Sidewall functionalization
As the carbon-carbon bonds in the sidewall of the CNTs are of lower reactivity than the bonds in fullerenes, reagents with high chemical reactivity have been involved in the sidewall functionalization of CNTs (Fig. 7). Such reagents include carbeneS’[9'11'13] fluorine,1-12-1 aryl radi-calS’[17'18'55] and azomethine ylides.[16- Several methods for the metal reduction of SWNTs have also been re-ported'[9'15] and SWNTs with alkyl substituents were obtained by treating the fluorinated SWNTs with nucle-ophilic reagents such as alkyllithium and alkyl magnesium bromides.[56,57-
Once the sidewall functionalization of SWNTs had been accomplished, the properties of the SWNTs change dramatically. The carbon atoms on the sidewalls of SWNTs that react with functional groups are converted from sp2 into sp3 hybridization. The saturation of the p-network leads to a significant change in the electronic band structure of SWNTs that can be monitored by optical spectroscopy.[11'13'14'17'18'55] The presence of sp3-hybridized carbon atoms in the SWNT framework can be detected by Raman spectroscopy[12,14,17,18,55,58,59] and is reflected by an increase in the relative intensity of the disorder mode of the SWNTs after sidewall functional-ization. The solubility of sidewall-functionalized SWNTs in organic solvents or water is usually improved by the addition of different functional groups.
Fig. 7 Schematic representation of sidewall functionalization of SWNTs.
Phenyl(bromodichloromethyl)-mercury has been used to add dichlorocarbene to both soluble and insoluble SWNTs (Fig. 8).[9,11,13,14] The degree of dichlorocarbene functionalization of soluble HiPco SWNTs can be as high as 23%, which is sufficient to almost completely eliminate all vestiges of the interband transitions in the SWNTs. The transitions at the Fermi level in the metallic SWNTs that appear in the far-infrared (FIR) region of the spectrum (Fig. 9) show a dramatic decrease of intensity on dichlorocarbene functionalization, which indicates that the dichlorocarbene addition significantly perturbs the conjugated p-network and converts a metal into semiconductor. On the other hand, the reaction of the SWNTs with oxidants causes an increase in the intensity of absorption at the Fermi level that was observable in the FIR due to hole doping of the semiconducting SWNTs.[13,32] Therefore the FIR region of the spectrum allows a clear differentiation between covalent and the ionic chemistry of SWNTs.[14]
The fluorination of SWNTs can produce a high degree of functionalization (up to the composition C2F) by using elemental fluorine at temperatures from 150°C to 600°C.[12] FT-IR spectra confirmed that the fluorine was covalently bonded to the carbon on the sidewall (VC-F at 1220-1250 cm"1). TEM images of these highly function-alized SWNTs showed that the nanotubes were significantly degraded. The electrical conductivity of the functionalized SWNTs (resistance >20 MO) is greatly decreased in comparison with the pristine SWNTs (resistance 10-15 O), which indicates a disruption of the conjugated p network. Hydrazine was used to defluorinate the SWNTs.
Fig. 8 Schematic representation of dichlorocarbene addition to soluble SWNTs.
When the temperature of fluoridation was 400°C or higher, most of the SWNT structure was destroyed. This was confirmed by Raman spectroscopy that showed that the radial (186 cm"1) and tangential (1580 cm"1) modes decreased, whereas the disorder mode (1340 cm"1) increased.
Fig. 9 Absorption spectra of films of soluble SWNTs functionalized with dichlorocarbene (right). The curves are labeled with the degree of functionality. Spectra are normalized to the value of the absorption at the p-plasmon peak at 37,000 cm"1.
The fluorinated SWNTs are soluble in alcohols after ultrasonication.[58- Nucleophilic reagents such as alkyl-lithium and alkyl magnesium bromides have been used to convert fluorinated SWNTs to alkylated SWNTs.[56 The alkylated SWNTs, such as hexyl-SWNTs, were soluble in THF and methylene chloride. Pristine SWNTs could be recovered from hexyl-SWNTs by heating in air at 250°C for 1 hr. The weight loss of these derivatized tubes was about 35%, corresponding to the attachment of a hexyl group to 1 in every 10 sidewall carbon atoms.
