Development of an amperometric biosensor based on covalent immobilization of tyrosinase on a boron-doped diamond electrode

An amperometric enzyme electrode based on direct covalent immobilization of tyrosinase on a boron-doped diamond (BDD) electrode has been developed for the detection of phenolic compounds. Combined chemical and electrochemical modifications of the BDD film with 4-nitrobenzenediazonium tetrafluoroborate, an aminophenyl-modified BDD (AP–BDD) surface was produced, and then the tyrosinase was covalently immobilized on the BDD surface via carbodiimide coupling. The response dependences of the enzyme electrode (Tyr–AP–BDD electrode) on pH of solution, applied potential, oxygen level and phenolic compounds diffusion were studied. The Tyr–AP–BDD electrode shows a linear response range of 1–200, 1–200 and 1–250 μM and sensitivity of 232.5, 636.7 and 385.8 mA M⁻¹ cm⁻² for phenol, p-cresol and 4-chlorophenol, respectively. 90 percent of the enzyme activity of the Tyr–AP–BDD electrode is retained for 5 weeks storing in 0.1 M PBS (pH 6.5) at 4 °C.     

Electroanalysis of myoglobin and hemoglobin with a boron-doped diamond electrode

The electrochemical behavior of myoglobin (Mb) and hemoglobin (Hb) was investigated with a boron-doped diamond (BDD) electrode by cyclic voltammetry. In acetate buffer solutions, the oxygen reduction at the BDD electrode showed a very high overpotential while the reduction of Mb or Hb was observed in the more positive potential region. Owing to the electrocatalytic reaction of O₂ and the participation of H⁺ following the electrochemical reduction of ferric proteins, the voltammetric responses for Mb and Hb on the BDD electrode in the negative going scans became remarkable in acidic buffer solutions in air. The peak current was linearly proportional to the concentration of Mb in the range 1×10⁻⁶–2×10⁻⁵ M or the concentration of Hb from 1×10⁻⁶ to 1×10⁻⁵ M.     

Composite hollow fiber membranes for CO2 capture

Composite hollow fibers membranes were prepared by coating poly(phenylene oxide) (PPO) and polysulfone (PSf) hollow fibers with high molecular polyvinylamine (PVAm). Two procedures of coating hollow fibers outside and respective inside were investigated with respect to intrinsic PVAm solution properties and hollow fibers geometry and material.The influence of operating mode (sweep or vacuum) on the performances of membranes was investigated. Vacuum operating mode gave better results than using sweep because part of the sweep gas permeated into feed and induced an extra resistance to the most permeable gas the CO₂. The composite PVAm/PSf HF membranes having a 0.7–1.5 μm PVAm selective layer, showed CO₂/N₂ selectivity between 100 and 230. The selectivity was attributed to the CO₂ facilitated transport imposed by PVAm selective layer. The CO₂ permeance changed from 0.006 to 0.022 m³(STP)/(m² bar h) in direct correlation with CO₂ permeance and separation mechanism of the individual porous supports used for membrane fabrication. The multilayer PVAm/PPO membrane using as support PPO hollow fibers with a 40 nm PPO dense skin layer, surprisingly presented an increase in selectivity with the increase in CO₂ partial pressure. This trend was opposite to the facilitated transport characteristic behaviour of PVAm/porous PSf. This indicated that PVAm/PPO membrane represents a new membrane, with new properties and a hybrid mechanism, extremely stable at high pressure ratios. The CO₂/N₂ selectivity ranged between 20 and 500 and the CO₂ permeance from 0.11 to 2.3 m³(STP)/(m² bar h) depending on the operating conditions.For both PVAm/PSf and PVAm/PPO membranes, the CO₂ permeance was similar with the CO₂ permeance of uncoated hollow fiber supports, confirming that the CO₂ diffusion rate limiting step resides in the properties of the relatively thick support, not at the level of 1.2 μm thin and water swollen PVAm selective layer. A dynamic transfer of the CO₂ diffusion rate limiting step between PVAm top layer and PPO support was observed by changing the feed relative humidity (RH%). The CO₂ diffusion rate was controlled by the PPO support when using humid feed. At low feed humidity the 1.2 μm PVAm top layer becomes the CO₂ diffusion rate limiting step.     

