Surface activation of plasma-patterned carbon nanotube based DNA sensing electrodes

We have studied the effect of treatment of multiwalled carbon nanotubes (MWCNTs) for use in DNA-based biosensors with oxygen plasma. Well-patterned MWCNT electrodes were photolithographically fabricated on glass substrates. Pure oxygen was used for etching and functionalization of the MWCNT film in a lab-made plasma chamber. The resulting electrodes exhibited a dramatic change in the morphology of their surface, the chemical composition, and the electrochemical properties in terms of peak current and peak potential separation. The electrodes also showed increased DNA immobilization efficiency and much higher sensitivity in the detection of target DNA as compared to non-treated MWCNT electrodes. Plasma treatment was optimized and electrodes were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and differential pulse voltammetry.

New strategies for the chemical characterization of multi-walled carbon nanotubes and their derivatives

Industrially relevant characterization of multi-walled carbon nanotubes (MWCNT) is still a challenging task. The aim of this work is to show novel and fast concepts for the chemical characterization of carbon nanotubes (CNT) by a combination of analytical techniques. Information obtained by individual tools like Fourier transform infrared spectroscopy (FTIR), attenuated total reflection infrared spectroscopy or Raman spectroscopy is not providing a full picture of the functionalization of MWCNTs. However, a combination of tools such as FTIR or mass spectrometry with thermogravimetric methods proved to be very useful. Sample preparation for FTIR and Raman spectroscopy is another focus of this contribution because of its strong effect on the results obtained. We also are suggesting methods for sample preparation that lead to highly reproducibility results. Measurements have been carried out on typical CNT samples such as commercially available pristine, carboxylated and amino-functionalized MWCNTs, and on polystyrenegrafted MWCNTs. The results may serve as a guidance for the qualitative and quantitative characterization of CNT.

Electrocatalytic oxidation of hydrazine at a glassy carbon electrode modified with nickel ferrite and multi-walled carbon nanotubes

A hydrothermal technique was used to synthesize nickel ferrite nanoparticles (NF-NPs) deposited on multi-walled carbon nanotubes (MWCNTs). The material was characterized by scanning electron microscopy, energy dispersive spectrometry, and X-ray powder diffraction which showed that the NF-NPs are located on the surface of the carboxylated MWCNTs. The material was used to modify a glassy carbon electrode which then was characterized via cyclic voltammetry, electrochemical impedance spectroscopy, and amperometry. The electrode displays strong electrochemical response to hydrazine. A potential hydrazine sensing scheme is suggested.

Preparation and characterization of methylene blue-SDS- multiwalled carbon nanotubes nanocomposite for the detection of hydrogen peroxide

A nanocomposite was prepared by physical adsorption of?(cationic) methylene blue (MB) on (anionic) sodium dodecylsulfate (SDS) that was wrapped on multiwalled carbon nanotubes (MWCNTs) on the surface of a glassy carbon electrode. This electrostatic interaction enables electrical communication between the electrode and analyte. Horseradish peroxidase was then immobilized in a film of gelatin on the nanocomposite to form a biosensor for hydrogen peroxide. Scanning electron microscopy, transmission electron microscopy, Fourier transform infrared and UV?Cvis spectrometry, and cyclic voltammetry were applied to characterize the electrode. The addition of both MWCNTs and MB causes a synergistic effect and leads to a large signal enhancement. The prepared nanocomposite material modified sensor shows better response in presence of several interferences. The biosensor has detection limit of 5 nM of hydrogen peroxide (at S/N?=?3) with a linear response between 0.2???M and 1.4?mM. Its lifetime is >4?months under dry conditions at 4?°C.

