Role of Carbon Nanotubes in Analytical Science - ACS Publications

Anal. Chem. 2007, 79, 4788-4797

Perspectives

Role of Carbon Nanotubes in Analytical Science M. Valca´rcel,* S. Ca´rdenas, and B. M. Simonet

Department of Analytical Chemistry, University of Co´ rdoba, Campus de Rabanales, E-14071 Co´ rdoba, Spain

Analytical science has gone through several turning points, one of the most decisive of which was signaled by the development and massive use of instruments for analytical purposes. One other pivotal turning point was the inception of computer science, which not only enabled the automatic control of analytical systems but also facilitated the acquisition of vast amounts of data and their processing with the aid of chemometrics. Following the growing significance of automation and miniaturization in recent times, the early 21st century is witnessing the rise of nanotechnology as a new, increasingly important, revolutionary trend in science in general and analytical science in particular. The ability to exploit molecular interactions between analytes and nanoparticles has opened up new, challenging prospects in this area. Good proof of the interest aroused by nanoparticles is the large number of papers on their use in quantum dots, fullerene, aurum nanoparticles, or carbon nanotubes published in recent years. Carbon nanotubes (CNTs) have received much attention from scientists ever since their discovery. This is somewhat surprising in a time where genomics and proteomics are the hottest research topics since CNTs are based on nonbiological compounds. In fact, the interest of CNTs lies in their unusually small size and excellent properties. Carbon nanotubes belong to the fullerene family of carbon allotropes. They are tubular in shape as implied by their name and consist entirely of covalently bonded carbon atoms. Carbon nanotubes were first described by Iijima in a paper published in Nature.1 His initial designation, helical microtubes of graphitic carbon, soon evolved into multiwalled carbon nanotubes (MWNTs), which reflects the fact that their coaxial packing contains several graphene sheets. A number of subsequent papers published in 1993 reported similar structures consisting of a single graphene cylinder2 that were called single-walled carbon nanotubes (SWNTs) and constitute the origin of modern nanotechnology. Figure 1 shows the structure, characteristic electronic microscopy photos, and characteristic Raman spectrum for a SWNT and a MWNT. As can be seen in the microscopy photo, the strong * To whom correspondence should be addressed. [email protected]. (1) Iijima, S. Nature 1991, 354, 56-58. (2) Iijima, S.; Ichichashi, T. Nature 1993, 363, 603-605.

4788 Analytical Chemistry, Vol. 79, No. 13, July 1, 2007

E-mail:

van der Waals forces among SWNTs cause the SWNT packing into thick bundles or ropes. PROPERTIES OF CNTS Carbon nanotubes have been the subject of a host of theoretical and experimental studies.3,4 Most, however, have focused on the properties of the supramolecular assemblies (aggregates) rather than on those of individual, isolated CNTs. This has been propitiated by the difficulty of making accurate measurements at the nanoscale (molecular) level. In any case, the high potential of CNTs has promoted much study aimed at elucidating their properties (particularly those of SWNTs). By contrast, accurately establishing the properties of MWNTs will necessitate further research into the interactions between layers. The most salient properties of SWNTs are summarized below; those of MWNTs can easily be inferred from them. (1) Based on structure, CNTs possess nonpolar bonds and high aspect ratios (length to diameter ratio) and are thus insoluble in water. This results in the spontaneous aggregation of CNTs. This affinity to aggregation combined with their high flexibility increases the possibility of bundling and close packing. This fact diminishes and degrades the real surface/area ratio.5 A prerequisite for the useful realization of CNT properties is then the effective utilization of their high aspect ratio, for which their disaggregation is essential. In spite of their insolubility, CNTs can be solubilized in aqueous medium by adding a chemical modifier such as a surfactant.5 The high insolubility of CNTs in water, however, makes them difficult to purify and characterize. (2) As opposed to aqueous solutions, hydrophobic CNTs are expected to be wetted by organic solvents and, therefore, to assemble less in bundles and ropes. However, CNTs were shown to exhibit a sufficient solubility only in a limited number of solvents, such as dimethyl formamide, dimethyl acetamide, and dimethyl pyrrolidone.6 (3) Like other organic molecules, CNTs can be functionalized covalently.7 In general, CNTs are not especially reactive but can be made to react under strong chemical conditions (e.g., by (3) Wei, B.; Vajtai, R.; Jung, Y.; Ward, J.; Zhang, R.; Ramanath, G.; Ajayan, P. M. Nature 2002, 416, 495-496. (4) Dai, H. Surf. Sci. 2002, 500, 218-241. (5) Vaisman, L.; Wagner, H. D.; Marom, G. Adv. Colloid Interface Sci. 2006, 128-130, 37-46. (6) Kim, B.; Lee, Y. H.; Ryu, J. H.; Suh, K. D. Colloid Surf., A 2006, 273, 161164. (7) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760-761. 10.1021/ac070196m CCC: $37.00

© 2007 American Chemical Society Published on Web 06/02/2007

Figure 1. Comparison of structures, electronic microscopy images, and Raman spectrums for a single-walled and multiwalled carbon nanotube.

