Differential Pulse Voltammetry Determination of Ofloxacin in Human Serum and Urine Based on a Novel Tryptophan‐graphene Oxide‐carbon Nanotube Electrochemical Sensor

The highly sensitive determination of ofloxacin (OFL) in human serum and urine was achieved on a novel tryptophan‐graphene oxide‐carbon nanotube (Trp‐GO‐CNT) composite modified glassy carbon electrode (Trp‐GO‐CNT/GCE). The Trp‐GO‐CNT composite was fabricated, and its morphologies and surface functional groups were characterized by field emission scanning electron microscopy (FE‐SEM) and Fourier transform infrared (FT‐IR) spectroscopy. The electrochemical properties of Trp‐GO‐CNT/GCE were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The superior electrochemical behaviors of Trp‐GO‐CNT/GCE toward OFL can be mainly assigned to the excellent electrocatalytic activity of Trp, the great conductivity and high surface area of GO and CNT, and the synergistic effect between Trp, GO and CNT. Under optimum conditions, a wide and valuable linear range (0.01–100 μM), a low detection limit (0.001 μM, S/N=3), a good linear relationship (R2>0.999), good stability and repeatability were obtained for the quantitative determination of OFL. Furthermore, the Trp‐GO‐CNT electrochemical sensor was successfully applied to the determination of OFL in human serum and urine samples, and satisfactory accuracy and recovery could be obtained.     

Facile preparation of carbon nanotube/poly(ethyl 2-cyanoacrylate) composite electrode by water-vapor-initiated polymerization for enhanced amperometric detection

A carbon nanotube/poly(ethyl 2-cyanoacrylate) (CNT/PECA) composite electrode was developed for enhanced amperometric detection. The composite electrode was fabricated on the basis of water-vapor-initiated polymerization of a mixture of CNTs and ethyl 2-cyanoacrylate in the bore of a piece of fused silica capillary. The morphology and structure of the composite were investigated by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. The results indicate that the CNTs were well dispersed and embedded throughout the PECA matrix to form an interconnected CNT network. The analytical performance of this unique CNT-based detector has been demonstrated by separating and detecting six flavones in combination with capillary electrophoresis. The advantages of the CNT/PECA composite detector include lower operating potential, higher sensitivity, low expense of fabrication, satisfactory resistance to surface fouling, and enhanced stability; these properties indicate great promise for a wide range of applications.     

Carbon nanotube/poly(methyl methacrylate) (CNT/PMMA) composite electrode fabricated by in situ polymerization for microchip capillary electrophoresis

We describe the development and application of a novel carbon nanotube/poly(methyl methacrylate) (CNT/PMMA) composite electrode as a sensitive amperometric detector of microchip capillary electrophoresis (CE). The composite electrode was fabricated by the in situ polymerization of a mixture of CNTs and prepolymerized methyl methacrylate in the microchannel of a piece of fused silica capillary under heat. The performance of this unique system was demonstrated by the separation and detection of phenolic pollutants and purines. The new CNT-based CE detector offered significantly lower operating potentials, yielded substantially enhanced signal-to-noise characteristics, and exhibited resistance to surface fouling and, hence, enhanced stability. Long-term stability and reproducibility with relative standard deviations of less than 5 % for the peak current (n=20) were also demonstrated. The simplicity and significant performance exhibited by the CNT/PMMA composite electrode indicate great promise for conventional CE, flowing-injection analysis, and other microfluidic analysis systems.     

Graphene Nanoplatelets: Electrochemical Properties and Applications for Oxidation of Endocrine‐Disrupting Chemicals

In most graphene‐based electrochemical applications, graphene nanoplatelets (GNPs) have been applied. Now, for the first time, electrochemical properties of GNPs, namely, its electrochemical activity, potential window, and double‐layer capacitance, have been investigated. These properties are compared with those of carbon nanotubes (CNTs). GNP‐ and CNT‐coated electrodes were then applied for electrochemical oxidation of endocrine‐disrupting chemicals. The GNP‐coated electrode was characterized by atomic force microscopy and electrochemical techniques. Compared with the CNT‐coated electrode, higher peak current for the oxidation of 4‐nonylphenol is achieved on the GNP‐coated electrode, together with lower capacitive current. Electrochemical oxidation of 2,4‐dichlorophenol, bisphenol A, and octylphenol in the absence or presence of 4‐nonylphenol was studied on the GNP‐coated electrode. The results suggest that GNPs have better electrochemical performance than CNTs and are thus more promising for electrochemical applications, for example, electrochemical detection and removal of endocrine‐disrupting chemicals.     

