Electrochemical behavior of electrodeposited rutin film on a multi-wall carbon nanotubes modified glassy carbon electrode. Improvement of the electrochemical reversibility and its application as a hydrazine sensor

By immobilizing rutin at the surface of a glassy carbon electrode (GCE) modified with multi-wall carbon nanotubes (MWCNT), a new modified electrode has been fabricated and its electrochemical behavior was investigated by cyclic voltammetry. Cyclic voltammograms of the resulting modified electrode show stable and a well defined redox couple with surface confined characteristics. The results show that the reversibility of rutin is significantly improved at a MWCNT modified GCE in comparison with GCE alone. The charge transfer coefficient, α, was calculated to be 0.4, and charge transfer rate constant, k_s, was 46.7 s⁻¹ in pH 8, indicating great facilitation of the electron transfer between rutin and MWCNT deposited on the electrode surface. The rutin MWCNT (RMWCNT) modified GCE showed excellent mediation of hydrazine oxidation: a decrease in the overvoltage of hydrazine electrooxidation was observed as well as a dramatic increase in the peak current compared to that seen at a rutin modified GCE (RMGCE), activated GCE or bare GCE. Hydrazine was determined amperometrically at the surface of RMWCNT modified GCE in pH 8. Under the optimized conditions the calibration curve is linear in the concentration range 2.0–190.0 μM hydrazine. The detection limit and sensitivity are 0.61 μM and 0.0656 μA μM⁻¹, respectively. Finally the kinetic parameters of the electron transfer coefficient, α, the heterogeneous rate constant of dependent to different potentials, k′(E), and the standard heterogeneous rate constant, k⁰, for oxidation of hydrazine at the RMWCNT surface were determined using various electrochemical methods. The advantages of this modified electrode for hydrazine determination are high sensitivity, excellent catalytic activity, short response time, wide linear range, and high exchange current density.     

Studies towards the development of an ethanol sensor

An amperometric biosensor for ethanol is described. The sensor uses benzoquinone and ferrocene carboxylic acid as mediators for electron transfer between a quinoprotein, alcohol dehydrogenase and an edge plane pyrolytic graphite electrode. A linear current response proportional to ethanol concentration is observed in the range 1–10 mM.     

Second Order Rate Constants of Donor-Strand Exchange Reveal Individual Amino Acid Residues Important in Determining the Subunit Specificity of Pilus Biogenesis

P pili are hair-like adhesive structures that are assembled on the outer membrane (OM) of uropathogenic Escherichia coli by the chaperone-usher pathway. In this pathway, chaperone-subunit complexes are formed in the periplasm and targeted to an OM assembly platform, the usher. Pilus subunits display a large groove caused by a missing β-strand which, in the chaperone-subunit complex, is provided by the chaperone. At the usher, pilus subunits are assembled in a mechanism termed “donor-strand exchange (DSE)” whereby the β-strand provided by the chaperone is exchanged by the incoming subunit’s N-terminal extension (Nte). This occurs in a zip-in-zip-out fashion, starting with a defined residue, P5, in the Nte inserting into a defined site in the groove, the P5 pocket. Here, electrospray ionization-mass spectrometry (ESI-MS) has been used to measure DSE rates in vitro. Second order rate constants between the chaperone-subunit complex and a range of Nte peptides substituted at different residues confirmed the importance of the P5 residue of the Nte in determining the rate of DSE. In addition, residues either side of the P5 residue (P5 + 1 and P5 – 1), the side-chains of which are directed away from the subunit groove, also modulate the rates of DSE, most likely by aiding the docking of the Nte into the P5 pocket on the accepting subunit prior to DSE. The ESI-MS approach developed is applicable to the measurement of rates of DSE in pilus biogenesis in general and demonstrates the scope of ESI-MS in determining biomolecular processes in molecular detail.     

Growth of silicon thin film by LPE on porous silicon bilayers

The fabrication of solar cells based on the transfer of a thin silicon film on a foreign substrate is an attractive way to realise cheap and efficient photovoltaic devices. The aim of this work is to realise a thin mono-crystalline silicon film on a double porous silicon layer in order to detach and transfer it on mullite. The first step is the fabrication of a double porous silicon layer by electrochemical anodisation using two different current densities. The low current leads to a low porosity layer and during annealing, the recrystallisation of this layer allows epitaxial growth. The second current leads to a high porosity which permits the transfer on to a low cost substrate. Liquid Phase Epitaxy (LPE) performed with indium (or In+Ga) in the temperature range of 950–1050°C leads to almost homogeneous layers. Growth rate is about 0.35 μm min⁻¹. Crystallinity of the grown epilayer is similar on porous silicon and on single crystal silicon. In this paper, we focus on the realisation of porous silicon sacrificial layer and subsequent LPE growth. This revised version was published online in July 2006 with corrections to the Cover Date.     

