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The functionalization of carbon nanotubes (CNTs) is becoming significant in the growing field of nanotechnology and nanomaterials chemistry as it allows properties such as polarity and solubility to be manipulated. There have been a number of strategies to functionalization of single-walled carbon nanotubes (SWCNTs), including the targeting of defects, end caps, side walls and the hollow interior (Hirsch, 2002) (Sinnott, 2002) (Bahr & Tour, 2002). Actual chemical approaches include covalent chemistry of conjugated double bonds in the SWCNTs (Georgakilas, Kordatos, Prato, Guldi, Holzinger, & Hirsch, 2002) (Bahr, Yang, Kosynkin, Broniskowski, Smalley, & Tour, 2001) (Mickelson, Huffman, Rinzler, Smalley, Hauge, & Margrave, 1998); covalent interactions at functionalities at nanotube ends and defects (Chen, et al., 1998); non-covalent Ï€-stacking (Chen, Zhang, Wang, & Dai, 2001) as well as the wrapping of macromolecules (O'Connell, et al., 2001) (Riggs, Guo, Carroll, & Sun, 2000) (Star, et al., 2001). There are three different approaches to covalently functionalise SWCNTs; thermally activated chemical functionalization, electrochemical modification and photochemical functionalization (Balasubramanian & Burghard, 2005).
The above-mentioned approach of thermally activated chemical functionalization can be used to add both hydroxyl and carboxyl groups. The ends and sidewalls of CNTs can be functionalized with oxygen-containing groups (especially carboxyl groups) by thermal oxidation.
One method involves ultrasonic treatment in a mixture of sulfuric acid and nitric acid (Chen, et al., 1998). However these conditions result in holes at the tube ends and in the sidewalls, followed by the release of carbon dioxide. The nanotubes produced are fragments between 100 and 300 nm in length, and contain a variety of the oxygen-containing groups. If instead refluxed in nitric acid the fragments are not as short, and keep their electronic and mechanical properties (Zhang, et al., 2003). These oxidatively added carboxyl groups provide useful sites for further modification since the creation of amide and ester bonds allows for the covalent coupling of molecules (Balasubramanian & Burghard, 2005).
Instead of the above oxidative method, one can enable direct coupling of functional groups onto the Ï€-conjugated carbon framework of the actual nanotubes. At present the exact mechanism of this addition isn't completely understood (Balasubramanian & Burghard, 2005). Numerous groups can be added, including hydroxyl groups. This can be done by ozonolysis (i.e. treatment with O3 at -78Â°C (Wong, Banerjee, & Kahn, 2003)), followed by treatment with sodium borohydride (Balasubramanian & Burghard, 2005) (Wong, Banerjee, & Kahn, 2003), dimethyl sulfide or hydrogen peroxide (Wong, Banerjee, & Kahn, 2003). Although hydroxyl groups are generated, aldehyde, ketones and carboxylic moieties are also formed; and there is no way to ensure 100% of only one functional group. However, when the ozonised nanotubes were treated with hydrogen peroxide, greater than 50% of the functional groups were carboxylic acids. When treated with dimethyl sulfide was used keto groups were the preferred functional group with a yield of 40%. Sodium borohydride yielded 30% alcoholic species when ozonised (Wong, Banerjee, & Kahn, 2003).
(Chong, Jin, Chow, & Saint, 2010, p. 2999)
Nano-TiO2 plays the role of a photocatalyst in water treatment and is used to improve the efficiency of degradation of various contaminants in water (Kwon, Fan, Cooper, & Yang, 2008). The reason for this; as given by Nagaveni et al., 2004; is its large surface area-to-volume ratio which promotes better charge separation and trapping at the physical surface (cited in Chong, Jin, Chow, & Saint, 2010). Indeed, nano-TiO2 can also serve as both oxidative and reductive catalysts for organic and inorganic pollutants (Savage & Diallo, 2005), this property which is explained by the distinct lone electron pair in its outer orbital (Chong, Jin, Chow, & Saint, 2010). They have been reported by Kabra et al. (2004) to have been used in the degradation of organic compounds such as chlorinated alkanes, benzenes, dioxins, furans and polychlorinated biphenyls (PCBs), as well as in the reduction of toxic metal ions such as Cr(VI), Ag(I), and Pt(II), under UV light (cited by Savage & Diallo, 2005).