Aryl diazonium salts are known to react with olefins[60-and have been used for the sidewall functionalization of SWNTs.[17,18- A series of aryl radicals have been generated by electrochemical reduction of aryl diazonium salts to derivatize SWNTs. The estimated degree of aryl radical functionalization was as high as 1 out of every 20 carbons in the SWNTs. Such high functionalization significantly improved the solubility of the SWNTs in organic solvents (THF, DMF, chloroform). The Raman spectra showed a significant increase in the disorder mode of the SWNTs at 1290 cm"1, and a complete loss of structure in the solution-phase absorption spectra was observed. The functional groups can be removed by heating at 500°C in argon, restoring the pristine SWNTs. In a solvent-free sidewall functionalization of SWNTs, the SWNTs were mixed with 4-substituted aniline and vigorously stirred at 60°C in the presence of isoamyl nitrite or sodium nitrite.[55- The isoamyl nitrite or sodium nitrite converted the aniline into diazonium salts which then reacted with SWNTs. The products showed a significant loss of the fine structure of interband transitions in the NIR-VIS region of the spectrum and a strong increase in the disorder mode in the Raman spectra. The aryldiazonium salts have also been used to react with individual SDS-coated SWNTs in aqueous solution to form aryl functionalized SWNTs.[61- 4-(10-hydroxydec-yl)benzoate-SWNTs have been produced by the same method and used to make composite materials with polystyrene.[59]
Sidewall functionalization of SWNTs by using aryl-diazonium salts shows high chemoselectivity with metallic SWNTs vs. the semiconducting SWNTs.[19] The high selectivity is reflected in the optical spectra of the SWNTs that show a different degree of functionalization in the metallic and semiconducting SWNTs. The absorption intensity of the electronic interband transitions of the metallic SWNTs decreased with increasing of degree of functionalization, whereas the absorption intensity of semiconducting SWNTs was unaffected. The result was confirmed by Raman spectroscopy on a sample with 22.4 groups attached per 1000 carbon atoms, in which all of the metallic modes had disappeared but the semiconductor modes remained. The reaction was explained in terms of the formation of a charge-transfer salt involving the aryldiazonium salt at the SWNT surface. Because the metallic SWNTs are superior electron donors they react preferentially with the diazonium salts.
The 1,3-dipolar cycloaddition reaction of azomethine ylides is another important method used in sidewall functionalization of SWNTs.[16] This 1,3-dipolar cyclo-addition has been widely applied to the organic modification of fullerenes.[62,63] The 1,3-dipolar cycloaddition of azomethine ylides to SWNTs has been accomplished by using aldehyde and a N-substituted glycine derivative. The glycine derivative has a long ether chain to enhance the solubility of SWNTs in organic solvents such as chloroform, methylenechloride, acetone, alcohol, and even water. The degree of functionalization was estimated to be 1 in 95 SWNT carbon atoms. The NIR spectra of these pyrrolidine functionalized SWNTs showed some loss of the electronic properties of the parent SWNTs. The pyrrolidine-functionalized SWNTs associated in bundles with diameters of 100 nm. The authors found that the pyrene-modified tubes formed more compact bundles than other functionalized SWNTs.
Recently, SWNTs were functionalized with polystyrene via an in situ polymerization.[64] First, the SWNTs were treated with sec-butyllithium to form carbanions. When styrene monomer was added, both free sec-butyllithium and the SWNT carbanions initiated polymerization, resulting in an intimately mixed composite system. The loading of SWNTs in the graft polymer was about 0.05 wt.%. The composite was soluble in organic solvents such as dimethylformamide, chloroform, and tetrahydrofuran.
Noncovalent Functionalization
The advantage of noncovalent functionalization is that it does not destroy the conjugation in the CNTs. One strategy for noncovalent functionalization is based on the usage of an amphiphilic molecule or polymer to interact with CNTs via its hydrophobic part, whereas the hydrophilic part of this molecule can enhance the solubility of CNTs or react with other molecules. Another method is the ionic interaction of SWNTs, which leads to the formation of acid-base salts or charge-transfer complexes. For example, the acid form of SWNTs was heated with melted octadecylamine to form an SWNT-carboxylate zwitterion.[65] Such a form of SWNTs is soluble (>0.5 mg/mL) in tetrahydrofuran and 1,2-dichlo-robenzene. Generally, a 0.1 mg/mL solution is stable for more than 10 days and is visually nonscattering. The majority of the SWNT ropes were exfoliated into small ropes (2-5 nm in diameter) and individual SWNTs with length of several micrometers during the dissolution process.
