Carbon nanotube/gold nanoparticles/polyethylenimine-functionalized ionic liquid thin film composites for glucose biosensing

A novel glucose biosensor based on immobilization of glucose oxidase (GOD) in thin films of polyethylenimine-functionalized ionic liquid (PFIL), containing a mixture of carbon nanotubes (CNT) and gold nanoparticles (AuNPs) and deposited on glassy carbon electrodes, was developed. Direct electrochemistry of glucose oxidase in the film was observed, with linear glucose response up to 12mM. The PFIL-stabilized gold nanoparticles had a diameter of 2.4+/-0.8nm and exhibited favorable stability (stored even over one month with invisible change in UV-vis spectroscopic measurements). In addition, CNT were also well dispersed in the PFIL matrix, then, the resulting CNT/AuNPs/PFIL composites film showed high electrocatalytic activity toward reduction of hydrogen peroxide and oxygen. Here, PFIL, due to its high ionic conductivity, good solubility to CNT, and stability to nanoparticles, played an important role in constructing stable CNT/AuNPs/PFIL/GOD composites. And good biocompatibility of PFIL also offered a friendly environment for the immobilization of biomolecules.     

Immobilization of glucose oxidase onto gold nanoparticles with enhanced thermostability

Immobilized proteins and enzymes were widely investigated in medical field as well as in food and environmental fields. In this paper, glucose oxidase (GOD) monolayer was covalently immobilized on the surface of gold nanoparticles (AuNPs) to fabricate bioconjugate complex. The citrate-stabilized AuNPs were first functionalized by a carboxyl-terminated alkanethiol and the terminal carboxyl groups were subsequently bonded with side-chain amino groups of protein surface through EDC/NHS coupling reaction. The enzyme activity assays of the obtained bioconjugates display an enhanced thermostability and similar pH-dependence behavior in contrast with that of free enzyme. Such GOD/AuNPs bioconjugates can be considered as a catalytic nanodevice to construct nanoreactor based on glucose oxidation reaction for biotechnological purpose.     

Nanotube–polymer composites: insights from Flory–Huggins theory and mesoscale simulations

Carbon nanotube (CNT)-polymer composites, with potential applications in structural materials, optoelectronics, sensors, biocatalysis, and thermal and electromagnetic shielding are an important emerging area of nanotechnology. However, progress has been slow due to difficulties in dispersing CNTs into the polymer matrix. We attack the problem from a Flory-Huggins theory point of view, and present novel simulations of the dispersion process at the mesoscale. The solubility parameter of the CNTs is mapped out as a function of tube diameter, and compared with that of well-known polymers. Parallel alignment of CNTs with the application of shear, and dispersion by attaching organic functional groups are also investigated.     

Layer-by-layer fabrication and direct electrochemistry of glucose oxidase on single wall carbon nanotubes

Layer-by-layer assembly of glucose oxidase (GOx) with single-wall carbon nanotubes (SWCNTs) is achieved on the electrode surface based on the electrostatic attraction between positively charged GOx in pH 3.8 buffer and negatively charged carboxylic groups of CNTs. The cyclic voltammetry and electrochemical impedance spectroscopy are used to characterize the formation of multilayer films. In deaerated buffer solutions, the cyclic voltammetry of the multilayer films of {GOx/CNT}_n shows two pairs of well-behaved redox peaks that are assigned to the redox reactions of CNTs and GOx, respectively, confirming the effective immobilization of GOx on CNTs using the layer-by-layer technique. The redox peak currents of GOx increase linearly with the increased number of layers indicating the uniform growth of GOx in multilayer films. The dependence of the cyclic voltammetric response of GOx in multilayer films on the scan rate and pH is also studied. A linear decrease of the reduction current of oxygen at the {GOx/CNT}-modified electrodes with the addition of glucose suggests that such multilayer films of GOx retain the bioactivity and can be used as reagentless glucose biosensors.     