The above scheme (2.1 - 2.11) is the widely theorised series of oxidative-reductive reactions which occur at the photon activated surface (Chong, Jin, Chow, & Saint, 2010). These are supposedly initiated when photon energy (hÎ½) of greather than or equal to the bandgap energy of TiO2 strikes its surface (3.0 eV for rutile, 3.2 eV for anatase). This causes the lone pair to be photoexcited to the empty conduction band in femtoseconds (10-15 s), resulting in an empty unfilled valence band and the sunsequent formation of an electron-hole pair, as seen in Figure 1 above (Chong, Jin, Chow, & Saint, 2010). When there are no so-called "electron scavengers" (i.e. electron withdrawers) the photoexcited electron recombines with the valence band hole, with the simultaneous release of heat (this occurs in nanoseconds), represented by (2.4) above. Thus, by maintaining the presence of electron scavengers, such as oxygen, the recombination and sucessful functioning of the photocatalysis can be prolonged (this is represented by (2.5)). Besides preventing the recombination of the electron-hole pair, oxygen allows for the formation of the superoxide radical (O2Î‡-), which can be further protonated to form the hydroperoxyl radical (HO2Î‡) (which is thought to have an electron scavenging character); and subsequently H2O2 (this is shown by 2.9 and 2.10) (Chong, Jin, Chow, & Saint, 2010) (Kwon, Fan, Cooper, & Yang, 2008).
The heterogenous photocatalysis reaction can be summarised by:
(Chong, Jin, Chow, & Saint, 2010)
This can be split into 5 steps:
Mass transfer of an organic contaminant in the liquid phase to the TiO2 surface.
The adsorption of the organic contaminant onto the photon-activated TiO2 surface (this occurs at the same time as the surface activation by photon energy).
Photocatalysis reaction of the adsorbed phase to TiO2 surface, i.e. an intermediate is formed.
The desorption of the intermediated from the TiO2 surface.
Mass transfer of the intermediate from the interfacial region to the bulk fluid.
(Chong, Jin, Chow, & Saint, 2010)
Gallium nitride (GaN) and indium nitride (InN) are regarded as important optoelectric materials, and are used in the manufacture of p-n junctions, laser diodes, light emitting diodes (in which they are significantly brighter than conventional incandescent light bulbs or gallium phosphate-based devices) and full colour, flat-panel displays (Cumberland, Blair, Wallace, Reynolds, & Kaner, 2001).
One of the first syntheses of gallium nitride was done using GaP or GaAs in dry ammonia. If GaP was used, the conversion to GaN occurred after about 2 hours at 727 - 827 K (i.e. 1000 - 1100Â°C); while if GaAs was used GaN was formed after 1-2 hours at (827 K (1000Â°C). Since GaN is thermally unstable, the products have to be cooled in an ammonia environment (Addamiano, 1961). GaN is usually synthesised by heating (at high temperatures) either gallium, gallium oxide (Ga2O3), or gallium halides for an extended period of time in ammonia. However these methods often result in impurities as well as being poorly crystalline and weakly photoluminescent. Some modern approaches include single-source and polymeric precursors; plasma-assisted nitridation; microwave heating; chemical vapour deposition (CVD); solid state metathesis; treatment of GaCl3 with Li3N in benzene under solvothermal conditions (i.e. a non-aqueous solution in an autoclave (a thick-walled steel vessel) at high temperature (127 K, i.e. 400 Â°C) and pressure (Royal Society of Chemistry, 2011)); and the use of gallium chloride or cupferronate along with HMDS (1,1,1,3,3,3-hexamethyldisilazane), a nitriding agent and toluene as solvent (this method has also been used in the synthesis of InN nanocrystals, with an average diameter of 15 nm) (Cumberland, Blair, Wallace, Reynolds, & Kaner, 2001) (Rao, Vivekchand, Biswas, & Govindaraj, 2007) (Pearton, et al., 2008).
The synthesis of indium nitride is considered to be more difficult than for GaN because it decomposes at lower temperature of about 990 K, as compared to 1150 K, the temperature at which GaN decomposes. The commercial manufacture of indium nitride samples usually have a significant amount of impure In and many nitrogen vacancies in the InN lattice causing defects (Cumberland, Blair, Wallace, Reynolds, & Kaner, 2001). Nanocrystalline indium nitrate has also been synthesised by using a benzene thermal method, using NaNH2 and In2S3 at -93 - -73 K (i.e. 180 - 200Â°C) (Xiao, Xie, Tang, & Luo, 2003).
