**3. Carbon nanotubes: fascinating nano-objects**

In the last decade, the dramatic development of nanotechnology in material science and engineering has led to the study and the development of innovative nanostructured materials (Xu T. et al., 2007; Niemeyer, 2001; Cui & Gao, 2003; Whitesides, 2003): certain materials with delicate structures of "small" sizes, falling in the 1–100 nm range, and specific properties and functions related to the "size effect" (Safarik & Safarikova, 2002; Laval et al., 1999). In particular, extensive researches have been focused on medicine and biomedical engineering for the investigation of the interactions between nanomaterials and biological systems (Foldvari & Bagonluri, 2008; Bianco et al., 2005; Desai, 2000). Potential products of bionanotechnology in the pharmaceutical and biomedical industry are refered as nanomedicines, including materials to be employed as clinical bio-analytical diagnostics (Bianco et al., 2005; Pantarotto et al., 2003), carriers to improve controlled and targeted drug release (Leary et al., 2006; Sinha & Yeow, 2005) and additives to improve solubility and bioavailability of poorly soluble drugs (Ajayan et al., 1993), novel tissue engineered scaffolds and devices (Bauer et al., 2004; Mazzola, 2003; Kikuchi et al., 2004).

Within the realm of bionanotechnology, carbon nanotubes (CNTs), a major class of carbonbased tubular nanostructures have been denoted great interest in the scientific community (Ke & Qiao, 2007; Klingeler & Sim, 2011).

The walls of CNTs are made up of a hexagonal lattice of carbon atoms analogous to the atomic planes of graphite (Dresselhaus et al., 2004). CNTs can be imaginatively produced by rolling up a graphene sheet (a single layer of graphite) forming a single-walled CNTs (SWCNTs), or by rolling up many graphene layers to form concentric cylinders (multiwalled CNTs; MWCNTs). The ends of CNT may be closed with halfspheres of fullerens. MWNTs are comprised of two up to several to tens of concentric carbon cylinders and thus generally have a larger outer diameter (2.5–100 nm) than SWNTs (0.6–2.4 nm) (Joselevich, 2004) (Figure 2).

Fig. 2. Schematic representation of SWNTs and MWNTs.

It should also be mentioned that, as a control in each polymerization, a non-imprinted polymer (NIP) is also synthesised in the same way as the MIP but in absence of the template. To evaluate the imprinting effect, the selectivities of the NIP and MIP are then compared.

In the last decade, the dramatic development of nanotechnology in material science and engineering has led to the study and the development of innovative nanostructured materials (Xu T. et al., 2007; Niemeyer, 2001; Cui & Gao, 2003; Whitesides, 2003): certain materials with delicate structures of "small" sizes, falling in the 1–100 nm range, and specific properties and functions related to the "size effect" (Safarik & Safarikova, 2002; Laval et al., 1999). In particular, extensive researches have been focused on medicine and biomedical engineering for the investigation of the interactions between nanomaterials and biological systems (Foldvari & Bagonluri, 2008; Bianco et al., 2005; Desai, 2000). Potential products of bionanotechnology in the pharmaceutical and biomedical industry are refered as nanomedicines, including materials to be employed as clinical bio-analytical diagnostics (Bianco et al., 2005; Pantarotto et al., 2003), carriers to improve controlled and targeted drug release (Leary et al., 2006; Sinha & Yeow, 2005) and additives to improve solubility and bioavailability of poorly soluble drugs (Ajayan et al., 1993), novel tissue engineered scaffolds

Within the realm of bionanotechnology, carbon nanotubes (CNTs), a major class of carbonbased tubular nanostructures have been denoted great interest in the scientific community

The walls of CNTs are made up of a hexagonal lattice of carbon atoms analogous to the atomic planes of graphite (Dresselhaus et al., 2004). CNTs can be imaginatively produced by rolling up a graphene sheet (a single layer of graphite) forming a single-walled CNTs (SWCNTs), or by rolling up many graphene layers to form concentric cylinders (multiwalled CNTs; MWCNTs). The ends of CNT may be closed with halfspheres of fullerens. MWNTs are comprised of two up to several to tens of concentric carbon cylinders and thus generally have a larger outer diameter (2.5–100 nm) than SWNTs (0.6–2.4 nm) (Joselevich,

**3. Carbon nanotubes: fascinating nano-objects** 

and devices (Bauer et al., 2004; Mazzola, 2003; Kikuchi et al., 2004).

