Functionalized CNTs

Several studies concerning the biodistribution and clearance of CNTs have been reported [119-126] most of them investigated the biokinetics of covalently functionalized CNTs, whereas only a few evaluated the distribution of noncovalently modified CNTs in the body. 

At first glance, the in vitro studies on CNT toxicity appear to be confusing, inconclusive, or contradictory. However, if one considers interference with dye-based viability assays, agglomeration issues, and oxidative stress due to catalyst contamination, the data available to date seem to favor the conclusion that well-dispersed, purified, and/or functionalized CNTs exhibit relatively low toxicity. 

Chemical modification of CNTs is an essential step towards the fabrication of CNT-based electrochemical sensors. Raman spectroscopy provides an effective way to monitor the modification process and to characterize the functionalized CNTs. 

Bioactive-peptide-functionalized CNTs can cause strong anti-peptide immunological responses in mice, no cross-reactivity against CNTs was detected (Pantarotto et al., 2003 Salvetat et al., 2006). Employing fully the advantages of CNTs and proteins may fabricate a lot of nanomotor or nanoscale machinery with unique function. 

CNTs also showed toxic sign to cells. For example, Pantarotto and his colleagues (Pantarotto et al., 2004) reported that CNT-conjugated bioactive peptides can cross cell membranes, can accumulate in the cytoplasm, or reach the nucleus this kind of peptides-functionalized CNTs may exhibit cytotoxicity when the concentration inside the cells exceed 10 pM. 

Regarding the mechanism of biomolecules functionalized CNTs entering into cells, endocytosis mechanism may be responsible for the phenomena, a theory model is also suggested (Gao et al., 2005) the optimal size of particles entering into cells is between 20 nm and 700 nm or so, too small nanopaiticles are very difficult to enter into cells because of cellular surface tension force and adhesion. The further mechanism of effects of CNTs on human healthcare and environment is being investigated from the following four scales such as molecular, cellular, animals, and environment levels. 

CNTs can be processed such as purification based on oxidation, cutting, and activation by forming carboxylic acid and hydroxyl groups on the surface of CNTs, which can further be linked with other biomolecules to realize special function (Ajayan et al., 1994). As shown in Fig. 9.19, ferritin molecules attached to the surface of CNTs via covalent bond, the nanocomposites with ferritin molecules-functionalized CNTs own better mechanical, thermal, and electronic properties... 

CNTs show sign of toxicity. Although biomolecules functionalized CNTs can be cleaned from blood circulation system by renal secretion, so far the course of CNTs metabolism in cells or environment, and the potential measurements to reduce CNTs toxicity, is still not clear. How to clarity those mechanisms and reduced risk measurements associated with CNTs toxicity is a great challenge. 

Biomolecules-functionalized CNTs can result in characteristic electric conductivity changes of CNTs (Hou et al., 2003), which may be developed into specific biosensor for ultrasensitive detection of biomolecules such as DNA molecules, bacteria and vims, etc. We also observed that oligo DNA-filled SWCNTs appeared as characteristic Electric Resistance peaks as shown in Fig. 9.22, which also may be used as biosensor to detect biomolecules or sequence DNA sequences. 

These four studies on functionalized CNTs may differ from each other because their functionalized groups may have a strong influence on the behavior of CNTs in mice. Despite these differences, the pharmacokinetic profiles provided are valuable in the development of diagnostic and therapeutic applications for CNTs (Deng et al., 2007 Helland et al., 2007). 

More recently, microwave chemistry has also been used to achieve covalent functionalization. In particular, these treatments can functionalize CNTs with sulfonated and carboxylic groups using a mixture of nitric and sulfuric acid under microwave radiation for 3 min, thus resulting in highly dispersible CNTs in ethanol and water... 

Fig. 5.9 Covalent grafting of (a) linear polymer and (b) dendrimer-like hyperbranched polymer of azo-functional porphyrin groups from alkyne functionalized CNTs. Redrawn from [122].

Fig. 5.10 Synthesis strategy and TEM image of CNT-Pt NP hybrid via chemical reduction of Pt ions on PAH functionalized CNTs.

Pristine CNTs are chemically inert and metal nanoparticles cannot be attached [111]. Hence, research is focused on the functionalization of CNTs in order to incorporate oxygen groups on their surface that will increase their hydrophilicity and improve the catalyst support interaction (see Chapter 3) [111]. These experimental methods include impregnation [113,114], ultrasound [115], acid treatment (such as H2S04) [116— 119], polyol processing [120,121], ion-exchange [122,123] and electrochemical deposition [120,124,125]. Acid-functionalized CNTs provide better dispersion and distribution of the catalysts nanoparticles [117-120],... 

In this chapter, we will focus on CNTs as advanced materials for the design of electrochemical devices. The next section vdll be devoted to review the structure, electronic, chemical and electrochemical properties of CNTs. Section 3.3 will comprise an overview of the synthesis, purification and (bio)functionalization of CNT, as well as the modification of substrates with CNT. In Section 3.4, we will address the electrochemical applications of functionalized CNT electrodes... 

A further approach to electrically wire redox enzymes by means of supramolecular structures that include CNTs as conductive elements involved the wrapping of CNTs with water-soluble polymers, for example, polyethylene imine or polyacrylic acid.54 The polymer coating enhanced the solubility of the CNTs in aqueous media, and facilitated the covalent linkage of the enzymes to the functionalized CNTs (Fig. 12.9c). The polyethylene imine-coated CNTs were covalently modified with electroactive ferrocene units, and the enzyme glucose oxidase (GOx) was covalently linked to the polymer coating. The ferrocene relay units were electrically contacted with the electrode by means of the CNTs, and the oxidized relay mediated the electron transfer from the enzyme-active center to the electrode, a process that activated the bioelectrocatalytic functions of GOx. Similar results were observed upon tethering the ferrocene units to polyacrylic acid-coated CNTs, and the covalent attachment of GOx to the modifying polymer. 

Scheme 1.4 Amido-functionalized CNTs containing highly reactive isocyanate groups at the tube surface.

Prato and coworkers succeeded in the 1,3-dipolar addition of azomethine ylides to CNTs [161], Treatment of pristine SWCNTs with an aldehyde and an N-substi-tuted glycine derivative resulted in the formation of substituted pyrrolidine moieties on the SWCNT surface (Scheme 1.22). The approach works effectively with both SWCNTs, prepared by several different methods, and MWCNTs. The pyrro-lidino-functionalized CNTs were sufficiently soluble in common organic solvents and were characterized by several spectroscopic techniques and TEM [161]. The... 

Fig. 1.9 1,3-Dipolar cycloaddition of ferrocene-modified azomethine ylide to SWCNTs in order to functionalize CNT sidewalls with electron donor units.

---