The impedance caused by interfacial current constriction: a case study on BaF2/YSZ bicrystals

Abstract  

BaF₂/YSZ (yttria stabilized zirconia) bicrystals are investigated by means of impedance spectroscopy. The spectra show two semicircles in the complex impedance plane, and both arcs exhibit very similar temperature dependences. The high frequency semicircle can be attributed to the ideal (quasi one-dimensional) bulk resistance of BaF₂, and the low frequency arc is caused by an additional resistance in BaF₂ due to the current constriction taking place close to the contact spots at the imperfect bicrystal interface. This interpretation is supported by finite element calculations revealing that bicrystals with imperfect contacts indeed exhibit an additional semicircle in the complex impedance plane even without any ion transfer resistance between the two crystals. The low frequency arc of the BaF₂/YSZ bicrystal drastically increases upon bias voltage, but relaxes to its original value after removing the bias. This phenomenon can be associated with a strong decrease of the vacancy concentration in YSZ (close to the interfacial contact spots), which is caused by F⁻ ions being pumped into YSZ and acting as a counter dopant to Y³⁺.     

Electrochemical behavior of hydrogen peroxide sensor based on new methylene blue as mediator

A novel amperometric hydrogen peroxide sensor was proposed by co-immobilizing new methylene blue (NMB) and Horseradish peroxidase (HRP) on glassy carbon electrode through covalent binding. The electrochemical behavior of the sensor was studied extensively in 0.1 mol/L phosphate buffering solution (pH = 7.0). The experiments showed NMB could effectively transfer electrons between hydrogen peroxide and glassy carbon electrode. The electron transfer coefficient and apparent reaction rate constant were determined to be 0.861 and 1.27 s⁻¹. The kinetic characteristics and responses of sensor on H₂O₂ were investigated. The Michaelis constant is 8.27 mol/L and the linear dependence of current on H₂O₂ is in the range of 2.5–100 μmol/L. At the same time, the effects of solution pH, buffer capacity, and temperature on the sensor were examined. Translated from Chemistry, 2006, 23(8): 916–920 [译自: 化学通报]     

Sequence dependence of DNA radioprotection by the thiols WR-1065 and WR-151326

Sequence-dependent variations of DNA structure modulate radiation-induced strand breakage. Thiols reduce breakage by scavenging damaging radiolytic OH ^. and repairing sugar radicals. As shown by sequencing gel electrophoresis, WR-1065 radioprotection is modulated by sequence, whereas that of WR-151326, a larger thiol, is more evenly distributed. Molecular modelling was performed on complexes of a 53 bp oligonucleotide (belonging to a natural restriction fragment) with one molecule of WR-1065 or WR-151326. Energy minimised structures exhibit a broadening of the minor groove of an AAATT motif upon WR-1065 binding, and a narrowing of the groove upon WR-151326 binding. Consequently, the accessibility to OH^˙ of H4′ (whose abstraction leads to strand breakage) increases near WR-1065, whereas it decreases near WR-151326. This modifies locally the otherwise homogeneous radioprotection. The effect of WR-151326 strengthens the protection at all tested binding sites, whereas that of WR-1065 diminishes it in some regions, in good agreement with the observed radioprotection distribution. Received: 24 April 1998 / Accepted: 4 August 1998 / Published online: 11 November 1998     

Influence of the nature of contacts on the dark and photovoltaic characteristics of the Schottky junction between n-doped silicon and poly(4,4′-dipentoxy-2,2′-bithiophene)

The dark and photovoltaic characteristics of the Schottky junction between n-doped silicon and a conducting polymer in its oxidised form, poly(4,4′-dipentoxy-2,2′-bithiophene), have been determined as a function of the nature of the electrical contact on the polymer side. It was found that the dark and photovoltaic performances of the device depended strongly on this contact. An aluminium contact is oxidised by the polymer and an aluminium oxide film forms between the polymer and the contact through which the forward current is controlled by charge injection. The devices assembled with indium tin oxide, platinum and gold contacts show better characteristics than the ones with aluminium contact; in the last three cases, however, a faradic charge transfer reaction on the contact, probably the oxidation of some amount of water contained in the polymer, affects the characteristics of the junction. Received: 27 December 1997 / Accepted: 24 April 1998     