CVD has been done using electron-gun evaporation of a gold film onto a pre-coated Si wafer. The result is GaN nanowires with diameters of about 100 nm (Pearton, et al., 2008).
Metathesis reactions (i.e. exchange reactions) have been used to synthesise both GaN and InN. Solid state metathesis has been used to synthesis GaN (as mentioned above), since the reaction is rapid and is driven by the formation of stable salt byproducts, e.g. GaI3 + Li3N à GaN + 3LiI. However, 4.5 GPa has to be applied, since the calculated adiabatic temperature of 1443 K for this reaction is above the decomposition temperature of GaN (i.e. 1150 K). Instead of this large pressure, additives, for instance LiNH2, can be added to reduce the reaction temperature; however this does result in a decrease in yield. This reaction is given by: 2GaI3 + Li3N + 3LiNH2 à 2GaN + 6Lil + 2NH3 (the analogous In reaction is given by: 2InI3 + Li3N + 3LiNH2 + NH4Cl à 2InN + 6Lil + 3NH3 + HCl). The salt byproducts are removed under an inert atmosphere and washed with water or ethanol, and the isolation of the powder is done by vacuum filtering or centrifugation (Cumberland, Blair, Wallace, Reynolds, & Kaner, 2001). InN has been synthesised by the metathesis reaction of InBr3 and NaN3 in superheated toluene and refluxing hexadecane solvents close to 553 K (Rao, Vivekchand, Biswas, & Govindaraj, 2007).
GaN nanowires can be used to detect hydrogen by coating them with Pd or Pt, which results in them being significantly environmentally stable and having high sensitivity (ppm range) and fast response (10 s in some cases); and those coated with Pd can detect hydrogen at room temperature (298 K or 25Â°C). The analogous Pt-coated InN and Pd-coated InN can also be used to selectively detect hydrogen at the 10 ppm level at 298 K (25Â°C) (Pearton, et al., 2010) (Pearton, et al., 2008). It is thought that if Pt-coated undoped GaN film is used, the Pt on the film dissociates oxygen molecules, water molecules and other gas molecules into more reactive species, which facilitates the electron transfer process as well increasing their surface coverage, i.e. their adsorption amounts on the surface. The high sensitivity of GaN to gas molecules is thought to be due to Ga or N vacancies or dislocations which result in defects and subsequently act as sorption sites for gas molecules on the surface; and the addition of Pt provides additional sorption sites which increase the GaN's sensitivity to alcohols (Lee, Lee, Lee, & Lee, 2003). Functionalization of the surface of GaN nanowires with species such as antibodies, enzymes or other receptors forms can turn the GaN nanowires into very specific sensors. For instance, functionalization with an amine and a carboxylic acid allows for biotinylation (the process of covalently attaching biotin, for detection purposes). Another example is in glucose detection; AlGaN/GaN can be functionalized with glucose oxidase attached to ZnO nanorods.
Raman spectroscopy is a popular technique used to characterise sp2 carbons from zero to three dimensions, e.g. 0D fullerenes, 1D carbon nanotubes, 2D graphene and 3D graphite. Raman spectra provide vibrational and crystallographic information (e.g. size) as well giving information about the presence of sp2-sp3 hybridisation, impurities, elastic constants, strain, number of graphene layers, chirality and curvature. When analysis of Raman spectra is combined with theory, information such as the electronic states, phonon energy dispersion and electron-phonon interaction can be determined. Due to an effect known as the resonance effect a Raman signal can even be observed from a single layer of graphene, or from an isolated single-wall carbon nanotube (SWCNT). This effect is due to the enhancement of the Raman intensity (about 1000 times) which is caused by a Raman process being combined with the optical absorption to or emission from an excited state (Dresselhaus, Jorio, & Saito, 2010) (Dresselhaus, Jorio, Hofmann, Dresselhaus, & Saito, 2010). Among the advantages of Raman spectroscopy is its high-speed, its non-destructiveness, its high resolution and the structural and electronic information it provides (Ferrari, 2007).
