Direct electrochemistry of horseradish peroxidase on graphene-modified electrode for electrocatalytic reduction towards H2O2

Graphene was synthesized by a chemical method to reduce graphite oxide and well characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and Fourier transform infrared (FTIR) spectra. Horseradish peroxidase (HRP) immobilized on a graphene film glassy carbon electrode was found to undergo direct electron transfer and exhibited a fast electron transfer rate constant of 4.63 s⁻¹. The HRP-immobilized electrode was investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The CV results showed that the modified electrode gave rise to well-defined peaks in phosphate buffer, corresponding to the electrochemical redox reaction between HRP–Fe(III) and HRP–Fe(II). The obtained electrode also displayed an electrocatalytic reduction behavior towards H₂O₂. The new H₂O₂ sensor shows a linear range of 0.33–14.0 μM (R² = 0.9987) with a calculated detection limit of 0.11 μM (S/N = 3). Furthermore, the biosensor exhibits both good operational storage and storage stability.     

Fabrication of conducting polymer-gold nanoparticles film on electrodes using monolayer protected gold nanoparticles and its electrocatalytic application

We wish to report a simple and new strategy for the fabrication of gold nanoparticles-conducting polymer film on glassy carbon (GC) and indium tin oxide (ITO) surfaces using 5-amino-2-mercapto-1,3,4-thiadiazole capped gold nanoparticles (AMT-AuNPs) in 0.01 M H₂SO₄ by electropolymerization. The presence of amine groups on the surface of the AuNPs was responsible for the deposition of the AMT-AuNPs film on the electrode surface. The atomic force microscopy (AFM) studies reveal that the fabricated p-AMT-AuNPs film showed homogeneously distributed AuNPs with a spherical shape of ∼8 nm diameter. The XPS spectrum shows the binding energies at 83.8 and 87.5 eV in the Au 4f region corresponding to 4f_7/2 and 4f_5/2, respectively. The position and difference between these two peaks (3.7 eV) exactly match the value reported for Au⁰. The N1s XPS showed three binding energies at 396.7, 399.6 and 403.3 eV, corresponding to the NH, –NH– and –N⁺H–, respectively, confirming that the electropolymerization proceeded through the oxidation of –NH₂ groups present on the periphery of the AMT-AuNPs. The application of the present p-AMT-AuNPs modified electrode was demonstrated by studying the electro reduction of oxygen at pH 7.2. The p-AMT-AuNPs film enhanced the oxygen reduction current more than three times than that of p-AMT film prepared under identical conditions.     

Electrochemical behaviors of graphene-ZnO and graphene-SnO2 composite films for supercapacitors

Graphene, graphene-ZnO and graphene-SnO₂ films were successfully synthesized and used as electrode materials for electrochemical supercapacitors, respectively. The screen-printing approach was employed to fabricate graphene film on graphite substrate while the ZnO and SnO₂ were deposited on graphene films by ultrasonic spray pyrolysis. The electrochemical performances of these electrodes were comparatively analyzed through electrochemical impedance spectrometry, cyclic voltammetry and chronopotentiometry tests. The results showed that the incorporation of ZnO or SnO₂ improved the capacitive performance of graphene electrode. Graphene-ZnO composite electrode exhibited higher capacitance value (61.7 F/g) and maximum power density (4.8 kW/kg) as compared with graphene-SnO₂ and pure graphene electrodes.     