A High‐Efficiency Sulfur/Carbon Composite Based on 3D Graphene Nanosheet@Carbon Nanotube Matrix as Cathode for Lithium–Sulfur Battery

Carbon materials have attracted extensive attention as the host materials of sulfur for lithium–sulfur battery, especially those with 3D architectural structure. Here, a novel 3D graphene nanosheet–carbon nanotube (GN–CNT) matrix is obtained through a simple one‐pot pyrolysis process. The length and density of CNTs can be readily tuned by altering the additive amount of carbon source (urea). Specifically, CNTs are in situ introduced onto the surface of the graphene nanosheets (GN) and show a stable covalent interaction with GN. Besides, in the GN–CNT matrix, cobalt nanoparticles with different diameters exist as being wrapped in the top of CNTs or scattering on the GN surface, and abundant heteroatoms (N, O) are detected, both of which can help in immobilizing sulfur species. Such a rationally designed 3D GN–CNT matrix makes much more sense in enhancing the electrochemical performance of the sulfur cathode for rapid charge transfer and favorable electrolyte infiltration. Moreover, the presence of dispersed cobalt nanoparticles is beneficial for trapping lithium polysulfides by strong chemical interaction, and facilitating the mutual transformation between the high‐order polysulfides and low‐order ones. As a result, the S/GN–CNT composite presents a high sulfur utilization and large capacity on the basis of the S/GN–CNT composite as active material.     

An amperometric biosensor based on a composite of single-walled carbon nanotubes, plasma-polymerized thin film, and an enzyme

We report on an amperometric biosensor that is based on a nanocomposite of carbon nanotubes (CNT), a nano-thin plasma-polymerized film (PPF), and glucose oxidase (GOx) as an enzyme model. A mixture of the GOx and a CNT film is sandwiched with 10-nm-thick acetonitrile PPFs. Under PPF layer was deposited onto a sputtered gold electrode. To facilitate the electrochemical communication between the CNT layer and GOx, CNT was treated with nitrogen or oxygen plasma. The resulting device showed that the oxidizing current response due to enzymatic reaction was 4-16-fold larger than that with only CNT or PPF, showing that the PPF and/or plasma process is an enzyme-friendly platform for designing electrochemical communication from the reaction center of GOx to the electrode via CNTs. The optimized glucose biosensor showed high sensitivity (sensitivity of 42 microA mM(-1)cm(-2), correlation coefficient of 0.992, linear response range of 0.025-2.2 mM, and a detection limit of 6 microM at signal/noise ratio of 3, +0.8 V versus Ag/AgCl), high selectivity (almost no interference by 0.5 mM ascorbic acid) for glucose quantification, and rapid response (<4 s to reach 95% of maximum response). Additionally, the devices showed a small and stable background current (0.35+/-0.013 microA) compared with the glucose response (ca. 10 microA at 10mM glucose) and suitable reproducibility from sample-to-sample (<3%, n=4).     

Modification of carbon nanotubes with redox hydrogel: improvement of amperometric sensing sensitivity for redox enzymes

This study demonstrated that redox hydrogel-modified carbon nanotube (CNT) electrodes can be developed as an amperometric sensor that are sensitive, specific and fast and do not require auxiliary enzymes. A redox polymer, poly(vinylimidazole) complexed with Os(4,4'-dimethylbpy)(2)Cl (PVI-dmeOs) was electrodeposited on Ta-supported multi-walled CNTs. The resulted PVI-dmeOs thin film did not change the surface morphology of the CNTs, but turned the CNT surface from hydrophobic to hydrophilic, as studied by scanning electron microscopy (SEM) and static water contact angle measurements. Cyclic voltammetry measurements in a Fe(CN)(6)(3-) solution and electrochemical impedance measurements in an equimolar Fe(CN)(6)(3-/4-) solution demonstrated that the PVI-dmeOs hydrogel thin film was electronic conductive with a resistance of about 15Omega. The PVI-dmeOs/CNT electrodes sensed rapidly, sensitively and specifically to model redox enzymes (glucose oxidase (GOD) and lactate oxidase (LOD)) in amperometric experiments in electrolyte solutions containing the substrates of the measured redox enzymes. Both the CNT substrate and the thin PVI-dmeOs film enhanced the sensing sensitivities. Exploration of the mechanisms suggests that the PVI-dmeOs film may enhance the sensing sensitivities by wiring the enzyme molecules through the redox centers tethered on the mobile redox polymer backbones to the CNT electrodes.     