Influencing factors of venlafaxine degradation at boron-doped diamond anode

Degradation of the antidepressant venlafaxine by an effective electrocatalytic process, boron-doped diamond (BDD) electrode, was study. The BDD electrode was selected as the anode, and the degradation efficiency of venlafaxine under different influencing factors was systematically investigated. The preliminary grasp of the degradation law of venlafaxine by anodic electro-degradation using BDD electrode was obtained. The experimental results showed that the electrochemical oxidation technology using BDD anode can effectively degrade venlafaxine and remove total organic carbon (TOC) from the solution, complete venlafaxine degradation and TOC elimination could be achieved within 30 and 120 min of BDD oxidation process, respectively, and it has good stability and reusability. Increasing the electrolyte concentration (≤0.1 mol/L) and current density (≤100 mA/cm²) within a certain range could accelerate the degradation of venlafaxine. HCO₃^– and PO₄^3? could inhibit the degradation efficiency of venlafaxine through of competing for free radicals. It is interesting that the presence of Cl^? significantly promoted the degradation efficiency of venlafaxine. The results of this study suggest that the Electro-degradation treatment may provide a promising way to treat venlafaxine contaminated water.     

Electrochemically induced mineralization of organics by molecular oxygen on boron-doped diamond electrode

The complete mineralization of organic pollutants present in wastewater is usually achieved via thermally activated oxygen/air. This process occurs at high temperatures and pressures (300 °C, 200 atm) and often gives small amount of acetic acid as a final product. In this work, we demonstrate using acetic acid as a model compound that organic molecules can be activated electrochemically such that they react at room temperature with oxygen, resulting in mineralization of even acetic acid present in 1 M HClO₄ supporting electrolyte. This electrochemically induced activation occurs during anodic polarization of boron-doped diamond electrodes (BDD) in air/oxygen-saturated solutions. The direct evidence for this process was found during electro-oxidation of acetic acid saturated with isotopically labelled ¹⁸O₂ resulting in evolution of C¹⁸O₂ and C¹⁶O¹⁸O. We suggest that the mechanism of activation on BDD is initiated by hydroxyl radicals formed on the electrode surface.     

A novel paper-based device coupled with a silver nanoparticle-modified boron-doped diamond electrode for cholesterol detection

A novel paper-based analytical device (PAD) coupled with a silver nanoparticle-modified boron-doped diamond (AgNP/BDD) electrode was first developed as a cholesterol sensor. The AgNP/BDD electrode was used as working electrode after modification by AgNPs using an electrodeposition method. Wax printing was used to define the hydrophilic and hydrophobic areas on filter paper, and then counter and reference electrodes were fabricated on the hydrophilic area by screen-printing in house. For the amperometric detection, cholesterol and cholesterol oxidase (ChOx) were directly drop-cast onto the hydrophilic area, and H₂O₂ produced from the enzymatic reaction was monitored. The fabricated device demonstrated a good linearity (0.39 mg dL⁻¹ to 270.69 mg dL⁻¹), low detection limit (0.25 mg dL⁻¹), and high sensitivity (49.61 μA mM⁻¹ cm⁻²). The precision value for ten replicates was 3.76% RSD for 1 mM H₂O₂. In addition, this biosensor exhibited very high selectivity for cholesterol detection and excellent recoveries for bovine serum analysis (in the range of 99.6–100.8%). The results showed that this new sensing platform will be an alternative tool for cholesterol detection in routine diagnosis and offers the advantages of low sample/reagent consumption, low cost, portability, and short analysis time.     

Novel co-extruded electrolyte–anode hollow fibres for solid oxide fuel cells

Novel CGO/NiO–CGO dual-layer hollow fibres (HFs) have been fabricated in a single-step co-extrusion and co-sintering process. LSCF–CGO cathodes layers were then deposited onto the dual-layer HFs to construct micro-tubular SOFCs. The NiO in the micro-tubular HF–SOFCs was reduced at 550 °C using hydrogen gas to form Ni anodes. Scanning electron microscope images showed that the dual-layer HFs have porous anodes and dense electrolyte layers. Preliminary measurements with a HF–SOFC fed with H₂ and atmospheric oxygen, produced maximum power densities of 420 W m⁻² and 800 W m⁻² at 450 °C and 550 °C, respectively.     

Electrochemical oxidation of phenol on boron-doped diamond electrode

The process of phenol oxidation on a boron-doped diamond electrode (BDD) is studied in acidic electrolytes under different conditions of generation of active oxygen forms (AOFs). The scheme of phenol oxidation known from the literature for other electrode materials is confirmed. Phenol is oxidized through a number of intermediates (benzoquinone, carboxylic acids) to carbon dioxide and water. Comparative analysis of phenol oxidation rate constants is performed as dependent on the electrolysis conditions: direct anodic oxidation, with oxygen bubbling, and addition of H₂O₂. A scheme is confirmed according to which active radicals (OH^·, HO₂^·, HO₂⁻) are formed on a BDD anode that can oxidize the substrate which leads to formation of organic radicals interacting with each other and forming condensation products. Processes with participation of free radicals (chain-radical mechanism) play an important role in electrochemical oxidation on BDD. Intermediates and polymeric substances (polyphenols, quinone structures, and resins) are formed. An excess of the oxidant (H₂O₂) promotes a more effective oxidation of organic radicals and accordingly inhibition of the condensation process.     

