Carbon nanotubes are intriguing structures that have sparked much excitement in recent years and a large amount of research has been dedicated to their understanding.
Carbon nanotubes are unique cylindrical carbon-based, seamless tubes of graphite sheets having molecular structures that exhibit interesting and novel properties that make them potentially useful in a wide variety of applications (e.g., nano-electronics, optics, materials applications, etc.). They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Two general types of carbon nanotubes are referred to as multi-wall carbon nanotubes (MWCNTs) and single-wall carbon nanotubes (SWCNTs). SWCNTs have a cylindrical sheet-like, one-atom-thick shell of hexagonally-arranged carbon atoms, and MWCNTs are typically composed of multiple coaxial cylinders of ever-increasing diameter about a common axis. Thus, SWCNTs can be considered to be the structure underlying MWCNTs and also carbon nanotube ropes, which are uniquely-arranged arrays of SWCNTs. Single-Wall Carbon Nanotubes (SWCNTs) have shown promising sensing properties in terms of sensitivity.
The above carbon nanotubes which are also known as buckytubes are members of the fullerene structural family, which also includes buckyballs. Buckyballs are spherical in shape, while a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several centimeters in length.
Single-wall carbon nanotubes (SWCNTs), have unique and very attractive physical properties, such as high strength, stiffness, thermal and electrical conductivity. SWCNTs are hollow, tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms similar to graphite and typically arranged in hexagons and pentagons. The sp2 bonding structure is stronger than the sp3 bonds found in diamond therefore providing the molecules with their unique strength. Nanotubes naturally align themselves into “ropes” held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp2 bonds for sp3 bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking. Single-wall carbon nanotubes typically have diameters in the range of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm. Useful information on single-wall carbon nanotubes can be found in B. I. Yakobson and R. E. Smalley, American Scientist, Vol. 85, July-August, 1997, pp. 324-337.
In 1991, Sumio Iijima, a researcher with the NEC Laboratory in Tsukuba, Japan, observed that carbon fibers produced with a carbon arc were hollow. This feature of nanotubes is of great interest to physicists because it permits experiments in one-dimensional quantum physics.
The diameter of most SWCNTs is a few nanometers, with a tube length that can be many thousands of times larger. The structure of a SWCNT can be conceptualized by wrapping a one-atom-thick layer of graphite (called graphene) into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n,m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m=0, the nanotubes are called “zigzag”. If n=m, the nanotubes are called “armchair”. Otherwise, they are called “chiral”.
A very important variety of carbon nanotube is a SWCNT because they exhibit important electric properties that are not shared by the multi-wall carbon nanotube (MWCNT) variants. SWCNTs are the most likely candidate for miniaturizing electronics past the microelectromechanical scale that is currently the basis of modern electronics. The most basic building block of these systems is the electric wire, and SWCNTs can be excellent conductors. One useful application of SWCNTs is in the development of the intramolecular field effect transistors (FETs).
The synthesis of fullerenes is typically accomplished by the condensation of vaporized carbon at high temperature. Fullerenes, such as C60 and C70, may be made by carbon arc methods using vaporized carbon at high temperature. Carbon nanotubes have also been produced as one of the deposits on the cathode in carbon arc processes.
Single-wall carbon nanotubes have been produced in very low yields in a DC arc discharge apparatus by simultaneously evaporating carbon and a small percentage of Group VIIb transition metal from the anode of an arc discharge apparatus. The above technique produces a population of carbon nanotubes which exhibits significant variations in structure and size.
An alternative method to synthesize carbon nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules dissociate on the metal particle surface and the resulting carbon atoms combine to form nanotubes. The method typically produces imperfect multi-wall carbon nanotubes, but under certain reaction conditions, can produce excellent single-wall carbon nanotubes. One example of this method involves the disproportionation of CO to form single-wall carbon nanotubes and CO2 catalyzed by transition metal catalyst particles comprising Mo, Fe, Ni, Co, or mixtures thereof residing on a support, such as alumina. The method often results in tangled carbon nanotubes and also requires the removal of the support material for many applications.