Graphene wrapped copper–nickel nanospheres on highly conductive graphene film for use as counter electrodes of dye-sensitized solar cells

A novel architecture of graphene wrapped copper–nickel (Cu–Ni) nanospheres (NSs)/graphene film was proposed to be TCO- and Pt-free counter electrode (CE) with high electrocatalytic activity for dye-sensitized solar cells (DSSCs). The novel architecture CE is composed of highly conductive graphene film, Cu–Ni alloy NSs and the wrapping graphene on the surface of alloy NSs. The graphene film as an electrically conductive layer was synthesized by chemical vapor deposition (CVD) on the insulating SiO₂ substrate, and graphene wrapped Cu–Ni alloy catalyst NSs on the graphene film were in situ formed by the reduction of Cu–Ni acetate and graphene growth using CVD. The graphene wrapped Cu–Ni NSs/graphene film CE shows much superior electrocatalytic activity, compared with graphene film, and the power conversion efficiency of 5.46% was achieved in DSSC devices, which is close to that of Pt/FTO electrode (6.19%). Therefore, the novel architecture of graphene wrapped Cu–Ni NSs/graphene film CE may be used as Pt- and TCO-free CEs for low-cost, high performance DSSCs.     

Multiwall carbon nanotube supported poly(3,4-ethylenedioxythiophene)/manganese oxide nano-composite electrode for super-capacitors

MWCNT-PSS/PEDOT/MnO₂ nano-composite electrodes were fabricated by generating pseudo-capacitive poly(3,4-ethylenedioxythiophene) (PEDOT)/MnO₂ nano-structures on poly(styrene sulfonate) (PSS) dispersed multiwalled carbon nanotubes (MWCNTs). PSS dispersed MWCNTs (MWCNT-PSS) facilitated the growth of PEDOT and MnO₂ into nano-rods with large active surface area and good electrical conductivity. The ternary MWCNT-PSS/PEDOT/MnO₂ nano-composite electrode was studied for the application in super-capacitors, and exhibited excellent capacitive behavior between −0.2 V and 0.8 V (vs. saturated Ag/AgCl electrode) with high reversibility. Specific capacitance of the nano-composite electrode was found as high as 375 F g⁻¹. In contrast, specific capacitance of MWCNT-PSS/MnO₂ and MWCNT-PSS nano-composite electrodes is 175 F g⁻¹ and 15 F g⁻¹, respectively. Based on cyclic voltammetric studies and cycle-life tests, the MWCNT-PSS/PEDOT/MnO₂ nano-composite electrode gave a highly stable and reversible performance up to 2000 cycles. Our studies demonstrate that the synergistic combination of MWCNT-PSS, PEDOT and MnO₂ has advantages over the sum of the individual components.     

TiO2-graphene nanocomposite for electrochemical sensing of adenine and guanine

TiO₂-graphene nanocomposite was prepared by hydrolysis of titanium isopropoxide in colloidal suspension of graphene oxide and in situ hydrothermal treatment. It provides an efficient and facile approach to yield nanocomposite with TiO₂ nanoparticles uniformly embedded on graphene substrate. The electrochemical behavior of adenine and guanine at the TiO₂-graphene nanocomposite modified glassy carbon electrode was investigated. The results show that the incorporation of TiO₂ nanoparticles with graphene significantly improved the electrocatalytic activity and voltammetric response towards these species comparing with that at the graphene film. The TiO₂-graphene based electrochemical sensor exhibits wide linear range of 0.5–200 μM with detection limit of 0.10 and 0.15 μM for adenine and guanine detection, respectively. The excellent performance of this electrochemical sensor can be attributed to the high adsorptivity and conductivity of TiO₂-graphene nanocomposite, which provides an efficient microenvironment for electrochemical reaction of these purine bases.     

Seed-mediated growth of platinum nanoparticles on carbon nanotubes for the fabrication of electrochemical biosensors

Platinum nanoparticles (PtNPs) were prepared by seed-mediated growth method with Au nanoparticles (AuNPs) playing the role of seeds. Carbon nanotubes (CNTs) and AuNPs were first dropped onto the surface of glassy carbon (GC) electrode, and then the electrode was immersed into growth solution which contains H₂PtCl₆ and ascorbic acid. PtNPs were successfully grown onto the CNT surface due to the chemical reduction of Pt(IV). The electrode modified with AuNP_seed/PtNP/CNT film displayed excellent electrochemical response to H₂O₂ at 0.45 V versus saturated calomel electrode (SCE) with sensitivity much larger than that of PtNP/CNT and AuNP_seed/PtNP modified electrodes. Glucose oxidase was selected as a model enzyme and electrodeposited onto the AuNP_seed/PtNP/CNT modified electrode in the presence of a detergent. The resulting biosensor enabled selective determination of glucose with high sensitivity of 4.49 μA mM⁻¹, quick response time about 2 s, low-detection limit of 0.5 μM and wide linear range from 1 μM to 4 mM with a correlation coefficient 0.9998. Thus, the modified electrode proved to be a nice electrochemical biosensing platform for the fabrication of oxidase-based biosensors.     