incorporating hydroxyl or carboxyl groups onto their side walls).8 In addition, CNTs can be covalently immobilized onto solid supports such as silica or steel. (4) SWNTs possess a good surface area to volume ratio. The fact that they contain no atoms in their lumina, combined with their excellent van der Waals physisorption properties, make them especially good candidates for gas filtration, sensing, and energy storage devices.9 This property can be exploited to develop gas sensors or to extract analytes from their matrixes (solid-phase extraction). (5) CNTs exhibit excellent thermal stability in inert atmospheres, where they remain stable at up to 1200 °C. This property is the basis of their use as stationary phases in gas chromatography. (6) SWNTs possess a π complex both above and below the plane containing the carbon atoms that is the origin of their high electron mobility and electrical conductivity.10 The extent to which the π complex is formed depends on the degree of overlap between pz orbitals in the carbon atoms, which in turn is influenced by the cylindrical shape of SWNTs. In fact, on the curved surface of the tube, the hexagonal arrays of carbon atoms wind around (8) Wang, Y.; Iqbal, Z.; Malhotra, S. V. Chem. Phys. Lett. 2005, 402, 91-101. (9) Raffaelle, R. P.; Landi, B. J.; Harris, J. D.; Bailey, S. G.; Hepp, A. F. Mater. Sci. Eng. B 2005, 116, 233-243. (10) Baughman, R. H.; Lui, C.; Zakhidov, A. A.; Iqbal, Z.; Baisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; de Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Hertesz, M. Science 1999, 284, 1340-1344.

in a helical fashion, introducing helicity to the structure.11 Therefore, the electronic properties of SWNTs depend on how distorted the planar graphene sheet is. As a result, some SWNTs are metallic and some are semiconducting. There are three different crystallographic types of CNTs depending on the way the graphene sheet is distorted, namely, armchair, zigzag, and chiral CNTs. In any of these three forms, SWNTs play a crucial role as molecular wires. This property has been used in to develop electrochemical (bio)sensors. (7) The strain energy associated with placing a SWNT in axial tension is somehow related to the dispersion of the longitudinal acoustic phonon since both involve atomic displacements along the tube axis. Ab initio methods have provided a modulus of elasticity for SWNT of ∼1000 GPa (1 TPa), which is 5 times higher than that for carbon steel.12 (8) There is empirical evidence that SWNTs can withstand strong bending and deformation and that the ensuing changes are fully reversible. The ability of SWNTs to accommodate large strains has been ascribed to their atomic monolayer (shell thickness), which allows sp2 carbons to rehybridize upon subjection to out-of-plane distortion. In nanotechnology, this property is often termed plasticity. (9) The sharp tip of the end of a SWNT results in a locally boosted electrical field upon application of a potential. In fact, a nanotube cathode surrounded by vacuum can release electrons (11) Ajayan, P. M. Chem. Rev. 1999, 19, 1787-1799. (12) Dai, H. Acc. Chem. Res. 2002, 35, 1035-1044.

Analytical Chemistry, Vol. 79, No. 13, July 1, 2007

4789

in a process known as field emission. This property is also the result of the intrinsic ability of SWNTs to carry large electric currents for their size with minimal resistance and enables their use as highly efficient, bright electron sources.13 As noted earlier, CNTs can be modified covalently. Attaching ligands or inserting functional groups in SWNTs can facilitate selfassembly into more complex structures. Although this paper is concerned mainly with the uses of CNTs, modified CNTs can be anticipated to open up very promising prospects for new applications in the future. SYNTHESIS AND PURIFICATION OF CNTS The conduct of basic research and development of practical applications frequently require devising new preparation methods in order to obtain improved, more homogeneous materials. The extreme dependence of the properties of CNTs on structure, described in the previous section, has promoted research that has no doubt helped enrich science and is bound to provide a wide range of attractive applications in the future. However, the need to control nanotube diameter and chirality has raised a major challenge to their chemical synthesis. Most CNTs reported to date have been prepared by using arc discharge,14 laser ablation,15 or chemical vapor deposition (CVD) techniques.16 The former two use a solid-state carbon precursor for nanotube growth. On the other hand, CVD uses hydrocarbon gases as sources for carbon atoms and metal catalyst particles as seeds for CNT growth. These well-established methods produce high-quality, nearly perfect nanotube structures despite the large amounts of byproducts also formed in the process. While arc discharge and laser ablation methods produce only tangle nanotubes randomly mixed with byproducts, CVD methods can be used on catalytic patterned substances to obtain CNT arrays at controllable locations and with the desired orientation on surfaces. Regularly positioned arrays of CNT towers grown normal to the surface of a substrate such as silicon can be used to develop a wide array of analytical applications based on nano- and micropreconcentrators, highly sensitive nanosensors, and heat sinks for chips, or CNT-modified monolithic columns.17,18 As noted earlier, one critical disadvantage of carbon nanotubes is inefficient conversion. Thus, the presence of unwanted byproducts in CNTs requires their purification. In fact, raw carbon nanotube samples often contain as little as 50% CNTs, the remainder consisting of impurities such as fullerenes, amorphous carbon, and catalyst residues. Therefore, isolating carbon nanotubes from carbon impurities is one of the greatest challenges and can be used a prerequisite for their further application. The product can be separated from its impurities by using an electrophoretic or chromatographic technique or size-exclusion (13) Yahachi, S. J. Nanosci. Nanotechnol. 2003, 3, 39-50. (14) Journet, C.; Matser, W. K.; Bernier, P.; Laiseau, L.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Nature 1997, 388, 756-758. (15) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Smalley, R. E. Science 1996, 276, 483-487. (16) Flahaut, E.; Laurent, C.; Peigney, A. Carbon 2005, 43, 375-383. (17) Saridara, C.; Mitra, S. Anal. Chem. 2005, 77, 7094-7097. (18) Chen, Y. L. Y.; Xiang, R.; Ciuparu, D.; Pfefferle, L. D.; Horva´th, C.; Wilkins, J. A. Anal. Chem. 2005, 77, 1398-1406. (19) Doorn, S. K.; Felds, R. E.; Hu, H.; Hamon, M. A.; Haddon, R. C.; Selegue, J. P.; Majidi, V. J. Am. Chem. Soc. 2002, 124, 3169-3174.