Electrochemical behavior of L-cysteine and its detection at carbon nanotube electrode modified with platinum

The electrochemical behavior of L-cysteine (CySH) on platinum (Pt)/carbon nanotube (CNT) electrode was investigated by cyclic voltammetry. CNTs used in this study were grown directly on graphite disk by chemical vapor deposition. Pt was electrochemically deposited on the activated CNT/graphite electrode by electroreduction of Pt(IV) complex ion on the surface of CNTs. Among graphite, CNT/graphite, and Pt/CNT electrodes, improved electrochemical behavior of CySH oxidation was found with Pt/CNT electrode. On the other hand, a sensitive CySH sensor was developed based on Pt/CNT/graphite electrode. A linear calibration curve can be observed in the range of 0.5 microM-0.1 mM. The detection limit of the Pt/CNT electrode is 0.3 microM (signal/nose=3). Effects of pH, scan rate, and interference of other oxidizable amino acids were also investigated and discussed. Additionally, the reproducibility, stability, and applicability of the Pt/CNT electrode were evaluated.     

Advances in carbon nanotube based electrochemical sensors for bioanalytical applications

Electrochemical (EC) sensing approaches have exploited the use of carbon nanotubes (CNTs) as electrode materials owing to their unique structures and properties to provide strong electrocatalytic activity with minimal surface fouling. Nanofabrication and device integration technologies have emerged along with significant advances in the synthesis, purification, conjugation and biofunctionalization of CNTs. Such combined efforts have contributed towards the rapid development of CNT-based sensors for a plethora of important analytes with improved detection sensitivity and selectivity. The use of CNTs opens an opportunity for the direct electron transfer between the enzyme and the active electrode area. Of particular interest are also excellent electrocatalytic activities of CNTs on the redox reaction of hydrogen peroxide and nicotinamide adenine dinucleotide, two major by-products of enzymatic reactions. This excellent electrocatalysis holds a promising future for the simple design and implementation of on-site biosensors for oxidases and dehydrogenases with enhanced selectivity. To date, the use of an anti-interference layer or an artificial electron mediator is critically needed to circumvent unwanted endogenous electroactive species. Such interfering species are effectively suppressed by using CNT based electrodes since the oxidation of NADH, thiols, hydrogen peroxide, etc. by CNTs can be performed at low potentials. Nevertheless, the major future challenges for the development of CNT-EC sensors include miniaturization, optimization and simplification of the procedure for fabricating CNT based electrodes with minimal non-specific binding, high sensitivity and rapid response followed by their extensive validation using “real world” samples. A high resistance to electrode fouling and selectivity are the two key pending issues for the application of CNT-based biosensors in clinical chemistry, food quality and control, waste water treatment and bioprocessing.     

A glucose biosensor based on electrodeposition of palladium nanoparticles and glucose oxidase onto Nafion-solubilized carbon nanotube electrode

Electrodeposition was used for the co-deposition of glucose oxidase (GOx) enzymes and palladium nanoparticles onto a Nafion-solubilized carbon nanotube (CNT) film. The co-deposited Pd-GOx-Nafion CNT bioelectrode retains its biocatalytic activity and offers an efficient oxidation and reduction of the enzymatically liberated H2O2, allowing for fast and sensitive glucose quantification. The combination of Pd-GOx electrodeposition with Nafion-solubilized CNTs enhances the storage time and performance of the sensor. An extra Nafion coating was used to eliminate common interferents such as uric and ascorbic acids. The fabricated Pd-GOx-Nafion CNT glucose biosensor exhibits a linear response up to 12 mM glucose and a detection limit of 0.15 mM (S/N = 3).     

Amperometric glucose biosensor based on adsorption of glucose oxidase at platinum nanoparticle-modified carbon nanotube electrode

A new amperometric biosensor, based on adsorption of glucose oxidase (GOD) at the platinum nanoparticle-modified carbon nanotube (CNT) electrode, is presented in this article. CNTs were grown directly on the graphite substrate. The resulting GOD/Pt/CNT electrode was covered by a thin layer of Nafion to avoid the loss of GOD in determination and to improve the anti-interferent ability. The morphologies and electrochemical performance of the CNT, Pt/CNT, and Nafion/GOD/Pt/CNT electrodes have been investigated by scanning electron microscopy, cyclic voltammetry, and amperometric methods. The excellent electrocatalytic activity and special three-dimensional structure of the enzyme electrode result in good characteristics such as a large determination range (0.1-13.5mM), a short response time (within 5s), a large current density (1.176 mA cm(-2)), and high sensitivity (91mA M(-1)cm(-2)) and stability (73.5% remains after 22 days). In addition, effects of pH value, applied potential, electrode construction, and electroactive interferents on the amperometric response of the sensor were investigated and discussed. The reproducibility and applicability to whole blood analysis of the enzyme electrode were also evaluated.     