Electrochemical detection of hydroquinone by graphene and Pt-graphene hybrid material synthesized through a microwave-assisted chemical reduction process

We have synthesized graphene and Pt-graphene hybrid material by a microwave-assisted chemical reduction process and evaluated their application as electrode materials towards the electrochemical detection of hydroquinone. Graphene modified glass carbon electrode (GCE) showed a good performance for detecting hydroquinone due to the unique properties of graphene which increased the active surface area of the electrode and accelerated the electron transfer. The linear detection range of hydroquinone concentration was 20–115 μM with a sensitivity of 1.38 μA μM⁻¹ cm⁻²; the detection limit was estimated to be 12 μM (S/N = 3). The electrocatalytic activity of the Pt-graphene modified GCE was further improved due to the enhanced electron transfer and the linear detection range was 20–145 μM with the sensitivity of 3.56 μA μM⁻¹ cm⁻², detection limit 6 μM (S/N = 3).     

Covalently anchored metal complexes onto silica as selective catalysts for the liquid phase oxidation of benzoins to benzils with air

Benzoins are selectively oxidized to benzils in the presence of covalently anchored metal complexes onto silica by stirring in toluene at 100 °C in air atmosphere. Silica functionalized metal complexes have been prepared by the complexation of organically modified silica (imine) with Pd(OAc)₂ (Cat 1), Co(OAc)₂ (Cat 2) and Ni(OAc)₂ (Cat 3). Cat 1, 2 and 3 have been characterized by FTIR, thermal analysis and AAS analysis. Cat 1 was found to be most active, stable and recyclable under the reaction conditions. It was also characterized by SEM and TEM.     

Potentiostatically deposited polypyrrole/graphene decorated nano-manganese oxide ternary film for supercapacitors

A simple method based on potentiostatic polymerization was developed for the preparation of ternary manganese oxide-based nanocomposite films. The ternary nanocomposites, which were characterized using x-ray diffraction spectroscopy and x-ray photoelectron spectroscopy, showed that the manganese oxide within the film consisted of MnO₂ and Mn₂O₃. Electrochemical measurements showed that the ternary nanocomposite electrode exhibited high specific capacitance (up to 320.6 F/g), which was attributed to the morphology of a polypyrrole/graphene/manganese-oxide (PPy/GR/MnO_x) ternary nanocomposite. The experimental approach maximized the pseudocapacitive contribution from redox-active manganese oxide (MnO_x) and polypyrrole (PPy), as well as the electrochemical double layer capacitive (EDLC) characteristic from graphene (GR) sheets. Long cyclic measurements indicated that the specific capacitance of the ternary nanocomposite film could retain 93% of its initial value over 1000 charge/discharge cycles, in the potential range of −0.2 to 0.7 V versus silver/silver chloride electrode (Ag/AgCl).     

Microwave synthesis of magnetically separable ZnFe2O4-reduced graphene oxide for wastewater treatment

A magnetically separable ZnFe₂O₄-reduced graphene oxide (rGO) nano-composite was synthesised via a microwave method. Field emission scanning electron microscopy images of the nano-composite showed a uniform dispersion of nanoparticles on the rGO sheets. The performance of the nano-composite in wastewater treatment was assessed by observing the decomposition of methylene blue. The nano-composite showed excellent bifunctionality, i.e. adsorption and photocatalytic degradation of methylene blue, for up to five cycles of water treatment when illuminated with light from a halogen bulb. In contrast, water treatment with the nano-composite without illumination and the illuminated rGO, with no decoration of nanoparticles, diminished significantly after the first treatment. The reclamation of the ZnFe₂O₄-rGO nano-composite from treated water could be easily achieved by applying an external magnetic field.     