Fabrication and Characterization of Carbon Nanotube‐Hydroxyapatite Nanocomposite: Application to Anodic Stripping Voltammetric Determination of Cadmium

A novel chemically modified electrode for stripping determination of cadmium is presented in this paper, based on carbon nanotube‐hydroxyapatite (CNT‐HAP) nanocomposite, which can be prepared by an easy and effective one‐step sonication. The newly synthesized nanocomposite was characterized with FTIR, TEM, and electrochemical methods. Due to the combination of the strong absorption ability of HAP and excellent electroanalytical properties of CNTs, the GC/CNT‐HAP electrode has been successfully used for determination of Cd²⁺ by anodic stripping voltammetry with a linear range of 20 nM–3 μM. The sensitivity and detection limit are 25.6 μA/μM and 4 nM, respectively. The practical application of the proposed electrode has been carried out for the determination of trace levels of Cd²⁺ in real water samples.     

Carbon-nanotube-alginate composite modified electrode fabricated by in situ gelation for capillary electrophoresis

This report describes the development and the application of a novel carbon-nanotube (CNT)-alginate composite modified electrode as a sensitive amperometric detector for capillary electrophoresis (CE). The composite electrode was fabricated on the basis of in situ gelation of a mixture of CNTs and sodium alginate on the surface of a carbon disc electrode in aqueous calcium chloride solution. SEM, energy-dispersive spectroscopy, XRD, and FTIR spectroscopy offered insights into the nature of the novel composite. The results indicated that the CNTs were well dispersed and embedded throughout the alginate matrix to form an interconnected carbon-nanotube network on the base electrode. The performance of this unique CNT-based detector has been demonstrated, in conjunction with CE, by separating and detecting five caffeic acid derivatives. The new CNT-based CE detector offered significantly lower operating potentials, substantially enhanced signal-to-noise characteristics, and a lower expense of operation. The simplicity and significant performance exhibited by the CNT-alginate composite modified electrode also indicate great promise for the use of this electrode in microchip CE, flowing-injection analysis, and other microfluidic analysis systems.     

Graphene/poly(ethylene‐co‐vinyl acetate) composite electrode fabricated by melt compounding for capillary electrophoretic determination of flavones in Cacumen platycladi

In this report, a graphene/poly(ethylene‐co‐vinyl acetate) composite electrode was fabricated by melt compounding for the amperometric detection of capillary electrophoresis. The composite electrode was fabricated by packing a mixture of graphene and melted poly(ethylene‐co‐vinyl acetate) in a piece of fused silica capillary under heat. The structure of the composite was investigated by scanning electron microscopy and Fourier transform infrared spectroscopy. The results indicated that graphene sheets were well dispersed in the composite to form an interconnected conducting network. The performance of this unique graphene‐based detector has been demonstrated by separating and detecting rutin, quercitrin, kaempferol, and quercetin in Cacumen platycladi in combination with capillary electrophoresis. The four flavones have been well separated within 9 min in a 50‐cm‐long capillary at a separation voltage of 12 kV using a 50 mM sodium borate buffer (pH 9.2). The graphene‐based detector offered significantly lower operating potentials, substantially enhanced signal‐to‐noise characteristics, lower expense of operation, high resistance to surface fouling, and enhanced stability. It showed long‐term stability and repeatability with relative standard deviations of <5% for the peak current (n = 15).     

Synthesis,characterization, and application of silver nanoparticle‐thiophenol nanocomposite film on the glassy carbon surface

In this study, 4‐thiophenol modified glassy carbon electrode was prepared by the reduction of 4‐diazothiophenol tetrafluoroborate salt. Silver nanoparticles were attached to the thiophenol modified surface to prepare a thiophenol‐silver nanoparticle composite film. 4‐Aminothiopenol molecules were deposited by self‐assembling technique to form multi‐layered nanofilms of TP/SNP/PhNH₂ on glassy carbon substrate. These surfaces were characterized by cyclic voltammetry, electrochemical impedance spectroscopy, X‐ray photoelectron spectroscopy, reflectance‐absorption infrared spectroscopy, and ellipsometry at each multilayer film growth process. Atomic force microscopic images of GC/TP/SNP/PhNH₂ surfaces were also acquired. The characterization methods show that the amine group containing surface permits the subsequent modification by a variety of coupling reactions for the immobilization of more complex systems. An application of the electrode modification for the determination of uric acid with a significantly lower detection limit is described. Copyright © 2013 John Wiley & Sons, Ltd.     

