**2. Molecularly Imprinted Polymers (MIPs): highly selective materials**

Molecular imprinting is a very useful technique to incorporate specific substrate recognition sites into polymers (Puoci et al., 2010). In particular, by this polymer synthesis technique, the formation of macromolecular networks characterized by specific recognition sites for a desired target molecule, named template could be obtained (Byrne & Salian, 2008). The specific binding properties of MIP must be attributed to specific interactions between the template and the functional groups in the polymeric network, thus the choice of the functional monomers is of primary importance to obtain performing imprinted materials (Puoci et al., 2007; Cirillo et al, 2010, 2011a)

In the synthesis of these macromolecular system, many parameters involved can affect the information associated with the binding sites, such as functional monomers/polymers, crosslinkers and solvents/porogens. Thus, both the feasibility of imprinting and the proper preparation conditions need exploration for the preparation of efficient imprinted materials (Liu Z. et al., 2010). It is important to state that MIP can be obtained in different formats, depending on the preparation method followed. To date, the most common polymerizations for preparing MIPs involve conventional solution, suspension, precipitation, multi-step swelling and emulsion core-shell. There are also other methods, such as aerosol or surface rearrangement of latex particles, but they are not used routinely (Puoci et al., 2011).

MIPs are stable polymers with molecular recognition abilities and resistant to a wide range of conditions (pH, organic solvents, temperature, pressure), and were used for several different applications, such as chromatographic stationary phases (Hishiya et al., 2003), enantiomeric separation (Adbo & Nicholls, 2001), solid-phase extraction (SPE) (El-Sheikh et al., 2010), and catalysis (Anderson et al., 2005); they were also used as receptors (Haupt, 2003), antibodies (Svitel et al., 2001), enzyme mimics (Nicholls et al., 1996), affinity and sensing materials (Syu et al., 2006), and, in recent years, pharmaceutical applications, such as drug discovery, drug purification, or drug delivery (Mosbach, 2006; Yu Y. et al., 2002; Caldorera-Moore & Peppas, 2009; Hilt & Byrne, 2004).

CNTs represent a new carbon material that has been widely recognized as the quintessential nanomaterial and, because the hexagonal arrays of carbon atoms of the CNTs surface have a strong interaction with other molecules or atoms, CNTs show a great analytical potential as a solid-phase extraction adsorbent. Additionally, CNTs' surfaces can be modified by introducing various organic functional groups, thus providing a strongly physic sorbing surface area, adjustable surface charge, and a source of protons for chemical ionization. It has been demonstrated that the surfaces of CNTs can be easily modified in numerous ways, either by covalent or non-covalent functionalization. All the facts mentioned before reveal that carbon nanotubes, and specially multiwalled carbon nanotubes, have great analytical potential as an effective solid-phase extraction adsorbent for chelates or ion pairs of metal

Based on these considerations, it could be summarized that MIPs-CNTs composites represent useful innovative materials for analytical determination of target analytes in

Molecular imprinting is a very useful technique to incorporate specific substrate recognition sites into polymers (Puoci et al., 2010). In particular, by this polymer synthesis technique, the formation of macromolecular networks characterized by specific recognition sites for a desired target molecule, named template could be obtained (Byrne & Salian, 2008). The specific binding properties of MIP must be attributed to specific interactions between the template and the functional groups in the polymeric network, thus the choice of the functional monomers is of primary importance to obtain performing imprinted materials

In the synthesis of these macromolecular system, many parameters involved can affect the information associated with the binding sites, such as functional monomers/polymers, crosslinkers and solvents/porogens. Thus, both the feasibility of imprinting and the proper preparation conditions need exploration for the preparation of efficient imprinted materials (Liu Z. et al., 2010). It is important to state that MIP can be obtained in different formats, depending on the preparation method followed. To date, the most common polymerizations for preparing MIPs involve conventional solution, suspension, precipitation, multi-step swelling and emulsion core-shell. There are also other methods, such as aerosol or surface rearrangement of latex particles, but they are not used routinely

MIPs are stable polymers with molecular recognition abilities and resistant to a wide range of conditions (pH, organic solvents, temperature, pressure), and were used for several different applications, such as chromatographic stationary phases (Hishiya et al., 2003), enantiomeric separation (Adbo & Nicholls, 2001), solid-phase extraction (SPE) (El-Sheikh et al., 2010), and catalysis (Anderson et al., 2005); they were also used as receptors (Haupt, 2003), antibodies (Svitel et al., 2001), enzyme mimics (Nicholls et al., 1996), affinity and sensing materials (Syu et al., 2006), and, in recent years, pharmaceutical applications, such as drug discovery, drug purification, or drug delivery (Mosbach, 2006; Yu Y. et al., 2002;

**2. Molecularly Imprinted Polymers (MIPs): highly selective materials** 

ions, organic compounds, and organometallic compounds.

(Puoci et al., 2007; Cirillo et al, 2010, 2011a)

Caldorera-Moore & Peppas, 2009; Hilt & Byrne, 2004).

complex matrices.

(Puoci et al., 2011).

MIPs can be synthesized following three different imprinting approaches named covalent, non-covalent and semi-covalent procedure, according to the kind of interaction between a template and functional groups during the synthesis and recognition phases (Caro et al., 2002).

In the non-covalent procedure (Figure 1), non-covalent interactions (hydrogen bonding, p-p interactions, Van der Waals forces, etc.) are involved in both the synthesis and the recognition step (Joshi et al., 1998). This method is still the most widely used method to prepare MIP because of the advantages that it offers from the point of view of synthesis. The covalent protocol requires the formation of covalent bonds between the template and the functional monomer prior to polymerization, as well as between template and functional group in the imprinted cavities during the re-binding process (Ikegami et al., 2004). Finally, the semi-covalent approach is a hybrid of the two previous methods. Specifically, covalent bonds are established between the template and the functional monomers before polymerization, while, once the template has been removed from the polymer matrix, the subsequent re-binding of the analyte to the MIP exploits non-covalent interactions (Curcio et al., 2010).

Fig. 1. Schematic representation of Non-Covalent Molecular Imprinting Process. Adapted from Liu Z. et al., 2010.

The binding sites obtained by molecular imprinting show different characteristics, depending on the type of imprinted approach. The average affinity of binding site prepared using bonding by non-covalent forces is generally weaker than those prepared using covalent methods because electrostatic, hydrogen bonding, π-π and hydrophobic interactions, between the template and the functional monomers, are used exclusively in forming the molecular assemblies (Hwang & Lee, 2002). However, when covalent bonds are established between the template and the functional monomer prior to polymerization, this gives rise to better defined and more homogeneous binding sites than the non-covalent approach, since the template-functional monomer interactions are far more stable and defined during the imprinting process.

Carbon Nanotubes – Imprinted Polymers: Hybrid Materials for Analytical Applications 185

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

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

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

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

pulsed arc discharge (Sugai et al., 1999, 2000, 2003; Shimada et al., 2004).

rate (Zhao X. et al., 2003; Kanai et al., 2001).

al., 2004).

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.