Electron transfer kinetics on composite diamond (sp3)–graphite (sp2) electrodes

The concept of non-diamond sp² impurity states as charge transfer mediators on boron-doped diamond (BDD) surface was suggested as an explanation for the electrochemical behavior of synthetic diamond based electrodes. In order to verify this concept, graphite particles (sp²) were deposited on diamond electrodes (sp³) by mechanical abrasion. The behavior of the so prepared diamond–graphite composite electrodes were compared with those of as-grown (BDD_ag) and those after mild anodic polarization (BDD_mild).Outer-sphere electron transfer processes such as ferri/ferrocyanide (Fe(CN)₆III/II) and inner-sphere charge transfer reactions such as 1,4-benzoquinone/hydroquinone (Q/H₂Q) were chosen in order to investigate the electrochemical properties of these composite electrodes. Both redox systems became more reversible as the graphite (sp²) loading increased. A strong analogy existed between as-grown diamond electrodes and diamond–graphite composite electrodes.Finally a model is proposed which describes the BDD electrode surface as a diamond matrix in which non-diamond (sp²) impurity states are dispersed. These non-diamond sp² states on BDD surface acts as charge mediators for both inner-sphere and outer-sphere reactions.     

Direct and mediated electrochemical oxidation of ammonia on boron-doped diamond electrode

Direct (non-mediated) electrochemical oxidation of ammonia on boron-doped diamond (BDD) electrode proceeds mainly at high pH (> 8) via free ammonia (NH₃) oxidation. To enhance ammonia oxidation on BDD at low pH (< 8), where mainly ammonium (NH₄⁺) is present, oxidation of ammonia was mediated by active free chlorine. In this process, electro-generated in situ active chlorine rapidly reacts with ammonia instead of being further electro-oxidized to chlorate at the electrode surface. Thus, active chlorine effectively removes ammonia from an acidic solution, while the formation of by-products such as chlorate and possibly perchlorate is minimized.     

Electrochemical Detection of Catechol on Boron‐doped Diamond Electrode Modified with Au/TiO2 Nanorod Composite

Au/TiO₂ nanorod composites with different ratios of [TiO₂]:[Au] have been prepared by chemically reducing AuCl₄^‐ on the positively charged TiO₂ nanorods surface and used to modify boron‐doped diamond (BDD) electrodes. The electrochemical behaviors of catechol on the bare and different Au/TiO₂ nanorod composites‐modified BDD electrodes are studied. The cyclic voltammetric results indicate that these different Au/TiO₂ nanorod composites‐modified BDD electrodes can enhance the electrocatalytic activity toward catechol detection, as compared with the bare BDD electrode. Among these different conditions, the Au/TiO₂‐BDD3 electrode (the ratio of [TiO₂]:[Au] is 27:1) is the most choice for catechol detection. The electrochemical response dependences of the Au/TiO₂‐BDD3 electrode on pH of solution and the applied potential are studied. The detection limit of catechol is found to be about 1.4 × 10^‐6 M in a linear range from 5 × 10^‐6 M to 200 × 10^‐6 M on the Au/TiO₂‐BDD3 electrode.     

Removal of pesticide chlorobenzene by anodic degradation: Variable effects and mechanism

The oxidation of chlorobenzene (CB) was studied by electrochemical electrolysis using boron-doped diamond (BDD), PbO₂ or platine (Pt) as anode and graphite bar as cathode. The effect of applied current density, supporting electrolyte and initial pH value were also studied. The results demonstrated that BDD anode had the best effectiveness and accomplishment of electrochemical degradation of CB compared to PbO₂ and Pt anodes. For a current density of 20 mA/cm² and at pH = 3, the elimination of COD and TOC were about 97% and 98%, respectively, after 360 min of electrolysis with the BDD anode. Pseudo-first order kinetics appears to be the most appropriate to describe the degradation of chlorobenzene. The electrochemical mechanism of chlorobenzene on BDD was proposed based on the identified intermediates.     