4790


methodology.20,21 However, their usefulness for the intended purpose is limited by the lack of effective methods for solubilizing CNTs or obtaining stable suspensions of isolated CNTs. Probably, size exclusion is the most obvious choice for purifying CNTs as most of the impurities are no larger than 10 nm in size. As a result, purified CNTs are often obtained by filtration under lamellar or tangential flow conditions. The principal constraint on this approach is posed by impurities adhering to the top of the tubes, removal of which requires using size-exclusion methodology in combination with oxidation, dissolution, or suspension.22,23 Oxidation schemes for this purpose rely on the fact that carbon atoms in the impurities are often more reactive than those in the CNTs. Thus, fullerene isomers and amorphous carbon can be easily digested with concentrated acids or strong oxidants at moderate temperatures. One disadvantage of this procedure is that CNTs also have reactive sites at defects and caps that can be functionalized as well during the process. Nanotubes can also be purified by filtration and centrifugation; these operations, however, can introduce new contaminants. Alternative separation techniques such as electrophoresis and chromatography require the use of a surfactant to disperse or solubilize nanotubes or an organic mobile phase (e.g., dimethyl formamide, tetrahydrofuran), respectively. A need therefore exists for more efficient CNT synthesis and purification procedures. While commercially available CNTs are supplied in a high purity, they are very expensive to purchase. In this way, it should be remembered that the prices for carbon fibers and fullerenes were also prohibitively high during their initial development. However, they have come down significantly with time. It should also be noted that CNTs are also used to prepare other materials such as fiber-reinforced structural composites. In fact, CNTs improve resin-dominant properties such as interlaminate strength, toughness, and thermal and environmental durability. This makes it especially important to use reliable methods for their purification and characterization. CHARACTERIZATION OF CNTS Carbon nanotubes have been examined with a variety of techniques. Thus, their structure has been studied by transmission electron microscopy and powder X-ray diffraction and elastic neutron diffraction scattering spectroscopies. These techniques provide complementary information since electrons and neutrons interact with nuclei (in the electron microscopy and neutron diffraction techniques), whereas X-rays interact with bound electrons (in the X-ray diffraction technique, which probes electron density). Raman spectroscopy is the usual choice for establishing the diameter distribution within a CNT bulk sample. In particular, Raman spectroscopy is a powerful tool for characterizing SWNTs where characteristic peaks occur due to the radial breating mode, disordered carbon, and out-of-phase grapheme sheetlike vibrations. In addition, it constitutes a unique tool for characterizing CNTs in terms of the amount of ordering and degree of sp2 and (20) Duesberg, G. S.; Muster, J.; Krstic, V.; Burghard, M.; Roth, S. Appl. Phys. A. Mater. Sci. Processes 1998, A67, 117-119. (21) Duesberg, G. S.; Blau, W.; Byrne, H. J.; Muster, J.; Burghard, M.; Roth, S. Synth. Met. 1999, 103, 2484-2485. (22) Hou, P. X.; Bai, S.; Yang, Q. H.; Liu, C.; Cheng, H. M. Carbon 2002, 40, 81-85. (23) Strong, K. L.; Anderson, D. P.; Lafdi, K.; Kuhn, J. N. Carbon 2003, 4, 14771488.

sp3 bonding in pristine SWNTs. In some cases, however, covalent functionalization of an MWNT causes no obvious spectral changes and quantifying Raman spectral changes is difficult as a result. The use of chemometrics (e.g., principal component analysis) can be an effective alternative.24 Despite the strong efforts made in recent years toward developing new, effective characterization methodologies for CNTs, straightforward, inexpensive alternatives are still required with a view to elucidating their structure and electronic properties. Carbon nanotubes can also be characterized by using other techniques such as size-exclusion chromatography,20,21 gel permeation chromatography, capillary electrophoresis,19,25 or fieldflow fractionation.26,27 Gel permeation chromatography can be used to purify and characterize SWNTs, which, however, must previously be shortened and solubilized by attachment of long-chain amines. Capillary electrophoresis also requires the prior solubilization of the nanotubes (e.g., by forming a surfactant coating).25 Combinations of molecular probes have been used to extract additional information about CNT electronic properties. The principal shortcoming of capillary electrophoresis as applied to CNTs is the correlation between migration times and CNT structure, which is related to the dimensions and electrophoretic mobility of surfactant-coated SWNTs; the last additionally depends on the ease with which the particular CNT can be coated by the surfactant. Neither gel permeation chromatography nor capillary electrophoresis has proved effective for extracting particle size distribution information from functionalized or soluble carbon nanotubes (e.g., carboxylated CNTs).28 By contrast, field-flow fractionation is an attractive choice for fractionating a broad distribution range of CNT sizes. The externally applied field drives unlike CNT particles to different average positions across the thin channel, where they are caught up at different flow velocities and thus eluted at different times. The greatest shortcoming of this technique is the difficulty of extrapolating its theory, which is almost entirely based on the assumption that particles are spherical rather than cylindrical as in CNTs.

