A Selective Simultaneous Determination of Levodopa and Serotonin Using a Glassy Carbon Electrode Modified with Multiwalled Carbon Nanotube/Chitosan Composite

A glassy carbon electrode (GCE) modified with carbon nanotube/chitosan (MWCNTs‐CHT/GCE) was used for the sensitive voltammetric determination of levodopa (Lev) and serotonin (Ser). The measurements were carried out using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronoamperometry (CA). Under the optimum conditions the electrode provides a linear response versus Lev and Ser concentrations in the range of 2.0–220.0 µM and 0.5–130.0 µM, respectively, using DPV. The modified electrode was satisfactorily used for determination of Lev and Ser in human serum and urine with satisfactory results.     

Selective Simultaneous Determination of Paracetamol and Uric Acid Using a Glassy Carbon Electrode Modified with Multiwalled Carbon Nanotube/Chitosan Composite

A multiwalled carbon nanotube/chitosan modified glassy carbon electrode (MWCNTs‐CHT/GCE) has been used for simultaneous determination of paracetamol (PAR) and uric acid (UA). The measurements were carried out using differential pulse voltammetry (DPV), cyclic voltammetry (CV) and chronoamperometry (CA). DPV measurements showed a linear relationship between oxidation peak current and concentration of PAR and UA in phosphate buffer (pH 7) over the concentration range 2 µM to 250 µM, and 10 µM to 400 µM, respectively. The analytical performance of this sensor has been evaluated for detection of PAR and UA in human serum and human urine with satisfactory results.     

Residual Metal Impurity Aids Facile In Situ Electrochemical Surface Derivatization of Single‐Walled Carbon Nanotubes

Residual metal impurities were exploited as reactants in the functionalization of the surface of single‐walled carbon nanotubes (SWCNT) with nickel hexacyanoferrate (NiHCF) by simple electrochemical cycling in ferricyanide solutions. This facile in situ electrochemical modification process provides intimate contact between NiHCF and SWCNTs that improves the stability of the redox property and reactivity of NiHCF. The characteristic redox behavior of NiHCF on SWCNT surfaces can be used as an electrochemical probe to access qualitative and quantitative information on unknown electroactive metal impurities in SWCNTs. Significantly, the NiHCF‐modified SWCNTs exhibit pseudocapacitive behavior, and the calculated specific capacitances are 710 and 36 F g^?1 for NiHCF‐SWCNTs and SWCNTs respectively. Furthermore, NiHCF‐SWCNTs were transformed into Ni(OH)₂/SWCNTs and used for enzymeless glucose oxidation.     

Adsorptive Stripping Voltammetric Detection of Tea Polyphenols at Multiwalled Carbon Nanotubes‐Chitosan Composite Electrode

This study reports the catalytic oxidation and detection of tea polyphenols (TPs) at glassy‐carbon electrode modified with multiwalled carbon nanotubes‐chitosan (MWCNTs‐CS) film. The adsorption of TPs at the surface of the MWCNTs through π–π conjugation prevents the aggregation of nanotubes and induces a stable MWCNTs suspension in water over 30 days. Based on the adsorptive accumulation of polyphenols at MWCNTs, TPs is sensitively and selectively detected by adsorptive stripping voltammetry. The accumulation conditions and pH effect on the adsorptive stripping detection were examined. The linear range was found to be 100 to 1000 mg L^?1 with a detection limit of 10 mg L^?1 (S/N=3) for 2.5 min accumulation. Additionally, the MWCNTs‐CS electrode is easily renewed by applying positive potential to remove the adsorbed TPs. This method was successfully applied to determine TPs in commercially available teas with satisfied result compared with that of conventional spectrometric analysis.     

