International Journal of Chemical and Analytical Science ISSN: Research Article

Transcription

1 Carbon Nanotubes International Journal of Chemical and Analytical Science ISSN: Research Article Ghosh Rajesh*, Roshan Makam, Pritpal Kaur, Department of Biotechnology, PESIT, 100 feet Ring Road, Banashankari 3 rd stage, Bangalore , Karnataka, India Carbon nanotubes are molecular-scale tubes of graphitic carbon with outstanding properties. The small dimensions, strength and the remarkable physical properties of these structures make them a very unique material with a whole range of promising applications. Carbon nanotubes are predicted to have interesting mechanical properties in particular, high stiffness and axial strength as a result of their seamless cylindrical graphitic structure. The last decade of research in this field points to several possible applications for these materials; electronic devices and interconnects, field emission devices, electrochemical devices, such as supercapacitors and batteries, nanoscale sensors, electromechanical actuators, separation membranes, filled polymer composites, and drug-delivery systems are some of the possible applications that have been demonstrated in the laboratories. The objective of this report is to study the different techniques used to synthesize carbon nanotubes, to study how chirality (twist) of the tubes affect the physical and electrical properties and also to study how carbon nanotubes play a significant role in treating cancer and its drug delivery pathway. Its applications in capacitors and solar cells along with the future trends in the field of carbon nanotubes have also been described. Keywords: Graphite, Nanotubes, Nano Sensors, Drug Delivery, Carbon Electrodes, Arc Discharge, Chemical Vapor Deposition, Laser Ablation INTRODUCTION Carbon nanotubes are allotropes of carbon with a cylindrical nano structure. Carbon nanotubes are one of the most commonly mentioned building blocks of nanotechnology. Carbon nanotubes were 'discovered' in 1991 by Sumio Iijima of NEC and are effectively long, thin cylinders of graphite Carbon nanotubes exist as a macro-molecule of carbon, analogous to a sheet of graphite (the pure, brittle form of carbon in the pencil lead) rolled into a cylinder. Graphite is made up of layers of carbon atoms arranged in a hexagonal lattice, like chicken wire. Though the chicken wire structure itself is very strong, the layers themselves are not chemically bonded to each other but held together by weak forces called Van der Waals. However, when coiled, the carbon arrangement becomes very strong. The main reason why these early tubes did not excite wide interest is that they were structurally rather imperfect, so did not have particularly interesting properties. Recent research has focused on improving the quality of catalyticallyproduced nanotubes. Singled walled nano-tube: Singled walled NTs: Single walled carbon nanotubes (SWCNTs) can be considered to be formed by the rolling of a single layer of graphite (called a graphene layer) into a seamless cylinder. SWCNT is one atom thick. The tube length may be many thousands of times larger and up to orders of cm. SWNT consists of two separate regions with different physical and chemical properties. The first is the sidewall of the tube and the second is the end cap of the tube. Fig. 1: Rolling of graphene sheet Nanotubes generally have a length to diameter ratio of about 1000 so they can be considered as nearly one-dimensional structures. The typical diameter of a carbon nanotube is 1nm. These nanotubes have a hemispherical "cap" at each end of the cylinder. They are light, flexible, thermally stable, and are chemically inert. They have the ability to be either metallic or semi-conducting depending on the "twist" of the tube. The twist of the tube depends upon the chirality of CNTs. The chirality in turn affects the conductance of the nanotube, its density, its lattice structure, and other properties. It is important to note, that nanoscale tubes of carbon, produced catalytically, had been known for many years before Iijima s discovery [1, 2]. Fig. 2: Types of SWNTs. The end cap structure is similar to or derived from a smaller fullerene, such as C60. C-atoms placed in hexagons and pentagons form the end cap structures. It can be easily derived from Euler s theorem that twelve pentagons are needed in order to obtain a closed cage structure which consists of only pentagons and hexagons. The combination of a pentagon and five surrounding hexagons results in the desired curvature of the surface to enclose a volume. A second rule is the isolated pentagon rule that states that the Corresponding Author: Ghosh Rajesh, Department of Biotechnology, PESIT, 100 feet Ring Road, Banashankari 3 rd stage, Bangalore , Karnataka, India Received ; Accepted March, 2012 International Journal of Chemical and Analytical Science, 2012, 3(3),

