Review
A review of DNA functionalized/grafted carbon nanotubes and their characterization

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Abstract

The functionalized carbon nanotubes (CNTs) are believed to be very promising in the fields such as preparation of functional and composite materials and biological technologies. Immobilization of nanotubes with specific recognition biosystems indeed provides ideal miniaturized biosensor. A prerequisite for the search in this area is the development of chemical methods to immobilize biomolecules onto carbon nanotubes in a reliable manner. The DNA-based biomolecular recognition principle has been applied to CNTs to constant nanotube electronic devices as well as CNT–DNA electrochemical sensors. The sp2 hybridization and the outstanding electronic properties of the nanotubes coupled with their specific recognition properties of the immobilized system indeed make CNTs, an ideal biosensor. DNA immobilization has been paid great attention and considered as a fundamental methodology for the construction of DNA biosensors. Successful integration of CNTs in electronic devices and sensors requires controlled deposition at well-defined locations and appropriate electrical contacts to metal leads. Different methods for achieving this goal are directional growth of the tubes, alignment by mechanical forces, alignment by electric and magnetic fields, patterned- and self-assembly.

Also the concept of using DNA to direct the assembly of nanotubes into nanoscale devices is attracting attention because of its potential to assemble a multicomponent system in one step by using different base sequence for each component. Thus, DNA functionalization of CNTs holds interesting prospects in various fields including solubilization in aqueous media, nucleic acid sensing, gene-therapy and controlled deposition on conducting or semiconducting substrates. This review highlights the functionalization/grafting of DNA onto single walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) with or without self-assembly which can be employed in fabricating biosensors for selective recognition of DNA. The review also addresses various characterization techniques that have been employed by various researchers to give the readers an insight into the planning of experiments and subsequent interpretation.

Introduction

Carbon nanotubes (CNTs) are a new allotrope of carbon originated from fullerene family, which will revolutionalize the future nanotechnological devices [1]. CNTs can be imagined as rolled up graphite sheets held together by van der Waal's bonds. These nanosized near perfect whiskers were first noticed and characterized in 1991 by Iijima [2] of NEC Corporation in Japan. CNTs are elongated fullerenes that can be produced with exactly 12 pentagons and millions of hexagons [3]. They have a long cylinder made of hexagonal honey comb of lattice of carbon, bound by two pieces of fullerenes at the ends. The first nanotubes observed were multiwalled CNTs (MWCNT) which consist of two or more concentric cylindrical shells of graphene sheets coaxially arranged around a central hollow area with a spacing between the layers which is close to that of the interlayer separation as in graphite (0.34 nm) [4]. In contrast, single shell or single walled nanotubes [5], [6] (SWCNT) are made of single grapheme (one layer of graphite) cylinders and have a very narrow size distribution(1–2 nm). Both types of nanotubes have the physical characteristics of solids and are microcrystals, although their diameters are close to molecular dimensions.

Carbon nanotubes have aroused increasing interests of many researchers due to their remarkable tensile strength, high resilence, flexibility and other unique structural, mechanical, brilliant electrical and physicochemical properties [7], [8]. Carbon nanotubes (CNTs) can be either metallic or semiconductive depending on their helicity and diameter, i.e. depending on the atomic structure, the CNTs behave as a metal or semiconductor. The exciting electronic properties suggest that CNTs have the ability to promote the electron transfer reactions when used as an electrode in electrochemical reactions [9]. The electrical conductivity of CNTs can be altered by modifying the parent structure. Thus, a novel strategy of altering the electronic properties of nanotubes are done either by chemically functionalizing them with a moiety or altering the structure whose intrinsic properties are electrically configurable. By doping the CNT with nitrogen, the semiconducting nanotubes becomes metallic [10]. This change in electronic properties makes CNTs an ideally suited material for the inter-connects in nanoelectronic devices. Also the electrical transport studies on CNTs reveal that they can behave as quantum wires at low temperature [11].

The nanodimension of the CNTs provide very large surface area. The surface area of the SWCNTs were found to be one magnitude higher than that of graphite but smaller compared to the activated porous carbon. Similarly, due to the relatively large hollow channels in the centre of nanotubes, their density is very low compared to graphite. Rough estimates suggest that SWCNT density could be as small as 0.6 g/cm3 and MWCNT density could range from 1 to 2 g/cm3 depending on the constitution of the samples. The porosity and the reactivity of the CNTs make them an ideal candidate for the storage of neutral species as well as electron donors [12]. Also since CNTs are having high surface area, they are better substrates than amorphous glassy carbon in studying the immobilization or growth of biomolecules [13]. The incorporation of superstrong light weight CNT structures into organic and inorganic polymeric matrices offers a novel approach to the design of high performance composite and ceramic materials with super mechanical properties [14], [15].

Like graphite, CNTs were relatively non-reactive, except at the nanotube caps which are more reactive due to the presence of the dangling bonds. The reactivity of the sidewalls of the carbon nanotube π-system can be influenced by the tube curvature or chirality. Loung et al. [16] reported that when CNTs were sonicated in organic solvents, they produce dangling bonds that will undergo further chemical reactions. Due to the less solubility of CNTs in any of the solvents, it is also very difficult to isolate one carbon nanotube from the other. The organic functional groups chemically attached to the CNTs assist effectively in unroping the nanotube bundles and improving their solubility. The reactivity of CNTs depends on the curvature of their surface. This review highlights the functionalization/grafting of DNA onto SWCNTs and MWCNTs with or without self-assembly which can be employed in fabricating biosensors for selective recognition of DNA.

