Carbon nanotubes (CNTs) have been the focal point of many studies since their discovery two decades ago (Iijima, 1991). They are allotropes of carbon with cylindrical shape and sp 2 hybridization, which are formed by benzene-type hexagonal rings of carbon atoms. CNTs can be classified as either single-walled (SWNTs) or multi-walled carbon nanotubes (MWNTs) depending on their structure. They possess a very high aspect ratio (i.e. length/diameter), with diameters in the range of nanometers and lengths that can reach microns. Additionally, CNTs can be divided according to their conductivity, into metallic or semiconducting tubes. Nanotubes possess outstanding structural, mechanical and electronic properties due to the unique combination of their dimension, structure and topology. For example, CNTs are extremely strong, can be highly conducting, and exhibit high thermal and chemical stability (Mintmire et al., 1993; Mintmire & White, 1998). Also, they are elastic and have many times the tensile strength of steel (Service, 1998). Due to these unique properties, CNTs have been proposed for many applications, ranging from scanning probes (Dai et al., 1996) and hydrogen storage (Dillon et al., 1997), to biosensors (Nguyen et al., 2002). Several methods have been commonly used to synthesize CNTs, including arc discharge, chemical vapor deposition and laser ablation, among others (Ando et al., 2004). After the growth process, CNTs contain impurities such as metal catalyst nanoparticles, amorphous carbon, carbon nanoparticles, and fullerenes, which must be removed. Typical CNTs purification techniques include ultrasonication (Shelimov, et al., 1998), microfiltration (Bandow et al., 1997), chromatographic techniques (Nigoyi et al., 2001), microwave heating (Harutyunyan et al., 2002), gas-phase oxidation (Ebbesen et al., 1994), and acid oxidation (Rinzler et al., 1998). Nevertheless, their purification is not a trivial task, and either improvement on the existing techniques or development of new purification methodologies are required in order to facilitate the use of CNTs and their integration into nanodevices and composite materials. Another main challenge to overcome is the need for the development of new functionalization chemistries that can increase the solubility of CNTs without altering their 35 www.intechopen.com

Iron oxide nanoparticles were coated with the biocompatible, biodegradable, non-immunogenic polysaccharide inulin by introduction of carboxyl groups into the inulin structure and conjugation with amine groups on the surface of iron oxide nanoparticles grafted with 3-aminopropyltriethoxysilane. The resulting nanoparticles were characterized by FT-IR spectroscopy, transmission electron microscopy, dynamic light scattering, zeta potential, SQUID magnetometry, and with respect to their energy dissipation rate in applied alternating magnetic fields. The nanoparticles had a hydrodynamic diameter in the range of 70 ± 10 nm and were superparamagnetic, with energy dissipation rates in the range of 58–175 W/g for an applied field frequency of 233 kHz and an applied field amplitude in the range of 20–48 kA/m. The nanoparticles were stable in a range of pH, at temperatures between 23°C and 53°C, and in short term storage in water, PBS, and culture media. The particles were non-cytotoxic to the immortalized human cancer cell lines Hey A8 FDR, A2780, MDA 468, MCF-7 and Caco-2. The nanoparticles were readily taken up by Caco-2 cells in a time and concentration dependent fashion, and were found to have a pharmacokinetic time constant of 47 ± 3 min. The small size, non-cytotoxicity, and efficient energy dissipation of the particles could make them useful for biomedical applications such as magnetic fluid hyperthermia.

Carbon nanotubes (CNTs) have been broadly studied due to their exceptional structural, electronic and mechanical properties, and their use have been proposed for many applications. The number of biosensing applications of CNTs has increased in the past few years. Nevertheless, in order to use CNTs as standard materials in the biosensing field and to take full advantage of their unique properties, several problems must be solved. For example, the solubility of CNTs, especially in aqueous solvents, needs to be improved. Furthermore, reliable methodologies for the easy, reproducible and efficient incorporation of CNTs into solid substrates must be developed. Chemical functionalization of CNTs with biomolecules, and specifically with DNA, provides an alternative to the previous challenges. This review will present a brief overview of the methods available for the chemical functionalization of CNTs with DNA by either covalent or non-covalent means. Moreover, the main scope will be on describing the use of DNA-CNT complexes in biosensing applications. Detection of ions, glucose, peroxide and DNA hybridization using the DNA-CNT hybrids will be discussed.

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