Functionalized carbon nanotubes and nanofibers for biosensing applications

This review summarizes recent advances in electrochemical biosensors based on carbon nanotubes (CNTs) and carbon nanofibers (CNFs) with an emphasis on applications of CNTs. CNTs and CNFs have unique electric, electrocatalytic and mechanical properties, which make them efficient materials for developing electrochemical biosensors.We discuss functionalizing CNTs for biosensors. We review electrochemical biosensors based on CNTs and their various applications (e.g., measurement of small biological molecules and environmental pollutants, detection of DNA, and immunosensing of disease biomarkers). Moreover, we outline the development of electrochemical biosensors based on CNFs and their applications. Finally, we discuss some future applications of CNTs.     

Carbon nanotube dielectrophoresis: Theory and applications

Carbon nanotubes (CNTs) are one of the most extensively studied nanomaterials in the 21st century. Since their discovery in 1991, many studies have been reported advancing our knowledge in terms of their structure, properties, synthesis, and applications. CNTs exhibit unique electrothermal and conductive properties which, combined with their mechanical strength, have led to tremendous attention of CNTs as a nanoscale material in the past two decades. To introduce the various types of CNTs, we first provide basic information on their structure followed by some intriguing properties and a brief overview of synthesis methods. Although impressive advances have been demonstrated with CNTs, critical applications require purification, positioning, and separation to yield desired properties and functional elements. Here, we review a versatile technique to manipulate CNTs based on their dielectric properties, namely dielectrophoresis (DEP). A detailed discussion on the DEP aspects of CNTs including the theory and various technical microfluidic realizations is provided. Various advancements in DEP-based manipulations of single-walled and multiwalled CNTs are also discussed with special emphasis on applications involving separation, purification, sensing, and nanofabrication.     

Carbon nanotube/poly(methyl methacrylate) composite electrode for capillary electrophoretic measurement of honokiol and magnolol in Cortex Magnoliae Officinalis

This paper describes the development and the application of a novel carbon nanotube/poly(methyl methacrylate) (CNT/PMMA) composite electrode as a sensitive amperometric detector of CE. The composite electrode was fabricated on the basis of the in situ polymerization of a mixture of CNT and prepolymerized methylmethacrylate in the microchannel of a piece of fused-silica capillary under heat. The performance of this unique system has been demonstrated by separating and detecting honokiol and magnolol in traditional Chinese medicine, Cortex Magnoliae Officinalis. Factors influencing their separation and detection processes were examined and optimized. Honokiol and magnolol were well separated within 7 min in a 40 cm long capillary at a separation voltage of 15 kV using a 50 mM borate buffer (pH 9.2). The new CNT-based CE detector offered significantly lower operating potentials, yielded substantially enhanced S/N characteristics, and exhibited resistance to surface fouling and hence enhanced stability. It demonstrated long-term stability and reproducibility with RSDs of less than 5% for the peak current (n = 9) and should also find a wide range of applications in microchip CE, flowing injection analysis, and other microfluidic analysis systems.     

Carbon nanotube field-effect transistor detector associated to gas chromatography for speciation of benzene,toluene, ethylbenzene, (o-, m- and p-)xylene

An analytical methodology based on a field-effect transistor detector using carbon nanotubes (NTFET) coupled to a gas chromatograph has been developed for the speciation of the following aromatic compounds: benzene, toluene, ethylbenzene, m-xylene, p-xylene and o-xylene (BTEX). This methodology combines the proven separation capability of gas chromatography (GC) with the potential for detection of a NTFET. The developed analyzer shows a high and stable analytical response upon repeated analysis of BTEX during 4 weeks, with detection limit less than 4 μg/L. The GC–NTFET system also shows a great suitability for actual monitoring of indoor atmospheres and no significant difference was observed between the results obtained by the developed analyzer and a more classical analytical methodology, namely gas chromatography–flame ionization detection (GC–FID).     