Acetylcholinesterase Immobilization on Polyacrylamide/Functionalized Multi-walled Carbon Nanotube Nanocomposite Nanofibrous Membrane

In this work, polyacrylamide/multi-walled carbon nanotubes (MWCNT) solution is electrospun to nanocomposite nanofibrous membranes for acetylcholinesterase enzyme immobilization. A new method for enzyme immobilization is proposed, and the results of analysis show successful covalent bonding of enzymes on electrospun membrane surface besides their non-covalent entrapment. Fourier transform infrared spectroscopy, mechanical and thermal investigations of nanofibrous membrane approve successful cross-linking and enzyme immobilization. The enzyme relative activity and kinetic on both pure and nanocomposite membranes is investigated, and the results show proper performance of designed membrane to even improve the enzyme activity followed by immobilization compared to free enzyme. Scanning electron microscopy images show nanofibrous web of 3D structure with a low shrinkage and hydrogel structure followed by enzyme immobilization and cross-linking. Moreover, the important role of functionalized carbon nanotubes on final nanofibrous membrane functionality as a media for enzyme immobilization is investigated. The results show that MWCNT could act effectively for enzyme immobilization improvement via both physical (enhanced fibers’ morphology and conductivity) and chemical (enzyme entrapment) methods.

Electrochemical tyrosine sensor based on a glassy carbon electrode modified with a nanohybrid made from graphene oxide and multiwalled carbon nanotubes

We report on a glassy carbon electrode that was modified with a composite made from graphene oxide (GO) and multiwalled carbon nanotubes (MWCNT) that enables highly sensitive determination of L-tyrosine. The sensor was characterized by transmission electron microscopy and electrochemical impedance spectroscopy, and its electrochemical properties by cyclic voltammetry, chronocoulometry and differential pulse voltammetry. The GO/MWCNT hybrid exhibits strong catalytic activity toward the oxidation of L-tyrosine, with a well defined oxidation peak at 761 mV. The respective current serves as the analytical information and is proportional to the L-tyrosine concentration in two ranges of different slope (0.05 to 1.0 μM and 1.0 to 650.0 μM), with limits of detection and quantification as low as 4.4 nM and 14.7 nM, respectively. The method was successfully applied to the analysis of L-tyrosine in human body fluids. The excellent reproducibility, stability, sensitivity and selectivity are believed to be due to the combination of the electrocatalytic properties of both GO and MWCNT. They are making this hybrid electrode a potentially useful electrochemical sensing platform for bioanalysis.

A nanocomposite made from conducting organic polymers and multi-walled carbon nanotubes for the adsorption and separation of gold(III) ions

We describe a new method for the separation and preconcentration of traces of Au(III) in environmental samples. Sorbents made from modified multiwalled carbon nanotubes and conducting polymers (PANI and PEDOT) were used for solid-phase extraction. The Au(III) ions are adsorbed as a result of the interaction with the electron pairs of =N- and -S- groups. Effects of pH value, flow rate and volume of sample, type, volume and concentration of eluent, and the adsorption capacity were investigated. The maximum adsorption capacity of MWCNTs/PANI and MWCNTs/PEDOT are 159 and 176?mg?g^?1, and the detection limits of this method are below 0.3 and 0.5?ng?mL^?1, respectively. The procedure was successfully applied to the determination of traces of Au(III) in a reference material and in environmental samples.

Gold nanoparticle-coated multiwall carbon nanotube-modified electrode for electrochemical determination of methyl parathion

We report on an electrochemical sensor for the determination of methyl parathion. It is based on an electrode modified with multi-walled carbon nanotubes that were covered with gold nanoparticles (Au-NPs). The vertically aligned array of MWCNTs on a tantalum substrate was coated with Au-NPs by overhead magnetron sputtering deposition. Scanning and transmission electron microscopy and XRD were used to characterize the Au-NP-MWCNTs composite. Cyclic voltammetry and differential pulse voltammetry were employed to evaluate the suitability of the new electrode for the determination of methyl parathion. Under the optimal conditions, the current response of the electrode to methyl parathion is linear in the range from 0.50 to 16.0 mg mL^-1, with a detection limit of 50 ??g mL^-1 (signal/noise = 3), and the sensitivity is 4.5 times better than that of the plain MWCNTs electrode. We conclude that this method represents a simple, rapid, effective and sensitive approach for the detection of methyl parathion.