Caffeic acid modified glassy carbon electrode for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH)

A new modified electrode was prepared by electrodeposition of caffeic acid (CFA) at the surface of an activated glassy carbon electrode. Cyclic voltammetry was used to investigate the redox properties of this electrode at various solution pH values and at various scan rates. The pH dependence of the electrode response was found to be 58.5 mV/pH, which is very close to the expected Nernstian value. The electrode was also employed to study electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH), using cyclic voltammetry, chronoamperometry and rotating disk voltammetry as diagnostic techniques. It was found that the modified electrode exhibits potent and persistent electrocatalytic properties toward NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 450 mV compared to the process at an unmodified electrode. The electrocatalytic current increases linearly with NADH concentration in the range tested from 0.05 to 1.0 mM. The apparent charge transfer rate constant and transfer coefficient for electron transfer between the electrode surface and immobilized CFA were calculated as 11.2 s⁻¹ and 0.43, respectively. The heterogeneous rate constant for oxidation of NADH at the CFA-modified electrode surface was also determined and found to be about 3 × 10³ M⁻¹ s⁻¹. Finally, the diffusion coefficient of NADH was calculated as 3.24 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometric results. Received: 6 January 1999 / Accepted: 11 May 1999     

The impedance caused by interfacial current constriction: a case study on BaF<Subscript>2</Subscript>/YSZ bicrystals

Abstract  BaF₂/YSZ (yttria stabilized zirconia) bicrystals are investigated by means of impedance spectroscopy. The spectra show two semicircles in the complex impedance plane, and both arcs exhibit very similar temperature dependences. The high frequency semicircle can be attributed to the ideal (quasi one-dimensional) bulk resistance of BaF₂, and the low frequency arc is caused by an additional resistance in BaF₂ due to the current constriction taking place close to the contact spots at the imperfect bicrystal interface. This interpretation is supported by finite element calculations revealing that bicrystals with imperfect contacts indeed exhibit an additional semicircle in the complex impedance plane even without any ion transfer resistance between the two crystals. The low frequency arc of the BaF₂/YSZ bicrystal drastically increases upon bias voltage, but relaxes to its original value after removing the bias. This phenomenon can be associated with a strong decrease of the vacancy concentration in YSZ (close to the interfacial contact spots), which is caused by F⁻ ions being pumped into YSZ and acting as a counter dopant to Y³⁺. Graphical Abstract        

Effects of pressure pulsation on oxygen transfer rate measured by sulfite method

Pressure pulsation (PP) was investigated for its effects on oxygen transfer rate (OTR) measured by sulfite oxidation. By manipulating airflow rate, 0.41–1.2 vvm, and a control valve in a 4-L bioreactor, the frequency of PP was varied at different gas pressures3–15 psig. A mathematical model of OTR was built and compared to experimental data. OTR was also examined at constant gas pressure, 4.5–15.0 psig. The results indicate a good agreement between measurement and model prediction. OTR above 9 psig during PP showed significant enhancement at 25°C. This proves that PP not only affects the elevation of DO level, but also increases the interfacial area and mass transfer coefficient.     

Step wise and dissociative mechanisms of the electron transfer in electrochemical reactions involving organosilicon compounds: Molecular-thermodynamic approach1

Electrochemical reduction of organochlorosilanes and oxidation of hexaorganodisilanes may occur via dissociative and stepwise mechanisms, the choice between which is determined by the balance between fundamental structural parameters of elementoorganic molecules. The formation of radical anions of silyl-substituted chloromethane in the conditions of an electrochemical experiment is shown. The formation is due to α-silicon stabilization of the intermediate during the electron transfer. The role of conjugation and hyperconjugation in the organosilicon compounds’ reactivity is analyzed. When employing terms “ stepwise ” and “ dissociative ” mechanisms of the electron transfer, we follow tradition introduced and developed by J.-M. Saveant’s group [1] and recommended by the IUPAC Commission on Electrochemical Nomenclature [2]. The terms refer to an electrochemically reversible transfer of electron whose kinetics does not limit the process rate as a whole and to an electrochemically irreversible transfer which involves a bond cleavage in an elementary act of electron transfer. The term “ activation ” mechanism, which is sometimes applied to electrochemically irreversible processes, is more universal; however, it is somewhat not unique as compared with the term “ dissociative.” Coupled with “ associative,” the latter may be used for indicating processes that involve the formation of a bond in an elementary act of electron transfer.     