Single‐Walled Carbon Nanotube/Metalloporphyrin Composites for the Chemiresistive Detection of Amines and Meat Spoilage

Chemiresistive detectors for amine vapors were made from single‐walled carbon nanotubes by noncovalent modification with cobalt meso‐arylporphyrin complexes. We show that through changes in the oxidation state of the metal, the electron‐withdrawing character of the porphyrinato ligand, and the counteranion, the magnitude of the chemiresistive response to ammonia could be improved. The devices exhibited sub‐ppm sensitivity and high selectivity toward amines as well as good stability to air, moisture, and time. The application of these chemiresistors in the detection of various biogenic amines (i.e. putrescine, cadaverine) and in the monitoring of spoilage in raw meat and fish samples (chicken, pork, salmon, cod) over several days was also demonstrated.     

Carbon Nanotube Electrodes for Hot‐Wire Electrochemistry

Hot‐wired electrodes : Thin metallic wires (d=25 μm) are covered with a 3 μm layer of single‐walled carbon nanotubes (SWCNTs; see image) by electrophoresis from a suspension containing excess ionic surfactant. A pure SWCNT surface is achieved by heating the electrode in air. Strong differences between covered and bare metallic electrodes occur with in situ heating during electrochemical experiments.

     

Fully Oriented Bilirubin Oxidase on Porphyrin‐Functionalized Carbon Nanotube Electrodes for Electrocatalytic Oxygen Reduction

The efficient immobilization and orientation of bilirubin oxidase from Myrothecium verrucaria on multi‐walled carbon nanotube electrodes by using π‐stacked porphyrins as a direct electron‐transfer promoter is reported. By comparing the use of different types of porphyrin, the rational effect of the porphyrin structure on both the immobilization and orientation of the enzyme is demonstrated. The best performances were obtained for protoporphyrin IX, which is the natural precursor of bilirubin. These electrodes exhibit full orientation of the enzyme, as confirmed by the observable non‐catalytic redox system corresponding to the T1 copper center associated with pure Nernstian electrocatalytic behavior with high catalytic currents of almost 5 mA cm^?2 at neutral pH.     

Inlaid Multi‐Walled Carbon Nanotube Nanoelectrode Arrays for Electroanalysis

The rapid development in nanomaterials and nanotechnologies has provided many new opportunities for electroanalysis. We review our recent results on the fabrication and electroanalytical applications of nanoelectrode arrays based on vertically aligned multi‐walled carbon nanotubes (MWCNTs). A bottom‐up approach is demonstrated, which is compatible with Si microfabrication processes. MWCNTs are encapsulated in SiO₂ matrix leaving only the very end exposed to form inlaid nanoelectrode arrays. The electrical and electrochemical properties have been characterized, showing well‐defined quasireversible nanoelectrode behavior. Ultrasensitive detection of small redox molecules in bulk solutions as well as immobilized at the MWCNT ends is demonstrated. A label‐free affinity‐based DNA sensor has shown extremely high sensitivity approaching that of fluorescence techniques. This platform can be integrated with microelectronics and microfluidics for fully automated microchips.     

Multiwalled Carbon Nanotubes‐Chitosan Modified Single‐Use Biosensors for Electrochemical Monitoring of Drug‐DNA Interactions

A multiwalled carbon nanotubes (CNT)‐chitosan (CHIT) modified pencil graphite electrode (CNT‐CHIT/PGE) was developed for the first time herein for electrochemical monitoring of the interaction of an anticancer drug, mitomycin C (MC) and DNA. The characterization of unmodified PGE, CHIT/PGE, CNT/PGE and CHIT‐CNT/PGE were performed by scanning electron microscopy and cyclic voltammetry techniques. The oxidation signals of MC and guanine were measured before and after interaction at the surface of CNT‐CHIT/PGEs using differential pulse voltammetry. Electrochemical impedance spectroscopy technique was also successfully utilized for monitoring of the interaction process at the surface of CNT‐CHIT/PGEs in different interaction times.     