Dye‐Sensitized Solar Cell Counter Electrodes Based on Carbon Nanotubes

Dye‐sensitized solar cells (DSSCs) have received significant attention from the scientific community since their discovery in 1991. However, the high cost and scarcity of platinum has motivated researchers to seek other suitable materials for the counter electrode of DSSCs. Owing to their exceptional properties such as high conductivity, good electrochemical activity, and low cost, carbon nanotubes (CNTs) have been considered as promising alternatives to expensive platinum (Pt) in the counter electrode of DSSCs. Herein, we provide a Minireview of the CNTs use in the counter electrode of DSSCs. A brief overview of Pt‐based counter electrodes is also discussed. Particular attention is given to the recent advances of counter electrodes with CNT‐based composite structures.     

A Self‐assembled L‐Cysteine and Electrodeposited Gold Nanoparticles‐reduced Graphene Oxide Modified Electrode for Adsorptive Stripping Determination of Copper

Glassy carbon electrode (GCE) modified with L‐cysteine and gold nanoparticles‐reduced graphene oxide (AuNPs‐RGO) composite was fabricated as a novel electrochemical sensor for the determination of Cu²⁺. The AuNPs‐RGO composite was formed on GCE surface by electrodeposition. The L‐cysteine was decorated on AuNPs by self‐assembly. Physicochemical and electrochemical properties of L‐cysteine/AuNPs‐RGO/GCE were characterized by scanning electron microscopy, atomic force microscopy, energy dispersive spectroscopy, Raman spectroscopy, X‐ray diffraction, cyclic voltammetry and adsorptive stripping voltammetry. The results validated that the prepared electrode had many attractive features, such as large electroactive area, good electrical conductivity and high sensitivity. Experimental conditions, including electrodeposition cycle, self‐assembly time, electrolyte pH and preconcentration time were studied and optimized. Stripping signals obtained from L‐cysteine/AuNPs‐RGO/GCE exhibited good linear relationship with Cu²⁺ concentrations in the range from 2 to 60 μg L⁻¹, with a detection limit of 0.037 μg L⁻¹. Finally, the prepared electrode was applied for the determination of Cu²⁺ in soil samples, and the results were in agreement with those obtained by inductively coupled plasma mass spectrometry.     

The preparation and performance of flocculent polyaniline/carbon nanotubes composite electrode material for supercapacitors

Polyaniline (PANI)/carbon nanotubes (CNTs) composite electrode material was prepared by in situ chemical polymerization. The structure and morphology of PANI/CNTs composite are characterized by Fourier infrared spectroscopy, scanning electron microscope, and transmission electron microscopy. It has been found that a flocculent PANI was uniformly deposited on the surface of CNTs. The supercapacitive behaviors of the PANI/CNTs composite materials are investigated with cyclic voltammetry, galvanostatic charge/discharge, impedance, and cycle life measurements. The results show that the PANI/CNTs composite electrodes have higher specific capacitances than CNT electrodes and better stability than the conducting polymers. The capacitance of PANI/CNTs composite electrode is as high as 837.6 F g⁻¹ measured by cyclic voltammetry at 1 mV s⁻¹. Besides, the capacitance retention of coin supercapacitors remained 68.0% after 3,000 cycles.     

Electroanalytical Detection of n‐Butylamine at a Nickel/Carbon Nanotube Composite

We report the electroanalytical detection of n‐butylamine at a nickel/carbon nanotube (Ni/CNT) composite. Scanning Electron Microscopy (SEM) characterisation of the composite demonstrated that it consisted of bulk nickel particles ca. 2 μm in diameter entangled in CNT bundles. The spontaneous formation of Ni(OH)₂ was optimised, and comparison with a 3 mm nickel electrode showed that ca. 4 μg of the Ni/CNT composite cast on a 3 mm GC electrode possessed bulk nickel characteristics while also having higher activity and higher sensitivity towards the electrochemical detection of n‐butylamine. However, the Ni/CNT composite showed no response to ammonia, in contrast to the macro‐nickel‐electrode.     