Carbon nanotubes for electrochemical biosensing

The aim of this review is to summarize the most relevant contributions in the development of electrochemical (bio)sensors based on carbon nanotubes in the last years.Since the first application of carbon nanotubes in the preparation of an electrochemical sensor, an increasing number of publications involving carbon nanotubes-based sensors have been reported, demonstrating that the particular structure of carbon nanotubes and their unique properties make them a very attractive material for the design of electrochemical biosensors.The advantages of carbon nanotubes to promote different electron transfer reactions, in special those related to biomolecules; the different strategies for constructing carbon nanotubes-based electrochemical sensors, their analytical performance and future prospects are discussed in this article.     

Phosphoryl choline-grafted water-soluble carbon nanotube

Water-soluble property is the precondition of biomedical evaluation and application of carbon nanotube （CNT）. Novel water- soluble CNT was synthesized in this letter by grafting phosphoryl choline （PC） onto multi-wall CNTs. Utilizing FTIR, XPS, TGA and TEM, the title CNTs were characterized and it was found that the target products could facilely dissolve in water. 2007 Tao Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.     

Flow injection based microfluidic device with carbon nanotube electrode for rapid salbutamol detection

A microfabicated flow injection device has been developed for in-channel electrochemical detection (ECD) of a β-agonist, namely salbutamol. The microfluidic system consists of PDMS (polydimethylsiloxane) microchannel and electrochemical electrodes formed on glass substrate. The carbon nanotube (CNT) on gold layer as working electrode, silver as reference electrode and platinum as auxiliary electrode were deposited on a glass substrate. Silver, platinum, gold and stainless steel catalyst layers were coated by DC-sputtering. CNTs were then grown on the glass substance by thermal chemical vapor deposition (CVD) with gravity effect and water-assisted etching. 100-μm-deep and 500-μm-wide PDMS microchannels fabricated by SU-8 molding and casting were then bonded on glass substrate by oxygen plasma treatment. Flow injection and ECD of salbutamol was performed with the amperometric detection mode for in-channel detection of salbutamol. The influences of flow rate, injection volume, and detection potential on the response of current signal were optimized. Analytical characteristics, such as sensitivity, repeatability and dynamic range have been evaluated. Fast and highly sensitive detection of salbutamol have been achieved. Thus, the proposed combination of the efficient CNT electrode and miniaturized lab-on-a-chip is a powerful platform for β-agonists detection.     

碳纳米管薄膜晶体管中的接触电阻效应

利用不同功函数的金属作为接触电极,研究了网络状碳纳米管薄膜晶体管(CNT-TFT)的接触电阻效应。研究表明金属Pd与碳纳米管薄膜形成良好的欧姆接触, Au则形成近欧姆接触,这两种接触的器件的开态电流和迁移率较高。Ti和Al都与碳纳米管薄膜形成肖特基接触,且Al接触比Ti接触的势垒更高,接触电阻也更大,相应器件的开态电流和迁移率都较低。该结果表明对于CNT-TFT仍然可以通过接触来调控器件的性能,这对CNT-TFT的实用化进程具有重要的促进作用。     

Carbon nanotube and diamond as electrochemical detectors in microchip and conventional capillary electrophoresis

As two important polymorphs of carbon, carbon nanotube (CNT) and diamond have been widely employed as electrode materials for electrochemical sensing. This review focuses on recent advances and the key strategies in the fabrication and application of electrochemical detectors in microchip and conventional capillary electrophoresis (CE) using CNT and boron-doped diamond. The subjects covered include CNT-based electrochemical detectors in microchip CE, CNT-based electrochemical detectors in conventional CE, boron-doped diamond electrochemical detectors in microchip CE, and boron-doped diamond electrochemical detectors in conventional CE. The attractive properties of CNT and boron-doped diamond make them very promising materials for the electrochemical detection in microchip and conventional CE systems and other microfluidic analysis systems.     

Nanomaterial-based electrochemical biosensors for medical applications

With recent advances in nanotechnology, great progress has been made in biosensors based on nanomaterials, but there are still numerous challenges to overcome. We describe nanomaterial-based biosensors for researchers new to the field, paying particular attention to metal nanoparticles and carbon nanotube (CNT)-based label-free approaches. Label-free monitoring of biorecognition events provides a promising platform, which is simple, cost-effective, and requires no external modification to biomolecules. Using examples from recent reports, we illustrate the diversity of biological recognition events and the range of experimental techniques employed for metal-nanoparticle-based and label-free characterization.     

