Multilayer structured carbon nanotubes/poly-l-lysine/laccase composite cathode for glucose/O2 biofuel cell

Multilayer film of laccase, poly-l-lysine (PLL) and multi-walled carbon nanotubes (MWNTs) were prepared by a layer-by-layer self-assembly technique. The results of the UV–vis spectroscopy and scanning electron microscopy studies demonstrated a uniform growth of the multilayer. The catalytic behavior of the modified electrode was investigated. The (MWNTs/PLL/laccase)_n multilayer modified electrode catalyzed four-electron reduction of O₂ to water, without any mediator. The possible application of the laccase-catalyzed O₂ reduction at the (MWNTs/PLL/laccase)_n multilayer modified ITO electrode was illustrated by constructing a glucose/O₂ biofuel cell with the (MWNTs/thionine/AuNPs)₈ GDH film modified ITO electrode as a bioanode and the (MWNTs/PLL/laccase)₁₅ film modified ITO electrode as a biocathode. The open-circuit voltage reached to 700 mV, and the maximum power density achieved 329 μW cm⁻² at 470 mV of the cell voltage.     

Direct Electrochemistry of Multi‐Copper Oxidases at Carbon Nanotubes Noncovalently Functionalized with Cellulose Derivatives

This study describes the direct electron transfer of multi‐copper oxidases, i.e., laccase (from Trametes versicolor) and bilirubin oxidase (BOD, from Myrothecium verrucaria) at multiwalled carbon nanotubes (MWNTs) noncovalently functionalized with biopolymers of cellulose derivatives, i.e., hydroxyethyl cellulose (HEC), methyl cellulose (MC), and carboxymethyl cellulose (CMC). The functionalization of the MWNTs with the cellulose derivatives is found to substantially solubilize the MWNTs into aqueous media and to avoid their aggregation on electrode surface. Under anaerobic conditions, the redox properties of laccase and BOD are difficult to be defined with cyclic voltammetry at either laccase/MWNT‐modified or BOD/MWNT‐modified electrodes. The direct electron transfer properties of laccase and BOD are thus studied in terms of the bioelectrocatalytic activities of the laccase/MWNT‐modified and BOD/MWNT‐modified electrodes toward the reduction of oxygen and found to be facilitated at the functionalized MWNTs. The possible application of the laccase‐catalyzed O₂ reduction at the laccase/MWNT‐modified electrode is illustrated by constructing a CNT‐based ascorbate/O₂ biofuel cell with the MWNT‐modified electrode as the anode for the oxidation of ascorbate biofuel.     

Carbon Nanotubes‐Based Amperometric Cholesterol Biosensor Fabricated Through Layer‐by‐Layer Technique

A carbon nanotubes‐based amperometric cholesterol biosensor has been fabricated through layer‐by‐layer (LBL) deposition of a cationic polyelectrolyte (PDDA, poly(diallyldimethylammonium chloride)) and cholesterol oxidase (ChOx) on multi‐walled carbon nanotubes (MWNTs)‐modified gold electrode, followed by electrochemical generation of a nonconducting poly(o‐phenylenediamine) (PPD) film as the protective coating. Electrochemical impedance measurements have shown that PDDA/ChOx multilayer film could be formed uniformly on MWNTs‐modified gold electrode. Due to the strong electrocatalytic properties of MWNTs toward H₂O₂ and the low permeability of PPD film for electroacitve species, such as ascorbic acid, uric acid and acetaminophen, the biosensor has shown high sensitivity and good anti‐interferent ability in the detection of cholesterol. The effect of the pH value of the detection solution on the response of the biosensor was also investigated. A linear range up to 6.0 mM has been observed for the biosensor with a detection limit of 0.2 mM. The apparent Michaelis‐Menten constant and the maximum response current density were calculated to be 7.17 mM and 7.32 μA cm^?2, respectively.     