2 distance between pentagons on the fullerene shell is maximised in order to obtain a minimal local curvature and surface stress, resulting in a more stable structure. The smallest stable structure that can be made this way is C60, the one just larger is C70 and so on. Another property is that all fullerenes are composed of an even number of C-atoms because adding one hexagon to an existing structure means adding two C-atoms [2]. The other structure of which a SWNT is composed is a cylinder. It is generated when a graphene sheet of a certain size that is wrapped in a certain direction. Since the cylinder is symmetric we can only roll in a discreet set of directions in order to form a closed cylinder. Special properties of Carbon Nanotubes: Chemical reactivity: Carbon nanotube reactivity is directly related to the pi-orbital mismatch caused by an increased curvature. Electrical conductivity: Depending on their chiral vector, carbon nanotubes with a small diameter are either semiconducting or metallic. Optical activity: Theoretical studies have revealed that the optical activity of chiral nanotubes disappears if the nanotubes become larger. Mechanical strength: Carbon nanotubes have a very large Young modulus in their axial direction [3]. Synthesis of Carbon Nanotubes: There are three techniques for the synthesis of carbon nanotubes 1. Arc discharge 2. Laser ablation 3. Chemical vapour deposition Arc discharge method : The carbon arc discharge method is the most common and easiest way to produce carbon nanotubes as it is rather simple to undertake. However, it is a technique that produces a mixture of components and requires separating nanotubes from the soot and the catalytic metals present in the crude product. by approximately 1mm, in an enclosure that is usually filled with inert gas (helium, argon) at low pressure (between 50 and 700 mbar). Liquid nitrogen can also be used with this method. A direct current of 50 to 100A driven by approximately 20V creates a high temperature discharge between the two electrodes. The discharge vaporises one of the carbon rods and forms a small rod shaped deposit on the other rod. Producing nanotubes in high yield depends on the uniformity of the plasma arc and the temperature of the deposit form on the carbon electrode [5,6]. Factors affecting Arc Discharge Method Different diameter distribution depends upon the mixture of Argon and helium. These mixtures have different diffusion co-efficient and thermal conductivities. These properties affect the speed with which carbon and the catalyst molecules diffuse and cool in turn affecting the nanotube diameter. Single layer tubules nucleate and grow on metal particles in different sizes depending on the quenching rate in the plasma. Temperature, metal catalysts and carbon densities affect the diameter distribution of the nanotubes. Laser vaporization: Synthesis of SWNT: Uniform SWNT s could be synthesised if a mixture of graphite with Co, Ni, Fe or Y is used instead of pure graphite [2, 4]. SWNT s synthesised this way exists as ropes. Laser vaporisation results in a higher yield for SWNT synthesis and the nanotubes have better properties and a narrower size distribution than SWNTs produced by arc-discharge. Nanotubes produced by laser ablation are purer (up to about 90 % purity) than those produced in the arc discharge process. The Ni/Y mixture catalyst (Ni/Y is 4.2/1) gives the best yield. The Ni/Co catalyst with a pulsed laser at 1470 C gives SWNTs with a diameter of nm. In case of a continuous laser at 1200 C and Ni/Y catalyst (Ni/Y is 2:0.5 at. %), SWNTs with an average diameter of 1.4 nm were formed with % yield. Different techniques in synthesizing SWNT 1. Ultra fast pulses from a free electron laser method 2. Continuous wave laser powder method Chemical vapour deposition Fig. 3: Arc discharge Synthesis: This method creates nanotubes through arcvaporisation of two carbon rods placed end to end, separated Fig. 4: Chemical Vapour Process Synthesis: Chemical vapor deposition (CVD) is the means of synthesis that is of interest for this study. CVD synthesis is achieved by taking a carbon species in the gas phase and March, 2012 International Journal of Chemical and Analytical Science, 2012, 3(3),

3 using an energy source, such as plasma or a resistively heated coil, to impart energy to a gaseous carbon molecule. Commonly used gaseous carbon sources include methane, carbon monoxide, and acetylene. The energy source is used to "crack" the molecule into a reactive radical species. These reactive species then diffuse down to the substrate, which is heated and coated in a catalyst (usually a first row transition metal such as Ni, Fe, or, Co) where it will bond. The result is that carbon nanotubes will form if the proper parameters are maintained. CVD carbon nanotube synthesis is essentially a two step process. A catalyst preparation step followed by actual synthesis of the nanotube. The catalyst particles must exist in order for one to grow carbon nanotubes by CVD. The catalyst is generally prepared by sputtering a transition metal onto a substrate and then using either a chemical etch or thermal annealing to induce catalyst particle nucleation. Ammonia may be used as the etchant. The thickness of the transition metal layer before the annealing step ranges from nanometers. Reported temperatures for the synthesis of nanotubes by CVD vary somewhat, but are generally within the ⁰C range. Common carbon sources are methane and acetylene. Hydrogen must also be present during the synthetic step in order for nanotube synthesis. It has been shown that the size and material of the catalyst particles play a vital role during synthesis. CVD allows the experimenter to avoid the process of separating nanotubes from the carbonaceous particulate that often accompanies the other two methods of synthesis. Excellent alignment, as well as positional control on the nanometer scale, can be achieved by the use of CVD. Control over the diameter, as well as the growth rate of the nanotube can also be maintained. The appropriate metal catalyst can preferentially