Label-free electrochemical impedance detection of kinase and phosphatase activities using carbon nanofiber nanoelectrode arrays

We demonstrate the feasibility of a label-free electrochemical method to detect the kinetics of phosphorylation and dephosphorylation of surface-attached peptides catalyzed by kinase and phosphatase, respectively. The peptides with a sequence specific to c-Src tyrosine kinase and protein tyrosine phosphatase 1B (PTP1B) were first validated with ELISA-based protein tyrosine kinase assay and then functionalized on vertically aligned carbon nanofiber (VACNF) nanoelectrode arrays (NEAs). Real-time electrochemical impedance spectroscopy (REIS) measurements showed reversible impedance changes upon the addition of c-Src kinase and PTP1B phosphatase. Only a small and unreliable impedance variation was observed during the peptide phosphorylation, but a large and fast impedance decrease was observed during the peptide dephosphorylation at different PTP1B concentrations. The REIS data of dephosphorylation displayed a well-defined exponential decay following the Michaelis–Menten heterogeneous enzymatic model with a specific constant, k_cat/K_m, of (2.1 ± 0.1) × 10⁷ M⁻¹ s⁻¹. Consistent values of the specific constant was measured at PTP1B concentration varying from 1.2 to 2.4 nM with the corresponding electrochemical signal decay constant varying from 38.5 to 19.1 s. This electrochemical method can be potentially used as a label-free method for profiling enzyme activities in fast reactions.     

Dielectrophoretic trapping of single bacteria at carbon nanofiber nanoelectrode arrays

We present an ac dielectrophoretic (DEP) technique for single-cell trapping using embedded carbon nanofiber (CNF) nanoelectrode arrays (NEAs). NEAs fabricated by inlaying vertically aligned carbon nanofibers in SiO2 matrix are applied as "points-and-lid" DEP devices in aqueous solution. The miniaturization of the electrode size provides a highly focused electrical field with the gradient enhanced by orders of magnitude. This generates extremely large positive DEP forces near the electrode surface and traps small bioparticles against strong hydrodynamic forces. This technology promises new capabilities to perform novel cell biology experiments at the nanoscale. We anticipate that the bottom-up approach of such nano-DEP devices allows the integration of millions of nanolectrodes deterministically in lab-on-a-chip devices and will be generally useful for manipulating submicron particles.     

Protein electrochemistry using aligned carbon nanotube arrays

The remarkable electrocatalytic properties and small size of carbon nanotubes make them ideal for achieving direct electron transfer to proteins, important in understanding their redox properties and in the development of biosensors. Here, we report shortened SWNTs can be aligned normal to an electrode by self-assembly and act as molecular wires to allow electrical communication between the underlying electrode and redox proteins covalently attached to the ends of the SWNTs, in this case, microperoxidase MP-11. The efficiency of the electron transfer through the SWNTs is demonstrated by electrodes modified with tubes cut to different lengths having the same electron-transfer rate constant.     

Polycarbonate carbon nanofiber composites

Carbon nanofiber (CNF) composites have the potential for creating inexpensive, semiconducting polymers. These composites require a homogeneous dispersion within the polymer. Many groups have focused on high shear methods such as twin screw extrusion. Although high shear methods produce a homogeneous dispersion, the aspect ratio of the nanofibers is reduced by the mechanical force. In this report, we present results for low shear composite formation via in situ polymerization of cyclic oligomeric carbonates. The composites were characterized by thermal gravimetric analysis, electrical conductivity, scanning electron microscopy and transmission electron microscopy. The composites exhibit minimal aggregation of the carbon nanofibers even at high weight percents. The polycarbonate/CNF composites exhibit an electrical conductivity percolation threshold of 6.3 wt% which is higher compared with similar CNF composites. The composites also show an increase in thermal stability of 40 °C as the CNF loading increases from 0 to 9 wt%.     

Fabrication of carbon nanofiber by pyrolysis of freeze-dried cellulose nanofiber

The possibility of fabricating carbon nanofibers from cellulose nanofibers was investigated. Cellulose nanofiber of ~50 nm in diameter was produced using ball milling in an eco-friendly manner. The effect of the drying techniques of cellulose nanofibers on the morphology of carbon residue was studied. After pyrolysis of freeze-dried cellulose nanofibers below 600 °C, amorphous carbon fibers of ~20 nm in diameter were obtained. The pyrolysis of oven-dried precursors resulted in the loss of original fibrous structures. The different results arising from the two drying techniques are attributed to the difference in the spatial distance between cellulose nanofiber precursors.     

Templated carbon nanofiber with mesoporosity and semiconductivity

We have newly fabricated a long one-dimensional (1D) mesoporous carbon nanofiber by using a mesoporous silica nanofiber template. The resulting mesoporous carbon nanofiber shows the unique mesoporous structure of circularly wound nanochannel alignment perpendicular to the long fiber axis and the high gas sorption property, which interestingly presents the p-type semiconducting behavior.     

