Enhanced Hydrogen Peroxide Sensing Based on Tetraruthenated Porphyrins/Nafion/Glassy Carbon‐modified Electrodes via Incorporating of Carbon Nanotubes

The incorporation of carbon nanotubes to a Nafion/tetraruthenated cobalt porphyrin/ glassy carbon electrode (GC/Nf/CoTRP vs GC/Nf/CNTCoTRP) enhanced the amperometric determination of hydrogen peroxide. Both electrodes produced a decrease in the overpotential required for the hydrogen peroxide oxidation in about 100 mV compared to glassy carbon under the same experimental conditions. Nevertheless, for GC/Nf/CNT/CoTRP, the increase in the current is remarkable. The GC/Nf/CoTRP modified electrode gave no significant analitycal signal for hydrogen peroxide reduction. Moreover, a great increase in current is observed with GC/Nf/CNT/CoTRP at ?150mV which suggests a significant increase in the sensitivity of the modified electrode. Scanning electrochemical microscopy (SECM) revealed an enhancement in the electroactivity of the GC/Nf/CNT/CoTRP modified electrode. This fact has been explained in terms of enhanced homogeneity of the electrodic surface as a consecuence of better dispersibility of CNT‐CoTRP produced by a Nafion polyelectrolyte.     

Critical evaluation of electrode design and matrix effects on monitoring organophosphate pesticides using composite carbon nanotube-modified electrodes

Carbon nanotubes (CNT)/Nafion-modified glassy carbon (GC) electrodes were used to immobilize the enzyme acetylcholinesterase (AChE) by crosslinking with glutaraldehyde. The CNT-modified electrodes exhibited a sensitive and stable electrocatalytic behavior towards thiocholine (TCh). Compared to ordinary GC electrodes modified with Nafion, a substantial (500-mV) decrease in the overvoltage of the TCh oxidation reaction is observed, along with a tenfold enhancement in the amperometric response. The CNT/Nafion/AChE electrode has very good stability of at least a month compared to surfaces made without crosslinking in the absence and presence of Nafion. Under optimal loadings of CNT, Nafion, AChE, and glutaraldehyde, a solution of CNT/Nafion in N,N-dimethylformamide (DMF) containing 4 mg/mL CNT and 0.01% Nafion was used to construct the electrodes in order to maximize the sensitivity of the biosensor for inhibition studies. An optimal enzyme loading of 0.137 U and crosslinking in 0.01% glutaraldehyde for 1 h was also needed to achieve this goal. The prepared electrodes had very good reproducibility to 1.0 mM acetylthiocholine (ATCh) (relative standard deviation [RSD] <5% for eight electrodes). Using paraoxon as a model pesticide, the biosensor was able to detect as low as 1.0 nM after 30 min of incubation at 30 °C. Using a log scale, the biosensor had good linearity in the concentration range 50?C800 nM, with a correlation coefficient of 0.99. The prepared biosensor was used to test real water samples spiked with paraoxon and showed good correlation with a calibration curve using phosphate buffer.     

Carbon nanotube detectors for microchip CE: comparative study of single-wall and multiwall carbon nanotube, and graphite powder films on glassy carbon, gold, and platinum electrode surfaces

The performance of microchip electrophoresis/electrochemistry system with carbon nanotube (CNT) film electrodes was studied. Electrocatalytic activities of different carbon materials (single-wall CNT (SWCNT), multiwall CNT (MWCNT), carbon powder) cast on different electrode substrates (glassy carbon (GC), gold, and platinum) were compared in a microfluidic setup and their performance as microchip electrochemical detectors was assessed. An MWCNT film on a GC electrode shows electrocatalytic effect toward oxidation of dopamine (E(1/2) shift of 0.09 V) and catechol (E(1/2) shift of 0.19 V) when compared to a bare GC electrode, while other CNT/carbon powder films on the GC electrode display negligible effects. Modification of a gold electrode by graphite powder results in a strong electrocatalytic effect toward oxidation of dopamine and catechol (E(1/2) shift of 0.14 and 0.11 V, respectively). A significant shift of the half-wave potentials to lower values also provide the MWCNT film (E(1/2) shift of 0.08 and 0.08 V for dopamine and catechol, respectively) and the SWCNT film (E(1/2) shift of 0.10 V for catechol) when compared to a bare gold electrode. A microfluidic device with a CNT film-modified detection electrode displays greatly improved separation resolution (R(s)) by a factor of two compared to a bare electrode, reflecting the electrocatalytic activity of CNT.     

