Electrochemiluminescent behavior of luminol on the glassy carbon electrode modified with CoTPP/MWNT composite film

A glassy carbon electrode (GCE) modified with cobalt(II) meso-tetraphenylporphrine/multiwall-carbon nanotube (CoTPP/MWNT) was applied to investigate the electrochemiluminescent (ECL) behavior of luminol. The ECL intensity of luminol was found to be increased greatly on this modified electrode. The presence of cobalt(II) meso-tetraphenylporphrine (CoTPP) can catalyze the reduction of oxygen on the electrode surface to produce HOO⁻, which can increase the ECL intensity of luminol. Moreover, MWNT can provide the more effective area of the electrode, and can act as a promoter to enhance the electrochemical reaction. The proposed method enables a detection limit for luminol of 1.0 × 10⁻⁸ mol/L in the neutral solution. Under the optimum condition, the enhanced ECL intensity of luminol by H₂O₂ had a linear relationship with the concentration of H₂O₂ in the range of 1.0 × 10⁻⁷ to 8.0 × 10⁻⁸ mol/L with the detection limit of 5.0 × 10⁻⁹ mol/L.     

Enhancement of electrochemiluminesence of lucigenin by ascorbic acid at single-wall carbon nanotube film-modified glassy carbon electrode

The electrochemiluminescent behavior of lucigenin on a single-wall carbon nanotube/DMF film-modified glassy carbon electrode was studied in this paper. Comparing with the bare glassy carbon electrode, the electrochemiluminescent of lucigenin at modified electrode is more stable and without tedious procedure for clean-up the surface of modified electrode. It has been found that ascorbic acid could enhance the electrochemiluminescent intensity of lucigenin greatly at this modified electrode. Based on which, a new sensitive and simple electrochemiluminescent method for determination of ascorbic acid could be developed. The condition for the determination of ascorbic acid was optimized. Under the optimized condition, the enhanced electrochemiluminescent intensity versus ascorbic acid concentration was linear in the range of 1.0 × 10⁻⁸ to 4.0 × 10⁻⁶ mol/L with a detection limit of 2.0 × 10⁻¹⁰ mol/L, and the relative standard derivation for 1.0 × 10⁻⁷ mol/L ascorbic acid was 3.8% (n = 8). The possible mechanism was also discussed.     

Capacitive performance of an ultralong aligned carbon nanotube electrode in an ionic liquid at 60 °C

An ultralong (1.0 mm) aligned carbon nanotube (ACNT) electrode was fabricated by a cut/paste method. The electrode retains the intrinsic properties, including robust mechanical property, high surface area, and regular pore structure, of individual nanotubes. Electrochemical properties of the ACNT electrode in an ionic liquid (IL) electrolyte were studied by cyclic voltammetry, galvanostatic charge/discharge, and ac impedance spectroscopy. The ACNT electrode achieved a specific capacitance of 27 F/g, had excellent rate capability, and a long cycle life at 60 °C, indicating that an ACNT electrode/IL electrolyte electrochemical double layer capacitor is promising for high temperature (60 °C) applications. The capacitive performance of ACNT electrode is excellent, because it possesses large pores and regular pore structures, which is revealed by N₂ adsorption and scanning electron microscopy.     

Improving the electrocapacitive properties of mesoporous CMK-5 carbon with carbon nanotubes and nitrogen doping

Nitrogen-containing carbon composite materials composed of mesoporous carbon CMK-5 and carbon nanotubes (CNTs) were prepared by the chemical vapor deposition method with Fe(NO₃)₃-impregnated SBA-15 as template and pyridine as the carbon precursor. The Fe nanoparticles confined in the channels of SBA-15 induced the formation of mesoporous carbon characteristic of CMK-5, whereas Fe particles homogeneously dispersed on the external surface of SBA-15 served as catalysts for CNTs growth. The contents of CNTs, the N doping level and the microstruture of the carbon composite were closely related to the initial Fe/Si atomic ratio in SBA-15 template. Incorporation of CNTs in the composite was found to substantially reduce the electric resistance, leading to the composite materials exhibiting excellent rate-performance. A maximum specific capacitance of 208 F/g and a power density of 10 kW/kg were achieved in 6.0 mol/L KOH aqueous electrolyte when these carbon composites were applied as supercapacitor electrodes. Moreover, the composite electrode also exhibited good electrochemical stability with no capacitance loss after 1000 cycles of galvanostatic charge-discharge process.     