Direct Electrochemistry and Electrocatalysis of Hemoglobin Based on Nafion‐Room Temperature Ionic Liquids‐Multiwalled Carbon Nanotubes Composite Film

A robust and effective composite film combined the benefits of Nafion, room temperature ionic liquid (RTIL) and multi‐wall carbon nanotubes (MWNTs) was prepared. Hemoglobin (Hb) was successfully immobilized on glassy carbon electrode surface by entrapping in the composite film. Direct electrochemistry and electrocatalysis of immobilized Hb were investigated in detail. A pair of well‐defined and quasi‐reversible redox peaks of Hb was obtained in 0.10 mol·L^?1 pH 7.0 phosphate buffer solution (PBS), indicating that the Nafion‐RTIL‐MWNTs film showed an obvious promotion for the direct electron transfer between Hb and the underlying electrode. The immobilized Hb exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis current was linear to H₂O₂ concentration in the range of 2.0×10^?6 to 2.5×10^?4 mol·L^?1, with a detection limit of 8.0×10^?7 mol·L^?1 (S/N=3). The apparent Michaelis‐Menten constant (K_m^app) was calculated to be 0.34 mmol·L^?1. Moreover, the modified electrode displayed a good stability and reproducibility. Based on the composite film, a third‐generation reagentless biosensor could be constructed for the determination of H₂O₂.     

Oxygen reduction on polycobaltprotoporphyrin film: II. pH effect and the mechanism of oxygen reduction

The oxygen reduction kinetics on polycobaltprotoporphyrin (PCoPP) film has been examined with the rotating disc electrode technique in O₂-saturated solution of various pH. PCoPP film promotes the oxygen reduction process via two electrons to produce peroxide over wide pH range. When pH value of solution decreases, catalytic activity of PCoPP film increases. Possible catalytic mechanisms have been proposed for oxygen reduction.     

A Simple Layer‐by‐Layer Assembly Strategy for a Reagentless Biosensor Based on a Nanocomposite of Methylene Blue‐Multiwalled Carbon Nanotubes

A simple layer‐by‐layer (LBL) assembly strategy was established for constructing a novel reagentless biosensor based on a nanocomposite of methylene blue multiwalled carbon nanotubes (MB‐MWNTs). A nanocomposite of MB‐MWNTs was obtained by direct premixing and possessed good dispersion in barbital‐HCl buffer. Through electrostatic interactions, the nanocomposite of MB‐MWNTs could alternately be assembled with horseradish peroxidase (HRP) on the Au electrode modified with precursor films. UV/Vis spectra and scanning electron microscopy (SEM) were applied to reveal the formation of the nanocomposite of MB‐MWNTs. The LBL assembly process was also verified by electrochemical impedance spectroscopy (EIS). The MB is a well‐established mediator and efficiently facilitated the electron shuttle between the HRP and the electrode, as demonstrated by the cyclic voltammetry (CV) measurements. The as‐prepared reagentless biosensor exhibited a fast response for the determination of hydrogen peroxide (H₂O₂) and reached 95% of the steady‐state current within 3 s. It was found that the linear response range of the reagentless biosensor for H₂O₂ was from 4.0 μM to 3.78 mM with a detection limit of 1.0 μM and a sensitivity of 22.5 μA mM⁻¹. The biosensor exhibited a high reproducibility and stability.     

Preparation and Characterization of GCE Coatings Combining DDDMAB and PDADMAC with FAD and HCM for Electrocatalytic Detection of Oxygen and Sulfur Oxoanions

Electrochemically active hybrid coatings based on cationic films, didodecyldimethylammonium bromide (DDDMAB), and poly(diallyldimethylammonium chloride) (PDADMAC) are prepared on glassy carbon electrode surface by cycling the film‐covered electrode repetitively in a pH 7 solution containing flavin adenine dinucleotide (FAD), and anionic hexacyanometalate (HCM) complexes, Fe(CN)₆^3? and Ru(CN)₆^4?. Cyclic voltammetric features of hybrid coatings resemble that of electron transfer process of surface‐confined redox species. Electrochemical quartz crystal microbalance (EQCM) was used to monitor the deposition of FAD on DDDMAB film. Cyclic voltammetric peak potentials of modified electrode were found to be shifted to more negative region with increasing pH of contacting solution with a slope value of 63.3mV per pH unit. The electrocatalytic behavior of FAD‐modified DDDMAB‐coated GCE and hybrid film electrodes was tested towards reduction of oxygen, S₂O₈^2?, SO₅^2? and oxidation of SO₃^2?. The application of FAD‐modified DDDMAB‐coated GCE for S₂O₈^2? estimation was demonstrated in amperometric mode. The sensitivity and detection limit (S/N=3) were 267.6 μA mM^?1 and 2×10^?6 M, respectively.     