Determination of some trace elements by flame atomic absorption spectrometry after preconcentration and separation by Escherichia coli immobilized on multiwalled carbon nanotubes

We have immobilized living and non-living Escherichia coli (E. coli) bacteria on multiwalled carbon nanotubes (MWCNT) and used such materials as a biosorbent for the separation and preconcentration of copper, cobalt, cadmium and nickel prior to their determination by flame atomic absorption spectrometry (FAAS). E. coli bacteria cells were mixed with MWCNTs in a 1:1 ratio, dried and placed at the tip of a 50-mL syringe. The ions were retained on the sorbent and then eluted by drawing and ejecting back the sample (or standard solution) and an eluent, respectively. The effects of various experimental parameters on the sorption and elution were investigated. The analytes were quantitatively retained (at pH values of 7) and eluted (with 0.5 M nitric acid) with high precision, the RSD being <5%. The performances of the new sorbents were compared using certified reference materials. The sorbent modified with living E. coli has a higher adsorption capacity and displays somewhat better recoveries compared to sorbent based on non-living E. coli. Both sorbents were successfully used for the separation and preconcentration of copper, cobalt, cadmium and nickel prior to their determination by flame atomic absorption spectrometry.

Double layer enzyme modified carbon nanotubes as label for sandwich-type immunoassay of tumor markers

An immunosensor was prepared for the determination of carcinoembryonic antigen (CEA). It is based on the use of multiwalled carbon nanotubes (MWCNTs) along with horseradish peroxidase-labeled antibody. The enzyme was assembled onto MWCNTs templates using the layer-by-layer technique and then conjugated to carcinoembryonic secondary antibodies (Ab₂) as the enzyme label. The resulting assembly results in a largely amplified sensitivity. The response is linear in the range of 0.05 to 45?ng?mL^-1, with a detection limit of 16.0?pg?mL^-1. The immunosensor possesses good stability and good reproducibility.

Sputtering deposition of Pt nanoparticles on vertically aligned multiwalled carbon nanotubes for sensing L-cysteine

A highly sensitive electrochemical sensor for determination of L-cysteine (CySH) is presented. It is based on vertically aligned multiwalled carbon nanotubes modified with Pt nanoparticles by magnetron sputtering deposition. The morphology of the nanocomposite was characterized by scanning electron microscopy, transmission electron microscopy and energy-dispersive. The electrochemistry of CySH was investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The mechanism for the electrochemical reaction of CySH at the modified electrode at different pH values is discussed. The electrode exhibits a higher electrocatalytic activity towards the oxidation of CySH than comparable other electrodes. It displays a linear dependence (R ²?=?0.9980) on the concentration of CySH in the range between 1 and 500 μM and at an applied potential of +0.45 V, a remarkably low detection limit of 0.5 μM (S/N?=?3), and an outstandingly high sensitivity of 1.42?×?10³ μA?mM^?1?cm^?2, which is the highest value ever reported. The electrode also is highly inert towards other amino acids, creatinine and urea. The sensor was applied to the determination of CySH in urine with satisfactory recovery, thus demonstrating its potential for practical applications.

Isothermal crystallization kinetics of poly(trimethylene terephthalate)/multiwall carbon nanotubes composites

The isothermal crystallization kinetics of poly(trimethylene terephthalate) (PTT) in the presence of varying amounts of multiwall carbon nanotubes (MWCNT) have been investigated using differential scanning calorimetry (DSC) and analyzed using Avrami and secondary nucleation theory. Polarized light microscopy (PLM) was used to study the crystal morphology of PTT/MWCNT composites. The results showed that the presence of MWCNTs in PTT acted as an effective nucleating agents and lead to the spherulitic morphology. The decrease in the spherulites size on MWCNT addition was observed by polarized light microscopy. Using values of transport parameters ( $ U* = 1500{\text{ cal mol}}^{ - 1} , \, \Updelta T =T_{\text{g}} - 30\, $ °C) together with experimentally determined values of equilibrium melting temperature [ $ T^{\text{o}}_{\text{m}} $ (245.2 °C)] and glass transition temperature [ $ T_{\text{g}} $ (45 °C)], the nucleation parameter, $ K_{\text{g}} $ and $ \sigma_{\text{e}} $ were determined for PTT and PTT/MWCNT composites according to Lauritzen–Hoffman theory. The decrease in the values of these parameters on MWCNT addition is in agreement with the fact that the rate of crystallization of PTT increased in the presence of MWCNTs.     

