectance spectra were obtained for 30, 54, and 64 wt % aqueous HNO3 and for four different H2SO4/HNO3/H2O mixtures in the temperature region from 293 to 183 K. The complex index of refraction was obtained from the reflectance spectra with the Kramers-Kronig transformation. The optical constants of the binary and ternary mixtures vary with the acid concentration and the temperature. The results demonstrate that vibrational bands originating from the sulfate species are more sensitive to changes in temperature than the bands originating from vibrations in the nitrate species; only minor changes in the nitrate vibrational bands are observed as the temperature decreases below 248 K.     

On the mechanisms of oxidation of organic sulfides by H2O2 in aqueous solutions

The mechanism of oxidation of organic sulfides in aqueous solutions by hydrogen peroxide was investigated via ab initio calculations. Specifically, two reactions, hydrogen transfer of hydrogen peroxide to form water oxide and the oxidation of dimethyl sulfide (DMS) by hydrogen peroxide to form dimethyl sulfoxide, were studied as models of these processes in general. Solvent effects are included both via including explicitly water molecules and via the polarizable continuum model. The former was found to have a much more significant effect than the latter. When explicit water molecules are included, a mechanism different from those proposed in the literature was found. Specific interactions including hydrogen bonding with 2-3 water molecules can provide enough stabilization for the charge separation of the activation complex. The energy barrier of the oxidation of DMS by hydrogen peroxide was estimated to be 12.7 kcal/mol, within the experimental range of the oxidation of analogous compounds (10-20 kcal/mol). The major reaction coordinates of the reaction are the breaking of the O-O bond of H2O2 and the formation of the S-O bond, the transfer of hydrogen to the distal oxygen of hydrogen peroxide occurring after the system has passed the transition state. Reaction barriers of the hydrogen transfer of H2O2 are an average of 10 kcal/mol or higher than the reaction barriers of the oxidation of DMS. Therefore, a two-step oxidation mechanism in which, first, the transfer of a hydrogen atom occurs to form water oxide and, second, the transfer of oxygen to the substrate occurs is unlikely to be correct. Our proposed oxidation mechanism does not suggest a pH dependence of oxidation rate within a moderate range around neutral pH (i.e., under conditions in which hydronium and hydroxide ions do not participate directly in the reaction), and it agrees with experimental observations over moderate pH values. Also, without including a protonated solvent molecule, it has activation energies that correspond to measured activation energies.