Self‐Assembly of Platinum Nanoparticle/Multiwalled Carbon Nanotube Multilayer Film on Au Substrate Electrode

A novel approach to assemble multilayer films of Pt nanoparticle/multiwalled carbon nanotube (MWNTs) composites on Au substrate has been developed for the purpose of improving the methanol oxidation efficiency by providing high catalytic surface area. MWNTs were firstly functionalized with 4‐mercaptobenzene and then assembled on an Au substrate electrode. Pt nanoparticles were fabricated and attached to the surface of the functionalized MWNTs subsequently. Thus a layer of Pt/MWNT composites were assembled on the Au substrate electrode. Repeating above process can assemble different layers of film of Pt/MWNTs composites on the Au electrode. Cyclic voltammetry shows that the Au electrode modified with two layers of film of Pt/MWNT composites exhibits high catalytic ability and long‐term stability for methanol oxidation. The layer‐by‐layer self‐assembly technique provides an efficient strategy to construct complex nanostructure for improving the methanol oxidation efficiency by providing high catalytic surface area.     

Electrocatalytic Dioxygen Reduction at Glassy Carbon Electrode Modified with Polyaniline Grafted Multiwall Carbon Nanotube Film

The work details the electrocatalysis of oxygen reduction reaction (ORR) in 0.5 M H₂SO₄ medium on a modified electrode containing a film of polyaniline (PANI) grafted multi‐wall carbon nanotube (MWNT) over the surface of glassy carbon electrode. We have fabricated a novel modified electrode in which conducting polymer is present as connected unit to MWNT. The GC/PANI‐g‐MWNT modified electrode (ME) is fabricated by electrochemical polymerization of a mixture of amine functionalized MWNT and aniline with GC as working electrode. Cyclic voltammetry and amperometry are used to demonstrate the electrocatalytic activity of the GC/PANI‐g‐MWNT‐ME. The GC/PANI‐g‐MWNT‐ME exhibits remarkable electrocatalytic activity for ORR. A more positive onset potential and higher catalytic current for ORR are striking features of GC/PANI‐g‐MWNT‐ME. Rapid and high sensitivity of GC/PANI‐g‐MWNT‐ME to ORR are evident from the higher rate constant (7.92×10² M^?1 s^?1) value for the reduction process. Double potential chronoamperometry and rotating disk and rotating ring‐disk electrode (RRDE) experiments are employed to investigate the kinetic parameters of ORR at this electrode. Results from RDE and RRDE voltammetry demonstrate the involvement of two electron transfer in oxygen reduction to form hydrogen peroxide in acidic media.     

A Facile Preparation of H2O2 Sensors Using Layer‐by‐Layer Deposited Thin Films Composed of Poly(ethyleneimine) and Carboxymethyl Cellulose as Matrices for Immobilizing Hemin

A layer‐by‐layer (LbL) thin film composed of poly(ethyleneimine) (PEI) and carboxymethyl cellulose (CMC) was prepared on the surface of a gold (Au) disk electrode and the LbL layer was impregnated with hemin to fabricate amperometric hydrogen peroxide (H₂O₂) sensors. Hemin can be easily immobilized in the LbL layer by immersing the LbL film‐coated electrode in the hemin solution. The hemin‐modified electrode thus prepared exhibited an amperometric response to H₂O₂ on the basis of the electrochemical reduction catalyzed by hemin. The output current of the hemin‐modified electrode depended on the concentration of H₂O₂ over the range of 0.005–1.0 mM. Thus, the LbL film composed of PEI and CMC was found to be an excellent material for the facile preparation of hemin‐based H₂O₂ sensors.     

