1. Introduction

1. Introduction 1-1 Developing bio-sensor for application in medical diagnosis Recently, bio-molecular analysis has become one of the most important tools in medical diagnosis. Highly sensitive and accurate methods for molecular diagnosis are urgently needed. Currently, commercial diagnostic kits are not efficient for quantitative analysis and their applications are frequently limited by their sensitivities. Recent development in biochip technology has greatly reduced the size (from ml to µl) of samples to be examined. At the same time, the sensitivity of the sensor chip needs to be significantly enhanced. Applications of nanoscale sensor chip are a major research direction to produce semiconductor sensor with the required sensitivity for a biosensor. Over the past few years, a lot of researches focused on specific properties of carbon nanotube (CNT) (Odom et al., 1998), particularly on the nano-scale structure and on the one-dimensional electronic conduction, rendering it became appropriate in a variety of nano-electronic devices. One (Tans et al., 1998) proposed that a promising application is on field effect transistors (FET). Since the first fabrication of carbonnanotube field effect transistor (CNT FET) was developed, performance of this mannered device has been significantly improved in many aspects, e.g., the contact between the CNT and the metal electrodes, on-off ratio of channel current, and hole mobility. In recent years, the highly charge sensitive property of 1 CNT FET rendered it an excellent candidate of being a nano-scale sensing device. Kong et al. (2000) were the first that built a single-walled nanotube (SWNT) chemical sensor for the detection of NO2 and NH3 gas. Electrochemical gating was studied on single multi-walled nanotube (MWNT) by Krüger et al. (2000) and SWNT by Rosenblatt et al.(Rosenblatt et al., 2002). Related reports have demonstrated a few highly sensitive biosensors fabricated by distinct configurations, procedures, and biomolecule-coupling techniques (Besteman et al., 2003). Generally, biological applications are performed in aqueous solution where an electronic device may exhibit an obvious leakage current across the two electrodes, namely the source and drain, in the FET device. In sensing circumstances, this leakage current contributes to background interference (or named background current) (Someya et al., 2003) of a sensor hampering detections toward a lower sensing limit and a higher sensitivity, crucial parameters of any sensor, and limiting the effective working range. This significant background signal, primarily ascribed to the electrochemical electrolysis of aqueous solution on the electrode surface, dominated the conduction across the two electrodes. Furthermore, the significant background current may degrade the thin-film metal electrode under a high applied bias across the source and drain. Therefore, a useful solution effectively diminishing the leakage current is valuable and worth to be developed. In this study we propose a method 2 solving this concern and investigate the effect of leakage current of the CNT device the biological sensing application. Further, we explore the relationship between deionized water (resistivity～18.2 ΜΩ－cm) and NaCl solution (1x10-4M) by the passivated CNT device. Currents through the CNTs are obviously different between two liquid samples: deionized water and NaCl solution. We also examine the characters in different concentration of NaCl solution (from 1x10-6M to 1M). It also indicates that the device can distinguish the differentiation of concentration of NaCl solution by high voltage of liquid-gate. Furthermore, the result indicates that the presented CNTs devices at certain bias conditions can be environmentally sensitive. The result also indicates that the CNT device is a potential candidate for developing a bio-sensor. 1-2 Discovery of the carbon nanotube (CNT) Crystalline (diamond and graphite) and amorphous (carbon black, pyrocarbon, etc.) forms of carbon have pride of place among technological materials – think of the abrasive properties of diamonds, the lubricating properties of graphite, and the performance of carbon fibers, with a micro-graphite structure, used in many applications. Two new forms of carbon have recently been discovered and application. Fullerenes were discovered in 1985, by an Anglo-American team (Kroto et al., 1985), and nanotubes in 1991, by a Japanese team (Iijima et al., 1991). A nanotube is a 3 cylinder with a graphite structure (curved, like a roll of chicken wire) which closed at both ends by a fullerene type cap (Odom et al., 1998). These cylinders can be a few microns or even millimeters long, with a diameter of the order of a nanometer (10-9 m) – hence their name. They were discovered by a Japanese electron microscopist Sumio Iijima, who was studying the material deposited on the cathode during the arc-evaporation synthesis of fullerenes (Iijima et al., 1991). He found that the central core of the cathodic deposit contained a variety of closed graphitic structures including nanoparticles and nanotubes, of a type which had never been observed previously. A short time later, Thomas Ebbesen and Pulickel Ajayan, from Iijima's laboratory, showed how nanotubes could be produced in bulk quantities by changing the arc-evaporation conditions. This paved the way to a research explosion into the physical and chemical properties of carbon nanotubes in laboratories all over the world. 