Carbon nanotube/metal oxide dispersed poly(ortho‐aminophenol) as a ternary nanocomposite film: Facile electrosynthesis,surface characterization,and electrochemical pseudocapacitive performance

In this work, CNT‐NiCo₂O₄ was first synthesized via a chemical strategy in order to fabricate the ternary nanocomposite of a p‐type conductive polymer. Subsequently, hybrid poly(o‐aminophenol) POAP/CNT‐NiCo₂O₄ ternary composite films were prepared via the electropolymerization of POAP in the presence of CNT‐NiCo₂O₄ to be used in electrochemical storage devices as the active electrode. Electrochemical analyses including galvanostatic charge–discharge experiments, cyclic voltammetry, and electrochemical impedance spectroscopy were conducted to study the system performance. Furthermore, surface analyses were carried out to characterize the POAP/CNT‐NiCo₂O₄ composite film. Novel nanocomposite materials with the merits of extraordinarily high active surface area, ease of fabrication, and superior stability in aqueous electrolytes are presented for use in electrochemical redox capacitors.     

Effects of CNT‐film Pretreatment on the Characteristics of NiCo2O4/CNT Core‐shell Hybrids as Electrode Material for Electrochemistry Capacitor

NiCo₂O₄/CNT nanocomposite films were fabricated by in‐situ growing ultrafine NiCo₂O₄ nanoparticles on acid‐modified carbon nanotube (CNT) films. The effects of CNT‐film pretreatment were investigated thoroughly by various characterization outfits including Fourier Transform Infrared spectroscopy (FT‐IR), X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, RTS‐9 four‐point probes resistivity measurement system, X‐ray powder diffraction (XRD), scanning electron microscopy (SEM) and CHI660D electrochemical workstation. These results suggested that carbon nanotubes were uniformly wrapped by NiCo₂O₄ nanoparticles forming a hierarchical core‐shell structure. And the crystallinity, conductivity of the CNTs and detail structure (both morphology and size) of the NiCo₂O₄ nanoparticles varied with prolonged acid treatment time which resulted in increased functional groups and defects on CNT films and further affected the electrochemical properties. The composite film composed of the CNT film pretreated by mixed acid for 12 h exhibited excellent electrochemical properties: 828 F/g at 1 A/g and 656 F/g at 20 A/g, and maintained over 99 % of its capacitance after 3000 cycles of charge/discharge at 5 A/g. Acid treatment for either too long or too short is detrimental to the electrochemical properties of the composite films. Such work should be of fundamental importance for tailoring electrochemical properties by elaborate design of acid treatment on CNTs.     

Pyrene‐Anderson‐Modified CNTs as Anode Materials for Lithium‐Ion Batteries

An organo‐functionalized polyoxometalate (POM)–pyrene hybrid (Py‐Anderson) has been used for noncovalent functionalization of carbon nanotubes (CNTs) to give a Py‐Anderson‐CNT nanocomposite through π–π interactions. The as‐synthesized nanocomposite was used as the anode material for lithium‐ion batteries, and shows higher discharge capacities and better rate capacity and cycling stability than the individual components. When the current density was 0.5 mA cm^?2, the nanocomposite exhibited an initial discharge capacity of 1898.5 mA h g^?1 and a high discharge capacity of 665.3 mA h g^?1 for up to 100 cycles. AC impedance spectroscopy provides insight into the electrochemical properties and the charge‐transfer mechanism of the Py‐Anderson‐CNTs electrode.     

Facile preparation of carbon nanotube-conducting polymer network for sensitive electrochemical immunoassay of Hepatitis B surface antigen in serum

A novel electrochemical immunosensor built on three dimensional carbon nanotube-conducting polymer (CNT-CP) network is reported for detection of Hepatitis B surface antigen (HBsAg) in human serum. The CNT-CP network is prepared by drop-drying of CNT solution on glassy carbon electrode, followed by electrochemical polymerization of poly (pyrrole propionic acid) (pPPA) film to crosslink and stabilize the CNTs, wherein the CNTs form the backbone of the network, and offer great specific surface areas for antibody attachment, and confer good conductivity for electrochemical detection, while the conducting film integrates the carbon nanotubes into a stable network due to its self-limiting growth behavior and provides abundant carboxyl groups for covalent immobilization of probe proteins. As a unique matrix, the CNT-CP network enables sensitive electrochemical detection of HBsAg biomarker by using alkaline phosphatase (ALP)-conjugated secondary antibodies under sandwich format coupling with the ALP substrate solution, p-aminophenyl phosphate (PAPP), reaching a detection limit of 0.01ng/mL with a dynamic range of 5 orders of magnitude.     