Fig. 2. Schematic representation of SWNTs and MWNTs.

(Ke & Qiao, 2007; Klingeler & Sim, 2011).

2004) (Figure 2).

Although there have been many interesting and successful attempts to grow CNTs by various methods (Klingeler et al., 2008; Vyalikh et al., 2008), the three most widely used techniques are: arc discharge, laser ablation, and chemical vapor deposition (CVD) (Ando et al., 2004).

The arc-discharge method is the one by which CNTs were first produced and recognized. In a model system, a dc arc voltage is applied between two graphite rods in the presence of an appropriate ambient gas. This method is useful for the production of both CNTs and fullerenes. In particular, when pure graphite rods are used, fullerenes are deposited in the form of soot in the chamber (Saito et al., 1992). However, a small part of the evaporated anode is deposited on the cathode, which includes CNTs (Iijima, 1991). As described by Ando et al., 2004, these CNTs represent MWNTs. They are found not only on the top surface of the cathode deposit (Ando, 1993) but also deep inside the deposit (Ando & Iijima, 1993). Large-scale synthesis of MWNTs by arc discharge has been achieved (Ebbesen & Ajayan, 1992; Colbert et al., 1994) in He gas. The same methodology could be applied to the synthesis of SWNTs if a graphite rod containing metal catalysts (Fe, Co, etc.) is used as the anode with a pure graphite cathode (Iijima & Ichihashi, 1993; Bethune et al., 1993). By using a dc pulsed arc discharge inside a furnace, homogeneous conditions in arc discharge are achieved and high-quality DWNTs are synthesized by a method called high-temperature pulsed arc discharge (Sugai et al., 1999, 2000, 2003; Shimada et al., 2004).

The laser ablation, based on the high energy density of lasers (typically a YAG or CO2 laser) which is suitable for materials with a high boiling temperature such as carbon, was developed for fullerene and CNT production by Smalley's group (Guo T. et al., 1992; Thess et al., 1996). The laser has sufficiently high energy density not to cleave the target into graphite particles but to vaporize it at the molecular level converting the graphite vapor into amorphous carbon as the starting material of SWNTs (Puretzky et al., 2000; Sen et al., 2000; Kokai et al., 2000). The annealing conditions of the amorphous carbon in the laser ablation method are more homogeneous than those of the arc-discharge method, in which the electrodes and the convection flow disturb the homogeneity of the temperature and flow rate (Zhao X. et al., 2003; Kanai et al., 2001).

Chemical Vapor Deposition (CVD) is a simple and economic technique for synthesizing CNTs at low temperature and ambient pressure. Usually, a carbon feedstock is thermally decomposed in the presence of a metal catalyst (Cirillo et al., 2011b). The generated carbon dissolves in the catalyst particles and, after saturation, is deposited in the shape of CNTs.

To be distinguished from the many kinds of CVD used for various purposes, the method is also known as thermal or catalytic. Compared with arc-discharge and laser methods, CVD is more versatile because it offers better control over growth parameters. Furthermore, it harnesses a variety of hydrocarbons in any state (solid, liquid, or gas), enables the use of various substrates, and allows CNTs growth in a variety of forms, such as powder, thin or thick films, aligned or entangled, straight or coiled, or even a desired architecture of nanotubes at predefined sites on a patterned substrate. MWNTs were grown from benzene, ethylene, methane, and many other hydrocarbons (Endo et al., 1993; José-Yacamán et al., 1993; Satiskumar et al., 1999; Hernadi et al., 1996). SWNTs were first produced by Dai et al. from disproportionation of CO, and SWNTs were also produced from benzene, acetylene, ethylene, and methane using various catalysts. (Cheng et al., 1998; Satishkumar et al., 1998; Hafner et al., 1998; Kong J. et al., 1998; Flahaut et al., 1999). Due to lower synthesis

