Yi Tu

Developing a sensor, actuator, and nanoskin based on carbon nanotube arrays

This paper describes progress in development of a sensor-actuator-nanoskin material based on multi-wall carbon nanotube arrays. This material can have individual sensing, actuation, or reinforcement properties, or the material may have combined multi-functional properties. The sensing and actuation properties are based on the theoretical telescoping property of multi-wall carbon nanotubes. The sensing property has been demonstrated in the literature. The actuation property is modeled in this paper but not demonstrated. Work is described that later may verify the actuation. Nanoskin samples are also fabricated and tested for mechanical, hydrophobicity, and capillarity properties. Overall, synthesis of dense arrays of long multi-wall carbon nanotubes is opening the door for the development of novel sensors, actuators, and multifunctional smart materials.

Substrate and Process Interplay During Synthesis of Millimeter Long Multi-Wall Carbon Nanotube Arrays

This paper reports data related to the effects of the substrate design and the deposition parameters on the growth of millimeter long multi-wall carbon nanotube (MWCNT) arrays by CVD (Chemical Vapor Deposition). Iron catalyst was formed on top of multilayered substrates of Si/SiO2and Si/SiO2/Al2O3by e-beam evaporation. The CNT synthesis was carried on in a hydrogen/ethylene/water/argon environment at 750 °C for different periods of deposition time. Atomic Force Microscopy (AFM), Environmental Scanning Electron Microscopy (ESEM), High Resolution Transmission Electron Microscopy (HRTEM), and Micro-Raman Spectroscopy were employed to characterize the substrates before and after nanotube growth. The study shows that for specific processing conditions, the length of highly oriented carbon nanotube arrays depends on the annealing temperature of the catalyst which determines the size of the catalyst particle. Other factors affecting the length are: substrate design and size and water concentration. The array nanotubes mitigate the limitations of the powdered “spaghetti type” CNT, and this is expected to open up new applications for them.

Carbon nanotube array smart materials

Highly aligned multi-wall carbon nanotube arrays up to 4 mm tall were synthesized on Si wafers using a chemical vapor deposition process with water delivery. Based on the long nanotube arrays, several prototype smart materials were developed including a biosensor, electrochemical actuator, and nanotube probes. The biosensor was formed by casting epoxy into a nanotube array and polishing the ends of the nanotubes. This electrode produced a near ideal sigmoidal cyclic voltammogram. Nanotube electrodes were then used to form a label-free immunosensor based on electrochemical impedance spectroscopy. The nanotube array immunosensor has good sensitivity, but decreasing the array size and improving the biofunctionalization is expected to dramatically increase the reproducibility and sensitivity. The electrochemical actuator was formed by bonding an electrode to a 1mm square by 4 mm long as-grown nanotube array post. The nanotube array actuator operated up to 10 Hz in a 2 M NaCl solution. With a driving voltage of 2 volts, the actuator produced 0.15% strain. Finally, nanotube bundles are being welded to tungsten tips and put inside glass needles for use as probes for biosensors and electrophysiology applications. All the smart materials applications discussed are recent, and further development is expected to yield improved performance and commodity level practical devices.

A New Intelligent Material Based on Long Carbon Nanotube Arrays

Highly aligned multi-walled carbon nanotube (MWCNT) arrays were synthesized on Si wafers. Water vapor was used to enhance the catalyst performance, which enabled continuous growth of MWCNT arrays for up to 3 hours. Various types of Fe patterning on a Si substrate with a multilayered structure were tested. MWCNT arrays up to 4 mm long were grown by Chemical Vapor Deposition (CVD). Environmental scanning electron microscopy was used to characterize the MWCNT morphology and showed that the nanotubes typically reveal a 20 nm outer diameter and 8 nm inner diameter. To investigate applications, a nanotube tower 1 mm × 1 mm × 4 mm in size was grown and peeled off the Si wafer. Each tower contains millions of individual nanotubes with 20–30 nm diameters. Electrochemical actuation of one MWCNT tower was demonstrated in a 2M NaCl solution. The MWCNT tower actuator operated up to 10 Hz without significantly decreasing strain. Only 1-2 volts was needed to obtain 0.2% strain. The aligned nanotube morphology of the tower is the reason for the high strain in the axial direction, which is an improvement compared to previously tested entangled buckypaper actuators. Cyclic voltammetry (CV) was performed to analyze the redox behavior of the nanotube tower used as an electrode. The CV response showed a sigmodal shape in a 6 mM K3(CN)6 ferrocyanide solution. This behavior provides ideal characteristics for biosensor development and application. Also, the measured electrical volume resistivity of the material was in the range of 0.1 ohm · cm. The overall improvement in the electrochemical actuation and electrical conductivity was much greater than previous nanocomposites obtained by dispersing powdered nanotubes into polymers. The demonstrated good properties suggest nanotube array towers can be considered a novel intelligent material.

Fabrication and Characterization of a Multiwall Carbon Nanotube Needle Biosensor

A nanotube electronic needle biosensor was developed to provide fast, low cost, accurate detection of biomolecules. The sensor was formed by synthesizing highly aligned multi-wall carbon nanotube arrays. Nanotube bundles from the array were welded onto the tips of tungsten needles using a microscope. The needles were then encased in glass and a polymer coating. Cyclic voltammetry (CV) for the respective reduction of 6 mM K3Fe(CN)6in a 1.0 M KNO3was performed to examine the redox behavior of the nanotube needle. The CV results showed a steady-state response attributable to radial diffusion with a high steady-state current density. An amperometric sensor was then developed for glucose detection by physical attachment of glucose oxidase on the nanotube needle. A label-free immunosensor based on electrochemical impedance spectroscopy was also formed. The nanotube needle amperometric have good sensitivity with a low detection limit, and the possibility exists to keep decreasing the size of the needle to increase the sensitivity.

The Columbi Eggs of Nanotechnology

The use of nanoscale materials to form larger size materials and devices is limited by many processing problems. These problems must be overcome to achieve many of the possible benefits of nanotechnology. In the context of finding seemingly simple solutions to complex problems, this paper looks at several of the barrier problems in nanoscale materials processing for different applications and proposes possible solutions to the problems. It is also noted that interdisciplinary and inter-institutional collaboration has made possible the progress reported in this paper.