Electrochemical sensing based on graphene oxide/Prussian blue hybrid film modified electrode

A novel graphene oxide (GO)/Prussian blue (PB) hybrid film was constructed by electropolymerizing Prussian blue onto the GO modified glassy carbon electrode, and its electrochemical behaviors were studied. Raman spectra were used to investigate the successful formation of the GO/PB hybrid film. Electrochemical experiments showed that the graphene oxide greatly enhanced electrochemical reactivity of the PB. Moreover, a much higher Prussian blue (PB) loading (6.388 × 10⁻⁸ mol cm⁻²) is obtained as compared to the bare glass carbon surface (3.204 × 10⁻⁹ mol cm⁻²). The GO/PB hybrid film modified electrode was used for the sensitive detection of hydrogen peroxide. The sensor exhibited a wide linearity range from 5.0 × 10⁻⁶ to 1.2 × 10⁻³ M with a detection limit of 1.22 × 10⁻⁷ M (S/N = 3), high sensitivity of 408.7 μA mM⁻¹ cm⁻² and good reproducibility. Furthermore, with glucose oxidase (GOD) as a model, the GO/PB/GOD/chitosan composite-modified electrode was also constructed.The resulting biosensor exhibited good amperometric response to glucose with linear range from 0.1 to 13.5 mM at 0.1 V, good reproducibility and detection limit of 3.43 × 10⁻⁷ M (S/N = 3). In addition, the biosensor presented high selectivity and long-term stability. Therefore, the PB/GO hybrid films-based modified electrode may hold great promise for electrochemical sensing and biosensing applications.     

Amperometric biosensor for nitrite and hydrogen peroxide based on hemoglobin immobilized on gold nanoparticles/polythionine/platinum nanoparticles modified glassy carbon electrode

BACKGROUND: This paper describes a convenient and effective strategy to construct a highly sensitive amperometric biosensor for nitrite (NO₂^?) and hydrogen peroxide (H₂O₂). First, Pt nanoparticles (PtNPs) were electrodeposited on a glassy carbon electrode (GCE) surface, which promoted electron transfer and enhanced the loading of poly‐thionine (PTH). Subsequently, thionine (TH) was electropolymerized on the PtNPs/GCE, and gold nanoparticles (AuNPs) were assembled onto the PTH film to improve the absorption capacity of hemoglobin (Hb) and further facilitate electron transfer. Finally, Hb was immobilized onto the electrode through the AuNPs. RESULTS: Cyclic voltammetry (CV) and scanning electron microscopy (SEM) were used to characterize the fabrication process of the sensing surface. Under optimum conditions, the biosensors can be used for the determination of NO₂^? in the concentration range 70 nmol L^?1 to 1.2 mmo L^?1 and of H₂O₂ in the range 4.9 µmol L^?1 to 6.8 mmol L^?1. The detection limits (S/N = 3) were 20 nmol L^?1 and 1.4 µmol L^?1, respectively. CONCLUSION: The biosensor exhibits good analytical performance, acceptable stability and good selectivity. Copyright © 2011 Society of Chemical Industry     

A novel method for direct growth of a few-layer graphene on Al2O3 film

Direct growth of graphene on Al₂O₃ film is successfully achieved assisted with NiAl₂O₄ film on a SiO₂ substrate by chemical vapor deposition at 800 °C. The Ni particles are first uniformly separated out on the substrate, and play an important role in capturing carbon atoms and accelerating the nucleation to grow high quality graphene rooting on insulating Al₂O₃ film. The thickness of graphene films can be tuned from two layers to few layers (<10) by changing growth time. The continuous graphene films exhibit extremely excellent electrical transport properties with a sheet resistance of down to 18.5 Ω sq⁻¹. The graphene/Ni/Al₂O₃/SiO₂ is used as the counter electrode of dye sensitized solar cell which achieves a photovoltaic efficiency of 7.62%.     