Direct Electrochemistry of Catalase at a Gold Electrode Modified with Single‐Wall Carbon Nanotubes

The direct electrochemistry of catalase (Ct) was accomplished at a gold electrode modified with single‐wall carbon nanotubes (SWNTs). A pair of well‐defined redox peaks was obtained for Ct with the reduction peak potential at ?0.414 V and a peak potential separation of 32 mV at pH 5.9. Both reflectance FT‐IR spectra and the dependence of the reduction peak current on the scan rate revealed that Ct adsorbed onto the SWNT surfaces. The redox wave corresponds to the Fe(III)/Fe(II) redox center of the heme group of the Ct adsorbate. Compared to other types of carbonaceous electrode materials (e.g., graphite and carbon soot), the electron transfer rate of Ct redox reaction was greatly enhanced at the SWNT‐modified electrode. The peak current was found to increase linearly with the Ct concentration in the range of 8×10^?6–8×10^?5 M used for the electrode preparation and the peak potential was shown to be pH dependent. The catalytic activity of Ct adsorbates at the SWNTs appears to be retained, as the addition of H₂O₂ produced a characteristic catalytic redox wave. This work demonstrates that direct electrochemistry of redox‐active biomacromolecules such as metalloenzymes can be improved through the use of carbon nanotubes.     

Electrocatalysis of Cyclic Voltammetrically Prepared Co‐MnO2 Composite towards Oxygen Reduction Reaction (ORR)

For the first time, cobalt particles were electrodeposited on the surface of manganese oxides by cyclic voltammetry (CV) from an aqueous solution of 0.1 M Na₂SO₄ containing 5 mM CoSO₄, and then the samples obtained were characterized by scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDAX), respectively. And then, the as‐prepared Co/MnO₂‐coated graphite electrode was employed to the oxygen reduction reaction (ORR). Interestingly, the reduction peak potential of ORR on a Co/MnO₂‐modified graphite electrode was positively shifted for about 100 mV as compared with that on a MnO₂‐modified graphite electrode, indicating that the electrocatalysis of Co/MnO₂ composite towards ORR is superior to that of pure MnO₂.     

Functionalization of Single‐Walled Carbon Nanotubes with Cubic Prussian Blue and Its Application for Amperometric Sensing

In this work, we synthesized electroactive cubic Prussian blue (PB) modified single‐walled carbon nanotubes (SWNTs) nanocomposites using the mixture solution of ferric‐(III) chloride and potassium ferricyanide under ambient conditions. The successful fabrication of the PB‐SWNTs nanocomposites was confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV‐vis absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and cyclic voltammetry (CV). PB nanocrystallites are observed to be finely attached on the SWNTs sidewalls in which the SWNTs not only act as a carrier of PB nanocrystallites but also as Fe(III)‐reducer. The electrochemical properties of PB‐SWNTs nanocomposites were also investigated. Using the electrodeposition technique, a thin film of PB‐SWNTs/chitosan nanocomposites was prepared onto glassy carbon electrode (GCE) for the construction of a H₂O₂ sensor. PB‐SWNTs/chitosan nanocomposites film shows enhanced electrocatalytic activity towards the reduction of H₂O₂ and the amperometric responses show a linear dependence on the concentration of H₂O₂ in a range of 0.5–27.5 mM and a low detection limit of 10 nM at the signal‐to‐noise ratio of 3. The time required to reach the 95% steady state response was less than 2 s. CV studies demonstrate that the modified electrode has outstanding stability. In addition, a glucose biosensor is further developed through the simple one‐step electrodeposition method. The observed wide concentration range, high stability and high reproducibility of the PB‐SWNTs/chitosan nanocomposites film make them promising for the reliable and durable detection of H₂O₂ and glucose.     

Amperometric Glucose Biosensors Based on Integration of Glucose Oxidase onto Prussian Blue/Carbon Nanotubes Nanocomposite Electrodes

Nanosized Prussian blue (PB) particles were synthesized with a chemical reduction method and then the PB nanoparticles were assembled on the surface of multiwall carbon nanotubes modified glassy carbon electrode (PB/MWNTs/GCE). The results showed that the PB/MWNTs nanocomposite exhibits a remarkably improved catalytic activity towards the reduction of hydrogen peroxide. Glucose oxidase (GOD) was immobilized on the PB/MWNTs platform by an electrochemically polymerized o‐phenylenediamine (OPD) film to construct an amperometric glucose biosensor. The biosensor exhibited a wide linear response up to 8 mM with a low detection limit of 12.7 μM (S/N=3). The Michaelis–Menten constant K_m and the maximum current i_max of the biosensor were 18.0 mM and 4.68 μA, respectively. The selectivity and stability of the biosensor were also investigated.     

