Spongelike Structures of Hexa-peri-hexabenzocoronene Derivatives

Apr 3, 2009 - In this study, we have achieved a marked improvement of the sensitivity and selectivity of random networks (RNs) of CNT chemiresistors t...
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Spongelike Structures of Hexa-peri-hexabenzocoronene Derivatives Enhance the Sensitivity of Chemiresistive Carbon Nanotubes to Nonpolar Volatile Organic Compounds of Cancer :: Yael Zilberman,†,§ Ulrike Tisch,†,§ Wojciech Pisula,‡ Xinliang Feng,‡ Klaus Mullen,‡ and ,† Hossam Haick* † Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, Technion - Israel Institute of Technology, Haifa 32000, Israel, and ‡Max-Planck-Institute for Polymer Research, Postfach 3148, D-55021 Mainz, Germany. § Authors have equal contribution to the manuscript

Received December 29, 2008. Revised Manuscript Received March 2, 2009 Cancer is a leading health hazard, and lung cancer is its most common form. Breath testing is a fast, noninvasive diagnostic method which links specific volatile organic compounds (VOCs) in exhaled breath to medical conditions. Arrays of sensors based on carbon nanotubes (CNTs) could in principle detect cancer by differentiating between the VOCs found in the breath of healthy and sick persons, but the notoriously low sensitivity of CNT sensors to nonpolar VOCs limits their accuracy. In this study, we have achieved a marked improvement of the sensitivity and selectivity of random networks (RNs) of CNT chemiresistors to nonpolar VOCs by functionalizing them with self-assembled, spongelike structures of discotic hexa-peri-hexabenzocoronene (HBC) derivatives. We observed swelling of the organic film by monitoring the changes of organic film thickness during exposure and propose that the expansion of the spongelike organic overlayer creates scattering centers in the underlying RN-CNTs by physically distancing the CNTs at their intersections. The results presented here could lead to the development of robust sensors for nonpolar VOCs of cancer breath, which have hitherto been difficult to trace.

1. Introduction Cancer is rapidly becoming the greatest health hazard of our days. Over 12 million new cancer cases were diagnosed in 2007 worldwide, and over 7.6 million lives were lost from the disease, with lung cancer being its most common form.1 The conventional diagnostic methods for lung cancer occasionally miss tumors and are costly and unsuitable for widespread screening.2 Breath testing is a recognized diagnostic method which links specific volatile organic compounds (VOCs) in exhaled breath to medical conditions.3,4 Exhaled breath is composed mainly of nitrogen, oxygen, carbon dioxide, water, and inert gases. Trace VOCs that are generated by the body or absorbed from the environment make up the rest of the breath.4,5 The exogenous VOCs are either directly absorbed through the lung via the inhaled breath or indirectly through the blood or through the skin.6 The endogenous VOCs, which are mostly nonpolar compounds, are generated by the cellular biochemical processes of the body and, thus, may provide an insight into its functioning.6 Several classes of nonpolar VOCs can be measured in the exhaled breath.7 For example, saturated hydrocarbons, such as ethane, pentane, and aldehydes, are formed during lipid peroxidation of fatty acid components of

cell membranes, triggered by reactive oxygen species.7 They are considered markers of oxidative stress. Smaller quantities may be produced by protein oxidation and colonic bacterial metabolism. They have a low solubility in the blood and hence are excreted in the breath within minutes of their formation. Another example for the nonpolar endogenous VOCs in the breath is the unsaturated hydrocarbons. Isoprene, for example, is formed along the mevalonic pathway of cholesterol synthesis.8,9 Patterns that include combinations of saturated and unsaturated hydrocarbons, most of which are nonpolar in their nature, could, via breath samples, characterize cancer.10-12 For example, recent studies using gas chromatography/mass spectroscopy (GC-MS) linked with a (103-104) preconcentrator have shown that several breath VOCs appear to be elevated in instances of (lung) cancer.10-12 The compounds of interest are generally to be found at 1-20 parts per billion (ppb) in healthy human breath but can be seen in distinctive mixture compositions and at elevated levels of 10-100 ppb (picomolar-nanomolar concentrations) in the breath of diseased patients.11 Discrimination between patients with and without (lung) cancer, regardless of the disease stage, was achieved via a combination of 22 VOCs.10-12 Chemical sensors based on random networks (RNs) of carbon nanotubes (CNTs) have several technological advantages

*Corresponding author. E-mail: [email protected]. Fax: +972-48295672. (1) American Cancer Society, ScienceDaily, 18 December 2007. (2) Banerjee, A. K.; Rabbitts, P. H.; George, J. Thorax 2003, 58, 266. (3) Breath analysis for clincal diagnosis and therapeutic monitoring; Amann, A., Smith, D., Eds.; World Scientific Publishing Co. Pte. Ltd.: Singapore, 2005. (4) Amann, A.; Spanel, P.; Smith, D. Mini-Rev. Med. Chem. 2007, 7, 115. (5) Buszewski, B.; Kesy, M.; Ligor, T.; Amann, A. Biomed. Chromatogr. 2007, 21, 553. (6) Baubach, J. I.; Vautz, W.; Ruzsanyi, V. Metabolites in human breath: Ion mobility spectrometers as diagnostic tools for lung diseases; World Scientific Publishing Co. Pte. Ltd.: Toh Tuck Link, Singapore, 2005. (7) Kneepkens, C. M. F.; Lepage, G.; Roy, C. C. Free Radical Biol. Med. 1994, 17, 127.