Carbon nanotube array: A new MIP platform

Here we demonstrate that a free-standing carbon nanotube (CNT) array can be used as a large surface area and high porosity 3D platform for molecular imprinted polymer (MIP), especially for surface imprinting. The thickness of polymer grafted around each CNT can be fine-tuned to imprint different sizes of target molecules, and yet it can be thin enough to expose every imprint site to the target molecules in solution without sacrificing the capacity of binding sites. The performance of this new CNT–MIP architecture was first assessed with a caffeine-imprinted polypyrrole (PPy) coating on two types of CNT arrays: sparse and dense CNTs. Real-time pulsed amperometric detection was used to study the rebinding of the caffeine molecules onto these CNT-MIPPy sensors. The dense CNT-MIPPy sensor presented the highest sensitivity, about 15 times better when compared to the conventional thin film, whereas an improvement of 3.6 times was recorded on the sparse CNT. However, due to the small tube-to-tube spacing in the dense CNT array, electrode fouling was observed during the detection of concentrated caffeine in phosphate buffer solution. A new I–V characterization method using pulsed amperometry was introduced to investigate the electrical characterization of these new devices. The resistance value derived from the I–V plot provides insight into the electrical conductivity of the CNT transducer and also the effective surface area for caffeine imprinting.     

A super highly sensitive glucose biosensor based on Au nanoparticles-AgCl@polyaniline hybrid material

Gold nanoparticles (AuNPs) with an average diameter of 5nm were assembled on the surface of silver chloride@polyaniline (PANI) core-shell nanocomposites (AgCl@PANI). Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) suggested that AuNPs were incorporated on AgCl@PANI through coordination bonds instead of electrostatic interaction. The resulting AuNPs-AgCl@PANI hybrid material exhibited good electroactivity at a neutral pH environment. An amperometric glucose biosensor was developed by adsorption of glucose oxidase (GOx) on an AuNPs-AgCl@PANI modified glassy carbon (GC) electrode. AuNPs-AgCl@PANI could provide a biocompatible surface for high enzyme loading. Due to size effect, the AuNPs in the hybrid material could act as a good catalyst for both oxidation and reduction of H(2)O(2). As the measurement of glucose was based on the electrochemical detection of H(2)O(2) generated by enzyme-catalyzed-oxidation of glucose, the biosensor exhibited a super highly sensitive response to the analyte with a detection limit of 4 pM. Moreover, the biosensor showed good reproducibility and operation stability. The effects of some factors, such as temperature and pH value, were also studied.     

Preparation of sulfonated poly(ether–ether–ketone) functionalized ternary graphene/AuNPs/chitosan nanocomposite for efficient glucose biosensor

A facile method of preparing water-dispersible sulfonated graphene (SPG) using sulfonated poly(ether–ether–ketone) organic polymer as a modifier was realized. A glucose biosensor was fabricated by immobilizing glucose oxidase (GO_x) on the surface of AuNPs used to modify SPG and chitosan (CH) deposited on an indium tin-oxide (ITO) glass electrode by a solution casting method. Morphological and structural characterizations confirm that the AuNPs can be efficiently applied to the SPG–CH matrix. The amperometric response of the GO_x/SPG–AuNPs–CH/ITO bioelectrode shows a broad linear range of 0.5 to 22.2 mM, with a limit of detection of 0.13 mM and a high sensitivity of 6.51 μA/(mM cm²). The excellent performance of the constructed biosensor is attributed to the large surface-to-volume ratio and electron transfer ability of SPG, the high catalytic activity of the AuNPs, and the good biocompatibility of CH. In addition, the sensor has important advantages, such as its simple preparation, fast response time (10 s), good stability (70 days), and high reproducibility. Favorable results upon examining the electrochemical response for the determination of glucose in human blood serum were obtained, without the assistance of a negligible effect of interfering bio-analytes. The results of studies show that the ternary SPG–AuNPs–CH nanocomposite may offer a new approach for developing novel types of highly sensitive and stable electrochemical biosensors.     