Micropatterning of active enzyme with a high-resolution by scanning electrochemical microscopy coupled with a nanometer-sized carbon fiber disk tip

We developed a new method for fabrication of nanometer-sized carbon fiber disk electrodes and applied them to micropattern active horseradish peroxidase (HRP) with a high-resolution by scanning electrochemical microscopy (SECM). In order to pattern active HRP, except for active HRP micropatterns predesigned other regions on a HRP-immobilized substrate was deactivated by a reactive species generated at the electrode as the tip of SECM held at 1.7 V through oxidation of Br⁻ in 0.20 mol/L phosphate buffer (PB) containing 2.5 × 10⁻² mol/L KBr and 2.0 × 10⁻³ mol/L BQ (pH 7.0). The micropatterns of active HRP were characterized using the feedback mode of SECM in PB containing 2.0 × 10⁻³ mol/L BQ and 2.0 × 10⁻³ mol/L H₂O₂, when the tip potential was held at −0.2 V.     

Bimodal, templated mesoporous carbons for capacitor applications

Several high capacitance ordered mesoporous carbon (OMC) materials, containing a bimodal pore distribution, were synthesized directly using hexagonal mesoporous silicas (HMS) as the template material. The HMS templates were formed using amine surfactants (C_nH_2n+1NH₂) with hydrophobic chain lengths containing 8-16 carbons (n = 8-16). These HMS structures were found to have an interconnected wormhole structure, high textural mesoporosity, a surface area ranging from 910 to 1370 m²/g, and a total pore volume of 1.09-1.83 cm³/g. Also, evidence for a change in structure from hexagonally ordered to layered (for surfactants of chain length with n > 12) was found. The resulting OMCs, formed using sucrose as the carbon precursor, contain bimodal pores 1.6-1.8 and 3.3-3.9 nm in diameter and have a very high surface area (980-1650 m²/g). The OMCs were evaluated as electrode materials for electrochemical capacitors using cyclic voltammetry in 0.5 M H₂SO₄ solution, giving a tunable gravimetric capacitance that increased linearly with BET area (and surfactant chain length), up to 260 F/g, among the highest yet reported for ordered carbon formed from an HMS templated precursor. All OMCs studied in this work displayed a specific capacitance of ∼0.15 F/m².     

Chemically immobilised carbon nanotubes on silicon: Stable surfaces for aqueous electrochemistry

Diazonium ion chemistry has been used to electrochemically graft aminophenyl layers onto p-type silicon (1 0 0) substrates. A condensation reaction was used to immobilise single-walled carbon nanotubes with high carboxylic acid functionality directly to this layer. Electrochemical monitoring of the aminophenyl groups confirmed the formation of an amide linkage between the single-walled carbon nanotubes and the aminophenyl layer. The carbon nanotube electrode showed high stability and good electrochemical performance in aqueous solution. At moderate scan rates the Ru(NH₃)₆^+3/+2 couple exhibited quasi-reversible electron transfer kinetics with a standard heterogenous rate constant of 1.2 × 10⁻³ cm s⁻¹ at the covalently-linked carbon nanotube surface. The electrode thus combines the advantages of a silicon substrate for easy integration into sophisticated electrical and electronic devices, carbon nanotubes for desirable electrochemical properties, and stability in aqueous medium for future applications in environmental sensing.     

Amperometric bienzyme glucose biosensor based on carbon nanotube modified electrode with electropolymerized poly(toluidine blue O) film

The amperometric bienzyme glucose biosensor utilizing horseradish peroxidase (HRP) and glucose oxidase (GOx) immobilized in poly(toluidine blue O) (PTBO) film was constructed on multi-walled carbon nanotube (MWNT) modified glassy carbon electrode. The HRP layer could be used to analyze hydrogen peroxide with toluidine blue O (TBO) mediators, while the bienzyme system (HRP + GOx) could be utilized for glucose determination. Glucose underwent biocatalytic oxidation by GOx in the presence of oxygen to yield H₂O₂ which was further reduced by HRP at the MWNT-modified electrode with TBO mediators. In the absence of oxygen, glucose oxidation proceeded with electron transfer between GOx and the electrode mediated by TBO moieties without H₂O₂ production. The bienzyme electrode offered high sensitivity for amperometric determination of glucose at low potential, displaying Michaelis-Menten kinetics. The bienzyme glucose biosensor displayed linear response from 0.1 to 1.2 mM with a sensitivity of 113 mA M⁻¹ cm⁻² at an applied potential of −0.10 V in air-saturated electrolytes.     