Enhanced Electrocatalytic Activity of Ni‐CNT Nanocomposites for Simultaneous Determination of Epinephrine and Dopamine

A promising electrochemical sensor based nickel‐carbon nanotube (Ni‐CNT) modified on glassy carbon (GC) electrode had been developed and the properties of the modified electrode were characterized by multispectroscopic analysis. The fabricated sensor (GC/Ni‐CNT) electrode was utilized to determine the catecholamines such as epinephrine and dopamine simultaneously. Differential pulse voltammetry and amperometry were used to verify the electrochemical behavior of the studied compounds. The GC/Ni‐CNT based amperometric sensor showed a wide linear range and low detection limit with high analytical sensitivity of 8.31 and 6.61 μA μM^?1 for EP and DA, respectively which demonstrates better characteristics compared to other electrodes reported in the literature. Further, no significant change in amperometric current response was observed in presence of biological interference species such as glucose, cysteine, citric acid, uric acid and ascorbic acid in the detection of EP and DA. The utility of this GC/Ni‐CNT electrode was well established for the determination of EP and DA in human urine samples.     

Influence of Metal Nanoparticles on the Electrocatalytic Oxidation of Glucose by Poly(NiIIteta) Modified Electrodes

Conductive polymeric [Ni^II(teta)]²⁺ (teta=C‐meso‐5,5,7,12,12,14‐hexamethyl‐1,4,8,11‐tetra‐azacyclotetradecane) films (poly(Ni)) have been deposited on the surface of glassy carbon (GC), Nafion (Nf) modified GC (GC/Nf) and Nf stabilized Ag and Au nanoparticles (NPs) modified GC (GC/Ag‐Nf and GC/Au‐Nf) electrodes. The cyclic voltammogram of the resulting electrodes, show a well defined redox peak due to oxidation and reduction of poly(Ni) system in 0.1 M NaOH. They show electrocatalytic activity towards the oxidation of glucose. AFM studies reveal the formation of poly(Ni) film on the modified electrodes. Presence of metal NPs increases electron transfer rate and electrocatalytic oxidation current by improving the communication within the Nf and poly(Ni) films. In the presence of metal NPs, 4 fold increase in current for glucose oxidation was observed.     

Dispersion of multi-wall carbon nanotubes in polyethylenimine: A new alternative for preparing electrochemical sensors

In this work we report on the analytical performance of glassy carbon electrodes modified with a dispersion of multi-wall carbon nanotubes in polyethylenimine (GCE/(PEI/CNT)). The resulting electrodes show an excellent electrocatalytic activity toward different bioanalytes like ascorbic acid, dopamine, 3,4-dihydroxyphenylacetic acid (dopac) and hydrogen peroxide. An important decrease in the overvoltages for the oxidation of ascorbic acid (505 mV) and hydrogen peroxide (350 mV) and for the reduction of hydrogen peroxide (450 mV), as well as a dramatic improvement in the reversibility of the electrochemical behavior of dopamine and dopac is obtained. The currents are higher than those obtained with other dispersant agents like Nafion, concentrated acids or chitosan, evidencing the high efficiency of the dispersion in PEI. The GCE/(PEI/CNT) demonstrated to be highly reproducible, with 3.0% RSD for the sensitivity of hydrogen peroxide for 10 electrodes prepared with five different dispersions. Differences in sensitivity of 10.0% were obtained for hydrogen peroxide with electrodes prepared using the same dispersion even after 14 days preparation. The CNT/PEI layer immobilized on glassy carbon electrodes has been also used as a platform for building supramolecular architectures based on the self-assembling of polyelectrolytes without any pretreatment of the electrode surface, oxidation or derivatization of the carbon nanotubes, just taking advantages of the polycationic nature of the polymer used for dispersing the nanotubes. The self-assembling of glucose oxidase has allowed us to obtain a supramolecular multistructure for glucose biosensing, with detection limits of 11 μM (0.02 g/L). Such an excellent performance of GCE/(PEI/CNT) toward hydrogen peroxide and the effectiveness of the use of CNT/PEI as a platform for obtaining supramolecular multistructures, represents a very good alternative for developing other enzymatic biosensors.     