grow single rather than multi-walled nanotubes. [7,8,9] Applications of Carbon Nanotube: Energy storage: Carbon nanotubes are being considered for energy production and storage. Graphite, carbonaceous materials and carbon fiber electrodes have been used for decades in fuel cells, battery and several other electrochemical applications. Nanotubes are special because they have small dimensions, a smooth surface topology, and perfect surface specificity. The rate of electron transfer at carbon electrodes ultimately determines the efficiency of fuel cells and this depends on various factors, such as the structure and morphology of the carbon material used in the electrodes. Nanotube microelectrodes have been constructed using a binder and have been successfully used in bioelectrochemical reactions (e.g., oxidation of dopamine). Their performance has been found to be superior to other carbon electrodes in terms of reaction rates and reversibility. Nanotubes can be excellent replacements for conventional carbon based electrodes [10]. The properties of catalytically grown carbon nanofibers (which are basically defective nanotubes) have been found to be desirable for high power electrochemical capacitors. Hydrogen storage: The advantage of hydrogen as energy source is that its combustion product is water. In addition, hydrogen can be easily regenerated. For this reason, a suitable hydrogen storage system is necessary, satisfying a combination of both volume and weight limitations. The two commonly used means to store hydrogen are gas phase and electrochemical adsorption. Because of their cylindrical and hollow geometry, and nanometre-scale diameters, it has been predicted that carbon nanotubes can store a liquid or a gas in the inner cores through a capillary effect. Another possibility for hydrogen storage is electrochemical storage [11]. In this case not a hydrogen molecule but an H atom is adsorbed. This is called chemisorption. Metal hydrides and cryo-adsorption are the two commonly used means to store hydrogen, typically at high pressure and/or low temperature. In metal hydrides, hydrogen is reversibly stored in the interstitial sites of the host lattice. The electrical energy is produced by direct electrochemical conversion. Hydrogen can also be stored in the gas phase in the metal hydrides. The relatively low gravimetric energy density has limited the application of metal hydride batteries. Because of their cylindrical and hollow geometry, and nanometer-scale diameters, it has been predicted that the carbon nanotubes can store liquid and gas in the inner cores through a capillary effect. Nano probes and Nano sensors: Because of their flexibility, nanotubes can also be used in scanning probe instruments. Advantages are the improved resolution in comparison with conventional Si or metal tips and the tips do not suffer from crashes with the surfaces because of their high elasticity. However, nanotube vibration, due to their large length, will remain an important issue until shorter nanotubes can be grown controllably. Nanotube tips can be modified chemically by attachment of functional groups. Because of this, nanotubes can be used as molecular probes, with potential applications in chemistry and biology [11]. Other applications are the following: A pair of nanotubes can be used as tweezers to move nanoscale structures on surfaces. Sheets of SWNTs can be used as electromechanical actuators, mimicking the actuator mechanism present in natural muscles. SWNTs may be used as miniaturised chemical sensors. On exposure to environments, which contain NO 2, NH 3 or O 2, the electrical resistance changes. Biomedical applications of Carbon Nanotubes: CNTs have very interesting physicochemical properties, such as ordered structure with high aspect ratio, ultra-light weight, high mechanical strength, high electrical conductivity, high thermal conductivity, metallic or semi-metallic behavior and high surface area. The combinations of these characteristics make CNTs unique materials with the potential for diverse applications. To date, there has been an increasing interest among biomedical scientists in exploring all of the above-mentioned properties that CNTs possess for nanobiotechnology applications. For example, CNTs are currently being considered to be a suitable substrate for the growth of cells for tissue regeneration, as delivery systems for a variety of diagnostic or therapeutic agents or as vectors for gene transfection [5]. March, 2012 International Journal of Chemical and Analytical Science, 2012, 3(3),

4 targeting nature and the protective environment they provide, these nanoscale drug carriers may reduce the adverse side effects which result from the current methods of delivery of anticancer agents. Furthermore, nanocapsules may improve drug delivery methods in many areas other than cancer treatment, such as infections, metabolic diseases, autoimmune diseases, pain treatment and gene therapy. Fig:5. Biomodification of carbon nanotubes The three main approaches for modifying carbon nanotubes with biomolecules: the covalent approach (step a), noncovalent approach (step b) and hybrid approach where a small molecule anchor is first non-covalently absorbed to the carbon nanotube (step c), followed by a chemical reaction between the anchor and the biomolecules of interest (step d). For biological and biomedical applications, the lack of solubility of carbon nanotubes in aqueous media has been a major technical barrier. The recent