Highly Catalytic Carbon Nanotube/Pt Nanohybrid‐Based Transparent Counter Electrode for Efficient Dye‐Sensitized Solar Cells

Low‐cost transparent counter electrodes (CEs) for efficient dye‐sensitized solar cells (DSSCs) are prepared by using nanohybrids of carbon nanotube (CNT)‐supported platinum nanoparticles as highly active catalysts. The nanohybrids, synthesized by an ionic‐liquid‐assisted sonochemical method, are directly deposited on either rigid glass or flexible plastic substrates by a facile electrospray method for operation as CEs. Their electrochemical performances are examined by cyclic voltammetry, current density–voltage characteristics, and electrochemical impedance spectroscopy (EIS) measurements. The CNT/Pt hybrid films exhibit high electrocatalytic activity for I^?/I₃^? with a weak dependence on film thickness. A transparent CNT/Pt hybrid CE film about 100 nm thick with a transparency of about 70 % (at 550 nm) can result in a high power conversion efficiency (η) of over 8.5 %, which is comparable to that of pyrolysis platinum‐based DSSCs, but lower cost. Furthermore, DSSC based on flexible CNT/Pt hybrid CE using indium‐doped tin oxide‐coated polyethylene terephthalate as the substrate also exhibits η=8.43 % with J_sc=16.85 mA cm^?2, V_oc=780 mV, and FF=0.64, and this shows great potential in developing highly efficient flexible DSSCs.     

The Heterogeneity of Multiwalled and Single‐Walled Carbon Nanotubes: Iron Oxide Impurities Can Catalyze the Electrochemical Oxidation of Glucose

It is well established that the heterogeneity of carbon nanotubes must be determined before the origin of the electrochemical performance can be attributed. Recently it has been diligently reported that for the case of multiwalled carbon nanotube modified electrodes, copper oxide impurities are responsible for the electrochemical activity facilitating a nonenzymatic sensing strategy towards glucose. We have explored both commercially available multiwalled and single‐walled carbon nanotubes for the sensing of glucose and find that iron oxide impurities remaining from the fabrication process are the electroactive sites facilitating the nonenzymatic detection of glucose. Given that the multiwalled carbon nanotubes in this work are purchased from the same leading supplier as that used recently, discrepancies in the fabrication process exist which clearly has implications in the commercialization of electrochemical sensors based on multiwalled carbon nanotubes.     

Nitrogen‐Doped Carbon Nanotube Arrays for High‐Efficiency Electrochemical Reduction of CO2: On the Understanding of Defects,Defect Density,and Selectivity

Nitrogen‐doped carbon nanotubes (NCNTs) have been considered as a promising electrocatalyst for carbon‐dioxide‐reduction reactions, but two fundamental chemistry questions remain obscure: 1) What are the active centers with respect to various defect species and 2) what is the role of defect density on the selectivity of NCNTs? The aim of this work is to address these questions. The catalytic activity of NCNTs depends on the structural nature of nitrogen in CNTs and defect density. Comparing with pristine CNTs, the presence of graphitic and pyridinic nitrogen significantly decreases the overpotential (ca. ?0.18 V) and increases the selectivity (ca. 80 %) towards the formation of CO. The experimental results are in congruent with DFT calculations, which show that pyridinic defects retain a lone pair of electrons that are capable of binding CO₂. However, for graphitic‐like nitrogen, electrons are located in the π* antibonding orbital, making them less accessible for CO₂ binding.     

Polypeptide Modification of Multiwalled Carbon Nanotubes by a Graft‐From Approach

Summary: Covalent surface functionalization of carbon nanotubes with polypeptides is promising for possible medical applications. This work presents a graft‐from approach to perform the polypeptide modification of multiwalled carbon nanotubes (MWNTs). The raw MWNTs are first amine‐functionalized. The amine‐functionalized MWNTs are then used as the initiator to initiate the ring‐opening polymerization of γ‐benzyl‐L ‐glutamate N‐carboxyanhydride (BLG‐NCA), to result in the polypeptide‐grafted MWNTs. FT‐IR, XPS, and TGA data demonstrate that the functionalization is successful. The TEM images of the products show that the thickness of the polypeptide shell of the PBLG‐MWNT is about 4.5–22 nm. Using the facile route developed here, carbon nanotubes functionalized with other types of polypeptides can be easily fabricated using the corresponding NCAs.

Strategy for the polypeptide modification of multiwalled carbon nanotubes by a graft‐from approach.     