Fabrication of polyaniline/carbon nanotube composite modified electrode and its electrocatalytic property to the reduction of nitrite

A polyaniline (PANI)/carbon nanotubes (CNTs) composite modified electrode was fabricated by galvanostatic electropolymerization of aniline on multi-walled carbon nanotubes (MWNTs)-modified gold electrode. The electrode thus prepared exhibits enhanced electrocatalytic behavior to the reduction of nitrite and facilitates the detection of nitrite at an applied potential of 0.0 V. Although the amperometric responses toward nitrite at MWNTs/gold and PANI/gold electrodes have also been observed in the experiments, these responses are far less than that obtained at PANI/MWNTs/gold electrode. The effects of electropolymerization time, MWNTs concentration and pH value of the detection solution on the current response of the composite modified electrode toward sodium nitrite, were investigated and discussed. A linear range from 5.0 × 10⁻⁶ to 1.5 × 10⁻² M for the detection of sodium nitrite has been observed at the PANI/MWNTs modified electrode with a sensitivity of 719.2 mA M⁻¹ cm⁻² and a detection limit of 1.0 μM based on a signal-to-noise ratio of 3.     

Review: Carbon nanotube based electrochemical sensors for biomolecules

Carbon nanotubes (CNTs) have been incorporated in electrochemical sensors to decrease overpotential and improve sensitivity. In this review, we focus on recent literature that describes how CNT-based electrochemical sensors are being developed to detect neurotransmitters, proteins, small molecules such as glucose, and DNA. Different types of electrochemical methods are used in these sensors including direct electrochemical detection with amperometry or voltammetry, indirect detection of an oxidation product using enzyme sensors, and detection of conductivity changes using CNT-field effect transistors (FETs). Future challenges for the field include miniaturizing sensors, developing methods to use only a specific nanotube allotrope, and simplifying manufacturing.     

Carbon nanotube disposable detectors in microchip capillary electrophoresis for water-soluble vitamin determination: analytical possibilities in pharmaceutical quality control

In this work, the synergy of one mature example from "lab-on-chip" domain, such as CE microchips with emerging miniaturized carbon nanotube detectors in analytical science, is presented. Two different carbon electrodes (glassy carbon electrode (GCE) 3 mm diameter, and screen-printed electrode (SPE) 0.3 mm x 2.5 mm) were modified with multiwalled carbon nanotubes (MWCNTs) and their electrochemical behavior was evaluated as detectors in CE microchip using water-soluble vitamins (pyridoxine, ascorbic acid, and folic acid) in pharmaceutical preparations as representative examples. The SPE modified with MWCNT was the best electrode for the vitamin analysis in terms of analytical performance. In addition, accurate determination of the three vitamins in four different pharmaceuticals was obtained (systematic error less than 9%) in only 400 s using a protocol that combined the sample analysis and the methodological calibration.     

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.     

生长中纳米碳管的电子结构

利用第一原理的离散变分方法计算了生长中纳米碳管团簇的电子结构,发现悬挂键的存在导致管口处的电子结构明显地不同于管体,对应的非键合价电子不仅直接改变管口处碳原子之间的键合模式,而且进一步增强管口处碳原子之间的结合,促使碳原子只向管芯移动,这与实验中观察到的封闭顶端的几何形态相一致;而且降低碳管的化学稳定性.与完整型纳米碳管截然不同的电子结构决定了生长中纳米碳管在合成纳米材料过程中具有不同的作用行为.     

Double toroids as model systems for carbon nanotube junctions: through-bond currents

We show that the through-bond currents in a closed molecular network originate from their topologically invariant edge-homologies. When applied to double toroidal structures they give rise to topologically induced currents whose 3D manifestations are highly sensitive to the way both loops are merged together i.e. on the nature of their junction.     