Direct electrochemistry of Cytochrome c on natural nano-attapulgite clay modified electrode and its electrocatalytic reduction for H2O2

Natural nano-structural attapulgite clay was purified by mechanical stirring with the aid of ultrasonic wave and its structure and morphology was investigated by XRD and transmission electron microscopy (TEM). Cytochrome c was immobilized on attapulgite modified glassy carbon electrode. The interaction between Cytochrome c and attapulgite clay was examined by using UV-vis spectroscopy and electrochemical methods. The direct electron transfer of the immobilized Cytochrome c exhibited a pair of redox peaks with formal potential (E0′) of about 17 mV (versus SCE) in 0.1 mol/L, pH 7.0, PBS. The electrode reaction showed a surface-controlled process with the apparent heterogeneous electron transfer rate constant (k_s) of 7.05 s⁻¹ and charge-transfer coefficient (α) of 0.49. Cytochrome c immobilized on the attapulgite modified electrode exhibits a remarkable electrocatalytic activity for the reduction of hydrogen peroxide (H₂O₂). The calculated apparent Michaelis-Menten constant was 470 μmol/L, indicating a high catalytic activity of Cytochrome c immobilized on attapulgite modified electrode to the reduction of H₂O₂. Based on these, a third generation of reagentless biosensor can be constructed for the determination of H₂O₂.     

Fabrication of CeO2 nanoparticles modified glassy carbon electrode and its application for electrochemical determination of UA and AA simultaneously

A glassy carbon electrode modified with CeO₂ nanoparticles was constructed and was characterized by electrochemical impedance spectrum (EIS) and cyclic voltammetry (CV). The resulting CeO₂ nanoparticles modified glassy carbon electrode (CeO₂ NP/GC electrode) was used to detect uric acid (UA) and ascorbic acid (AA) simultaneously in mixture. This modified electrode exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards UA and AA with activation overpotential. For UA and AA in mixture, one can well separate from the other with a potential difference of 273 mV, which was large enough to allow the determination of one in presence of the other. The DPV peak currents obtained in mixture increased linearly on the UA and AA in the range of 5.0 × 10⁻⁶ to 1.0 × 10⁻³ mol/L and 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol/L, with the detection limit (signal-to-noise ratio was 3) for UA and AA were 2.0 × 10⁻⁷ and 5.0 × 10⁻⁶ mol/L, respectively. The proposed method showed excellent selectivity and stability, and the determination of UA and AA simultaneously in serum was satisfactory.     

The structure and electrochemical performance of LiFeBO3 as a novel Li-battery cathode material

LiFeBO₃ cathode material has been synthesized successfully by solid-state reaction using Li₂CO₃, H₃BO₃ and FeC₂O₄·2H₂O as starting materials. The crystal structure has been determined by the X-ray diffraction. Electrochemical tests show that an initial discharge capacity of about 125.8 mAh/g can be obtained at the discharge current density of 5 mA/g. When the discharge current density is increased to 50 mA/g, the specific capacity of 88.6 mAh/g can still be held. In order to further improve the electrochemical properties, the carbon-coated LiFeBO₃, C-LiFeBO₃, are also prepared. The amount of carbon coated on LiFeBO₃ particles was determined to be around 5% by TG analysis. In comparison with the pure LiFeBO₃, a higher discharge capacity, 158.3 mAh/g at 5 mA/g and 122.9 mAh/g at 50 mA/g, was obtained for C-LiFeBO₃. Based on its low cost and reasonable electrochemical properties obtained in this work, LiFeBO₃ may be an attractive cathode for lithium-ion batteries.     