Electrodeposition of ZnO on carbon nanofiber buckypaper

ZnO nanostructures have been electrochemically synthesized on three-dimensional, interconnected, and porous carbon nanofiber Buckypaper substrates. Using potentiostatic deposition, wurtzite ZnO with controlled microstructure and morphology has been deposited. While all ZnO deposits exhibit a band gap value of around 3.2 eV, impurity states determined by photoluminescence (PL) measurements show strong deposition potential influences. Both the green and red emissions corresponding to respective oxygen vacancies and oxygen rich impurity states can be identified. Thermal annealing can greatly reduce oxygen vacancy concentration but has limited effects on the oxygen rich defects. This study suggests a cost-effective and high-throughput approach in deposition of ZnO nanostructures suitable for photovoltaic applications.     

碳纳米纤维增强钴酞菁催化氧化染料的性能研究

将氨基钴酞菁（CoTAPc）以共价键的形式负载到碳纳米纤维（CNF）上制备碳纳米纤维负载钴酞菁催化剂（CoTAPc-CNF）,利用原子吸收光谱、紫外-可见光吸收光谱、衰减全反射-红外光谱等方法对CoTAPc-CNF进行表征.选用具有氧杂蒽结构的罗丹明6G（Rh6G）为主要研究对象,研究CoTAPc-CNF对Rh6G的催化氧化性能,考察了温度、pH、NaCl、异丙醇等对CoTAPc-CNF催化性能的影响.结果表明,CoTAPc-CNF在常温中性条件下能有效催化氧化Rh6G;随着温度和pH的增加,CoTAPc-CNF催化氧化Rh6G速率逐渐提高;NaCl和异丙醇的加入,没有抑制催化氧化反应的进行,相反大大提高了Rh6G的降解速率,这与一般羟基自由基占主导的高级氧化体系完全不同;进一步采用电子顺磁共振波谱法证实CoTAPc-CNF/H2O2体系确实为非羟基自由基催化机理.另外,研究发现CoTAPc-CNF还能有效催化氧化其他共轭结构的染料,如偶氮染料酸性橙7（AO7）、三芳甲烷染料碱性绿1（BG1）.因此,本文探索的CoTAPc-CNF/H2O2非羟基自由基催化反应体系在处理成分复杂的实际印染废水中具有较好的应用前景.     

Role of counter-substrate surface energy in macroscale friction of nanofiber arrays

The effect of counter-substrate surface energy on macroscale friction of nanofiber array is studied. Low-density polyethylene (LDPE) fibrillar array fabricated from silicon nanowire template is tested against glass substrates modified with various self-assembled monolayers, which exhibit a wide range of surface energy. A large drop in friction over a narrow range of surface energy is observed and explained in terms of drastically reduced number of fibers in actual contact, in addition to the reduced surface energy. The relationship between surface energy and fiber engagement is discussed with Johnson-Kendall-Roberts (JKR) and elastic beam models.     

Surface control of activated carbon fiber by growth of carbon nanofiber

Carbon nanofiber/activated carbon fiber (CNF/ACF) composites with multifunctional surfaces were prepared through catalytic growth of CNFs on an ACF. Because of selective deposition of catalyst particles in ACF micropores, partial oxidation of ACF after catalyst impregnation was a critical step to control the surface area of the CNF/ACF composites, of which the surface functions can be synergistically performed by both the microporous surface of ACF and free edges of CNFs. CNF/ACF composites of this study are expected to provide an improved performance in SOx or NOx removal.     

Wettability of carbon nanofiber layers on nickel foils

Carbon nanofiber (CNF) layers have been directly synthesized on nickel foils by chemical vapor deposition at 450°C using different H(2) concentrations and reaction times. The addition of 5% H(2) produces thicker, rougher and more porous CNF layers than when 1% H(2) is used. The roughness and porosity increases with reaction time when 5%, 10% or 20% H(2) are used; however, this effect is less pronounced when 1% H(2) is used. CNFs are 50-55 nm in diameter and have a fishbone type structure. We have studied the influence of CNF layer thickness, porosity and surface roughness on the interaction with water by measuring the contact angle. The water wetting properties of the samples are more significantly influenced by the CNF layer thickness than both surface roughness and porosity. When the CNF layer is thicker than ca. 20 μm, the surface is hydrophobic and the contact angle increases with surface roughness and porosity. When the CNF layer is thinner than ca. 20 μm, the surface is hydrophilic and the contact angle decreases with increasing surface roughness and porosity. This behavior is attributed to penetration of water, making contact with the hydrophilic C layer between the CNF layer and the foil.     