Hydrogen peroxide sensor based on a stainless steel electrode coated with multi-walled carbon nanotubes modified with magnetite nanoparticles

Multi-walled carbon nanotubes (MWCNTs) were decorated with magnetite (Fe₃O₄) nanoparticles and then used to modify a stainless steel electrode. The Fe₃O₄/MWCNTs composite was characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and X-ray diffraction patterns. Electrochemical properties of the modified electrode revealed a substantial catalytic activity for the reduction of hydrogen peroxide. The relationship between peak current and the concentration of hydrogen peroxide was linear in the range from 0.06?mmol?L^?1 to 0.36?mmol?L^?1, and the lowest detectable concentration is 0.01?mmol·L^?1 (S/N?=?3). The modified stainless steel electrode displays excellent stability.

Non-enzymatic analysis of glucose on printed films based on multi-walled carbon nanotubes

We report on the fabrication of an enzyme–free electrochemical sensor for glucose based on a printed film consisting of multi–walled carbon nanotubes (MWCNTs). The MWCNT–based film can be produced by means of a flexographic printing process on a polycarbonate (PC) substrate. The electrochemical response of the MWCNT–based film (referred to as MWCNT–PC) towards the oxidation of glucose at pH 7 was studied by means of cyclic voltammetry and electrochemical impedance spectroscopy. The MWCNT–PC film exhibits substantial electrocatalytic activity towards the oxidation of glucose at an anodic potential of 0.30?V (vs. Ag/AgCl). The findings reveal that the MWCNT–PC film enables non–enzymatic sensing of glucose with a detection limit as low as 2.16?μM and a sensitivity of 1045?μA?mM^?1?cm^?2.

Examination of incubation time of bare gold electrode inside cysteamine solution for immobilization of multi-walled carbon nanotubes on a gold electrode modified with cysteamine

A cysteamine (CysAm) nanostructure was generated to act as an intermediate layer between gold electrode and carbon nanotubes. A bare gold electrode was placed in a solution of CysAm to create a self-assembled monolayer on its surface. The modified electrode was then incubated with a solution of multi-walled carbon nanotubes. Cyclic voltammetry and atomic force microscopy were used to characterize the modified electrode. The results indicated that the number of functionalized MWCNTs on the surface of the electrodes increased by enhancing incubation time.

Sensing Lorazepam with a glassy carbon electrode coated with an electropolymerized-imprinted polymer modified with multiwalled carbon nanotubes and gold nanoparticles

We report on an electrode for the amperometric determination of lorazepam. A glassy carbon electrode was coated with a molecular imprint made by electropolymerization of ortho-phenylenediamine and filled with multiwalled carbon nanotubes and gold nanoparticles, which enhances the transmission of electrons. The sensor was studied with respect to its response to hexacyanoferrate (III) as a probe and by electrochemical impedance spectroscopy, cyclic voltammetry and square wave voltammetry. The linear response range to Lorazepam is from 0.5 nM to 1.0 nM and from 1.0 nM to 10.0 nM, with a detection limit of 0.2 nM (at an S/N of 3). The electrode was successfully applied to determine Lorazepam in spiked human serum.

Correlation between wetting properties and electrical performance of solution processed PEDOT:PSS/CNT nano-composite thin films

Nano-composite thin films of poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS) with different loading concentrations of multi-walled carbon nanotubes (MWCNT) were deposited on glass substrates using inkjet printing and spin coating techniques. The surface energy of the substrate was modified using an oxygen plasma to achieve different degrees of wetting by the composite solution. We show that the electrical properties strongly depend on the wetting of the substrate and by controlling the wettability, the conductivity of the nano-composite samples can be improved. Based on polymer conductivity, the electrical conductivity of the composite film can be improved or degraded by orders of magnitude with the incorporation of the same concentration of MWCNT. Moreover, electrical measurements show strong correlation between the conductivity of the carbon nanotube network and the resulting nano-composite films. The dependence of electrical properties on the wettability and the conductivity of the composite components could explain the diversity in the electrical behaviour reported in the literature for PEDOT:PSS/MWCNT nano-composite thin films.