CNTs and can efficiently debundle raw CNTs.25 Dispersion in CNTs has been studied in water containing a variety of surfactants including octylphenol ethoxylate (Triton X-100),30 sodium dodecyl sulfate (SDS)25,31,32 and sodium dodecylbenzenesulfonate.30 These surfactants possess a long, hydrophobic chain tail that binds to CNTs and a hydrophilic headgroup that mediates the interaction with water. Recent interest in introducing CNTs in biological systems has led to the discovery that starch (amylose) can solubilize SWNTs via hydrophobic interactions,33 thereby facilitating their stabilization in aqueous media. Some of the most effective surfactants also contain benzene rings, which are expected to have a high affinity for CNT surfaces and establish π-π interactions with them. The same principle allows SWNT bundles to be coordinated by certain conjugated polymers such as poly(aryleneethynylene)34 and poly(m-phenylenevinylene).35 Even surfactants having no benzene rings can coat and disperse CNTs highly efficiently. Thus, SDS at levels below its critical micelle concentration can efficiently coat CNTs. The exact mechanism by which surfactant molecules organize on the CNT surface remains unclear. The surfactant has been assigned three tentative roles involving facilitating (a) structureless random adsorption of the CNT with no preferential arrangement of the head and tail, (b) micelle adsorption on the CNT surface or (c) encapsulation of the CNT in cylindrical surfactant micelles. As a rule, efficient debundling entails sonicating the CNT in the aqueous solution of surfactant. Although this approach is widely used, the mechanism behind the dispersing action is poorly understood. Too long or too strong sonication can lead to CNTs decomposing into amorphous carbon. Also, inefficient dispersion results in the formation of irreproducible aggregates that hinder further analysis. Recently, UV-vis spectroscopy has been proposed as a tool for controlling the dispersion efficiency.36 The authors have used capillary electrophoresis to study dispersion in CNTs. The principal advantage of the CE technique here is that it provides partial information about the number of aggregates and their distribution.

DISPERSION OF CNTS As noted earlier, the poor solubility and difficult manipulation of CNTs in most solvents have also severely restricted the uses of SWNTs and the development of effective procedures for their characterization. In response to this problem, a variety of covalent29 and noncovalent functionalization methods have been proposed to ensure efficient debundling and dispersion in CNTs. Because covalent functionalization has been found to impair the intrinsic properties of CNTs, noncovalent functionalization approaches, also known as “supramolecule formation”, have aroused much interest among analytical chemists. We should note that noncovalent procedures preserve both the integrity and intrinsic properties of

CARBON NANOTUBES AS ANALYTICAL TOOLS Carbon nanotubes possess useful features with a view to improving the analytical process. Figure 2 illustrates their potential roles in the development of new tools for analytical science, arranged in terms of complexity of design and integration. This and other aspects of the analytical uses of CNTs are discussed in the following sections. (1) Uses of CNTs Based on their Sorption Properties. The known ability of CNTs to establish π-π electrostatic interactions, and their large surface areas, can facilitate the adsorption of

(24) Sato-Berru´, R. Y.; Basiuk, E. V.; Saniger, J. M. J. Raman Spectrosc. 2006, 37, 1302-1306. (25) Sua´rez, B.; Simonet, B. M.; Ca´rdenas, S.; Valca´rcel, M. J. Chromatogr., A 2006, 1127, 278-285. (26) Kim, H. Y.; Choi, W. B.; Lee, N.; Chung, D. S.; Kang, J. H.; Han, I. T.; Kim, J. M.; Moon, M. H.; Kim, J. S. Mater. Res. Soc. Symp. Proc. 2000, 593, 123-127. (27) Chen, B.; Selegue, J. P. Anal. Chem. 2002, 74, 4774-4780. (28) Sua´rez, B.; Simonet, B. M.; Ca´rdenas, S; Valca´rcel, M. Submitted for publication. (29) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760-761.

(30) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269-273. (31) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Jounet, C.; Bernier, P.; Poulin, P. Science 2000, 290, 1331-1334. (32) Regev, O.; Elkati, P. N. B.; Loos, J.; Koning, C. E. Adv. Mater. 2004, 16, 248-251. (33) Kim, O. K.; Je, J.; Baldwin, J. W.; Kooi, S.; Pehrsson, P. E.; Buckley, L. J. J. Am. Chem. Soc. 2003, 125, 4426-4427. (34) Rice, N.; Soper, K.; Zhou, N.; Merschrod, E.; Zhao, Y. Chem. Commun. 2006, 47, 4937-4939. (35) Star, A.; Liu, Y.; Grant, K.; Ridvan, L.; Stoddart, J. F.; Steuerman, D. V.; Diehl, M. R.; Boukai, A.; Heath, J. R. Macromolecules 2003, 36, 553-560. (36) Attal, S.; Thiruvengadathan, R.; Regev, O. Anal. Chem. 2006, 78, 80988104.


