This paper describes the development of glucose biosensors based on carbon nanotube (CNT) nanoelectrode ensembles (NEEs) for the selective detection of glucose. Glucose oxidase was covalently immobilized on CNT NEEs via carbodiimide chemistry by forming amide linkages between their amine residues and carboxylic acid groups on the CNT tips. The catalytic reduction of hydrogen peroxide liberated from the enzymatic reaction of glucose oxidase upon the glucose and oxygen on CNT NEEs leads to the selective detection of glucose. The biosensor effectively performs a selective electrochemical analysis of glucose in the presence of common interferents (e.g., acetaminophen, uric and ascorbic acids), avoiding the generation of an overlapping signal from such interferers. Such an operation eliminates the need for permselective membrane barriers or artificial electron mediators, thus greatly simplifying the sensor design and fabrication.Because of the high demand for blood glucose monitoring, significant research and development efforts have been devoted to producing reliable glucose sensors for in vitro or in vivo applications. 1-2 The measurement principle of oxidase-based amperometric biosensors previously relied upon the immobilization of oxidase enzymes on the surface of various electrodes and the detection of the current associated with the redox product in the biological reaction. To increase the selectivity and sensitivity of amperometric biosensors, artificial mediators and permselective coatings are often used in biosensor fabrication. Artificial mediators are used to shuttle electrons between the enzyme and the electrode to allow operation at low potentials. 3-5 This approach can minimize interference with coexisting electroactive species, but the stability and toxicity of some mediators limit their in vivo applications. Permselective membranes are also used to eliminate interference. 6-7 Effective, but incomplete, rejection has been reported in most cases. A mediator-free and membrane-free biosensor was described by Wang's method provides a means for measuring the cathodic current of enzymatically liberated hydrogen peroxide in metal-dispersed carbon paste biosensors. The idea of a mediator-free and membrane-free biosensor based on the reduction of hydrogen peroxide has provided a new approach for biosensor development.Recently, electrochemical properties of carbon nanotubes (CNTs) have been unveiled, and their application toward electrochemical sensors and biosensors has gained interest. [10][11][12][13][14][15][16][17][18][19][20][21][22]

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The length and the spacing of carbon nanotube (CNT) films are varied independently to investigate their effect on the field-emission characteristics of the vertically aligned CNT films grown by plasma-enhanced hot filament chemical vapor deposition using pulsed-current electrochemically deposited catalyst particles. It is shown that, in general, the macroscopic electric field Emac,1, defined as the electric field when the emission current density reaches 1 mA/cm2, can be reduced by increasing the length and the spacing of CNTs. However, for the very-high-density CNT films, the increase of length increases Emac,1 slightly, whereas for the very short CNT films, the increase of spacing does not effectively reduce Emac,1.

Nanoelectrode arrays (NEAs) were fabricated from low site density aligned carbon nanotubes (CNTs). The CNTs were grown by plasmaenhanced chemical vapor deposition on Ni nanoparticles made by electrochemical deposition. Each nanotube is separated from the nearest neighbor by several micrometers. NEAs of 1 cm 2 consisting of up to millions of individual nanoelectrodes, each with a diameter of 100 nm, were made by this nonlithography method. These carbon NEAs could be used as templates to fabricate other metal NEAs. Electrochemical characterization including cyclic voltammetry and square wave voltammetry were performed.

Highly aligned arrays of multiwalled carbon nanotube (MWCNT) on layered Si substrates have been synthesized by chemical vapor deposition (CVD). The effect of the substrate design and the process parameters on the growth mechanism were studied. Adding water vapor to the reaction gas mixture of hydrogen and ethylene enhanced the growth which led to synthesis of longer CNT arrays with high density. Environmental scanning electron microscopy (ESEM), energy-dispersive spectroscopy (EDS), and atomic force microscopy (AFM) were used to analyze the CNT morphology and composition. Quadrupole mass spectroscopy (QMS) provided in-situ information on the gas spices within the reaction zone. On the basis of results, we verified the top growth mechanism and evaluated the reason of decline and stoppage of the CNT growth after extended period of deposition. Multilayered Si substrates with a top film of Al2O3, having appropriate roughness, provide favorable conditions to form catalyst islands with uniform distribution and size. Using water-assisted CVD process and optimized substrate design, our group succeeded to grow vertically aligned, patterned MWCNT up to 4-mm long. The arrays were of high purity and weak adhesion which allowed to be peeled off easily from the substrate.

Pulse-current electrochemical deposition has been used to prepare Ni nanoparticles that are used as the catalysts for the growth of aligned carbon nanotubes. The nucleation site density of the Ni nanoparticles was controlled by changing the magnitude and duration of the pulse current. The aligned carbon nanotubes from the nickel nanoparticles were grown by plasma enhanced hot filament chemical vapor deposition. The site density of the aligned carbon nanotubes varied from 10 5 to 10 8 cm Ϫ2. The achievement of controlling the site density is significant for applications of carbon nanotubes as field emitters, nanoelectrodes array, etc.

We describe an ultrasensitive voltammetric detection of trace heavy metal ions using nanoelectrode arrays (NEAs) that are based on low-site density carbon nanotubes (CNTs). The NEAs were prepared by sealing the side-walls of CNTs with an epoxy passive layer that reduces the current leakage and eliminates the electrode capacitance, leading to a low background current. This provides a high signal-to-noise ratio. The CNTs-NEAs coated with a bismuth film were used successfully for voltammetric detection of trace cadmium(II) and lead(II) at the sub-ppb level. The detection limit of 0.04 microg L(-1) was obtained under optimum experimental conditions. The attractive behavior of the new carbon NEA sensing platform holds great promise for onsite environmental monitoring and biomonitoring of toxic metals.

Patterned multiwall carbon nanotube arrays up to four millimeters long were synthesized using chemical vapor deposition. Electrochemical actuation of a nanotube array tower was demonstrated in a 2 M NaCl solution at frequencies up to 10 Hz with 0.15% strain using a 2 V square wave excitation. The synthesis and electrochemical modeling approach outlined in the paper provide a foundation for the design of nanotube smart materials that actuate and are load bearing.