Preparation of porous carbons from thermoplastic precursors and their performance for electric double layer capacitors

Porous carbons with high surface area were successfully prepared from thermoplastic precursors, such as poly(vinyl alcohol) (PVA), hydroxyl propyl cellulose and poly(ethylene terephthalate), by the carbonization of a mixture with MgO at 900 °C in an inert atmosphere. After carbonization the MgO was dissolved out using a diluted sulfuric acid and the carbons formed were isolated. The mixing of the PVA carbon precursor with the MgO precursors (reagent grade MgO, magnesium acetate or citrate) was done either in powder form or in an aqueous solution. The BET surface area of the carbons obtained via solution mixing could reach a very high value, such as 2000 m²/g, without any activation process. The pore structure of the resultant carbons was found to depend strongly on the mixing method; the carbons prepared via solution mixing were rich in mesopores, but those produced via powder mixing were rich in micropores. The size of mesopores was found to be almost the same as that of the MgO particles, suggesting a way of controlling the mesopore size in the resultant carbons. Measurement of capacitance was carried out in 1 mol/L H₂SO₄ electrolyte. The porous carbon with a BET surface area of 1900 m²/g prepared at 900 °C through solution mixing of Mg acetate with PVA showed a fairly high EDLC capacitance, about 250 F/g with a current density of 20 mA/g and 210 F/g with 1000 mA/g. The rate performance was closely related to the mesoporous surface area.     

Synthesis of carbon nanowalls by plasma-enhanced chemical vapor deposition in a CO/H2 microwave discharge system

Synthesis of carbon nanowalls was performed by a plasma-enhanced chemical vapor deposition in a CO/H₂ microwave discharge system. At the optimum CO/H₂ feed ratio of 46 sccm/4 sccm, the aligned carbon nanowalls with thickness of about several tens of nanometers were able to be synthesized. The extremely high growth rate of 1 μm min^− 1 was obtained in our system with a relatively low microwave discharge power of 60 W for a growth area of 1 cm². An optical emission spectroscopy was performed to clarify the characteristics of a CO/H₂ discharge system.     

High rate performance of highly dispersed C60 on activated carbon capacitor

Fullerene-activated carbon composite electrodes were prepared and their charge/discharge characteristics were studied for use in a high power electric double-layer capacitor. The capacitance of the C₆₀-loaded activated carbon fiber (ACF) electrodes became greater than that of the unloaded ACF at charge/discharge current densities above 50 mA/cm². In order to obtain a highly dispersed C₆₀-loaded electrode, an ultrasonic treatment was performed. The size of the C₆₀ agglomerate decreased from 1-2 to 0.1 μm or less, and the capacitance of the C₆₀-loaded ACF electrodes increased with an increase in the ultrasonic treatment time. A higher capacitance of 172 F/g was obtained at 50 mA/cm² on a 1 wt% C₆₀-loaded electrode with ultrasonic treatment, and the C₆₀-loaded ACF electrode also showed a higher cycle performance.     

Buckypaper-based catalytic electrodes for improving platinum utilization and PEMFC's performance

Platinum (Pt) catalytic electrode was developed by using carbon nanotube films (buckypaper) as supporting medium and electrodeposition method to deposit Pt catalyst. Buckypapers are free-standing thin films consisting of single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and/or carbon nanofibers (CNFs) held together by van der Waals forces without any chemical binders. Special mixed buckypapers was developed by layered microstructures with a dense and high-conducting SWNT networks at the surface, as well as large porous structures of CNF networks as back supports. This unique microstructure can lead to improve Pt catalyst accessibility and mass exchange properties. Pt particles of about 6 nm were uniformly deposited in porous buckypapers. A promising electrochemical surface area of ∼40 m²/g was obtained from these electrodes. A Pt utilization as low as 0.28 g_Pt/kW was achieved for the cathode electrode at 80 °C. Pt utilization efficiency can be further improved by optimization of the electrodeposition condition in order to reduce the Pt particle size.     

Controlled fabrication of nanosized TiO2 hollow sphere particles via acid catalytic hydrolysis/hydrothermal treatment

TiO₂ hollow spheres of controlled size were synthesized by combined acid catalytic hydrolysis and hydrothermal treatment, which involves the deposition of an inorganic coating of TiO₂ on the surface of carbon spheres prepared by a hydrothermal method and subsequent removal of the carbon spheres by calcination in air. The obtained TiO₂ hollow spheres were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and powder X-ray diffraction. The results revealed that the size and surface morphology of the TiO₂ hollow spheres can be controlled by adjusting the concentration of the aqueous solution of glucose used to produce the template carbon spheres. Increasing the concentration of the glucose solution increased the average diameter of the TiO₂ hollow spheres from 190 to 300 nm. TiO₂ hollow spheres prepared using a glucose solution with a concentration of 0.7 mol/L are uniform in size with a diameter of 220 nm and shell thickness of 28 nm. The phenol removal rate of the sample prepared by calcination at 600 °C is 1.35 times higher than that of TiO₂ made by the same method without using the carbon template.     