A photosynthetic biosensor with enhanced electron transfer generation realized by laser printing technology

One of the limits of current electrochemical biosensors is a lack of methods providing stable and highly efficient junctions between biomaterial and solid-state devices. This paper shows how laser-induced forward transfer (LIFT) can enable efficient electron transfer from photosynthetic biomaterial immobilized on screen-printed electrodes (SPE). The ideal pattern, in terms of photocurrent signal of thylakoid droplets giving a stable response signal with a current intensity of approximately 335 ± 13 nA for a thylakoid mass of 28 ± 4 ng, was selected. It is shown that the efficiency of energy production of a photosynthetic system can be strongly enhanced by the LIFT process, as demonstrated by use of the technique to construct an efficient and sensitive photosynthesis-based biosensor for detecting herbicides at nanomolar concentrations.     

Self-assembled monolayers on electrode surfaces: a probe for redox kinetics

The formation and characterization of self-assembled monolayers of organosulfur compounds like alkanethiols and dialkyl (di)sulfides on metal surfaces such as gold are areas of current research interest. The presence of an aromatic ring in a thiol molecule can enhance the binding between Au and the thiol, resulting in the formation of compact and impervious self-assembled monolayers. The formation of a monolayer of 2-mercaptobenzothiazole (MBT), containing an aromatic group with a fused thiazole ring but no long alkyl chain, is achieved on a gold electrode surface. Voltammetric investigations of ferro/ferricyanide and ferrous/ferric redox systems carried out on this Au|MBT electrode are reported. Further, the possibility of using such an Au|MBT electrode to distinguish between inner and outer sphere electron transfer reactions is indicated. Received: 2 January 1998 / Accepted: 14 May 1998     

Capillary electrophoresis-atmospheric pressure chemical ionization-mass spectrometry using an orthogonal interface: Set-up and system parameters

The feasibility of atmospheric pressure chemical ionization (APCI) as an alternative ionization technique for capillary electrophoresis-mass spectrometry (CE-MS) was investigated using a grounded sheath-flow CE-MS sprayer and an orthogonal APCI source. Infusion experiments indicated that highest analyte signals were achieved when the sprayer tip was in close vicinity of the vaporizer entrance. The APCI-MS set-up enabled detection of basic, neutral, and acidic compounds, whereas apolar and ionic compounds could not be detected. In the positive ion mode, analytes could be detected in the entire transfer voltage range (0–5 kV), whereas highest signal intensities were observed when the corona discharge current was between 1000 and 2000 nA. In the negative ion mode, the transfer voltage typically was 500 V and the optimum corona discharge current was 6000 nA. Analyte signals were raised with increasing nebulizing gas pressure, but the pressure was limited to 25 psi to avoid siphoning and current drops. Signal intensities appeared to be optimal and constant over a wide range of sheath liquid flow rate (5–25 μL/min) and vaporizer temperature (200–350 °C). APCI-MS signals were unaffected by the composition of the background electrolyte (BGE), even when it contained sodium phosphate and sodium dodecyl sulfate (SDS). Consequently, BGE composition, sheath-liquid flow rate, and vaporizer temperature can be optimized with respect to the CE separation without affecting the APCI-MS response. The analysis of a mixture of basic compounds and a steroid using volatile and nonvolatile BGEs further demonstrates the feasibility of CE-APCI-MS. Detection limits (S/N = 3) were 1. 6–10 μM injected concentrations.     

Molecular modeling of the binding mode of chiral metal complexes δ- and ∧-[Co(phen)2dppz]3+with B-DNA

Molecular modeling methods have been applied to the structural characterization of the interaction between chiral metal complexes [Co(phen)₂dppz]³⁺ (where phen = 1, 10-phenanthroline, dppz = dipyrido[3,2-a: 2′, 3′ -c]phenazine) and the oligonucleotide (B-DNA fragment). The natures of two kinds of the binding modes, which are currently intense controversy, have been explored. Barton proposed that there is enantio-selective DNA binding by the octahedral complexes and intercalative access by these complexes from the major groove; but Norden suggested that both enantiomers bind extremely strongly to DNA from the minor groove without any noticeable enantio-selectivity. Our results support and extend structural models based upon Norden’s studies, and conflict with Barton’ model.     