Determination of the Tafel slope for oxygen evolution on boron-doped diamond electrodes

It has been reported that the oxygen evolution reaction (OER) on boron-doped diamond (BDD) electrodes appears at high overpotential and results in unusually high Tafel slope. In this work, we have studied the OER in 1 M HClO₄ on BDD macroelectrode and microelectrodes-array (MEA). The correction of the anodic polarization curve for ohmic drop has been performed on BDD macroelectrode taking into account the total uncompensated resistance of the studied system. On BDD MEA, no correction of the polarization curve was necessary due to the small contribution of ohmic drop to the measured potential. At low overpotential (<1.2 V), abnormally high Tafel slopes (340 and 680 mV dec⁻¹ on BDD MEA and BDD, respectively) have been observed. Such high slopes may result from the presence of surface redox couples/functional groups which act as a barrier for OER on BDD. In this potential region, the Tafel slope depends strongly on the state of the electrode surface. In the high overpotential region (>1.2 V), the Tafel slope has been found equal to 120 mV dec⁻¹, which is the theoretical value considering a first or a second electron transfer step as the rate determining step.     

Enhanced ammonia oxidation on BDD induced by inhibition of oxygen evolution reaction

Electrochemical oxidation of ammonia (NH₃ and NH₄ ⁺ ) on boron-doped diamond (BDD) electrode was studied using differential electrochemical mass-spectrometry (DEMS) and chronoamperometry. Electro-oxidation of ammonia induces inhibition of the oxygen evolution reaction (OER) due to adsorption of the ammonia oxidation products on the BDD surface. The inhibition of the OER enhances ammonia electro-oxidation, which becomes the main reaction. The amino radicals, formed during ammonia oxidation, trigger a reaction chain in which molecular oxygen dissolved in solution is involved in the ammonia electro-oxidation. Nitrogen, nitrous oxide, and nitrogen dioxide were detected as the ammonia oxidation products, with nitrogen being the main gaseous product of the oxidation.     

Simultaneous reactive extraction separation of amino acids from water with D2EHPA in hollow fiber contactors

Reactive extraction separation of binary amino acids from water using a microporous hollow fiber has been studied, in which the acidic extractant di(2-ethylhexyl)phosphoric acid (D2EHPA) was selected as an active carrier dissolved in kerosene. l-Phenylalanine (Phe) was extracted from an aqueous solution through the shell side of module to the organic phase through the lumen of fiber in the extraction module, in which l-Phe was then back-extracted to stripping phase in stripping module. Experiments were conducted as a function of the initial feed concentration of equimolar Phe and l-aspartic acid (l-Asp) (5 mol/m³), feed pH (3–5), the carrier concentration (0.1–0.5 mol/dm³), and stripping acidity (0.1–2 mol/dm³). The effect of process variables on the separation factor of Phe/Asp and the possible transport resistances including aqueous-layer diffusion, membrane diffusion, organic-layer, and interfacial chemical reaction were quantitatively studied and discussed. The high separation factor (β) of Phe/Asp was obtained to be 18.5 at feed pH 5 and 2 mol/dm³ of strip solution (HCl). The extraction and stripping processes appear to rely on pH dependence of the distribution coefficient of amino acids in reactive extraction system. The separation factor (β) was enhanced in hollow fiber membrane (HFM) process compared with conventional solvent process, which was a result of the counter transport of hydrogen ions.     

Au-TiO2/Chit modified sensor for electrochemical detection of trace organophosphates insecticides

In this paper, Au–TiO₂/Chit modified electrode was prepared with Au–TiO₂ nanocomposite (Au–TiO₂) and Chitosan (Chit) as a conjunct. The Au–TiO₂ nanocomposite and the films were characterized by electrochemical and spectroscopy methods. A set of experimental conditions was also optimized for the film's fabrication. The electrochemical and electrocatalytic behaviors of Au–TiO₂/Chit modified electrode to trace organophosphates (OPs) insecticides such as parathion were discussed in this work. By differential pulse voltammetry (DPV) measurement, the current responses of Au–TiO₂/Chit modified electrode were linear with parathion concentration ranging from 1.0 ng/ml to 7.0 × 10³ ng/ml with the detection limit of 0.5 ng/ml. In order to evaluate the performance of the detection system, we also examined the real samples successfully in this work. It exhibited a sensitive, rapid and easy-to-use method for the fast determination of trace OPs insecticides.     

