Applications of CARBON NANOTUBES in Biotechnology

Abstract Carbon nanotubes (CNTs), first discovered by Iijima in 1991, are a remarkable class of nanomaterials within the fullerene family. These elongated, hollow cylindrical structures, often described as "nano test tubes," possess extraordinary mechanical strength, electrical conductivity, and thermal stability. Their nanoscale dimensions and unique properties make them indispensable in various scientific and industrial applications. Among the synthesis methods available, Chemical Vapor Deposition (CVD) is the most widely used and efficient. This process involves the application of a high electric current between graphite electrodes in a helium atmosphere, leading to the controlled deposition of CNTs on vessel walls and electrodes. The exceptional properties of CNTs have unlocked their potential in a wide range of fields. In electronics, they are utilized for developing high-performance transistors, sensors, and conductive materials. Their lightweight and high-strength characteristics make them ideal for reinforcing composite materials used in aerospace and automotive industries. Additionally, their high surface area and conductivity enable their use in energy storage devices, such as batteries and supercapacitors. The discovery of CNTs has revolutionized material science, paving the way for advanced technologies in medicine, energy, and environmental sustainability. Introduction: The third allotropic form of carbon buckminsterfullerene also known as buckyballs discovered in 1985 has ball form structure. There are 60 atoms of carbon arranged in icosahedral geometry in its structure. Smalley and kroto vaporize graphite with laser they got symmetrical closed cluster in which carbon atom are bonded to each other in spherical symmetry known as buckyballs which is greatly stable. Then for its commercial applications Wolfgang from max plank institute and Donald Huffman of the University of Arizona used slightly modified technique then described first. They vaporize graphite using carbon arc in a helium atmosphere. As a result of this experiment fullerenes were produced. [1][10] Carbon nanotube discovered by Iijima belongs to a class of fullerenes rich family that is elongated having hollow structure inside. They were discovered in 1991 at NEC laboratories in Japan. They are nanometers sized ropes and test tubes. Many methods are used for its synthesis but carbon vapor Deposition (CVD) is the best one, in which two graphite electrodes are used and current of 50 A is applied between them in the helium atmosphere. It causes the evaporation of the graphite. Some part of it is deposited on the wall of the vessel and some part is deposited on the cathode, which are mainly the carbon nanotubes. [1][8][10] They are of two types multi-walled tubes that were discovered first and single walled tubes that were discovered 2 years after the discovery of multi-walled tubes. Structure of CNTs: Carbon nanotubes are of two types: 1. Single-walled carbon nanotubes 2. Multi-walled carbon nanotubes Structure of SWCNTs: Single-walled carbon nanotubes are formed by the wrapping of single sheet of graphite to assume the shape of the cylinder. Their wrapping can be of three types: I. Arm chair nanotube II. Zigzag nanotube III. Chiral nanotube Structure of MWCNTs: MWCNTs are made up of multiple layers of graphite that rolled upon them to form a tube shaped with an interlayer spacing of 3.4 Å. The outer diameter of MWCNTs may range from 1 to 50 nm while the inner diameter can be varied. Two models are used to describe the structures of MWCNTs I. Russian Doll model where the sheets of graphite are arranged in concentric cylinders. II. Parchment model in which a single sheet of graphite is rolled around itself like a toilet tissue paper [1][3]. Applications: Properties like small sizes, high conductivity, high mechanical strength and high flexibility make nanotubes use in nanoprobes. These probes can be used in high resolution imaging, nano-lithography, nanoelectrodes, drug delivery, and sensors etc. High resolution imaging: Multi-walled CNTs are attached at the tip of the scanning probe microscope used for the purpose of imaging. These nanotubes tips can be used in SEM, AFM etc. The advantage of using CNTs tips is that they are conducting and can make image of extremely small size particle or structure, deep cracks which are impossible for the probe using silicon or other metal tips. Biological molecules, such as DNA can also be imaged using nanotube tips for example imaging of amyloid-b-protofibrils that is related to Alzheimer’s disease whose resolution was never achieved before. Due to the elasticity of the CNTs they don’t suffer crash on contacting with the substrate. To detect different things CNTs can be chemically modified. They can also be used as molecular probe. [6] DNA-CNT nano wire networks for DNA detection: DNA in a solution can be detected in the presence of oligo-nucleotide functionalization enzyme. The reactive site on the CNT is created by treating it with an acid for introducing a carboxyl group at its end. An ss-DNA is bridged between two SWCNTs through its amide groups in such a way that the two tubes are joined to each other at their terminus via covalent linkage through ssDNA. DNA analyte and oligo-nucleotide enzyme acting as a probe is immersed in a solution. DNA-CNT nano wire is kept between two electrodes in that solution. DNA to be analyzed has two recognition sites: one with the probe and the other with the bridging DNA strand. Thus in the presence of analyte DNA and probe the gap between two CNTs is filled by the complementary base pairing of probe. In the presence of suitable reagents, by enzymatic metallization conductive connections take place between two tubes upon the detection of DNA. So this technique can be helpful for detecting and study of various genetic diseases [5]. This method is also significant in the detection of matched and mismatched bases. When there is mismatching between the two strands, detection signal falls down slowly and becomes in sensitive. For just a single mismatch, there is a detection decline of about 70%. This device can be used for the detection of DNA at femto molar level and can be used for detection of DNA viruses such as Ebola virus [13]. CNT based electrodes with immobilized enzymes: CNT-modified electrodes are improved for their selectivity and sensitivity when enzymes are immobilized on their surfaces. The CNT then acts as the transducer that communicates the signal between the enzyme active sites and their specific substrates. Enzymatic interaction can be covalent or non covalent. Non covalent methods do not affect the activity and efficiency of enzyme. Non covalent methods include adsorption, entrapment and encapsulation. In first method, enzyme is attached with help of binder or ion exchange resin. In second technique, (electro) polymerization reaction is carried out and enzyme is dissolved in resulting matrix of polymer. In encapsulation, enzyme is immobilized with help of sol-gels or hydro gels. Non covalent method distributes the enzymes randomly. However, equal distribution takes place through covalent attachment method. It also helps in direct electron transfer to active center of enzyme. This technique is very helpful in sequencing of human genome project. SWCNT modified GCE and MWCNT can detect smaller sequence of about 24 basepairs. Immobilized enzymes as biosensors: Detection of glucose: For detection of glucose, glucose biosensors are used that incorporate an enzyme glucose oxidase immobilized on the surface of CNT. To stabilize the enzyme on CNT electrodes, phosphate buffer is used in presence of Argon conditions. It also prevents the denaturation of enzyme and contamination of biomaterial. Glucose oxidase (GOx) oxidizes β-D glucose to D-glucono 1,5-lectone using oxygen as electron acceptor and producing H2O2. The amount of this H2O2 produced is detected electrochemically at the electrodes and can be estimated for the level of glucose present inside the body. So it has beneficial application for diabetes diagnostics [14]. Detection of phenolic compounds: Some phenolic compounds and cresols are toxic, so their determination in food products and environment is very necessary. MWCNTs layered with Nafions are used for their detection. The enzyme tyrosinase is adsorbed on the surface of CNT modified electrode. In presence of oxygen, the oxidation of phenolic compounds in presence of tyrosinase converts them to quinone. The quinone produced at electrodes is detected electrochemically and its amount is then estimated for maintaining safe level. A CNT paste electrode is also modified with enzyme polyphenyloxidase (PPOx). It detects various phenolic compounds including phenols, catequine and catechols. These compounds are detected in various pharmaceutical products for maintaining their quality [12]. Dopamine sensing: Sensing of dopamine is very important in medical diagnoses. The first biosensor utilizing CNT electrode was the dopamine sensor. Dopamine sensor consists of a multi walled carbon nano tube paste electrode. This electrode is covered by Nafion membrane sometimes, which blocks invading ascorbates that exhibit the same properties as dopamine. MWCNT electrode was used for the detection of dopamine in goat by making its brain tissue homogenate. When a microelectrode of carbon fiber is modified with an MWCNT, this device gives a linear response by increasing concentrations of dopamine in brain. The detection of dopamine is also performed by PPOx-modified CNT paste electrodes, which is sensitive to about 2 µA/mmol/L [9]. Organo phosphorus compounds: Organo phosphorus agents are used as chemical warfare agents as they are neurotoxins. They are also used as common pesti