Amperometric and voltammetric detection of hydrazine using glassy carbon electrodes modified with carbon nanotubes and catechol derivatives

A simple procedure was developed to prepare a glassy carbon (GC) electrode modified with carbon nanotubes (CNTs) and catechol compounds. First, 25 microL of DMSO-CNTs solutions (0.4 mg/mL) was cast on the surface of GC electrode and dried in air to form a CNTs film. Then the GC/CNTs modified electrode immersed into a chlorogenic acid, catechine hydrate and caffeic acid solution (electroless deposition) for a short period of time (2-80s). The cyclic voltammogram of the modified electrode in aqueous solution shows a pair of well-defined, stable and nearly reversible redox couple (quinone/hydroquinone) with surface confined characteristics. The combination of unique electronic and electrocatalytic properties of CNTs and catechol compounds results in a remarkable synergistic augmentation on the response. The electrochemical reversibility and stability of modified electrode prepared with incorporation of catechol compound into CNTs film was evaluated and compared with usual methods for attachment of catechols to electrode surfaces. The transfer coefficient (alpha), heterogeneous electron transfer rate constants (k(s)) and surface concentrations (Gamma) for GC/CNTs/catechol compound modified electrodes were calculated through the cyclic voltammetry technique. The modified electrodes showed excellent catalytic activity, fast response time and high sensitivity toward oxidation of hydrazine in phosphate buffer solutions at pH range 4-8. The modified electrode retains its initial response for at least 2 months if stored in dry ambient condition. The properties of modified electrodes as an amperometric sensor for micromolar or lower concentration detection of hydrazine have been characterized.     

Voltammetric detection of synthetic water-soluble phenolic antioxidants using carbon nanotube based electrodes

Glassy carbon electrodes (GCE) modified with carbon nanotubes (CNT) have been created for detection of phenolic compounds—one of the important group of antioxidants in life sciences. The surface of electrode has been characterized by atomic force microscopy. The presence of CNT leads to an at least 20-fold increase in the surface roughness of the electrode. The CNT layer displays closely intertwined vermicular structures with high degree of homogeneity at CNT suspension concentration of 0.2–0.5 mg L⁻¹. Synthetic water-soluble antioxidants (hydroquinone, catechol, pyrogallol, and their derivatives) are electrochemically active on bare GCE and CNT-modified GCE in phosphate buffer solution pH 7.4. Effect of substitutes in molecular structure of phenolic antioxidants has been evaluated. In several cases, oxidation at CNT-modified GCE occurs at potentials that are less positive by 100–200 mV in comparison to bare GCE. The electrodes were studied with respect to their capability of phenols voltammetric sensing. CNT-modified GCE display an enlarged linear range in the calibration graphs and lower detection limits. Voltammetric method for determination of hydroquinone, catechol, pyrogallol, and their derivatives has been developed.     