Hollow fiber membrane of yttrium-stabilized zirconia and strontium-doped lanthanum manganite dual-phase composite for oxygen separation

The hollow fiber composite membrane involving Zr_0.84Y_0.16O_1.92 (YSZ) as an oxygen ionic conductor and La_0.8Sr_0.2MnO_3−δ (LSM) as an electronic conductor was explored for oxygen separation application. The hollow fiber precursor was prepared by the phase-inversion process, and transformed to a gas-tight ceramic by sintering at 1350 °C. The as-prepared fiber exhibited a thermal expansion coefficient of 11.1 × 10⁻⁶ K⁻¹ and a three-point bending strength of 152 ± 12 MPa. An oxygen permeation flux of 2.1 × 10⁻⁷ mol cm⁻² s⁻¹ was obtained under air/He gradient at 950 °C for a hollow fiber of length 57.00 mm and wall thickness 0.16 mm. The oxygen permeation flux remained unchanged when the sweeping gas was changed from helium to high concentration of CO₂. Considering the satisfactory trade-off between the permeability and stability, the YSZ–LSM hollow fiber is promising for oxygen production applications.     

CoMnO2-Decorated Polyimide-Based Carbon Fiber Electrodes for Wire-Type Asymmetric Supercapacitor Applications

In this work, we report the carbon fiber-based wire-type asymmetric supercapacitors (ASCs). The highly conductive carbon fibers were prepared by the carbonized and graphitized process using the polyimide (PI) as a carbon fiber precursor. To assemble the ASC device, the CoMnO₂-coated and Fe₂O₃-coated carbon fibers were used as the cathode and the anode materials, respectively. Herein, the nanostructured CoMnO₂ were directly deposited onto carbon fibers by a chemical oxidation route without high temperature treatment in presence of ammonium persulfate (APS) as an oxidizing agent. FE-SEM analysis confirmed that the CoMnO₂-coated carbon fiber electrode exhibited the porous hierarchical interconnected nanosheet structures, depending on the added amount of APS, and Fe₂O₃-coated carbon fiber electrode showed a uniform distribution of porous Fe₂O₃ nanorods over the surface of carbon fibers. The electrochemical properties of the CoMnO₂-coated carbon fiber with the concentration of 6 mmol APS presented the enhanced electrochemical activity, probably due to its porous morphologies and good conductivity. Further, to reduce the interfacial contact resistance as well as improve the adhesion between transition metal nanostructures and carbon fibers, the carbon fibers were pre-coated with the Ni layer as a seed layer using an electrochemical deposition method. The fabricated ASC device delivered a specific capacitance of 221 F g⁻¹ at 0.7 A g⁻¹ and good rate capability of 34.8% at 4.9 A g⁻¹. Moreover, the wire-type device displayed the superior energy density of 60.2 Wh kg⁻¹ at a power density of 490 W kg⁻¹ and excellent capacitance retention of 95% up to 3000 charge/discharge cycles.     

Investigation of Al diffusion in different oxide thin-film layers by SNMS/HFM method

Depth profiles of Ga₂O₃/a-SiO₂/Al₂O₃- substrate, Ga₂O₃/a-Si₃N₄/Al₂O₃- substrate, and Ga₂O₃/Al₂O₃ substrate thin layers were determined by the SNMS/HFM method. Al diffusion from the Al₂O₃ substrate was investigated after 50, and in some cases after 600 hours of heat treatment time at different temperatures (600 °C,850 °C,950 °C,1050 °C and 1150 °C). The diffusion coefficient of Al at 850 °C was found to be D _Al=8.7 * 10^–18 cm²/s in amorphous SiO₂; D _Al=1.5*10^–17 cm²/s in amorphous Si₃N₄ and D _Al=5.5* 10^–16 cm²/s in Ga₂O₃ at 600 °C, respectively. The possible diffusion mechanism is explained in terms of the metal-oxygen bond-strengths. Although the studied materials have high resistivity at room temperature, the applied SNMS/HFM method has proven to be an efficient surface analytical tool even in these cases.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

The Fabrication and Characterization of a Bismuth Nanoparticle Modified Boron Doped Diamond Electrode and Its Application to the Simultaneous Determination of Cadmium(II) And Lead(II)

We report the simultaneous electroanalytical determination of Pb²⁺ and Cd²⁺ by square‐wave anodic stripping voltammetry (SWASV) using a bismuth nanoparticle modified boron doped diamond (Bi‐BDD) electrode. Bi deposition was performed in situ with the analytes, from a solution of 0.1 mM Bi(NO₃)₃ in 0.1 M HClO₄ (pH 1.2), and gave detection limits of 1.9 μg L^?1 and 2.3 μg L^?1 for Pb(II) and Cd(II) respectively. Pb²⁺ and Cd²⁺ could not be detected simultaneously at a bare BDD electrode, whilst on a bulk Bi macro electrode (BiBE) the limits of detection for the simultaneous determination of Pb²⁺ and Cd²⁺ were ca. ten times higher.