Sexithiophene Encapsulated in a Single‐Walled Carbon Nanotube: An In Situ Raman Spectroelectrochemical Study of a Peapod Structure

The interaction of single‐walled carbon nanotubes (SWCNTs) and α‐sexithiophene (6T) was studied by Raman spectroscopy and by in situ Raman spectroelectrochemistry. The encapsulation of 6T in SWCNT and its interaction causes a bleaching of its photoluminescence, and also small shifts of its Raman bands. The Raman features of the SWCNT with embedded 6T (6T‐peapods) change in both intensity and frequency compared to those of pristine SWCNT, which is a consequence of a change of the resonant condition. Electrochemical doping demonstrated that the electrode potential applied to the SWCNT wall causes changes in the embedded 6T. The effects of electrochemical charging on the Raman features of pristine SWCNT and 6T@SWCNT were compared. It is shown that the interaction of SWCNT with 6T also changes the electronic structure of SWCNT in its charged state. This change of electronic structure is demonstrated both for semiconducting and metallic tubes.     

Large Deformable Multiwalled Carbon Nanotube Core–Shell Structure on Polystyrene Beads

Multiwalled carbon nanotube (MWCNT)‐coated polystyrene (PS) beads have been prepared by dispersion polymerization followed by a layer‐by‐layer self‐assembly method. The concentration of carboxylic acid groups on the MWCNTs increased from 1.81 × 10²¹ to 3.43 × 10²² COO⁻ per g as the treatment time was increased from 3 to 9 h. The sulfonated polystyrene (SPS) beads changed from being negatively charged to positively charged when the cationic polyelectrolyte was self‐assembled on their surface. The surface morphology of the adsorbed polyelectrolyte was smooth without any aggregation and the thickness of the polyelectrolyte coating on the SPS beads was ≈0.6 µm. The electrical conductivity and resistance of the MWCNT‐coated SPS beads were measured to be 4.0 × 10⁻² S · cm⁻¹ and 12.8 Ω at a volume fraction of 91%, respectively.

     

Ionic Dopant‐Encapsulating Single‐Walled Carbon Nanotube Films with Metal‐Like Electrical Conductivity

Heavy doping is inevitable for utilizing single‐walled carbon nanotubes for wiring. However, the electrical conductivity of their films is currently as low as one tenth of the films made from typical metal pastes. Herein we report on metal‐comparable electrical conductivity from single‐walled carbon nanotube network films. We use ionic liquids and crown ether complexes for p‐type and n‐type doping, respectively. The encapsulation of counterions into carbon nanotubes promotes the conductivities in the range of 7000 S cm^?1, approximately ten times larger than those of undoped films.     

Carbon Nanotube and Gold‐Based Materials: A Symbiosis

Carbon nanotubes constitute a novel class of nanomaterials with potential applications in many areas. The attachment of metal nanoparticles to carbon nanotubes is new way to obtain novel hybrid materials with interesting properties for various applications such as catalysts and gas sensors as well as electronic and magnetic devices. Their unique properties such as excellent electronic properties, a good chemical stability, and a large surface area make carbon nanotubes very useful as a support for gold nanoparticles in many potential applications, ranging from advanced catalytic systems through very sensitive electrochemical sensors and biosensors to highly efficient fuel cells. Here we give an overview on the recent progress in this area by exploring the various synthesis approaches and types of assemblies, in which nanotubes can be decorated with gold nanoparticles and explore the diverse applications of the resulting composites.     

A Mediator‐Free Bienzyme Amperometric Biosensor Based on Horseradish Peroxidase and Glucose Oxidase Immobilized on Carbon Nanotube Modified Electrode

Multiwalled carbon nanotube (CNT) modified glassy carbon electrode immobilized with horseradish peroxidase (HRP) in Nafion coating showed direct electron transfer between HRP enzyme and the CNT‐modified electrode. A mediator‐free bienzyme glucose biosensor based on horseradish peroxidase and glucose oxidase was constructed. The bienzyme biosensor exhibited a high sensitivity for glucose detection at zero applied potential.