Amperometric glucose biosensor based on electrodeposition of platinum nanoparticles onto covalently immobilized carbon nanotube electrode

A new amperometric biosensor for glucose was developed based on adsorption of glucose oxidase (GOx) at the gold and platinum nanoparticles-modified carbon nanotube (CNT) electrode. CNTs were covalently immobilized on gold electrode via carbodiimide chemistry by forming amide linkages between carboxylic acid groups on the CNTs and amine residues of cysteamine self-assembled monolayer (SAM). The fabricated GOx/Au_nano/Pt_nano/CNT electrode was covered with a thin layer of Nafion to avoid the loss of GOx in determination and to improve the anti-interferent ability. The immobilization of CNTs on the gold electrode was characterized by quartz crystal microbalance technique. The morphologies of the CNT/gold and Pt_nano/CNT/gold electrodes have been investigated by scanning electron microscopy (SEM), and the electrochemical performance of the gold, CNT/gold, Pt_nano/gold and Pt_nano/CNT/gold electrodes has also been studied by amperometric method. In addition, effects of electrodeposition time of Pt nanoparticles, pH value, applied potential and electroactive interferents on the amperometric response of the sensor were discussed.

The enzyme electrode exhibited excellent electrocatalytic activity and rapid response for glucose in the absence of a mediator. The linear range was from 0.5 to 17.5 mM with correction coefficient of 0.996. The biosensor had good reproducibility and stability for the determination of glucose.     

Electrochemical oxidation of catecholamines and catechols at carbon nanotube electrodes

The differences in the electrochemical oxidation of two commonly known catecholamines, dopamine and norepinephrine, and one catechol, dihydroxyphenylacetic acid (DOPAC), at three different types of carbon based electrodes comprising conventionally polished glassy carbon (GC), nitrogen-doped carbon nanotubes (N-CNTs), and non-doped CNTs were assessed. Raman microscopy and X-ray photoelectron spectroscopy (XPS) were employed to evaluate structural and compositional properties. Raman measurements indicate that N-CNT electrodes have ca. 2.4 times more edge plane sites over non-doped CNTs. XPS data show no evidence of oxygen functionalities at the surface of either CNT type. N-CNTs possess 4.0 at. % nitrogen as pyridinic, pyrrolic, and quaternary nitrogen functionalities that result in positively charged carbon surfaces in neutral and acidic solutions. The electrochemical behavior of the various carbon electrodes were investigated by cyclic voltammetry conducted in pH 5.8 acetate buffer. Semiintegral analysis of the voltammograms reveals a significant adsorptive character of dopamine and norepinephrine oxidation at N-CNT electrodes. Larger peak splittings, DeltaE(p), for the cyclic voltammograms of both catecholamines and a smaller DeltaE(p) for the cyclic voltammogram for DOPAC at N-CNT electrodes suggest that electrostatic interactions hinder oxidation of cationic dopamine and norepinephrine, but facilitate anionic DOPAC oxidation. These observations were supported by titrimetry of solid suspensions to determine the pH of point of zero charge (pH(pzc)) and estimate the number of basic sites for both CNT varieties. This study demonstrates that carbon purity, the presence of exposed edge plane sites, surface charge, and basicity of CNTs are important factors for influencing adsorption and enhancing the electrochemical oxidation of catecholamines and catechols.     

聚血红素修饰电极对儿茶酚类化合物和抗坏血酸氧化的电催化作用研究

电催化是化学修饰电极研究的中心课题之一,血红素是一种重要的铁卟啉化合物,其中的铁原子能够以两种价态存在．我们采用循环伏安法将血红素修饰于电极表面,得到了氧化还原体（ｒｅｄｏｘ）型化学修饰电极,并用于儿茶酚类化合物和抗坏血酸的电催化氧化研究．采用伏安法...     

Carbon-nanotube-modified glassy carbon electrodes for amplified label-free electrochemical detection of DNA hybridization

The preparation and attractive performance of carbon-nanotube modified glassy-carbon (CNT/GC) electrodes for improved detection of purines, nucleic acids, and DNA hybridization are described. The surface-confined multiwall carbon-nanotube (MWCNT) facilitates the adsorptive accumulation of the guanine nucleobase and greatly enhances its oxidation signal. The advantages of CNT/GC electrodes are illustrated from comparison to the common unmodified glassy carbon, carbon paste and graphite pencil electrodes. The dramatic amplification of the guanine signal has been combined with a label-free electrical detection of DNA hybridization. Factors influencing the enhancement of the guanine signal are assessed and optimized. The performance characteristics of the amplified label-free electrochemical detection of DNA hybridization are reported in connection to measurements of nucleic-acid segments related to the breast-cancer BRCA1 gene.     