Nano-composite of PtRu alloy electrocatalyst and electronically conducting polymer for use as the anode in a direct methanol fuel cell

Nano-composites comprised of PtRu alloy nanoparticles and an electronically conducting polymer for the anode electrode in direct methanol fuel cell (DMFC) were prepared. Two conducting polymers of poly(N-vinyl carbazole) and poly(9-(4-vinyl-phenyl)carbazole) were used for the nano-composite electrodes. Structural analyses were carried out using Fourier transform nuclear magnetic resonance spectroscopy, AC impedance spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Electrocatalytic activities were investigated by voltammetry and chronoamperometry in a 2 M CH₃OH/0.5 M H₂SO₄ solution and the data compared with a carbon-supported PtRu electrode. XRD patterns indicated good alloy formation and nano-composite formation was confirmed by TEM. Electrochemical measurements and DMFC unit-cell tests indicate that the nano-composites could be useful in a DMFC, but its performance would be slightly lower than that of a carbon-supported electrode. The interfacial property between the PtRu-polymer nano-composite anode and the polymer electrolyte was good, as evidenced by scanning electron microscopy. For better performance in a DMFC, a higher electric conductivity of the polymer and a lower catalyst loss are needed in nano-composite electrodes.     

Simultaneous determination of paracetamol and ascorbic acid using tetraoctylammonium bromide capped gold nanoparticles immobilized on 1,6-hexanedithiol modified Au electrode

Tetraoctylammonium bromide stabilized gold nanoparticles (TOAB-AuNPs) attached to 1,6-hexanedithiol (HDT) modified Au electrode was used for the simultaneous determination of paracetamol (PA) and ascorbic acid (AA) at physiological pH. The attachment of TOAB-AuNPs on HDT modified Au surface was confirmed by attenuated total reflectance (ATR)-FT-IR spectroscopy and atomic force microscope (AFM). The ATR-FT-IR spectrum of TOAB-AuNPs attached to the HDT monolayer showed a characteristic stretching modes corresponding to -CH₂ and -CH₃ of TOAB, confirming the immobilization of AuNPs with surface-protecting TOAB ions on the surface of the AuNPs after being attached to HDT modified Au electrode. AFM image showed that the immobilized AuNPs were spherical in shape and densely packed to a film of ca. 7 nm thickness. Interestingly, TOAB-AuNPs modified electrode shifted the oxidation potential of PA towards less positive potential by 70 mV and enhanced its oxidation current twice when compared to bare Au electrode. In addition, the AuNPs modified electrode separated the oxidation potentials of AA and PA by 210 mV, whereas bare Au electrode failed to resolve them. The amperometry current of PA was increased linearly from 1.50 × 10⁻⁷ to 1.34 × 10⁻⁵ M with a correlation coefficient of 0.9981 and the lowest detection limit was found to be 2.6 nM (S/N = 3). The present method was successfully used to determine the concentration of PA in human blood plasma and commercial drugs.     

Electro-oxidation nitrite based on copper calcined layered double hydroxide and gold nanoparticles modified glassy carbon electrode

In this paper, a novel nitrite sensor was constructed based on electrodeposition of gold nanoparticles (AuNPs) on a copper calcined layered double hydroxide (Cu-CLDH) modified glassy carbon electrode. Electrochemical experiments showed that AuNPs/CLDH composite film exhibited excellent electrocatalytic oxidation activity with nitrite due to the synergistic effect of the Cu-CLDH with AuNPs. The fabricated sensor exhibited excellent performance for nitrite detection within a wide concentration interval of 1–191 μM and with a detection limit of 0.5 μM. The superior electrocatalytic response to nitrite was mainly attributed to the large surface area, minimized diffusion resistance, and enhanced electron transfer of the Cu-CLDH and AuNPs composition film. This platform offers a novel route for nitrite sensing with wide analytical applications and will supply the practical applications for a variety of simple, robust, and easy-to-manufacture analytical approaches in the future.     