1-3 Types and properties of CNT Carbon nanotubes are cylindrical molecules of ~1 nm in diameter and 1-100 microns in length (Odom et al., 1998). They are constituted of carbon atoms only, and can essentially be thought of as a sheet of graphite (a hexagonal lattice of carbon) rolled into a cylinder. There are two kinds of nanotube –MWNT and SWNT. Most of these tubular fullerene molecules consisting of multiple shells are arranged in a 4 coaxial fashion. Individual MWNTs can be easily resolved. MWNTs, however, have many unexplained features, e.g. the multi-layered structure affects electrical conduction in the MWNT devices. On the other hand, SWNTs, discovered in 1993, serve as a model system for theoretical calculation and some key experiments because of their simple and well defined structure. Nevertheless, the SWNTs tend to form a bundled structure, which contains many SWNTs of various lengths. It is quite difficult to resolve the SWNT bundle into individual SWNT and it will be an important technique to isolate the bundle into individual SWNT effectly. Single-walled carbon nanotubes exist in a variety of structures corresponding to the many ways a sheet of graphite can be wrapped into a seamless tube. Each structure has a specific diameter and chirality, or wrapping angle. The “armchair” nanotubes, with a = 30°, have metallic characteristic. The “zigzag” nanotubes, for which a = 0°, can be either semimetallic or semiconductive, depending on the specific diameter. “Helical” nanotubes with chiral angles intermediate between 0 and 30° include both semimetals and semiconductors. (“Armchair” and “zigzag” refer to the pattern of carbon–carbon bonds along a tube’s circumference.) Therefore, carbon nanotubes, depending on their structure, can be used as metals or semiconductors (Mintmire et al., 1992; Saito et al., 1992). Understanding the 5 electronic properties of the graphene sheet helps to understand the electronic properties of carbon nanotubes. Graphene is a zero-gap semiconductor; for most directions in the graphene sheet, there is a band-gap, and electrons are not free flowing along those directions unless they are given extra energy. However, in certain special directions graphene is metallic, and electrons flow easily along those directions. This property is not obvious in bulk graphite, since there is always a conducting metallic path which can connect any two points, and hence graphite conducts electricity. However, when graphene is rolled up to make the nanotube, a special direction is selected along the axis of the nanotube. Sometimes it exhibits metallic property, and sometimes is semiconducting property. Therefore, both metals and semiconductors can be made from the same all-carbon system In addition to their interesting electronic structure, nanotubes have several other useful properties. Nanotubes are incredibly stiff and tough mechanically - the world's strongest fibers. Nanotubes conduct heat as well as diamond at room temperature. Nanotubes are very sharp, and thus can be used as probe tips for scanning-probe microscopes, and field-emission electron sources for lamps and displays. The above characteristics have generated strong interest in their possible use in nano-electronic and nano-mechanical devices. For example, they can be used as 6 nano-wires or as active components in electronic devices such as the field-effect transistor (Martel et al., 1998; Tans et al., 1998). Thus, carbon nanotubes are a novel material system whose unique properties offer intriguing possibilities for the fabrication of nanometre-scale molecular electronic device. Carbon nanotubes can be thought of as naturally occurring nano size bricks that could act as base components for nano-electronics. By building up the nanobricks, nano scale device structures, which cannot be fabricated from three-dimensional bulk materials, can be constructed. Prior to construction, there are a number of issues to be addressed. Thus, nanotubes are ideal candidates in molecular electronic technology. 1-4 Application of CNT The discovery of carbon nanotubes prompted

Load more