The Raman spectra consist of numerous modes: the G-band, the G'-band, the D-band and the radial breathing mode (RBM). The G-band and RBM contribute to first order scattering, while the G'-band and D-band contribute to second-order scattering. The first order scattering gives spectra involving only one-phonon emission; while second-order scattering can consist of either two-phonon scattering or one-phonon and one elastic scattering. This second-order scattering is dependent on the laser excitation energy (this is known as dispersive behavior) and results in several, weak Raman signals (Dresselhaus, Dresselhaus, Saito, & Jorio, 2004).
The RBM is important for the determination of the nanotube diameter and is also useful for understanding tube-tube interactions in multi-wall carbon nanotubes (Dresselhaus, Jorio, Hofmann, Dresselhaus, & Saito, 2010). It is a unique phonon mode that only appears in carbon nanotubes and the observation of these is direct evidence of SWCNTs. As the name suggests, it represents all the carbon atoms which are moving coherently in the radial direction, and is a bond-stretching out-of-plane mode. Also, the frequency of the RBM is inversely proportionate to the diameter of the nanotube (Dresselhaus, Dresselhaus, Saito, & Jorio, 2004).
The G-band for graphite corresponds to the longitudinal optical mode vibration of two neighbouring carbon atoms on the graphene layer and is due to the bond stretching of all pairs of sp2 atoms in both rings and chains. It occurs in the spectra around 1560 -1600 cm-1, but is usually shifted to about 1565 cm-1 in single-layer graphene (Dresselhaus, Dresselhaus, & Hofmann, 2008) (Ferrari, 2007) (Dresselhaus, Jorio, & Saito, 2010). The G-band is very sensitive to strain effects in sp2 nanocarbons, thus curvature effects will result in multiple peaks in the G-band spectrum of SWCNTs, and a single peak for a 2D graphene sheet (Dresselhaus, Jorio, Hofmann, Dresselhaus, & Saito, 2010).
The G'-band provides a way of characterising specific sp2 nanocarbons, i.e. it is dependent on the number of layers. The number of peaks and their frequencies vary as a result of curve-induced strain as well as quantum confinement effects. The G'-band can also be used to assign p- and n-type doping in SWCNTs (Dresselhaus, Jorio, Hofmann, Dresselhaus, & Saito, 2010). In graphite and SWCNTs, one- and two-phonon scattering results in the G'-band at about 2700 cm-1 (for a laser with Elaser = 2.414 eV) (Dresselhaus, Dresselhaus, Saito, & Jorio, 2004). (Dresselhaus, Jorio, & Saito, Characterizing Graphene, Graphite, and Carbon Nanotubes by Raman Spectroscopy, 2010). It is interesting to note that for monolayer graphene only one strong peak is seen in the Raman spectrum, while for graphite two distinct peaks are resolved (Dresselhaus, Dresselhaus, & Hofmann, 2008).
The other second-order mode is the D-band, which is observed around 1350 cm-1 (for a laser with Elaser = 2.414 eV) in graphite and SWCNTs, and like the G'-band is also due to one- and two-phonon, second-order Raman scattering (Dresselhaus, Dresselhaus, Saito, & Jorio, 2004) (Dresselhaus, Jorio, & Saito, 2010). The D-band is as a result of disorder in sp2-hybridised systems, i.e. defects which disturb the symmetry of the graphene sheet, porosity, or impurities (Costa, Borowiak-Palen, Kruszynska, Bachmatiuk, & Kalenczuk, 2008); and allows one to characterise these systems, since sharp dispersive bands at special frequencies are formed if defects are introduced. In order to quantify the disorder in a graphene monolayer, the ratio of the disorder-induced D-band intensity to the normal Raman allowed G-band intensity is analysed (i.e. ID/IG) (Dresselhaus, Jorio, Hofmann, Dresselhaus, & Saito, 2010) (Dresselhaus, Jorio, & Saito, 2010) (Dresselhaus, Dresselhaus, & Hofmann, 2008).
Ferrari and Robertson (2000-2001) described a three-stage system of classifying disorder based on the following criteria: clustering of the sp2 phase; bond disorder; presence of sp2 rings or chains and the sp2/sp3 ratio. The three stages are (1) graphite à nanocrystalline graphite; (2) nanocrystalline graphite à low sp3 amorphous carbon; (3) low sp3 amorphous carbon à high sp3 amorphous carbon (cited by Ferrari, 2007).