4791

Figure 2. Classification of analytical tools based on the use of carbon nanotubes according to the analytical complexity and the increasing level of system design and integration.

analytes in a selective, reproducible manner. Analytically, this ability has been used for three purposes, namely: (a) preparing samples for (micro)solid-phase extraction, (b) obtaining stationary or pseudostationary phases for analyte separation, and (c) developing piezoelectric detection systems for volatile analytes. Figure 3 shows the different configurations described in the literature. In addition to direct CNTs packing in a minicolumn, as can be seen in the microscopy photos, CNTs can be grown on steel capillary, silica capillary, or an organic monolithic polymer. The possibility to synthesize CNTs on several materials allows the development of microtrap or chromatographic columns. CNTs as (Micro)Solid-Phase Extraction Sorbents. The distortion of planar graphene into a cylinder considerably complicates the orbital overlapping, resulting in carbon atoms wound around in a helical fashion. As a consequence, nanotubes easily experience fluctuating and induced dipole moments, which results in excellent van de Waals adhesion to other molecules. In fact, they can be expected to exhibit a strong binding affinity for hydrophobic molecules relative to planar carbon surfaces. Surface areas are especially high on the outside and in interstitial spaces within nanotube bundles. As a rule, MWNTs possess a higher sorption capacity than do SWNTs. Thus, MWNTs have been used as sorbents in packed minicolumns of metal ions, organometals, and a wide variety of aromatic compounds such as bisphenols, 4-nonylphenol, 4-tert-octylphenol,37 dioxins,38 chlorobenzenes,39 (37) Cai, Y.; Jiang, G.; Liu, J.; Zhou, Q. Anal. Chem. 2003, 75, 2517-2521. (38) Long, R. Q.; Yang, R. T. J. Am. Chem. Soc. 2001, 123, 2058-2059. (39) Liu, G. H.; Wang, J. L.; Zhang, X. R. Anal. Lett. 2004, 37, 3085-3104.

4792


phthalate esters,40 atrazine, simazine,41 tetracyclines,42 nonsteroidal antiinflammatory drugs,43 and benzodiazepines.44 In our opinion, the possibility of adsorbing analytes on individual CNTs (via molecular interactions) rather than in a raw CNT mass where CNTs interact with one another will be exploited in the future to develop new analyte extraction modes. This type of interaction will be especially selective and facilitate the realization of the sizeand property-related advantages of CNTs. Our group conducted a study where CNTs dispersed and subsequently retained on a filter proved useful for preconcentrating tetracyclines by using only a few milligrams of nanotubes. In a subsequent experiment, we immobilized CNTs covalently on glass microparticles43 in order to suppress interactions between CNTs; immobilized CNTs exhibited an increased capacity to adsorb analytes. Miniaturization is a very interesting trend in analytical science inasmuch as the forces involved in sorption processes differ from those prevailing at a macroscopic level and provide clear advantages for new potential uses. Similarly to supramolecular chemistry, interactions between analytes and individual nanoparticles are bound to also open up new prospects here. (40) Cai, Y. Q.; Jiang, J. F.; Zhou, Q. X. Anal. Chim. Acta 2003, 494, 149-156. (41) Zhou, Q. X.; Wang, W. D.; Xiao, J. P.; Wang, J. H.; Liu, G. G.; Shi, Q.; Guo, G. L. Microchim. Acta 2006, 152, 215-224. (42) Sua´rez, B.; Santos, B.; Simonet, B. M.; Ca´rdenas, S.; Valca´rcel, M. Submitted for publication. (43) Sua´rez, B.; Simonet, B. M.; Ca´rdenas, S.; Valca´rcel, M. J. Chromatogr., A, In press. (44) Wang, L.; Zhao, H.; Qiu, Y.; Zhou, Z. J. Chromatogr., A 2006, 1136, 99105.

Figure 3. Different configurations used to preconcentrate or to separate analytes based on the sorption capabilities of carbon nanotubes. Figure adapted from refs 46-51.

Recently, MWNT-coated fibers have been used for the solidphase microextraction of polybrominated diphenyl ethers.45 In fact, CNTs are stable enough for this purpose and analytes can be thermally desorbed from them at the injection port of a gas chromatograph. The thermal stability of CNTs has also been (45) Wang, J. X.; Jiang, D. Q.; Gu, Z. Y.; Yan, X. P. J. Chromatogr., A 2006, 1137, 8-14.