Direct Electrochemistry of Glucose Oxidase at Carbon Nanotube‐gold Colloid Modified Electrode with Poly(diallyldimethylammonium chloride) Coating

Glucose oxidase showed direct electrochemical transfer at glassy carbon electrodes immobilized with carbon nanotube‐gold colloid (CNT‐Au) composites with poly(diallydimethylammonium chloride) (PDDA) coatings. The modified electrode (GC/CNT/Au/PDDA‐GOD) was employed for the amperometric determination of glucose. Under optimal conditions, the biosensor displayed linear response to glucose from 0.5 to 5 mM with a sensitivity of 2.50 mA M^?1 at an applied potential of ?0.3 V (vs. Ag|AgCl reference).     

Comparative study of multi walled carbon nanotubes-based electrodes in micellar media and their application to micellar electrokinetic capillary chromatography

This work reports on a comparative study of the electrochemical performance of carbon nanotubes-based electrodes in micellar media and their application for amperometric detection in micellar electrokinetic capillary chromatography (MEKC) separations. These electrodes were prepared in two different ways: immobilization of a layer of carbon nanotubes dispersed in polyethylenimine (PEI), ethanol or Nafion onto glassy carbon electrodes or preparation of paste electrodes using mineral oil as binder. Scanning electron microscopy (SEM) was employed for surface morphology characterization while cyclic voltammetry of background electrolyte was used for capacitance estimation. The amperometric responses to hydrogen peroxide, amitrol, diuron and 2,3-diclorophenol (2,3CP) in the presence and in the absence of sodium dodecylsulphate (SDS) were studied by flow injection analysis (FIA), demonstrating that the electrocatalytic activity, background current and electroanalytical performance were strongly dependent on the electrodes preparation procedure. Glassy carbon electrodes modified with carbon nanotubes dispersed in PEI (GC/(CNT/PEI)) displayed the most adequate performance in micellar media, maintaining good electrocatalytic properties combined with acceptable background currents and resistance to passivation. The advantages of using GC/(CNT/PEI) as detectors in capillary electrophoresis were illustrated for the MEKC separations of phenolic pollutants (phenol, 3-chlorophenol, 2,3-dichlorophenol and 4-nitrophenol) and herbicides (amitrol, asulam, diuron, fenuron, monuron and chlortoluron).     

Determination of Phenolic Compounds Based on the Tyrosinase‐ Single Walled Carbon Nanotubes Sensor

An amperometric sensor to phenolic compound was successfully constructed by immobilizing tyrosinase on the SWNTs modified glassy carbon (GC) electrode, which was covered with Nafion film. The sensitivity of the tyrosinase‐SWNTs sensor to phenol was 155 μA/mM. The tyrosinase‐SWNTs sensor also had good response to catechol, p‐chlorophenol and m‐cresol. Furthermore, benzoic acid could be detected based on the inhibition to tyrosinase activity.     

Electrochemical Preparation of Poly(Malachite Green) Film Modified Nafion‐Coated Glassy Carbon Electrode and Its Electrocatalytic Behavior Towards NADH,Dopamine and Ascorbic Acid

Poly(malachite green) film modified Nafion‐coated glassy carbon electrodes have been prepared by potentiodynamic cycling in malachite green solution. The pH of polymerisation solution has only minor effect on film formation. Electrochemical quartz crystal microbalance (EQCM) was used to monitor the growth of the poly(malachite green) film. Cyclic voltammogram of the poly(malachite green) film shows a redox couple with well‐defined peaks. The redox response of the modified electrode was found to be depending on the pH of the contacting solution. The peak potentials were shifted to a less positive region with increasing pH and the dependence of the peak potential was found to be 56 mV per pH unit. The electrocatalytic behavior of poly(malachite green) film modified Nafion‐coated glassy carbon electrodes was tested towards oxidation of NADH, dopamine, and ascorbic acid. The oxidation of dopamine and ascorbic acid occurred at less positive potential on poly(malachite green) film compared to bare glassy carbon electrode. In the case of NADH, the overpotential was reduced substantially on modified electrode. Finally, the feasibility of utilizing poly(malachite green) film electrode in analytical estimation of ascorbic acid was demonstrated in flow injection analysis.     