expansion in methods to chemically modify and functionalize carbon nanotubes has made it possible to solubilize and disperse carbon nanotubes in water, thus opening the path for their facile manipulation and processing in physiological environments [6,11]. Equally important is the recent demonstration that biological and bioactive species such as proteins, carbohydrates, and nucleic acids can be conjugated with carbon nanotubes. These nanotube bioconjugates will play a significant role in the research effort toward bioapplications of carbon nanotubes[5,12]. One focal point has been the development of nanoscale bioelectronics systems based on carbon nanotubes, which has been driven by the experimental evidence that biological species such as proteins and DNA can be immobilized either with the hollow cavity of or on the surface of carbon nanotubes. Factors on which carbon nanotubes depend upon: Particles must be biologically inert and biodegradable. Must have high sorption capacity Sorption selectivity must be adjustable Convenient binding with anti-bodies must be possible High magnetization and magnetic susceptibility in the relatively weak magnetic fields should also be achievable (particularly if such particles are designed for the guided drug delivery) [6]. Drug delivery: The ideal drug carrier may be something out of science fiction. In principle, it is injected into the body and transports itself to the correct target, such as a tumor, and delivers the required dose at this target [6, 11, 12, 13]. This idealized concept was first proposed by Paul Ehrlich at the beginning of the 20th century and was nicknamed the magic bullet concept. With the advent of nanotechnology and nanomedicine this dream is rapidly becoming a reality. Nanotechnology has already been applied to drug delivery and cosmetics through the use of liposomal technology, and now nanoparticles and nanotubes present an exciting and more promising alternative. Typically nanoparticles have been used for drug delivery and it has been only recently that nanotubes have gained attention as potential drug delivery vehicles. Due to their precise Carbon nanotubes offer a number of advantages which suggest that they may provide an improved result. They have a larger inner volume which allows more drug molecules to be encapsulated, and this volume is more easily accessible because the end caps can be easily removed, and they have distinct inner and outer surfaces for functionalization. Both nanoparticles and nanotubes have been shown to be readily taken up by cells, and nanotubes have been found to enter cell nuclei suggesting that they may be useful in gene therapy [13].The general process of drug delivery using a carbon nanotube proceeds as follows: a. The carbon nanotube, in this case in the form of a nano test tube, has its surface functionalized with some chemical receptor such in folate-targeting ("Folate receptor-mediated drug targeting: From therapeutics to diagnostics") and the drug molecules are encapsulated through the open end. b. The open end is capped with some chemically removable cap, such as biodegradable. Degradation may be sensitive to the environment such as ph, or may be initiated by an external source. c. The nanocapsule is then introduced into the body by intravenous injection or orally, whereby it locates to the target site through the use of the chemical receptors. For example often cancer tumors over express folate receptors and thus the nanocapsule selectively bind to these cells. d. The cell ingests the nanocapsule, for example by receptor-mediated endocytosis. e. The chemically removable cap is triggered and either falls off or biodegrades. REFERENCES 1. Harris PJF, Carbon nanotubes and other graphitic structures as contaminants on evaporated carbon films,journal of Microscopy, Vol.186, 1997,pg: Harris PJF,Carbon nanotubes and related structures,cambridge Press, (Cambridge, London) JP Salvetat et al., Mechanical properties of carbon nanotubes, Appl. Phys. A 69, 1999, Charanjeet Singh et al., Catalytic synthesis carbon nanotubes and nanofibers, Encyclopedia of nanoscience and nano technology, vol 10, pg: Priti Singh et al.,synthesis of carbon nanotubes and their biomedical applications, Journal of Optoelectronics and Biomedical Materials Vol. 2, Issue 2, 2010, p E.borowiak-palen et al., Iron filled carbon nanotubes for bio-applications, Materials Science-Poland, Vol. 26, No. 2, Al. Darabont et al., Synthesis of carbon nanotubes by spray pyrolysis, Journal of Optoelectronics and Advanced Materials Vol. 7, No. 2, 2005,p Allouch et al., Chemical vapor deposition of pyrolytic carbon on carbon nanotubes, Popovska et al., Catalytic growth of carbon nanotubes on zeolite supported iron, ruthenium and iron/ruthenium nanoparticles by chemical vapor deposition in a fluidized bed reactor, Sandeep Agnihotri et al., Energy and Environmental applications of carbon nanotubes, Fuel Chemistry Division Preprints, 47(2), 2002,p 474. March, 2012 International Journal of Chemical and Analytical Science, 2012, 3(3),

5 11. Pulickel M. Ajayan1et al., Applications of Carbon Nanotubes, Topics Appl. Phys. 80, 2001,p Wenrong Yang et al., Carbon nanotubes for biological and biomedical applications, Nanotechnology 18, Wenting Chen et al., Investigation of Functionalized Carbon Nanotubes as a Delivery System for Enhanced Gene Expression, Source of support: Nil, Conflict of interest: None Declared March, 2012 International Journal of Chemical and Analytical Science, 2012, 3(3),