Noncovalent functionalization of SWNTs has been achieved by using small molecules such as 1-pyrenebu-tanoic acid succinimidyl ester.[65] This amphiphilic molecule can attach to the surface of SWNTs by p-p interaction with the pyrene moiety, while the hydrophilic ester of this molecule enhances the solubility of SWNTs, and this approach has been used in protein immobilization, including the study of ferritin, streptavidin, and biotin-PEO-amine. These bioactive molecules have been attached to the surface of SWNTs via a nucleophilic substitution of N-hydroxysuccinimide by using the amine group on the proteins to form an amide.
Molecules of high molecular weight can wrap themselves around the surfaces of SWNTs. Polymers, including polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS),[66] poly(phenylacetylene) (PPA),[67] poly(metaphe-nylenevinylene) (PmPV),[68] poly(aryleneethynylene)s (PPE),[69] poly-anisidine (POAS),[70] amylose,[71] amphiphilic copolymer poly(styrene)-block-poly(acrylic ac-id),[72] and natural polymer,[73] have been used to wrap or encapsulate CNTs.
The interaction between the CNTs and the polymer hosts usually originates from the van der Waals interaction between the surface of the CNTs and the hy-drophobic part of the host polymer. The mechanism of these polymer-wrapping methods depends on both the interaction between SWNTs and polymers, and the structure of the polymers. A thermodynamically driven model for wrapping SWNTs has been suggested, wherein the polymer disrupted both the hydrophobic interface with water and nanotube-nanotube interactions in the CNT aggregates.[66] AFM investigation revealed that the individual PVP-SWNT displayed a uniform diameter along its length, which indicates that the polymers were uniformly wrapped along the surface of the SWNTs rather than being randomly attached. A helical wrapping model was used to explain the solubilization of SWNTs at the molecular level. In another case, a straight, rigid polymer, poly(aryleneethynylene)s (PPE) , has been used to interact with SWNTs.[69- Because the polymer is short and rigid, it attached to the surface of SWNTs in a nonwrapping form parallel to the nanotube axis.
APPLICATIONS OF CHEMICALLY FUNCTIONALIZED CARBON NANOTUBES
The functionalized CNTs can have higher solubility or differing molecular affinity or electronic response and this allows for more effective use of their outstanding electronic and mechanical properties. It is already clear that the CNTs will find applications in electronics, sensors, composite materials, biology, and medicine.
The strength and high aspect ratio of CNTs are particularly valuable in the design of probe tips for scanning probe microscopy,[74-76- electrochemistry,[77-and biological analysis.1-78,79-1 In 1998, a nanometer-size probe was made with covalently modified MWNTs.1-75-1 Using carbodiimide chemistry, carboxyl groups at the tip ends coupled with different amines to form amide-linked groups. By employing the characteristic properties of these amide groups, this chemically modified MWNT probe could be used for titrating acid and base groups, to image patterned samples based on molecular interactions, and for measuring the binding force between single protein-ligand pairs. Individual semiconducting SWNT-based chemical sensors are reliable for the detection of small amounts of gases such as NH3 and NO2.[80- The mechanism of this sensor action is based on charge transfer between the SWNTs and NO2 or NH3. Exposure to the gases affected the electronic properties of the SWNTs and led to a change in the conductivity. It was observed that the conductance of the SWNT sample dramatically increased in NO2 and decreased in NH3.
Because of their 1-D structure and excellent mechanical properties, CNTs are the ideal reinforcing fibers for composite materials. By the addition of CNTs, the strength, elasticity, toughness, durability, and conductivity of the composite material can be improved. The major challenge in using CNTs for composite materials lies in achieving a homogeneous dispersion of CNTs throughout the matrix without destroying their integrity, and an enhanced interfacial bonding with the matrix. Function-alized CNTs show great potentials in this field and CNT composite materials are under active investigation. Conducting polymers such as polypyrrole (PPy) has been used to grow composite films with MWNTs by electro-chemistry;1-81-1 a SWNT/PmPV composite has been used in making organic light-emitting diodes;1-82- poly(p-phenyl-ene benzobisoxazole) (PBO) has been applied to produce PBO/SWNT fibers.1-83-1
Recently, there has been intense interest in exploring the novel properties of CNTs and especially SWNTs for biological applications. The 1-D structure of SWNTs makes them an ideal candidate for the development of a new generation of biodevices. Because SWNTs are molecular wires with every carbon atom exposed on the surface, SWNTs are promising candidate for the development of extremely sensitive biosensors. Because of their small dimensions SWNTs can be easily introduced into cells with little or no disturbance of the cell function. Additionally, the outstanding electronic properties of SWNTs will allow biological events to be addressed electronically. The progress in understanding the interactions between CNTs and biomolecules has stimulated extensive research on the fabrication of bionanodevices and especially biosensors. SWNT devices have shown high sensitivity in the detection of redox enzymes,[84-86-DNA,[87- and proteins.[88-
Fig. 10 Schematic representation of functionalized SWNTs in biological application.