exploited to elute organic vapor from a preconcentration tube;46 the nanotubes, in the form of SWNTs, were used as packing sorption material.47 As stated above, CNTs can be assembled onto a substrate surface. This has been used in combination with the sorption capabilities of CNTs, and the ability to thermally desorb analytes from them, to develop a microtrap for small organic molecules. CNTs as Stationary or Pseudostationary Phases. Carbon-based sorbents are usually employed in gas chromatography to separate small organic and inorganic molecules. Typically, particles are packed into a tube. Because CNTs can be prepared in different shapes, sizes, or even functionalities, their affinity and selectivity can be adjusted as required to separate a wide range of solutes. These properties can be used to prepare highly selective and thermally stable chromatographic sorbents. MWNTs have been used mainly in packed columns in this context. However, powdered CNTs are known to easily form aggregates and lose many of their nanoscale characteristics as a result. This has led to using self-assembled CNTs as alternatives.48,49 To this end, nanotubes are deposited over long pieces of tubing in order to construct effective gas chromatographic columns.50 SWNTs have also been used as stationary phases in liquid chromatography. To this end, they have been incorporated into an organic polymer containing vinylbenzyl chloride and ethylene dimethacrylate in order to obtain a monolithic stationary phase.51 The strong hydrophobicity of SWNTs results in improved chromatographic retention of small neutral molecules in the reversedphased mode. Stationary phases of this type have also been used in electrochromatography. Surfactant-coated SWNTs (SC-CNTs) have been used as pseudostationary phases for improved resolution in capillary electrophoresis.52 This new electrophoretic mode, which is called micellar nanoparticle dispersion electrokinetic chromatography, has proved highly effective at the separation of various aromatic compounds including chlorophenols, penicillins, and nonsteroidal antiinflammatory drugs (see Figure 4).52 In addition to the intrinsic interactions of MEKC, this new mode introduces that between the analytes and CNT surfaces. The latter, which is strongly dependent on the properties on the particular analyte, results in increased resolution by virtue of the ability of CNTs to interact with aromatic and hydrophobic compounds. SC-SWNTs have also exhibited some capacity to resolve isomers.52,53 In the future, this property is bound to be further exploited by incorporating and using chiral CNTs. No analytical uses for chiral CNTs have to date been reported, however. Likewise, the potential of open CNTs to lodge molecules has scarcely been assessed. CNT-Based Piezoelectric Detection Systems. The sorption capabilities of CNTs can be used for the piezoelectric detection of volatile analytes.12 Thus, SWNTs are known to physisorb NO2 and (46) Saridara, C.; Brukh, R.; Iqbal, Z.; Mitra, S. Anal. Chem. 2005, 77, 11831187. (47) Zheng, F.; Baldwin, D. L.; Fifield, L. S.; Anheier, N. C.; Aardahl, C. L.; Grate, J. W. Anal. Chem. 2006, 78, 2442-2446. (48) Saridara, C.; Mitra, S. Anal. Chem. 2005, 77, 7094-7097. (49) Karwa, M.; Mitra, S. Anal. Chem. 2006, 78, 2064-2070. (50) Karwa, M.; Mitra, S. Anal. Chem. 2006, 78, 2064-2070. (51) Li, Y.; Chen, Y.; Xiang, R.; Ciuparu, D.; Pfefferle, L. D.; Horva´th, C.; Wilkins, J. A. Anal. Chem. 2005, 77, 1398-1406. (52) Sua´rez, B.; Simonet, B. M.; Ca´rdenas, S; Valca´rcel, M. Electrophoresis. In press. (53) Moliner, Y.; Ca´rdenas, S.; Valca´rcel, M. Submitted for publication.


4793

Figure 4. Effect of surfactant coated single-walled carbon nanotube on capillary electrophoresis ressolution. Adapted from ref 52.

O2 strongly54 and NH3 weakly.55 Volatile organic compounds can also be detected in this way. Covering a crystal microbalance with Langmuir-Blodgett films compressing tangled bundles of SWNTs allows alcohol vapor to be detected. CNT-based sensors can also be used to detect changes in their electronic properties resulting from the sorption of molecules on their surface. This is partly related to the sorption capabilities of nanotubes and is discussed in the following section. One interesting, recently described, but as yet analytically unexplored aspect of SWNTs is photoinduced molecular desorption from them.56 Molecules adsorbed on SWNTs can be desorbed by heating the nanotubes at a high temperature. Alternatively, UV light at a low photon flux can be used to effect the rapid molecular desorption of, for example, O2 at room temperature.56 Wavelength-dependent measurements have revealed that photodesorption is due to electronic excitation of nanotubes and is a nonthermal process. CNTs for Other Applications. One other analytical use of CNTs based on their surface properties is as matrixes in MALDI-TOFMS for the analysis of small molecules. Each CNT functions to trap analyte molecules by sorption and acts as an energy receptacle for laser ablation. The advantages of desorption/ionization on CNTs for biomolecular analysis arise from their simplifying sample preparation and avoiding interferences from matrix and background ions. This procedure has proved effective for the analysis of cyclodextrin, small peptides, and miscellaneous other organic compounds.57,58 (54) Collins, P. G.; Bradley, K.; Ishigami, M.; Zetti, A. Science 2000, 287, 18011804. (55) Kong, J.; Franklin, N.; Zhou, C.; Chapline, M.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622-625. (56) Chen, R.; Franklin, N.; Kong, J.; Cao, J.; Tombler, T.; Zang, Y.; Dai, H. Appl. Phys. Lett. 2001, 79, 2258-2260. (57) Ugarov, M. V.; Egan, T.; Khabashesku, D. V.; Schultz, J. A.; Peng, H.; Khabashesku, V. N.; Furutani, H.; Prather, K. S.; Wang, H. W. J.; Jackson, S. N.; Woods, A. S. Anal. Chem. 2004, 76, 6734-6742.