Enhanced inactivation of E. coli bacteria using immobilized porous TiO2 photoelectrocatalysis

Immobilized TiO₂ nanotube electrodes with high surface areas were grown via electrochemical anodization in aqueous solution containing fluoride ions for photocatalysis applications. The photoelectrochemical properties of the grown immobilized TiO₂ film were studied by potentiodynamic measurements (linear sweep voltammetry), in addition to the calculation of the photocurrent response. The nanotube electrode properties were compared to mesoporous TiO₂ electrodes grown by anodization in sulfuric acid at high potentials (above the microsparking potential) and to 1 g/l P-25 TiO₂ powder. Photocatalyst films were evaluated by high resolution SEM and XRD for surface and crystallographic characterization. Finally, photoelectrocatalytic application of TiO₂ was studied via inactivation of E. coli. The use of the high surface area TiO₂ nanotubes resulted in a high photocurrent and an extremely rapid E. coli inactivation rate of ∼10⁶ CFU/ml bacteria within 10 min. The immobilized nanotube system is proven to be the most potent electrode for water purification.     

Synthesis and characterization of ruthenium-containing ordered mesoporous carbon with high specific surface area

A Ru-containing ordered mesoporous carbon with a high specific surface area of 2186 m²/g was synthesized through evaporation-induced multi-constituent co-assembly method, wherein soluble resol polymer is used as the carbon precursor, silicate oligomers as the inorganic precursor, triblock copolymer as the template, and RuCl₃ · 3H₂O as the Ru precursor. The resultant sample was characterized by X-ray diffraction, nitrogen sorption, transmission electron microscopy and scanning electron microscopy. The results showed that the carbon material exhibited highly ordered mesoporous structure, and the ruthenium particles with sizes of ∼2 nm were uniformly distributed in the carbon matrix. The sample was used to catalyze benzene hydrogenation, which displayed high efficiency for this reaction.     

Synthesis and characterization of high-density LiFePO4/C composites as cathode materials for lithium-ion batteries

To achieve a high-energy-density lithium electrode, high-density LiFePO₄/C composite cathode material for a lithium-ion battery was synthesized using self-produced high-density FePO₄ as a precursor, glucose as a C source, and Li₂CO₃ as a Li source, in a pipe furnace under an atmosphere of 5% H₂-95% N₂. The structure of the synthesized material was analyzed and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The electrochemical properties of the synthesized LiFePO₄/carbon composite were investigated by cyclic voltammetry (CV) and the charge/discharge process. The tap-density of the synthesized LiFePO₄/carbon composite powder with a carbon content of 7% reached 1.80 g m⁻³. The charge/discharge tests show that the cathode material has initial charge/discharge capacities of 190.5 and 167.0 mAh g⁻¹, respectively, with a volume capacity of 300.6 mAh cm⁻³, at a 0.1C rate. At a rate of 5C, the LiFePO₄/carbon composite shows a high discharge capacity of 98.3 mAh g⁻¹ and a volume capacity of 176.94 mAh cm⁻³.     

An electrochemical biosensor based on Nafion-ionic liquid and a myoglobin-modified carbon paste electrode

An electrochemical biosensor was constructed based on the immobilization of myoglobin (Mb) in a composite film of Nafion and hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) for a modified carbon paste electrode (CPE). Direct electrochemistry of Mb in the Nafion-BMIMPF₆/CPE was achieved, confirmed by the appearance of a pair of well-defined redox peaks. The results indicate that Nafion-BMIMPF₆ composite film provided a suitable microenvironment to realize direct electron transfer between Mb and the electrode. The cathodic and anodic peak potentials were located at −0.351 V and −0.263 V (vs. SCE), with the apparent formal potential (Ep) of −0.307 V, which was characteristic of Mb Fe(III)/Fe(II) redox couples. The electrochemical behavior of Mb in the composite film was a surface-controlled quasi-reversible electrode process with one electron transfer and one proton transportation when the scan rate was smaller than 200 mV/s. Mb-modified electrode showed excellent electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) in a linear concentration range from 2.0 × 10⁻⁴ mol/L to 1.1 × 10⁻² mol/L and with a detection limit of 1.6 × 10⁻⁵ mol/L (3σ). The proposed method would be valuable for the construction of a third-generation biosensor with cheap reagents and a simple procedure.