Carbon nanotube/polystyrene composite electrode for microchip electrophoretic determination of rutin and quercetin in Flos Sophorae Immaturus

In this report, carbon nanotube/polystyrene (CNT/PS) composite electrodes have been fabricated as sensitive amperometric detectors of microchip capillary electrophoresis (CE) for the determination of rutin and quercetin in Flos Sophorae Immaturus. The composite electrode was fabricated on the basis of the in situ polymerization of a mixture of CNT and styrene in the microchannel of a piece of fused silica capillary under heat. The surface morphologies of the composite in the electrodes were observed by using a scanning electron microscope. The performance of this unique system has been demonstrated by separating and detecting rutin and quercetin. The new CNT-based CE detector offered significantly lower detection potentials, yielded substantially enhanced signal-to-noise characteristics, and exhibited resistance to surface fouling and hence enhanced stability. It demonstrated long-term stability and reproducibility with a relative standard deviation of less than 5% for the peak current (n = 20) and should also find a wide range of applications in conventional CE, flowing injection analysis, and other microfluidic analysis systems.     

Electrochemical reduction of oxygen on double-walled carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions

The oxygen reduction reaction has been investigated on double-walled carbon nanotube (DWCNT) modified glassy carbon (GC) electrodes in acid and alkaline media using the rotating disk electrode (RDE) method. The surface morphology and composition of DWCNT samples was examined by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Aqueous suspensions of DWCNTs were prepared using Nafion and non-ionic surfactant Triton X-100 as dispersing agents. The RDE results indicated that the DWCNT modified GC electrodes are active catalysts for oxygen reduction in alkaline solution. In acid media DWCNT/GC electrodes possess poor electrocatalytic properties for O₂ reduction which indicates lack of metal catalyst impurities in the DWCNT material studied. The oxygen reduction behaviour of DWCNTs was similar to that of multi-walled carbon nanotubes (MWCNTs) observed in our previous studies.     

Nicotinamide Adenine Dinucleotide Oxidation Studies at Multiwalled Carbon Nanotube/Polymer Composite Modified Glassy Carbon Electrodes

NADH oxidation has previously been investigated at carbon nanotube surfaces, although studies into the effect of the polymer binders are needed to fully understand whether the polymer binder affects the electrochemistry. This work details NADH oxidation at glassy carbon electrodes modified with composites containing multiwalled carbon nanotubes and selected polymer binders. NADH is shown to be oxidized at a lower potential than at glassy carbon electrodes and the oxidation potential is a function of the polymer binder. Hydrophobically modified Nafion, Nafion, linear poly(ethylenimine) (LPEI), octyl‐modified LPEI, and poly(vinylpyridine) binders were studied. Experiments showed the peak current and electrochemically assessible electrode area are dependent on the polymer binder. Overall, this paper shows that polymer binders affect NADH oxidation potential at carbon nanotube modified electrodes.     

Highly Sensitive and Selective Determination of Dopamine Based on Graphite Nanosheet‐Nafion Composite Film Modified Electrode

A graphite nanosheet (GNS)‐Nafion modified glassy carbon (GC) electrode was prepared and used for highly sensitive and selective determination of dopamine (DA). The GNS‐Nafion/GC electrode displayed excellent electrocatalytic activities towards DA and ascorbic acid (AA). The selective determination of DA was carried out successfully in the presence of AA by differential pulse voltammetry. High sensitivity (3.695 μA μM^?1) and low detection limit (0.02 μM, S/N=3) for the DA detection were obtained. These good properties can be attributed to a large amount of edge plane defects presented on GNSs and the charge‐exclusion and concentration features of Nafion.