Molecular dynamics simulation of doxorubicin adsorption on a bundle of functionalized CNT

In this study, molecular dynamics simulation is used to investigate the adsorption of an anticancer drug, doxorubicin, on bundles of functionalized single-walled carbon nanotubes (SWNTs) in an aqueous solution. Carboxylic group has been selected as the functional group. Molecular dynamics (MD) simulations are performed for both separated systems containing a SWNT bundle and a functionalized carbon nanotube bundle, and results are compared with existing experimental data. MD results show that doxorubicin can be adsorbed on CNTs using different methods such as entrapment within CNT bundle, attachment to the side wall of the CNT, and adsorption on the CNT inner cavity. For functionalized CNT, the adsorption of drugs on the functional groups is essential for predicting the enhancement of drug loading on the functionalized nanotubes. Furthermore, the adsorption behavior of doxorubicin on CNTs is fitted with Langmuir and Freundlich isotherm models. The results show that Langmuir model can predict the adsorption behavior of doxorubicin on CNTs more accurately than Freundlich model does. As predicted by this isotherm model, the adsorption process of doxorubicin on CNTs is relatively difficult, but it can be improved by increasing the functional groups on the CNTs surface.     

Amphiphilic helical peptide enhances the uptake of single-walled carbon nanotubes by living cells

The success of many projected applications of carbon nano-tubes (CNTs) to living cells, such as intracellular sensors and nanovectors, will depend on how many CNTs are taken up by cells. Here we report the enhanced uptake by HeLa cells of single-walled CNTs coated with a designed peptide termed nano-1. Atomic force microscopy showed that the dispersions were composed of individual and small bundles of nano-1 CNTs with 0.7- to 32-nm diameters and 100- to 400-nm lengths. Spectroscopic characterizations revealed that nano-1 disperses CNTs in a non-covalent fashion that preserves CNT optical properties. Elemental analyses indicated that our sample preparation protocol involving sonication and centrifugation effectively eliminated metal impurities associated with CNT manufacturing processes. We further showed that the purified CNT dispersions are taken up by HeLa cells in a time- and temperature-dependent fashion, and that they do not affect the HeLa cell growth rate, evidence that the CNTs inside cells are not toxic under these conditions. Finally, we discovered that approximately 6-fold more CNTs are taken up by cells in the presence of nano-1 compared with medium containing serum but no peptide. The fact that coating CNTs with a peptide enhances uptake offers a strategy for improving the performance of applications that require CNTs to be inside cells.     

Preparation of co‐Co3O4/carbon nanotube/carbon foam for glucose sensor

Co‐Co₃O₄/carbon nanotube/carbon foam (Co‐Co₃O₄/CNT/CF) nanocomposites were prepared by soaking melamine foam into a solution of Co(NO₃)₂·6H₂O, followed by calcination in N₂ and air in sequence. The obtained Co‐Co₃O₄/CNT/CF nanocomposites were characterized with scanning electron microscopy and cyclic voltammetry. It was found that Co₃O₄ nanoparticles were grown on the external of CF successfully, while CNTs were grown on the surfaces of CF in a large amount, which further improved the electrical conductivity of the. The prepared Co‐Co₃O₄/CNT/CF nanocomposites were then used to construct nonenzymatic sensor to detect glucose in alkaline solution. The sensor showed detection range from 1.2 μM to 2.29 mM with a detection limit of 0.4 μM (S/N =3) and a high sensitivity of 637.5 μA^?1 cm^?2. The developed sensor also showed an instant response, favorable reproducibility, and high selectivity. The results attest that Co‐Co₃O₄/CNT/CF composites have great potential in the development of nonenzymatic sensors for glucose.     