Section snippets

Chemical modification of CNTs

Experiments on opening multiwalled tubes with acids and other reagents could result in the formation of surface carboxylate and other groups on the nanotube surfaces. More recently attempts have been made to functionalize SWCNT. For example, in late 1998, a group from University of Kentucky [17] described a method for dissolving SWCNT in organic solutions by derivatization with thionyl chloride and octadecyl amine. This approach opened the way for solution phase chemistry to be carried out on

CNTs as sensors

The unique structural, electrical and mechanical properties of CNTs make them extremely attractive for the task of electrochemical sensing. Electrodes modified with CNTs have been employed for the detection of important biomolecules including Cytochrome c, ascorbic acid, NADH, etc. [20]. A wide range of inorganic and biological molecules can be adsorbed onto nanotube in the hope of using them as highly accurate tiny sensors [10]. CNTs have been recently used as transducers for enhanced

Functionalization/grafting of CNTs

The surface functionalization will aid the carbon nanotube materials in becoming biocompatible, improving their solubility in physiological solutions and selective binding to biotargets. The functionalized CNTs were believed to be very promising in the fields such as preparation of functional and composite materials and biological technologies [24].

The functionalization of CNTs may be separated into two categories—a noncovalent wrapping or adsorption and the covalent tethering. Strano [10]

DNA-based sensors

DNA is an important and promising molecule with all the basic properties necessary for the assembly of nanoscale electronic devices. The combination of CNT with DNA has attracted the attention of several research groups recently. DNA chains have been used to create various functional structures and devices through the sequence-specific pairing interactions. The DNA-based biomolecular recognition principle has been applied to CNTs to construct nanotube electronic devices as well as CNT–DNA

Characterization of functionalized CNTs

Belin and Epron [56] have recently reviewed various characterization methods for CNTs. As pointed out by these authors, a limited number of techniques are available for the investigation of the morphological and structural characterization of CNTs. Of these, scanning tunneling microscopy and transmission electron microscopy are only useful in characterizing CNTs at the individual level. X-ray photoelectron spectroscopy (XPS) is helpful in determining chemical structure of nanotubes, while

Summary

Ajayan [4] has vividly described the origin, development, distinguishing properties and futuristic scope of CNTs market place. The author has concluded that nanotubes as quantum wires in electronic devices is a promising field but most imminent applications are based on a combination of nanotubes and other materials such as filled polymer. The present review starts off with introduction giving concise account of CNTs and the use of carbon nanotubes as sensors followed by detailed account on

Sobhi Daniel received her MSc degree from the M.G. University in 1991 and MPhil degree from the Kerala University in 2001 in chemistry. She is currently pursuing the PhD in the Regional Research Laboratory, Trivandrum. Her research interest is focused on nanomaterials, especially ion imprinted polymers.

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  • Cited by (0)

    Sobhi Daniel received her MSc degree from the M.G. University in 1991 and MPhil degree from the Kerala University in 2001 in chemistry. She is currently pursuing the PhD in the Regional Research Laboratory, Trivandrum. Her research interest is focused on nanomaterials, especially ion imprinted polymers.

    Dr. T. Prasada Rao received his PhD degree in chemistry in 1981 from Indian Institute of Technology, Chennai after his postgraduation in analytical chemistry from Andhra University. He worked as scientist in different CSIR laboratories of India, such as IICT Hyderabad, CECRI Karaikudi, and RRL Trivandrum and currently is Deputy Director and scientist-in-charge of Inorganic Materials Group of RRL. He is recipient of Prof. T.L. Ramachar award and Andhra University medal in 1987 and 1994, respectively. Has to his credit 1 US patent, 6 Indian patents and published 175 research papers including 22 review articles in SCI journals. Eight students were awarded PhD degrees under his guidance. His current research interests are solid phase extraction, flow injection analysis, molecular/ion imprinted polymers and biomimetic sensors.

    Kota Sreenivasa Rao received his BSc degree in 1995 from Nagarjuna University, Guntur and obtained MSc degree and PhD in chemistry from Sri Venkateswara University, Tirupati in 1997 and 2001, respectively. He spent two and half years in Kyoto University, Kyoto, and later he joined in Osaka University, Japan in 2005 and is continuing his research activities. His main fields of current scientific interests are surface science of lipid membranes, development of electric nanobiosensors, synthesis and characterization of nanomaterials, and fuctionalization of biomolecules for analysis of target analytes.

    Sikhakolli Usha Rani received her BSc degree from P.B. Siddharadha college of Arts & Sciences in 2002 and MSc degree in biotechnology from Jawaharlal Nehru Technological University in 2004. Her main fields of current scientific interest are functionalization of nanomaterials by biomolecules and observation by AFM study.

    G.R.K. Naidu received his BSc and MSc degree in chemistry from Sri Venkateswara University, Tirupati in 1973 and 1975, respectively. He is presently Professor and Head in the department of Environmental Sciences and continuing his research activities. His main fields of current scientific interests are on development of chemical sensors for analysis of environmental and biological samples, flow injection analysis, molecular/ion imprinted polymers and biomimetic sensors.

    HeaYeon Lee received her BSc and MSc degree in applied chemistry from Pukyong National University, South Korea in 1987 and 1990, respectively. She received her PhD in chemistry in 1995 from Osaka University, Japan. Her main field of current scientific interest is nanoarray patterning and electrochemical bioassay for nanobioelectronic sensor.

    Tomoji Kawai received his BSc, MSc, and PhD degree in chemistry from Tokyo University, Japan in 1969, 1971, and 1974, respectively. He is presently Director in the Institute of Scientific and Industrial Research (ISIR-SANKEN), Osaka University, Japan. His research interests are DNA and Protein Nanotechnology for various medical applications.

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