The first two stages are the stages to be considered with regards to graphene. For stage 1, the D peak will appear and ID/IG will increase. Another D peak, D' will also appear at about 1620 cm-1; and all the peaks' FWHM (full width at half maximum) will broaden due to the disorder. Also, the doublet structure of the 2 D peaks will be lost. The position of the G peak will also move from about 1580 cm-1 to about 1600 cm-1 (Ferrari, 2007).
For stage 2, the amount of disorder increases, e.g. bond length and angle disorder on the atomic level, and at the end of stage two the graphite is completely disordered , consisting of either distorted sixfold rings or distorted rings of other orders. The noticeable changes in the Raman spectrum is the shift of the G peak from about 1600 cm-1 to about 1510 cm-1, as well an increase in its dispersion. Also, the ratio ID/IG will tend to zero, and most second-order peaks will be absent, except small a small bump near 2400 - 3100 cm-1. If only sp2 rings are present in the material, the G peak will have maximum dispersion around 1600 cm-1, whereas if only sp2 chains are present this peak will be between 1600 cm-1 and 1690 cm-1 (Ferrari, 2007).
The below diagram illustrates the differences in the Raman spectra that would be observed with increasing disorder as well as different single walled nanotubes.
In recent times the amount of research being done on nanomaterials and especially carbon nanotubes (CNTs) has opened up the door for their potential use in drug delivery and other medicinal applications. In order for CNTs to be used for these applications they require chemical modification. This is because in their pure forms, they tend to form bundles, are insoluble in most solvents, and are cytotoxic due to residual metal catalysts as well as their previously mentioned insolubility. Functionalization of the CNTs can be done using the methods outlined in Question 2 above. By oxidation followed by carboxyl-based coupling, CNTs can be functionalized with various functional groups, including bioactive agents such as peptides, proteins and nuclei acids; as well as therapeutic agents such as anti-cancer drugs (Tran, Zhang, & Webster, 2009).
Functionalized CNTs have the ability to cross plasma membranes making them very attractive targets for the purpose of drug delivery as well as gene delivery (Bianco, Kostarelos, & Prato, 2005) (Tran, Zhang, & Webster, 2009) (Ji, et al., 2010). In addition to this, the CNTs inherent spectroscopic properties, such Raman scattering and photoluminescence allow for its easy tracking and detection as well as a possible way of imaging diseases (Zhang, Bai, & Yan, 2010).
For their use in drug delivery (i.e. targeting drugs to a specific, desirable group of cells; especially for the treatment of cancer), a drug needs to be attached to the carrier (CNTs), either covalently or non-covalently. The drug-carrier complex is then directed to the targeted cells using either passive targeting methods (this is done primarily by reducing non-specific interactions with non-target organs, tissues and cells) or active targeting methods (in the case of cancer therapy, the therapeutic agent is attached with a ligand that binds to specific receptors that are over-expressed on target cells in tumours). Once at the targeted site (organs, tissues or cells) there are two ways that internalisation can take place. In the first possibility, both the drug and carrier are internalised. Of the two possibilities this way has the higher delivery efficacy, since drug molecules are released inside the cell by degradation of the drug-carrier complex by the intracellular environment. In contrast, in the second possibility, only the drug is internalised. In this method the extracellular environment assists in the degradation of the drug-carrier complex, with the result that only the drug will cross the lipid cell membrane to enter the cell.