A hyaluronic acid dispersed carbon nanotube electrode used for a mediatorless NADH sensing and biosensing

A biocompatible nanocomposite consisting of single-walled carbon nanotubes (CNTs) dispersed in a hyaluronic acid (HA) was investigated as a sensing platform for a mediatorless electrochemical detection of NADH. The device was characterised by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and extensively by electrochemistry. CNT-HA bionanocomposite showed more reversible electrochemistry, higher short-term stability of NADH sensing and higher selectivity of NADH detection compared to frequently used CNT-CHI (chitosan) modified GCE. Finally the performance of the sensor modified by CNT-HA was tested in a batch and flow injection analysis (FIA) mode of operation with basic characteristics revealed. The NADH sensor exhibits a good long-term operational stability (95% of the original sensitivity after 22 h of continuous operation). Subsequently a d-sorbitol biosensor based on such a nanoscale built interface was prepared and characterised with a d-sorbitol dehydrogenase used as a biocatalyst.     

Voltammetry and amperometric detection of tetracyclines at multi-wall carbon nanotube modified electrodes

The voltammetric behaviour and amperometric detection of tetracycline (TC) antibiotics at multi-wall carbon nanotube modified glassy carbon electrodes (MWCNT-GCE) are reported. Cyclic voltammograms of TCs showed enhanced oxidation responses at the MWCNT-GCE with respect to the bare GCE, attributable to the increased active electrode surface area. Hydrodynamic voltammograms obtained by flow-injection with amperometric detection at the MWCNT-GCE led us to select a potential value E _det = +1.20 V. The repeatability of the amperometric responses was much better than that achieved with bare GCE (RSD ranged from 7 to 12%), with RSD values for i _p of around 3%, thus demonstrating the antifouling capability of MWCNT modified electrodes. An HPLC method with amperometric electrochemical detection (ED) at the MWCNT-GCE was developed for tetracycline, oxytetracycline (OTC), chlortetracycline and doxycycline (DC). A mobile phase consisting of 18:82 acetonitrile/0.05 mol L⁻¹ phosphate buffer of pH 2.5 was selected. The limits of detection ranged from 0.09 μmol L⁻¹ for OTC to 0.44 μmol L⁻¹ for DC. The possibility to carry out multiresidue analysis is demonstrated. The HPLC-ED/MWCNT-GCE method was applied to the analysis of fish farm pool water and underground well water samples spiked with the four TCs at 2.0 × 10⁻⁷ mol L⁻¹. Solid-phase extraction was accomplished for the preconcentration of the analytes and clean-up of the samples. Recoveries ranged from 87 ± 6 to 99 ± 3%. Under preconcentration conditions, limits of detection in the water samples were between 0.50 and 3.10 ng mL⁻¹.     

碳纳米管激光等离子体的质谱研究

The time-of-flight mass spectra of carbon nanotube plasma produced by laser was first investigated in this paper. We found the hemi-spherical tips of carbon nanotube were easily fragmentated and aggregated into fullerenes between Coo arid C_(174) in high laser fluence.     

Carbon nanospheres enhanced electrochemiluminescence of CdS quantum dots for biosensing of hypoxanthine

This work developed a novel method to greatly enhance the electrochemiluminescence (ECL) of CdS quantum dots (QDs). The ECL amplification was achieved by the assembly of QDs on poly (diallyldimethylammonium chloride)-functionalized carbon nanospheres (PFCNSs), and successfully employed for sensitive ECL biosensing of oxidase substrates. The carbon nanospheres were prepared by a “green” method, and the high loading of QDs on carbon nanospheres led to a 4-times increased ECL intensity with dissolved O₂ as the coreactant. Using xanthine oxidase (XOD) as a model, an ECL biosensor was fabricated by immobilizing the enzyme on the mixing membrane of PFCNSs and QDs. The ECL biosensor showed a fast response to hypoxanthine with a linear concentration range from 2.5 × 10⁻⁸ to 1.4 × 10⁻⁵ M. The limit of detection was 5 nM at a signal-to-noise ratio of 3. The assay results of hypoxanthine in fish samples were in a good agreement with the reference values by amperometric technique. This facile approach to prepare the PFCNSs/QDs system for ECL biosensing could be of promising application in bioanalysis and electronic device.