Fabrication of Photoelectrochemical Glucose Biosensor in Flow Injection Analysis System Using ZnS/CdS‐Carbon Nanotube Nanocomposite Electrode

In this work, a photoamperometric glucose biosensor based on glucose oxidase (GODx) was developed in flow injection analysis (FIA) system using ZnS‐CdS quantum dot (QD) modified multiwalled carbon nanotube/glassy carbon electrode (ZnS‐CdS/MWCNT/GCE). Cyclic voltammograms of the proposed electrode (GODx/ZnS‐CdS/MWCNT/GCE) showed a pair of well‐defined reversible redox peak attributing that direct electron transfer between the protein and electrode. The current of the reduction peak became more cathodic in the presence of O₂ due to the electrocatalytic activity of the electrode towards the reduction of dissolved O₂, but reduction current shifted to a less negative value upon addition of glucose in the solution. The obtained CV currents were affected by the irradiation of the electrode surface. Thus, the photoelectrochemical biosensing of glucose in the FIA system was studied by monitoring of the changes in the electrocatalyzed reduction peak current of dissolved O₂ at the proposed electrode dependent on glucose concentration. The proposed photoelectrochemical FIA method has a linear response to glucose ranging from of 0.01 to 1.0 mM with detection limit of 3.0 μM under optimized conditions. Photoelectrochemical biosensor was successfully fabricated in FIA system for selective, sensitive and repeatable detection of glucose and has been satisfactorily applied to determination of glucose in real sample.     

Enhanced Electrocatalytic Performance of Anthraquinonemonosulfonate‐Doped Polypyrrole Composite: Electroanalysis for the Specific Roles of Anthraquinone Derivative and Polypyrrole Layer on Oxygen Reduction Reaction

To analyze the specific roles of anthraquinone‐2‐sulfonate (AQS) and polypyrrole (PPy) layer on oxygen reduction reaction (ORR), the electrocatalytic reduction of oxygen was investigated on the AQS/PPy composite modified graphite electrode. Results show that the enhanced electrocatalytic performance is attributed to the excellent electrocatalytic activity of the immobilized AQS functional groups to mediate two‐electron reduction of O₂ to H₂O₂. The PPy layer may not participate in ORR, but it can further catalyze the two‐electron reduction of H₂O₂ to produce H₂O in the potential range more negative than that the two‐electron reduction of oxygen proceeds efficiently on the AQS sites.     

Synthesis and Characterization of MWNTs/Au NPs/HS(CH2)6Fc Nanocomposite: Application to Electrochemical Determination of Ascorbic Acid

In this article, a detailed electrochemical study of a novel 6‐ferrocenylhexanethiol (HS(CH₂)₆Fc) self‐assembled multiwalled carbon nanotubes‐Au nanoparticles (MWNTs/Au NPs) composite film was demonstrated. MWNTs/Au NPs were prepared by one‐step in situ synthesis using linear polyethyleneimine (PEI) as bifunctionalizing agent. HS(CH₂)₆Fc, which acted as the redox mediator, was self‐assembled to MWNTs/Au NPs via Au‐S bond. Transmission electron microscopy (TEM), energy‐dispersive X‐ray analysis (EDX), Fourier transformed infrared absorption spectroscopy (FT‐IR), UV‐visible absorption spectroscopy, and cyclic voltammetry were used to characterize the properties of the MWNTs/Au NPs/HS(CH₂)₆Fc nanocomposite. The preparation of the nanocomposite was very simple and effectively prevented the leakage of the HS(CH₂)₆Fc mediator during measurements. The electrooxidation of AA could be catalyzed by Fc/Fc⁺ couple as a mediator and had a higher electrochemical response due to the unique performance of MWNTs/Au NPs. The nanocomposite modified electrode exhibited excellent catalytic efficiency, high sensitivity, good stability, fast response (within 3 s) and low detection limit toward the oxidation of AA at a lower potential.     