4794


(2) Uses of CNTs Based on Their Electronic Properties. The electronic properties of CNTs are suggestive of their potential ability as electrodes to mediate electron-transfer reactions with electroactive species in solution. In addition, CNTs exhibit a high ability to promote some types of electron-transfer reactions, minimize fouling of electrode surfaces, enhance electrocatalytic activity, and facilitate the immobilization of molecules such as enzymes or antibodies on their surface with a view to developing biosensors. A recent review of CNT uses by Trojanowicz places special emphasis on electroanalytical applications.59 This is unsurprising since the electronic properties of CNTs have to date been the most widely exploited for analytical purposes. Electrode configurations involving carbon nanotubes are constructed by (a) mixing CNTs with a binder in order to form a paste for packing into an electrode, (b) evaporating a solution of CNTs onto a glassy carbon electrode, (c) attaching individual CNTs to the end of a wire, (d) incorporating nanotubes onto a biosensing platform, or (e) microfabricating a nanoelectrode ensemble or array. Carbon nanotubes in both oriented and nonoriented configurations have been used to modify electrode surfaces.59-61 MWNTs in oriented configurations exhibit a high electron-transfer rates. On the other hand, SWNTs exhibit slow electron transfer and a low specific capacitance. Electrode surfaces can be easily modified with CNTs by evaporating a suspension of nanotubes in an appropriate solvent such as bromoform or N,N-dimethylformamide.62 A variety of CNT-modified, paste63 and composite elec(58) Xu, S.; Li, Y.; Zhou, H.; Qiu, J.; Guo, Z.; Guo, B. Anal. Chem. 2003, 75, 6191-6195. (59) Trojanowicz, M. Trends Anal. Chem. 2006, 25, 480-489. (60) Su, L.; Gao, F.; Mao, L. Anal. Chem. 2006, 78, 2651-2657. (61) Liu, G.; Lin, Y. Anal. Chem. 2006, 78, 835-843. (62) Luo, H.; Shi, Z.; Li, N.; Gu, Z.; Zhuang, Q. Anal. Chem. 2001, 73, 915-920. (63) Valentini, F.; Amine, A.; Orlanducci, S.; Terranova, M. L.; Palleschi, G. Anal. Chem. 2003, 75, 5413-5421.

Figure 5. SEM image of a multiwalled carbon nanotube nanoneedle electrode. (A) cicly voltagram of 1 mM dopamine; (B) differential pulse voltagramms of several dopamine concentrations at the nanoelectrode; and (C) detection of glutamate at the nanobiosensor (nanoelectrode modified with enzyme). Adapted from ref 69.

trodes64 have also been reported. Also, a number of biosensorswhere CNTs act as molecular wires bonded to biomolecules such as proteins, DNA, or antibodies have been constructed.65,66 The small size and excellent electronic properties of CNTs make them especially attractive for developing microelectrodes.67 In fact, several microelectrodes for electrophoretic microchips have already been reported. Electrode dimensions have been reduced not only to the micro but also to the nano level.68,69 The first attempt at using an individual CNT as an electrochemical probe was reported in 1999;70 the probe consisted of an MWNT assembled on a tungsten tip. Four years later, an SWNT nanoelectrode affording the determination of neurotransmitters in individual biological cells was developed. Finally, the first nano(64) Hrapovic, S.; Majid, E.; Liu, Y.; Male, K.; Luong, J. H. T. Anal. Chem. 2006, 78, 5504-5512. (65) Wang, J. Electroanalysis 2005, 17, 7-14. (66) Liu, G.; Lin, Y. Anal. Chem. 2006, 78, 835-843. (67) Wang, J.; Chen, G.; Chatrathi, M. P.; Musameh, M. Anal. Chem. 2004, 76, 298-302. (68) Chen, R. S.; Huang, W. H.; Tong, H.; Wang, Z. L.; Cheng, J. K. Anal. Chem. 2003, 75, 6341-6345. (69) Boo, H.; Jeong, R. A.; Park, S.; Kim, K. S.; An, K. H.; Lee, Y. H.; Han, J. H.; Kim, H. C.; Chung, T. D. Anal. Chem. 2006, 78, 617-620. (70) Campbell, J. K.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 37793780.

Figure 6. Comparison of AFM images obtained with a regular silicon probe and a multiwalled carbon nanotube probe. Adapted from ref 71.

biosensor, which consisted of glutamate oxidase immobilized on electrochemical nanoneedles, was reported as recently as 2006 (Figure 5).69 Carbon nanotubes have also been used to develop chemical sensors. Direct measurements of some electronic property such Analytical Chemistry, Vol. 79, No. 13, July 1, 2007

4795

Figure 7. Analytical applications of carbon nanotubes raising from the combination of their predominant and general properties.