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.     

Copper‐based Metal‐organic Framework for Non‐enzymatic Electrochemical Detection of Glucose

A non‐enzymatic electrochemical glucose sensor based on a Cu‐based metal‐organic framework (Cu‐MOF) modified electrode was developed. The Cu‐MOF was prepared by a simple ionothermal synthesis, and the characterizations of the Cu‐MOF were studied by Fourier transform infrared spectroscopy (FT‐IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), single‐crystal X‐ray powder diffraction (SCXRD), and X‐ray powder diffraction (XRD). Electrochemical behaviors of the Cu‐MOF modified electrode to glucose were measured by differential pulse voltammetry (DPV). The electrochemical results showed that the Cu‐MOF modified electrode exhibited an excellent electro‐catalytic oxidation towards glucose in the range of 0.06 μM to 5 mM with a sensitivity of 89 μA/mM cm² and a detection limit of 10.5 nM. Moreover, the fabricated sensor showed a high selectivity to the oxidation of glucose in coexistence with other interferences. The sensor was satisfactorily applied to the determination of glucose in urine samples. With the significant electrochemical performances, MOFs may provide a suitable platform in the construction of kinds of electrochemical sensors and/or biosensors and hold a great promise for sensing applications.     

Electrochemical Sensor Based on Thioether Oligomer Poly(N‐vinylpyrrolidone)‐modified Gold Electrode for Bisphenol A Detection

Presently, bisphenol A (BPA) has been added to the list of substances of very high concern as endocrine disruptors. According to the literature, exposure to bisphenol A even at low doses may result in adverse health effects. In this study, electrochemical sensor of Bisphenol A based on thioether DDT‐Poly(N‐vinylpyrrolidone) oligomer has been developed. The thioether oligomer, which is capable of recognizing BPA, was prepared and used for gold electrode modification. The characterization of the modified gold electrode and the synthesized thioether oligomer were carried out by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), Fourier‐transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (¹H NMR) and Size exclusion chromatography (SEC). Obtained results indicate that the modified electrode shows good electrochemical activity, good sensitivity and reproducibility for BPA detection. It exhibited a good linear relationship ranging from 1 to 20 pg/mL, and the detection limit was found to be 1.9 pg/mL at S/N=3. Several interfering species such as hydroquinone, phenol and resorcinol were used and their behaviors on the modified gold electrode were investigated.     

Graphene Paper with Sharp‐edged Nanorods of Fe−CuMOF as an Excellent Electrode for the Simultaneous Detection of Catechol and Resorcinol

A flexible composite paper Fe?Cu‐based metal‐organic framework (MOF)/reduced graphene oxide (rGO) (Fe?CuMOF/rGO) electrode was prepared by using a simple electrochemical method for the simultaneous detection of catechol (CC) and resorcinol (RC). Free‐standing, flexible and double‐sided Fe?CuMOF/rGO composite paper was obtained by applying the electrochemical deposition process on the rGO paper electrode in the solution containing Fe?CuMOF composite. The morphological analysis of Fe?CuMOF/rGO composite paper showed that sea urchin‐like structures formed on the rGO electrode surface consist of numerous sharp‐edged nanorods of Fe?CuMOF. Flexible Fe?CuMOF/rGO paper electrode exhibited high sensitivity, wide linear range and low detection limit for the simultaneous determination of CC and RC. The linear ranges of concentration for CC and RC were 0.1–800 and 0.1–720 μM, respectively, and the corresponding limits of detection (S/N=3) were 0.016 and 0.020 μM. The outstanding performance of this flexible electrode could be attributed to the sharp‐edged urchin‐like Fe?CuMOF structures which provide an increment of the surface area and the electrochemical activity of the composite paper electrode. Stability tests showed that Fe?CuMOF/rGO composite paper electrode has excellent flexibility, high durability, and good reproducibility. Furthermore, this electrode exhibited high sensitivity and selectivity for the determination of CC and RC in real sample analysis.