Acid 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],... 

A number of studies on CNT-polymer composites have focused on improving the dispersion and load transfer efficiency in other words the compatibility between the CNTs and polymer matrix through covalent chemical functionalization of CNT surface (12,40). Many of the studies reported above have used acid-functionalized CNTs to fabricate MWCNT-PMMA composites with improved mechanical properties using different processing methods (24,25,27,62). Yang et. al (68) modified the acid functionalized CNTs with octadecylam-ine (ODA) to obtain ODA-functionalized CNTs. These CNTs were reinforced in a copolymer P(MMA-co-EMA) to form composites with improved dispersion and mechanical properties. 

By a surface imprinting technique, in Zhang H. et al., 2011, a comfX)site imprinted material, on the basis of a MWCNTs-incorporated layer using melamine as a template, methacrylic acid as a functional monomer, and ethylene glycol dimethaciylate as a cross-linker, was synthesized. In this work, the poly(acrylic-acid)-functionalized CNTs were synthesized to increase the diameter of CNTs. Then, the vinyl group was introduced to the surface of poly(acrylic-acid)-functionalized CNTs by an amidation Using Melamine as a template molecule, imprinted CNT composite material was fabricated by a thermal fX)lymeiization. Applied as a sorbent, the imprinted materials were used for the determination of Melamine in the spiked sample by online SPE combined with HPLC. 

Alternatively, CNT-polymer composites have been utilized as polymer electrolyte membranes in fuel cell devices (PEMFCs) [111]. Sulfonic acid-functionalized CNTs were blended with Nafion polymer so that the proton transport capability of the polymer matrix could be enhanced appreciably. Ionic conductivity measurements of Nafion and CNT-Nafion membranes revealed almost one order of magnitude higher conductivity for the composite than that for neat matrix. 

The acid reflux procedure was first described by Rinzler el al. [28], in which raw nanotube materials are refluxed in nitric acid to oxidize the metals and carbon impurities. Acid-treated CNTs are considered to have carboxylic acid groups at the tube ends and, possibly, at defects on the side walls. The functionalized SWNTs have considerably different properties from those of the pristine tubes. 

Negatively charged species such as carboxylic acid group in acid-treated CNTs can attract positively charged enzymes from solution as long as the pH value of the enzyme solution is controlled to be lower than the iso-electric point of the enzyme thus, multilayer films of the enzyme can be formed by the layer-by-layer technique. For example, five layers of GOx can be immobilized on the electrode surface by alternatively dipping a poly(diallyldimethylammonium chloride (PDDA))-functionalized GC into a CNT solution and a GOx solution (pH 3.8). Figure 15.15 illustrates the preparation process for the formation of a multilayer film of GOx on the electrode. 

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... 

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... 

Composites made with carbon nanostructures have demonstrated their high performance as biomaterials, basically applied in the field of tissue regeneration with excellent results. For example, P.R. Supronowicz et al. demonstrated that nanocomposites fabricated with polylactic acid and CNTs can be used to expose cells to electrical stimulation, thus promoting osteoblast functions that are responsible for the chemical composition of the organic and inorganic phases of bone [277]. MacDonald et al. prepared composites containing a collagen matrix CNTs and found that CNTs do not affect the cell viability or cell proliferation [278]. 

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 3. Schematic representation of the immobilization of nucleic acid to the functionalized CNT array. From Reference 69.

The development and application of biosensors has been discussed in several reviews [125, 126]. For instance, fabrication of cholesterol biosensors relies on the immobilization of cholesterol oxidase and cholesterol esterase onto CNTs, while horseradish peroxidase and flavocytochrome P450scc are also used for the same reason [125, 126]. Beyond the biosensing field, carbon nanotubes are also used as carriers for peptide, nucleic acid and drug delivery, due to their intrinsic property to cross cell membranes [102, 124]. The fact that the functionalized CNTs (f-CNTs) are not immunogenic and low-toxic opens the pathway for more research in the field of CNT-abetted drug delivery [102]. 

Besides the classical nitric acid treatments, other oxidative agents have been used to prepare functionalized CNTs. Rao et al. [170] have compared the effects of concentrated nitric acid, aqua regia, HF-BF3, aqueous OSO4, and KMn04 (acid/alkali) solutions on MWCNT structures [170]. Indeed, all these oxidants... 

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