Electrochemical behavior of uric acid at a penicillamine self-assembled gold electrode

The fabrication and electrochemical characteristics of a penicillamine (PCA) self-assembled monolayer modified gold electrode were investigated. The electrode can enhance the electrochemical response of uric acid (UA), and the electrochemical reaction of UA on the PCA electrode has been studied by cyclic voltammetry and differential pulse voltammetry. Some electrochemical parameters, such as diffusion coefficient, standard rate constant, electron transfer coefficient and proton transfer number have been determined for the electrochemical behavior on the PCA self-assembled monolayer electrode. The electrode reaction of UA is an irreversible process, which is controlled by the diffusion of UA with two electrons and two protons transfer at the PCA/Au electrode. In phosphate buffer (pH 5.0), the peak current is proportional to the concentration of UA in the range of 6.0 × 10⁻⁵–7.0 × 10⁻⁴ mol L⁻¹ and 2.0 × 10⁻⁵–7.0 × 10⁻⁴ mol L⁻¹ for the cyclic voltammetry and differential pulse voltammetry methods with the detection limits of 5.0 × 10⁻⁶ and 3.0 × 10⁻⁶ mol L⁻¹, respectively. The method can be applied to determine UA concentration in real samples.     

Investigation of electrochemical reactions on copper electrode in Cu4RbCl3I2 solid electrolyte

Investigations are carried out by potentiostatic method. It is found that at potentials ϕ less than 100 mV, a reaction rate of copper deposition is limited by the formation and the three-dimensional growth of copper nuclei and the rate of copper dissolution is limited by a two-dimensional growth of holes in the metal. The rate of nucleus growth was evaluated at 10⁻⁹...10⁻⁶ μm s⁻¹ depending on the potential. At ϕ>120 mV, the reaction rate is limited by charge transport at the exchange current density of 2.7 mA cm⁻² and the anodic transfer coefficient α_a≈ 0.45. The accumulation of a divalent copper phase on Cu/Cu₄RbCl₃I₂ interface at anodic polarization is explained by a parallel course of Cu⁺ − e → Cu²⁺ reaction.     

Electrocatalytic reduction of oxygen on the surface of glassy carbon electrodes modified with plumbagin

The preparation and electrochemical characterization of glassy carbon electrodes modified with plumbagin were investigated by employing cyclic voltammetry, chronoamperometry and rotating disc electrode techniques. The cyclic voltammograms of the electroreduction of oxygen showed an enhanced current peak at approximately −0.289 V in air-saturated phosphate buffer pH = 7 and scan rate 10 mV s⁻¹. The thermodynamic and kinetic parameters of the reduction of oxygen at glassy carbon have been evaluated using cyclic voltammetry. The experimental parameters were optimized and the mechanism of the catalytic process was discussed. The obtained values of E°′ (V vs. Ag/AgCl), the apparent electron transfer rate constant k_s (s⁻¹), heterogeneous rate constant for the reduction of O₂ at the surface of the modified electrode k_h (M⁻¹ s⁻¹) and α (charge transfer coefficient of oxygen) were as follows: −0.146, 23.4, 9.9 × 10³ and 0.57, respectively. In addition, plumbagin exhibited strong catalytic activity toward the reduction of H₂O₂.     

Determination of Kinetic Parameters in the Biosorption of Cr (VI) on Immobilized <Emphasis Type="Italic">Bacillus cereus</Emphasis> M<Superscript>1</Superscript><Subscript>16</Subscript> in a Continuous Packed Bed Column Reactor

Due to technological advancement, environment suffers from untreated toxic heavy metal bearing effluent coming from different industries. Chromium (VI) is one of those heavy metals having adverse impact on ecological balance, human, and plant health because of its carcinogenic properties. Biosorption is presented as an alternative to traditional technologies which are costly and inefficient for treatment of industrial wastes containing low amount of heavy metals. In this study, bioremediation of Cr (VI) ions by immobilized Bacillus cereus M¹ ₁₆ was investigated in a laboratory scale packed bed up-flow column reactor. The effect of important parameters, such as the inlet flow rate, influent concentration, and effective bed height, has been studied. External mass transfer, surface adsorption, and intrabead mass transfer were also studied to conclude the rate limiting step for removal of Cr (VI) and to determine the process parameters which are important for biosorption optimization. The external mass transfer coefficient was calculated at different flow rates (6.51 × 10⁻² to 7.58 × 10⁻² cm/min). Using the model, the surface adsorption rate constant (k _ad) and the intrabead mass transfer coefficient (k _i) were predicted as 0.0267 × 10⁻³ and 0.7465 × 10⁻³ l/g/min, respectively. Both are much lower than the external mass transfer coefficient (k _e). The surface adsorption phenomenon is acting as the rate-limiting step due to its high resistance for removal of Cr (VI).