Another way of producing single-wall carbon nanotubes involves laser vaporization of a graphite substrate doped with transition metal atoms (such as nickel, cobalt, or a mixture thereof) to produce single-wall carbon nanotubes. The single-wall carbon nanotubes produced by this method tend to be formed in clusters, termed “ropes,” of about 10 to about 1000 single-wall carbon nanotubes in parallel alignment, held by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although certain structures may predominate. The laser vaporization process can produce improved yields of single-wall carbon nanotubes, however the resulting product is still heterogeneous, and the nanotubes tend to be too tangled for many potential uses of these materials. In addition, the laser vaporization of carbon is a high energy process.
The known methods of synthesizing single-carbon nanotubes also produce a distribution of reaction products, including, but not limited to, single-wall carbon nanotubes, amorphous carbon, metallic catalyst residues, and, in some cases, multi-wall carbon nanotubes. The distribution of reaction products will vary depending on the process and the operating conditions used in the process. In addition to the distribution of reaction products, the process type and operating conditions will also produce single-wall carbon nanotubes having a particular distribution of diameters and conformations.
The electronic properties of single-wall carbon nanotubes are very dependent on the conformation. For example, armchair tubes are metallic and have extremely high electrical conductivity. All single-wall carbon nanotubes can be categorized as metallic, semi-metals, or semiconducting depending on their conformation. In the present specification, both metallic tubes and semi-metal tubes will be referred to collectively as metallic nanotubes. For single-wall carbon nanotubes, about one-third of the tubes are metallic and about two-thirds are semiconducting. Metallic (n, m)-type nanotubes are those that satisfy the condition: 2n+m=3q, or n−m=3q where “q” is an integer. Metallic nanotubes are conducting with a zero band gap in energy states. Nanotubes not satisfying either condition are semiconducting and have an energy band gap. Generally, semiconducting nanotubes with smaller diameters have larger energy band gaps. Regardless of tube type, all single-wall nanotubes have extremely high thermal conductivity and tensile strength.
The properties of a collection of a particular (n, m) selected carbon nanotube will differ from those of a mixture of single-wall carbon nanotubes that are made by the different production processes. The properties of mixtures of nanotube types represent a composite value over a distribution of property values. This composite value is generally not “average” behavior. In fact, the properties of composites can not only be inferior to, but also lacking altogether in a mixture of single-wall carbon nanotubes compared to those of a particular selected (n, m) type nanotube. Additionally, in the diameter range of single-wall carbon nanotubes, generally about 0.5 nm to about 3.5 nm, the alignment of the nanotubes to each other in closely-packed arrays, such as the well-known single-wall carbon nanotube “ropes”, can be optimized when all the nanotubes are of the same diameter, minimizing misfits between tubes of different diameter.
No effective process for making single-wall carbon nanotubes is known whereby significant quantities of carbon nanotubes made are of a single (n, m) type. Furthermore, to date, no methods for separating quantities of single-wall carbon nanotubes by (n, m) type are known, and no macroscopic quantity of such single (n, m) type single-wall carbon nanotube material has been produced. Macroscopic amounts of type-sorted single-wall carbon nanotubes that would provide the broadest range of possible nanotube properties and applications are heretofore unknown.
The particular nanotube diameter and conformation affects the physical and electronic properties of the single-wall carbon nanotube. For example, the stiffness, strength, density, electrical conductivity, magnetic properties, crystallinity, thermal conductivity, absorption, response to doping, utility as semiconductors, optical properties such as absorption and luminescence, utility as emitters and detectors, energy transfer, heat conduction, reaction to changes in pH, buffering capacity, sensitivity to a range of chemicals, contraction and expansion by electrical charge or chemical interaction, nanoporous filtration membranes and many more properties are affected by the diameter and conformation of the single-wall carbon nanotube.
Little work has been done on the unique chemical properties of carbon nanotubes in solution, particularly, in aqueous solutions. As an extended conjugated double-bond system with high surface area, carbon nanotubes are expected to stabilize and accumulate charges far better than small molecules. The accumulated charges on a carbon nanotube can be potentially used for many applications including reactions that are hard to carry out using small molecules.