Simultaneous determination of ascorbic acid,dopamine, and uric acid using a carbon paste electrode modified with multiwalled carbon nanotubes,ionic liquid,and palladium nanoparticles

We describe the modification of a carbon paste electrode (CPE) with multiwalled carbon nanotubes (MWCNT) and an ionic liquid (IL). Electrochemical studies revealed an optimized composition of 60 % graphite, 20 % paraffin, 10 % MWCNT and 10 % IL. In a next step, the optimized CPE was modified with palladium nanoparticles (Pd-NPs) by applying a double-pulse electrochemical technique. The resulting electrode was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, cyclic voltammetry, and electrochemical impedance spectroscopy. It gives three sharp and well separated oxidation peaks for ascorbic acid (AA), dopamine (DA), and uric acid (UA), with peak separations of 180 and 200 mV for AA-DA and DA-UA, respectively. The sensor enables simultaneous determination of AA, DA and UA with linear responses from 0.6 to 112, 0.1 to 151, and 0.5 to 225 μM, respectively, and with 200, 30 and 150 nM detection limits (at an S/N of 3). The method was successfully applied to the determination of AA, DA, and UA in spiked samples of human serum and urine. Figure

The CPE was modified with multiwalled carbon nanotubes and an ionic liquid. After optimization the electrode was further modified with palladium nanoparticles. The resulting electrode gives three sharp and well separated oxidation peaks for ascorbic acid, dopamine and uric acid     

Electrodeposition of cobalt oxide nanoparticles on carbon nanotubes, and their electrocatalytic properties for nitrite electrooxidation

We describe a modified glassy carbon electrode (GCE) for the sensitive determination of nitrite in waste water samples. The GCE was modified by electrodeposition of cobalt oxide nanoparticles on multi-walled carbon nanotubes (MWCNTs) deposited on a conventional GCE. Scanning electron microscopy and electrochemical techniques were used for the characterization of the composite material which is very uniform and forms a kind of nanoporous structure. Electrochemical experiments showed that the modified electrode exhibited excellent electrocatalytic properties for nitrite. Amperometry revealed a good linear relationship between peak current and nitrate concentration in the 0.5 to 250???M range with a detection limit of 0.3???M (S/N?=?3). The method has been applied to the amperometric detection of nitrite. The modified electrode displays good storage stability, reproducibility, and selectivity for a promising practical application.

Direct electron transfer of glucose oxidase promoted by carbon nanotubes is without value in certain mediator-free applications

We have investigated the direct electron transfer (DET) promoted by carbon nanotubes (CNTs) on an electrode containing immobilized glucose oxidase (GOx) with the aim to develop a third-generation glucose biosensor and a mediator-free glucose biofuel cell anode. GOx was immobilized via chitosan (CS) on a glassy carbon electrode (GCE) modified with multi-walled carbon nanotubes (MWCNTs). Cyclic voltammetric revealed that the GOx on the surface of such an electrode is unable to simultaneously demonstrate DET with the electrode and to retain its catalytic activity towards glucose, although the MWCNTs alone can promote electron transfer between GOx and electrode. This is interpreted in terms of two types of GOx on the surface, the distribution and properties of which are quite different. The first type exhibits DET capability that results from the collaboration of MWCNTs and metal impurities, but is unable to catalyze the oxidation of glucose. The second type maintains its glucose-specific catalytic capability in the presence of a mediator, which can be enhanced by MWCNTs, but cannot undergo DET with the electrode. As a result, the MWCNTs are capable of promoting the electron transfer, but this is without value in some mediator-free applications such as in third-generation glucose biosensors and in mediator-free anodes for glucose biofuel cells.