Yet the interactions of biomolecules with SWNTs are poorly understood. Early studies have focused on the nonspecific interactions between proteins and CNTs arising from the hydrophobic nature of the nanotube walls. It has been demonstrated that streptavidin and HupR form highly ordered helical structures upon adsorption on MWNTs.[89- Nonspecific binding on SWNTs has been shown to be a general phenomenon with a wide range of proteins, including streptavidin, avidin, bovine serum albumin, glucosidase, staphylococ-cal protein A, and human IgG.[65,84,89-91- These nonspecific interactions could be explored for the controlled organization of SWNTS into useful architectures. It has been shown that specifically designed amphiphilic a-helical peptides wrap and solubilize nanotubes, enabling controlled assembly of peptide-wrapped nanotubes into macromolecular structures.1-92-1 The importance of the nonspecific interactions between SWNTs and biomole-cules has been recently demonstrated in a study on the separation of metallic from semiconducting nanotubes by DNA-assisted dispersion in aqueous solution.[93- It has been suggested that the highly efficient mechanism for dispersion of individual SWNTs in solution involves p-stacking interactions between the nanotube walls and the DNA-bases, resulting in helical wrapping of the nano-tubes, complemented by hydrophilic interactions between the sugar-phosphate groups in the backbone of DNA and water molecules, rendering the hybrid structure soluble in water. Strategies to prevent the nonspecific binding of proteins to SWNTs by immobilization of PEG in the presence of surfactant have also been developed.[90-
As the biocompatibility and biorecognition properties are key issues regarding biological applications of SWNTs, recent research focuses on the functionalization of SWNTs. Both covalent and noncovalent functionaliza-tion with proteins, enzymes, and DNA has been explored. The noncovalent approach involves p-stacking of 1-pyrenebutanoic acid succinimidyl ester onto the sidewalls of SWNTs.[65- This technique has enabled the immobilization of a wide range of biomolecules on the sidewalls of SWNTs including ferritin, streptavidin, and biotin-PEO-amine.[65- The immobilization is robust and its application has been demonstrated in biosensors based on glucose oxidase-functionalized SWNTs.[91-
An alternative approach to tethering biological molecules to SWNTs in a controlled manner is covalent functionalization. Covalent functionalization provides better integrity, stability, and reproducibility of the fabricated devices. Typically, covalent binding of proteins and enzymes utilizes the diimide-activated amidation of carboxylic acid-functionalized carbon nanotubes as schematically illustrated in Fig. 10 (paths a and b).[52,94-96- For covalent attachment of DNA, amine-terminated SWNTs are cross-linked with succinimidyl 4-(N-maleimidome-thyl)cyclohexane-1-carboxylate (SMCC) to produce mal-eimide groups further reacted with thiol-terminated DNA (path c).[97-
A method for functionalization of SWNTs with N-protected amino acids based on the 1,3-dipolar cycload-dition reaction to the external walls of SWNTs has also been reported.[98-
While in most cases the functionalization is essential to impart biorecognition properties to SWNTs, it should be noted that antifullerene IgG monoclonal antibody[99- and several peptides have been shown to bind specifically to SWNTs.[100-
The ultimate goal of this work is the utilization of the CNTs in medicine leading to the development of a broad-based effort in nanomedicine which offers the promise of conquering disease at the level at which it occurs—the molecular level. Thus it is important to begin to investigate the interaction of CNTs with live cells. The first efforts in this direction explored the use of CNTs as the substratum for neuron growth.1-101-1
CONCLUSION
The chemistry of CNTs has made enormous strides, and it is clear that this subject will drive the applications of carbon nanotubes. In order to further refine the chemically functionalized CNTs, it is important to begin the chemistry with high quality materials. Functionalization of individual CNT, and particularly CNTs of defined length, diameter, and chirality, is the next step that will lead to the control of CNT-based materials and devices at the molecular level.