There are two possible mechanisms by which the CNTs can be internalised: via the endocytosis pathway and via the endocytosis-independent pathway (Tran, Zhang, & Webster, 2009). Endocytosis is the process by which a material is engulfed by the cell membrane into the cell, forming a vesicle that fuses with other vesicles such as endosomes and lysosomes. The internalisation of CNTs or CNT conjugates by this process can be monitored by fluorescent labelling of the CNTs or CNT conjugates. It was found by some studies that SWCNTs conjugated with proteins (such as bovine serum albumin or biotin-streptavidin) or DNA (such as CATTCCGAGTGTCCA Cy3) were able to enter cells (human promyelocytic leukemia (HL60) cells or cervical cancer HeLa cells) after incubation at 37Â°C for 1 hour; but the protein or DNA alone, under the same conditions, wasn't found in the cells (Tran, Zhang, & Webster, 2009). The internalisation of CNTs by endocytosis-independent pathways has been observed in some experiments. In one experiment this was monitored by labelling the functionalized CNTs with a green fluorescent agent (fluorescein-isothiocyanate, otherwise known as FITC) and subsequently tracked by using epifluorescence and confocal microscopy. When the fluorescent-labelled CNTs were incubated with cells (human 3T6 and murine 3T3 fibroblasts) for an hour, they were internalised; whereas when just fluorescein (FITC) was incubated, it wasn't internalised. There are numerous factors which are important in cellular uptake of CNTs. These factors include: surface properties (such as hydrophobicity and hydrophilicity), size, shape, and whether the CNTs are in bundles or dispersed (Tran, Zhang, & Webster, 2009).
Some of the advantages of using CNTs in drug delivery include their effectiveness as well as their ability to carry drugs safely (since the cytotoxicity is reduced) to previously unreachable cells. Also, SWCNTs provide a higher drug loading capacity due to their high surface to volume ratio, as well as having an increased circulation time due to their stability and structural flexibility. Tumour-targeting drug delivery systems (DDS) proposed consist of three parts; the functionalized SWCNTs, tumour-targeting ligands and anti-cancer drugs. The principle behind this system is that when the DDS interacts with the cancer cells, they recognise cancer-specific receptors on the surface of the cells and thus induce receptor-mediated endocytosis, which has been reported to be specific and efficient, with the subsequent release of chemotherapeutic agents (Ji, et al., 2010). When it comes to drug delivery there is a desire to ensure that the correct amounts of drugs are transported to the correct target tissue, and to minimise unwanted effects of the drugs on healthy, normal tissue. The EPR (enhanced permeability and retention) effect is a useful strategy to specifically deliver drugs to a tumour site. This effect is caused by leaky vascular structures and an impaired drainage system of tumours, which results in blood vessels having a significantly larger pore size (100 to 800 nm) compared to the pore size of pores in healthy blood vessels (2 to 6 nm). Thus, nanoparticles (which have sizes between approximately 100 and 700 nm) will only be able to penetrate the pores of tumour blood vessels and not the pores of health blood vessels (Ji, et al., 2010) (Tran, Zhang, & Webster, 2009).
As already mentioned above, CNTs can also be used in gene delivery, to promote cells to generate their own therapeutic proteins. Additional potential medicinal applications of CNTs include: bone and neural regeneration (Tran, Zhang, & Webster, 2009); lymphatic targeting, thermal therapy, as diagnostic tools (Ji, et al., 2010); treatment of infectious diseases and central nervous system disorders and in tissue engineering (Zhang, Bai, & Yan, 2010); as well as in peptide and nucleic acid delivery (Bianco, Kostarelos, & Prato, 2005). However, at this point in time the full extent of CNTs toxicity is not completely understood and further research would be needed before full-scale clinical usage.
The principle behind homogeneous nucleation is that the probability of a so-called "critical nucleus" forming on a surface is the same at any point and thus more evenly distributed growth will be observed, with few defects, i.e. a homogeneous layer will be formed (Schmelzer, 2005). This is in contrast with heterogeneous nucleation which forms at "preferential" sites such as phase boundaries or impurities (e.g. dust), causing the effective surface energy to be lower, which will facilitate nucleation, resulting in a heterogeneous layer being formed (almost like "clumps"). Additional differences include the critical radius of a homogenous nucleus being much smaller than the critical radius of a heterogeneous nucleus (Monteiro); as well as heterogeneous nucleation having a lower energy barrier than homogeneous nucleation, meaning that heterogeneous nucleation occurs much more often than homogeneous nucleation. Also, all the reactants and catalysts in homogeneous nucleation from the vapour phase will be vapour, whereas in homogeneous nucleation vapour-liquid and vapour-solid phases can occur as well.
For the growth of a thin film, homogeneous nucleation would be favoured since homogeneous nucleation would produce a consistent layer, i.e. a thin film.
For the growth of nanoparticles, heterogeneous nucleation would be favoured since there would be more isolated and irregularly located nuclei.
Nucleation can be regarded as a bottom-up process since the layers are formed atom by atom. Also, nucleation is thermodynamically favoured.