as their resistance can be used to quantify molecular adsorption of target compounds on them. This principle has been used to develop miniature chemical sensors for detecting low concentrations of gas molecules at room temperature with a high sensitivity. Chemically and physically modified CNTs may facilitate the construction of highly sensitive, selective chemical sensors. For example, semiconducting CNTs coated with a noncontinuous thin layer of palladium metal are extremely sensitive to molecular hydrogen.4 (3) Uses of CNTs Based on their Thermal, Mechanical, and Field Emitter Properties. Carbon nanotubes possess a high thermal conduction capacity. This allows a high surface area for dissipating heat to be obtained by preparing them as molecular towers. This property has been used to dissipate heat from chips and also in small heat exchangers for nuclear reactors by virtue of their low absorbing power and high thermal conductivity of high-speed neutrons without direct exposure of the metal to water. Obviously, these CNT assemblies could be used to dissipate heat from electrophoretic chips or boost the performance of in-chip miniaturized PCR techniques. Somehow related to the sorption properties or molecular interactions of CNTs is their use as tips with a view to improving spatial resolution in atomic force microscopy (AFM) (see Figure 6).71,72 Also, these CNTs can be derivatized and functionalized at their ends. CNT tips were first prepared by mechanically attaching nanotubes to a commercial cantilever tip using acrylic adhesive. (71) Nguyen, C. V.; Chao, K. J.; Stevens, R. M. D.; Delzeit, L.; Cassell, A.; Han, J.; Meyyappan, M. Nanotechnology 2001, 12, 363-367. (72) Hafner, J. H.; Cheung, C. L.; Lieber, C. M. Nature 1999, 398, 761-762.

4796


At present, tips can also be constructed by in situ bonding CNTs onto AFM tips during observation or by chemical deposition.72 CNTs have also been used in scanning tunneling microscopy. CNT tips allow carboxylated SWNTs to be immobilized with a view to recognizing ether oxygens with a high resolution; this is a result of electron tunneling being facilitated by hydrogen-bonding interactions between the ether oxygens and carboxyl groups at SWNT apexes.73 One of the most interesting properties of CNTs is their capacity as field emitters.74 This property, which remains unexplored in analytical science, could be used to develop microfocused X-ray sources or even to obtain microplasmas. Although CNTs are robust and inert structures, it has been demonstrated that the electrical properties of CNTs are extremely sensitive to the effects of charge transfer and chemical doping by various molecules. This is the basis of most nanosensors based on CNT called field effect transitors. The electronic structures of target molecules near of the CNT cause measurable change of the electrical conductivity of the CNT.75,76 Based on a similar effect, the use of CNT tips to develop a nanotube ionization detector that could be used as a gas sensor in chromatography to develop field-portable gas chromatographs has also been described.77 (73) Nishino, T.; Ito, T.; Umezawa, Y. Anal. Chem. 2002, 74, 4275-4278. (74) Zhao, G.; Zhang, J.; Xhang, Q.; Zhan, H.; Zhou, O.; Oin, L. C. Appl. Phys. Lett. 2006, 89, 193113/1-193113/3. (75) Collins, P.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 18011804. (76) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Kyeongiae, S. P.; Dai, H. Science 2000, 287, 622-625.

COMBINING CNTS WITH NEW MATERIALS Quantum dots (QDs) are semiconducting nanocrystals with quite attractive optoelectronic properties including high emission profiles, quantum yields, size-tunable emission profiles, and narrow spectral bands. Therefore, coupling QDs with CNTs can be expected to provide composite materials affording selective wavelength sorption, charge transfer to CNTs, and efficient electron transport.78 Such materials would have especially attractive properties for use in a variety of analytical applications. None of the QD-CNT combinations obtained and examined in physical terms to date has yet found analytical use, however. The combination of carbon nanotubes and ionic liquids has alsoarousedmuchinterestinmanyareasincludingelectrochemistry.79-80 Ionic liquids hold much promise for use as electrolytes as they are nonvolatile and highly conductive and exhibit very large potential windows relative to conventional electrolytes. In addition, CNTs provide substantial advantages over conventional carbon electrodes used in electrochemical systems. One can anticipate the use of CNT electrodes in ionic liquid electrolytes for many purposes including the production of batteries, capacitors, pho(77) Modi, A.; Koratkar, N.; Lass, E.; Wei, B.; Ajayan, P. M. Nature 2003, 424, 171-174. (78) Ravindran, S.; Chaudharly, S.; Colburn, B.; Ozkan, M.; Ozkan, C. S. Nano Lett. 2003, 3, 447-453. (79) Maleki, N.; Safari, A.; Tajabadi, F. Anal. Chem. 2006, 78, 3820-3826. (80) Zhao, F.; Wu, X.; Wang, M.; Liu, Y.; Gao, L.; Dong, S. Anal. Chem. 2004, 76, 4960-4967.

tovoltaic systems, electronic displays, or even mechanical actuators. FUTURE TRENDS Carbon nanotubes can have a strong, symbiotic impact on analytical science. Thus, improving CNTs requires the concourse of analytical science to develop effective procedures for their purification and characterization in order to ensure efficient exploitation of their capabilities. Conversely, analytical science can be the principal beneficiary of a powerful tool for improving existing procedures or designing new instruments. In fact, CNTs hold strong promise for analytical science and nanotechnology. The possibility of preparing modified CNTs of even better, accurately controlled properties is just one of the hosts of opportunities these materials can provide. Figure 7 shows the present state of the art. As can be seen, there are a large number of CNT properties that still have not be exploited from the analytical point of view which could be interesting for a large number of analytical applications. ACKNOWLEDGMENT Financial support from Junta of Andalucı´a within the framework of project FQM-147 is gratefully acknowledged. Received for review January 31, 2007. Accepted April 16, 2007. AC070196M


4797

Role of Carbon Nanotubes in Analytical Science - ACS Publications

Recommend Documents