Improvement of Selectivity and Stability of Amperometric Detection of Hydrogen Peroxide Using Prussian Blue‐PAMAM Supramolecular Complex Membrane as a Catalytic Layer

A novel hydrogen peroxide (H₂O₂) sensor was fabricated by using a submonolayer of 3‐mercaptopropionic acid (3‐MPA) adsorbed on a polycrystalline gold electrode further reacted with poly(amidoamine) (PAMAM) dendrimer (generation 4.0) to obtain a film on which Prussian Blue (PB) was later coordinated to afford a mixed and stable electrocatalytic layer for H₂O₂ reduction. On the basis of the electrochemical behaviors, atomic force microscopy (AFM) and X‐ray photoelectron spectra (XPS), it is suggested that the PB molecules are located within the dendritic structure of the surface attached PAMAM dendrimers. It was found that the PB/PAMAM/3‐MPA/Au modified electrode showed an excellent electrocatalytic activity for H₂O₂ reduction. The effects of applied potential and pH of solution upon the response of the modified electrode were investigated for an optimum analytical performance. Even in the presence of dissolved oxygen, the sensor exhibited highly sensitive and rapid response to H₂O₂. The steady‐state cathodic current responses of the modified electrode obtained at ?0.20 V (vs. SCE) in air‐saturated 0.1 mol L^?1 phosphate buffer solution (PBS, pH 6.50) showed a linear relationship to H₂O₂ concentration ranging from 1.2×10^?6 mol L^?1 to 6.5×10^?4 mol L^?1 with a detection limit of 3.1×10^?7 mol L^?1. Performance of the electrode was evaluated with respected to possible interferences such as ascorbic acid and uric acid etc. The selectivity, stability, and reproducibility of the modified electrode were satisfactory.     

Immobilization of Enzymes on the Nano‐Au Film Modified Glassy Carbon Electrode for the Determination of Hydrogen Peroxide and Glucose

A new enzyme‐based amperometric biosensor for hydrogen peroxide was developed relying on the efficient immobilization of horseradish peroxidase (HRP) to a nano‐scaled particulate gold (nano‐Au) film modified glassy carbon electrode (GC). The nano‐Au film was obtained by a chitosan film which was first formed on the surface of GC. The high affinity of chitosan for nano‐Au associated with its amino groups resulted in the formation of nano‐Au film on the surface of GC. The film formed served as an intermediator to retain high efficient and stable immobilization of the enzyme. H₂O₂ was detected using hydroquinone as an electron mediator to transfer electrons between the electrode and HRP. The HRP immobilized on nano‐Au film maintained excellent electrocatalytical activity to the reduction of H₂O₂. The experimental parameters such as the operating potential of the working electrode, mediator concentration and pH of background electrolyte were optimized for best analytical performance of amperometry. The linear range of detection for H₂O₂ is from 6.1×10^?6 to 1.8×10^?3 mol L^?1 with a detection limit of 6.1 μmol L^?1 based on signal/noise=3. The proposed HRP enzyme sensor has the features of high sensitivity (0.25 Almol^?1cm^?2), fast response time (t_90%≤10 s) and a long‐term stability (>1 month). As an extension, glucose oxidase (GOD) was chemically bound to HRP‐modified electrode. A GOD/HRP bienzyme‐modified electrode formed in this way can be applied to the determination of glucose with satisfactory performance.     

Modification of vertically aligned carbon nanotube arrays with palladium nanoparticles for electrocatalytic reduction of oxygen

A novel type of palladium nanoparticles-modified multiwalled carbon nanotubes composite-electrode with electrocatalytic activity for oxygen reduction is presented. The nanocomposite was prepared by magnetron sputtering deposition with Pd in Ar atmosphere on MWNTs, which were synthesized on Ta plates by chemical vapor deposition. Both scanning electron microscopy and transmission electron microscopy were employed to observe the surface morphology. The Pd nanoparticles, with diameters around 5 nm, are dispersed at the tips and on the sidewalls of the MWNTs. Voltammetry, amperometry and electrochemical impedance measurements were used to demonstrate the strong electrocatalytic activity of the nanocomposite in acid solution. Compared to the bare MWNT electrode, the PdNPs/MWNT nanocomposite shows a positive shift of the O₂ reduction current at onset potentials from +400 to +500 mV, a concurrent 1.5-fold increase in the O₂ reduction peak current with high stability. The successful preparation of PdNPs/MWNTs nanocomposite by magnetron sputtering deposition opens a new path for an efficient dispersion of promising nanoparticles for fuel cells and O₂ sensors.