Influence of type and positioning of N-aryl substituents on vinyl polymerization of norbornene by Ni(II) α-diimine complexes

Effects of structural variations of the diimine ligand on catalyst activities for vinyl polymerization of norbornene (NB) have been investigated by a series of Ni(II) α-diimine catalysts of the general formula: [{ArN=C(Ac)-C(Ac)=NAr}]NiBr₂ (Ac=acenaphthyl) (Cat(H), Ar=C₆H₅; Cat(2,6-Me), Ar=2,6-C₆H₃Me₂; Cat(2,6-Et), Ar=2,6-C₆H₃Et₂; Cat(2,6- i Pr), Ar=2,6-C₆H₃ i-Pr₂; Cat(2,3-Me), Ar=2,3-C₆H₃Me₂; Cat(2,4-Me), Ar=2,4-C₆H₃Me₂; Cat(2,5-Me), Ar=2,5-C₆H₃Me₂; Cat(3,5-Me), Ar=3,5-C₆H₃Me₂; Cat(2,4,6-Me), Ar=2,4,6-C₆H₂Me₃). In situ reactions with methylaluminoxane generated the active catalysts, and they showed good activity towards NB polymerizations. As indicated by relatively higher activities of Cat(H) and Cat(3,5-Me), it can be generalized that catalysts having 2,6-substituents are less active due to steric interaction between monomer and substituents. In addition, electron donating methyl groups at 2-, 4-or 6-position on the N-aryl have a con effect and that at 3,5-position has a pro effect. This paper was presented at the 11th Korea-Japan Symposium on Catatysis held at Seoul, Korea, May 21–24, 2007.     

Synthesis of graphene-CoS electro-catalytic electrodes for dye sensitized solar cells

A large CoS-implanted graphene (G-CoS) film electrode was prepared using chemical vapor deposition followed by successive ionic layer absorption and reaction. HRTEM and AFM show that CoS nanoparticles are uniformly implanted on the graphene film. Furthermore, the G-CoS electro-catalytic electrode is characterized in a dye sensitized solar cells (DSSC) and is found to be highly electro-catalytic towards iodine reduction with low charge transfer resistance (R_ct ～5.05 Ω cm²) and high exchange current density (J₀～2.50 mA cm^?2). The improved performance compared to the pristine graphene is attributed to the increased number of active catalytic sites of G-CoS and highly conducting path of graphene. The comprehensive G-CoS synthesis process is a simple and scalable process which can easily adapt for large scale electro-catalytic film fabrication for several other electro-chemical energy harvesting and storage applications.     

Destruction of percolative network using green synthesized gold nanoparticles: Formation of high dielectric material

Gold nanoparticles (AuNPs) of about 5 nm in diameter were biosynthesized at room temperature (300 K). The PVA/2.5 wt% KH₂PO₄ or KDP composite film and PVA/2.5 wt% KDP/AuNPs nanocomposite films with different concentrations of AuNPs were prepared. Interestingly, addition of 0.05 wt% of AuNPs to the PVA/2.5 wt% KDP percolative composite film destroys percolative behavior of this composite film. Furthermore, the PVA/2.5 wt% KDP/0.05 wt% AuNPs nanocomposite film exhibited high room temperature dielectric permittivity (ε′ ∼ 590 at 1 kHz). The behavior of AC conductivity (σ_ac) of the nanocomposite films indicated correlated barrier hopping type of conduction mechanism. The Cole–Davidson dielectric response becomes evident as the interfacial polarization process acquires a more symmetric form, tending to Debye relaxation. High value of ε′ promises direct application in capacitors. Moreover, the novel feature of destroying the percolative behavior by AuNPs may be applied even in other systems.