Industry, academia, experimentalist and theoreticians have given a great deal of attention in trying to understand the electronic transport and optical properties of carbon nanotubes. For example, the plasmon excitation properties have been studied for singlewall and multiwall nanotubes as well as a linear array of nanotubes. Plasmon excitations display many new features in the presence of an applied magnetic field, such as many cusps in the plasmon spectrum as a function of the magnetic flux through the tubule. The flux-dependent plasmon frequency has been shown to be proportional to the induced persistent-current density in the tubule.
Despite their unique properties, the low solubility of carbon nanotubes in aqueous solution has impeded further development of useful applications in biologically related systems. The common approach to enhance their solubility in aqueous solution is to either physically or chemically incorporate charged groups into a carbon nanotube. Sonication has been a widely used physical method. Recently, Chiang et al. (J. Phys. Chem. B 105, 8297 (2001)) have reported a procedure to prepare protonated single-wall carbon nanotubes through sonication of carbon nanotubes in acidic solution. It is believed that the hydronium ions strongly intercalate into the ropes of carbon nanotubes, which cannot be removed by simple vacuum drying. Protonated single-wall carbonnanotubes may be considered as charged nanotubes.
In addition to positively charged nanotubes, negatively charged nanotubes have been obtained by sonication of the mixture of double stranded DNA and nanotubes in aqueous solution. Nakashima et al. (Chem. Lett. 32, 456 (2003)) have shown that DNA and nanotube complexes are water soluble. Meanwhile, the negative charge sign of DNA-nanotube complexes has been confirmed by Zheng et al. (Science 302, 1545 (2003)). In other words, both positively and negatively charged carbon nanotubes can be synthesized.
An alternative to add charges onto a carbon nanotube may be achieved by surface modification. Water-soluble SWCNTs have been obtained by surface modifications such as functionalization with carboxylate groups and surface coatings with surfactants or single stranded DNA. As a matter of fact, there has been great interest in using single-wall carbon nanotubes (SWCNTs) as nanoscale probes and sensors in biological electronics and optical devices because the electronic and optical properties of SWCNTs are extremely sensitive to the surrounding environmental changes. To date, most research on SWCNTs has focused on electronic devices, with relatively little work on optical biosensors.
SWCNTs possess unique optical properties as a result of their one-dimensional nature. Sharp peaks in the density of states, called van Hove singularities, arise from a quantization of the electronic wave vector in the 1-D system. As a result of these singularities, SWCNTs possess peaks in their optical spectra that correspond to interband transitions from the valence band to the conduction band. In addition, the transitions are found to be grouped in spectral space according to nanotube type (metallic vs. semiconducting) and band index, which are responsible for the observed sharp and pronounced optical absorption peaks in individual SWCNTs.
The abovementioned water-soluble SWCNTs, obtained by surface modifications, have displayed undisrupted characteristic optical absorption features. It has been observed that the optical characteristics of surface modified SWNTs are pH sensitive, which suggests new opportunities for SWCNTs based optical biosensor applications yet to be explored. Nanotubes may even be combined with recently developed nanolasers, nano waveguides and nano optical fibers, to make optical nanosensors in the near future.
Additionally, the probing, characterization, and manipulating of single polymers like DNA has been accomplished with the aid of optical methods, e.g., observing evanescent field fluorescence of dye molecules, deflecting light beams in atomic force microscopes, or trapping attached dielectric objects with optical tweezers. Also, there has been some progress at the molecular level in the study of the electrical ionic conduction signals from voltage biased nanoscale biopores. Recently, a voltage bias on an alpha hemolysin biopore has been shown to induce charged single-stranded DNA and RNA molecules to translocate through the pore. Each translocating molecule blocks the open pore ionic current providing an electrical signal that depends on several characteristics of the molecule. The system has limits for studies of biological molecules: the pore is of a fixed size, and its stability and noise characteristics are restricted by chemical, mechanical, electrical, and thermal constraints.
Therefore, a long felt need exists in the art to address the aforementioned deficiencies and inadequacies. The prior art is silent on the use of charged carbon nanotubes for applications such as optical pH sensors, biosensors and many other applications. Also, there is a need for new methods for studying polymer molecules such as DNA.
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