Carbon Nanotubes – Imprinted Polymers: Hybrid Materials for Analytical Applications 187

environment-friendly and energy-saving automotive parts, high-purity components in semiconductor manufacturing equipment for highly integrated chips, internal walls of vacuum vessels in the nuclear fusion reactors expected to be an energy source in the 21st century, and negative electrodes of lithium ion secondary batteries, of which demand is rapidly increasing with the popularization of mobile electronic instruments (Kazua, 1999). During the last years a great interest has been focused on the application of carbons (Flavel et al., 2009; Ma G.P. et al., 2009) as electrode materials because of their accessibility, easy processability and relatively low cost. They are chemically stable in different solutions (from strongly acidic to basic) and able to show performance in a wide range of temperatures. Already well-established chemical and physical methods of activation allow for the production of materials with a developed surface area and a controlled distribution of pores

that determine the electrode/electrolyte interface for electrochemical applications.

(Atesa & Sarac, 2009).

The possibility of using the activated carbon without binding substances (e.g. fibrous fabrics or felts) gives an additional advantage from the construction point of view. Unique properties of ultra-microelectrodes (UMEs) make them very attractive in electroanalytical measurements which require high spatial and temporal resolution (Wigthman, 1988). At ultra-microelectrodes, at typical slow scan rates of voltammetry, signal to noise ratios (S/N) can increase up to one decade due to efficient mass transportation to the electrode resulting from edge effects. In addition, because of the small dimensions and low IR drop, fast scan voltammetric measurements (Garreau et al., 1990; Hsueh et al., 1993) and measurements in highly resistive media are possible at UMEs (Howell & Wightman, 1984) while high spatial resolution and minimal tissue damage (Musallam et al., 2007). can be achieved in vivo analysis. Microelectrodes are useful for measurements for changes in easily oxidized neurotransmitter concentrations in extracellular brain fluid (Kawagoe et al., 1993) as they provide a way to observe the rapid chemical changes associated with the release of neurotransmitters from neurons and their subsequent removal from the extracellular fluid

The nanodimensions of CNTs guarantee a very large active surface area and are especially suited for the conception of miniaturized sensors. In addition to the high porosity and reactivity, this makes them ideal candidates for the storage of neutral species as well as electron donors, when used as electrodes in electrochemical reactions, for which they may also be called "nanoelectrodes" (Basabe-Desmonts et al., 2007; Daniel et al., 2007; Guo et al., 2004). Hence, it is not surprising that gas sensors made from individual nanotubes show good sensitivities at room temperature (J. Kong et al., 2001; Vasanth Kumar et al., 2010, 2011) in comparison to commercially available classical semiconductor sensors, which in general operate above 200°C. However, a necessary prerequisite is that the molecules to be detected must have a distinct electron donating or accepting ability, which is fulfilled, for example, by ammonia (NH3) as a donor and nitrogen dioxide (NO2) as an acceptor. The adsorption of these molecules on the nanotubes is associated with a partial charge transfer, which alters the charge-carrier concentration or, alternatively, the adsorbed molecules may affect the potential barriers present at the tube–electrode contacts. In any event, the resulting change in the electrical resistance of the nanotube is utilized as a sensor signal. However, for the detection of molecules that are only weakly adsorbed (e.g., carbon monoxide and hydrogen), the change in resistance is often too small. A possible method to overcome this drawback is accomplished by the modification of the nanotube sidewalls with nanoparticles made of a suitable metal. For instance, sensitive hydrogen sensors operating at room

temperatures CVD grown CNTs exhibit a lower crystallinity than tubes produced using the two alternative methods mentioned above.

When considering the whole of CNTs applications, the solubilization of pristine CNTs in aqueous solvents remains an obstacle to realizing their potential, due to the rather hydrophobic character of the graphene sidewalls. To successfully disperse CNTs the dispersing medium should be capable of both wetting the hydrophobic tube surfaces and modifying the tube surfaces to decrease tube aggregation. Four basic approaches have been used to obtain a dispersion: surfactant-assisted dispersion (Ham et al., 2005; Islam et al., 2003; Moore et al., 2003; Yurekli et al., 2004; Vaisman et al., 2006), solvent dispersion (Fu & Sun, 2003; Ausman et al., 2000; Kim D.S. et al., 2005), functionalization of CNT sidewalls (Dyke & Tour 2004; Fernando et al., 2004; Peng et al., 2003), and biomolecular dispersion (Gigliotti et al., 2006).