Effect of fiber geometry on macroscale friction of ordered low-density polyethylene nanofiber arrays

Ordered low-density polyethylene (LDPE) nanofiber arrays are fabricated from silicon nanowire (SiNW) templates synthesized by a simple wet-chemical process based on metal-assisted electroless etching combined with colloidal lithography. The geometrical effect of nanofibrillar structures on their macroscale friction is investigated over a wide range of diameters and lengths under the same fiber density. The optimum geometry for contacting a smooth glass surface is presented with discussions on the compromise between fiber tip-contact area and fiber compliance. A friction design map is developed, which shows that the theoretical optimum design condition agrees well with the LDPE nanofiber geometries exhibiting high measured friction.     

Rheological and electrical analysis in carbon nanofiber reinforced polypropylene composites

Two different types of carbon nanofibers (CNF) were incorporated in the same polypropylene (PP) matrix by twin‐screw extrusion. The electrical characterization of both CNFs/PP composites as a function of volume fraction show different electrical performance: conducting and nonconducting. The objective of this work is to study the rheological behavior of both composites with the aim of relating it to the electrical behavior. The results indicate that the rheological behaviors are different, suggesting that rheology differentiates the microstructural variations responsible for the electrical performance. Furthermore, the main rheological parameters were correlated to the electrical conductivity. The results show that G′/G″ and G′ are the most sensitive parameters when compared with the onset of electrical percolation. Finally, in spite of the intrinsic measuring differences between electrical and rheological analysis, the two calculated thresholds are very similar: ～0.5 for the rheological and ～0.4 for the electrical. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Quasi-continuous growth of ultralong carbon nanotube arrays

We report the growth of ultralong (>10 cm) multi-walled and single-walled carbon nanotubes such that the length is limited by the size of the furnace rather than by the termination of growth. The disturbance of microscale laminar flows results in disordered or shorter growth of carbon nanotubes. By downsizing reaction pipes, reaction gas flows are stabilized with low Reynolds numbers. In this way, the catalyst nanoparticles at the end of growing carbon nanotubes can travel a longer distance to grow ultralong nanotubes.     

Deposition precipitation for the preparation of carbon nanofiber supported nickel catalysts

Deposition precipitation of nickel hydroxide onto modified carbon nanofibers has been studied and compared to deposition onto silica. The carbon nanofiber support materials consisted of graphite-like material of the fishbone-type with a diameter of 20-50 nm and a specific surface area of 150 m2/g. Modification involved surface oxidation (CNF-O) optionally followed by partial reduction (CNF-OR) or thermal treatment (CNF-OT). Titration of the support materials showed the presence of 0.17 and 0.03 mmol/g carboxylic acid groups for CNF-O and CNF-OR, respectively. For the CNF-OT only basic groups were present. The deposition precipitation of 20 wt % nickel onto these supports has been studied by time dependent pH and nickel loading studies. With silica, nickel ion adsorption did not occur prior to nucleation of the nickel hydroxide phase at pH = 5.6. With CNF-O, nickel ion adsorption took place right from the start of the deposition process at pH = 3.5, and at pH = 5.6 already 4 wt % nickel was adsorbed. Nucleation of nickel hydroxide onto adsorbed nickel ion clusters proceeded subsequently. Characterization of the dried Ni/CNF-O samples with TEM and XRD showed well dispersed and thin (5 nm) platelets of nickel hydroxide adhering to the carbon nanofibers. After reduction at 773 K in hydrogen the Ni/CNF-O contained metallic nickel particles of 8 nm homogeneously distributed over the fibers. With CNF-OR and CNF-OT, precipitation of large platelets (> 500 nm) separate from the support took place. Clearly, the presence of carboxylic acid groups is essential to successfully deposit nickel hydroxide onto modified carbon nanofibers.     

Enhanced thermal conductivity of rheologically percolated carbon nanofiber reinforced polypropylene composites

Three different types of carbon nanofibers (CNF) were incorporated in the same polypropylene (PP) matrix by twin‐screw extrusion. The rheological and thermal properties were investigated. The rheological characterization of CNFs/PP composites as function of their volume fraction shows different microstructures: percolated and non‐percolated behaviors of their CNF's networks. In this work, the laser flash technique is employed in the experimental determination of the thermal diffusivity and conductivity of composites at room temperature. The ultimate aim is to correlate microstructure described by rheological analysis with final thermal properties. The results show that thermal diffusivity and conductivity are clearly higher for rheologically percolated composites suggesting that above certain critical content of nanofibers thermal transport is mainly controlled by percolated structures caused by interconnected CNFs' networks. Finally, thermal conductivity results are described by means of percolation theory from which an intrinsic thermal conductivity for the CNFs' network of approximately 6.5 W/m K, i.e. close to three times lower than some values reported in literature for SWCNTs' networks, was calculated. Copyright © 2015 John Wiley & Sons, Ltd.