Ultrasensitive luminol electrochemiluminescence for protein detection based on in situ generated hydrogen peroxide as coreactant with glucose oxidase anchored AuNPs@MWCNTs labeling

In this study, an ultrasensitive luminol electrochemiluminescence (ECL) immunosensor was constructed using carboxyl group functionalized multi-walled carbon nanotubes (MWCNTs) as platform and glucose oxidase (GOD) supported on Au nanoparticles (AuNPs) decorated MWCNTs (AuNPs@MWCNTs-GOD) as labels. Firstly, using poly(ethylenimine) (PEI) as linkage reagents, AuNPs@MWCNTs were prepared and introduced for binding of the secondary antibody (Ab(2)) and glucose oxidase (GOD) with high loading amount and good biological activity due to the improved surface area of AuNPs@MWCNTs and excellent biocompatibility of AuNPs. Then the GOD and Ab(2) labeled AuNPs@MWCNTs were linked to the electrode surface via sandwich immunoreactions. These localized GOD and AuNPs amplified luminol ECL signals dramatically, which was achieved by efficient catalysis of the GOD and AuNPs towards the oxidation of glucose to in situ generate improved amount of hydrogen peroxide (H(2)O(2)) as coreactant and the enhancement of AuNPs to the ECL reaction of luminol-H(2)O(2). The experimental results demonstrated that the proposed immunosensor exhibited sensitive and stable response for the detection of α-1-fetoprotein (AFP), ranging from 0.0001 to 80 ng mL(-1) with a limit of detection down to 0.03 pg mL(-1) (S/N=3). With excellent stability, sensitivity, selectivity and simplicity, the proposed luminol ECL immunosensor showed great potential in clinical applications.     

Platinum nanoparticles-doped sol-gel/carbon nanotubes composite electrochemical sensors and biosensors

Platinum nanoparticle-doped sol-gel solution is prepared and used as a binder for multi-walled carbon nanotubes (CNT) for the fabrication of electrochemical sensors. Amine group containing sol-gel solution is selected to utilize the affinity of -NH(2) groups toward metal nanoparticles for stabilization the nanoparticles in solution. The resulting CNT-silicate material brings new capabilities for electrochemical devices by using the synergistic action of the electrocatalytic activity of Pt nanoparticles and CNT. The combined electrocatalytic activity permits low-potential detection of hydrogen peroxide with remarkably improved sensitivity. With the incorporation of glucose oxidase within the Pt-CNT-silicate matrix, a Pt-CNT paste-based biosensor has been constructed that responds more sensitively to glucose than CNT-based biosensor. The influences of the composite of the sol-gel solution, the quantity of the solution and the Pt nanoparticles loading are examined. In pH 6.98 phosphate buffer, almost interference free determination of glucose is realized at 0.1 V versus SCE with a linear range from 1 to 25 mM, a response time <15s, and the sensitivity is 0.98 microA mM(-1)cm(-2). The sensitivity of the Pt-CNT paste-based biosensor is almost four times larger than that of the CNT-based biosensor (0.27 microA mM(-1)cm(-2) at 0.1 V). The improved electrocatalytic activity and surface renewability made the Pt-CNT-silicate system a potential platform to immobilize different enzymes for other bioelectrochemical applications.     

Sensitive electrochemical immunosensor for α-synuclein based on dual signal amplification using PAMAM dendrimer-encapsulated Au and enhanced gold nanoparticle labels

A novel electrochemical immunosensor for sensitive detection of α-synuclein (α-SYN), a very important neuronal protein, has been developed based on dual signal amplification strategy. Herein, G4-polyamidoamine dendrimer-encapsulated Au nanoparticles (PAMAM-Au nanocomposites) were covalently bound on the poly-o-aminobenzoic acid (poly-o-ABA), which was initially electropolymerized on the electrode surface to perform abundant carboxyl groups. The formed immunosensor platform, PAMAM-Au, was proved to provide numerous amino groups to allow highly dense immobilization of antigen, and facilitate the improvement of electrochemical responses as well. Subsequently, the enhanced gold nanoparticle labels ({HRP-Ab(2)-GNPs}) were fabricated by immobilizing horseradish peroxidase-secondary antibody (HRP-Ab(2)) on the surface of gold nanoparticles (GNPs). After an immunoassay process, the {HRP-Ab(2)-GNPs} labels were introduced onto the electrode surface, and produced an electrocatalytic response by reduction of hydrogen peroxide (H(2)O(2)) in the presence of enzymatically oxidized thionine. On the basis of the dual signal amplification of PAMAM-Au and {HRP-Ab(2)-GNPs} labels, the designed immunosensor displayed an excellent analytical performance with high sensitivity and stability. This developed strategy was successfully proved as a simple, cost-effective method, and could be easily extended to other protein analysis schemes.