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(8) Karl, T.; Prazeller, P.; Mayr, D.; Jordan, A.; Rieder, J.; Fall, R.; Lindinger, W. J. Appl. Physiol. 2001, 91, 762. (9) Kushch, I.; Arendacka, B.; Stolc, S.; Mochalski, P.; Filipiak, W.; Schwarz, K.; Schwentner, L.; Schmid, A.; Dzien, A.; Lechleitner, M.; Witkovsky, V.; Miekisch, W.; Schubert, J.; Unterkofler, K.; Amann, A. Clin. Chem. Lab. Med. 2008, 46, 1011. (10) Gordon, S. M.; Szidon, J. P.; Krotoszynski, B. K.; Gibbons, R. D.; Oneill, H. J. Clin. Chem. 1985, 31, 1278. (11) O’Neill, H. J.; Gordon, S. M.; O’Neill, M. H.; Gibbons, R. D.; Szidon, J. P. Clin. Chem. 1988, 34, 1613. (12) Phillips, M.; Gleeson, K.; Hughes, J. M. B.; Greenberg, J.; Cataneo, R. N.; Baker, L.; McVay, W. P. Lancet 1999, 353, 1930.

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(e.g., low power, simplicity, manufacturability, etc.).13,14 RN-CNT sensors share these technological advantages with the wellestablished carbon-black/polymer composite chemiresistors15 but hold greater potential for ultrasensitive chemical sensors, because their carrier concentration can be controlled also via the capacitance16 or gate voltage.17 Putting these advantages in a wider perspective reveals, however, that the sensitivity of RNCNT sensors to nonpolar molecules is still problematic to some extent.16,18 This can be attributed to the absence of either a single or a combination of the following effects when exposed to nonpolar VOCs: (i) charge transfer between the adsorbed molecules and CNTs,19,20 (ii) carrier pinning of CNT energy levels,21 and/or (iii) substrate interactions and/or modification of the Schottky barrier at the CNT/metal contact.22-25 Haick and co-workers have recently addressed finding reliable, yet simple ways to utilize RN-CNTs for detecting VOCs of cancer at relatively low concentrations.26,27 In this endeavor, they have constructed RN-CNT field effect transistors (FETs) with chemisensitive, nonpolymeric (mostly, hydrophobic) organic films. Monitoring the changes in conductance, work function, and organic film thickness during exposure revealed enhanced sensitivity to nonpolar VOCs of cancer breath.26 These findings were attributed to carrier scattering induced by swelling of the organic film. In conjugation with this finding, it was also shown that other effects, such as charge transfer between the adsorbed molecules and the CNTs, carrier pinning of CNT energy levels, and/or modification of the Schottky barrier at the CNT/metal contact, play a negligible role in the detection of nonpolar VOCs.26 Hence, controlling the carrier scattering in RN-CNTs via deliberate functionalization with suitable organic films was found to be an important factor in the design of sensors for nonpolar VOCs, which have hitherto been difficult to trace. Based on this knowledge, Haick and co-workers have further shown that an array of 10 RN-CNT sensors functionalized with different nonpolymeric organic layers exhibit discrimination between the VOCs found in the breath of patients with lung cancer, relative to healthy controls, especially if the sensor array is preceded with a water extractor.27 In this Article, we report a further approach to increase the sensitivity of RN-CNT based sensors to nonpolar VOCs. The approach relies on functionalizing the RN-CNTs with discontinuous layers of a certain type of discotic liquid crystal hexa-perihexabenzocoronene (HBC) derivatives having hydrophobic functional groups. These HBC derivatives are particularly interesting materials for sensors used for breath testing, because they selfassemble into 1D stacks via π-π interaction between the aro(13) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (14) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (15) Lewis, N. S. Acc. Chem. Res. 2004, 37, 663. (16) Snow, E. S.; Perkins, F. K.; Houser, E. J.; Badescu, S. C.; Reinecke, T. L. Science 2005, 307, 1942. (17) Kauffman, D. R.; Star, A. Angew. Chem., Int. Ed. 2008, 47, 2. (18) Li, J.; Lu, Y.; Ye, Q.; Cinke, M.; Han, J.; Meyyappan, M. Nano Lett. 2003, 3, 929. (19) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (20) Zhao, J.; Buldum, A.; Han, J.; Lu, J. P. Nanotechnology 2002, 13, 195. (21) Tchernatinsky, A.; Desai, S.; Sumanasekera, G. U.; Jayanthi, C. S.; Wu, S. Y.; Nagabhirava, B.; Alphenaar, B. J. Appl. Phys. 2006, 99, 034306. (22) Byon, H. R.; Choi, H. C. J. Am. Chem. Soc. 2006, 128, 2188. (23) Peng, S.; Cho, K. c. Nano Lett. 2003, 3, 513. (24) Cui, X. D.; Freitag, M.; Martel, R.; Brus, L.; Avouris, P. Nano Lett. 2003, 3, 783. (25) Heinze, S.; Tersoff, J.; Martel, R.; Derycke, V.; Appenzeller, J.; Avouris, P. Phys. Rev. Lett. 2002, 89, 106801. (26) Peng, G.; Tisch, U.; Haick, H. Nano Lett. 2009, 9, 1362. (27) Peng, G.; Trock, E.; Haick, H. Nano Lett. 2008, 8, 3601.

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matic cores, thus exposing only their hydrophobic side groups to the analyte vapors. Because of the hydrophobic nature of their side groups, the reported HBC derivatives are expected to be almost insensitive to the water vapors, which saturate the breath samples. With this in mind, we exposed HBC-functionalized RNCNTs to representative nonpolar cancer biomarkers and, for comparison, to water vapor and to polar organic molecules in the gas phase. The results indicate enhanced sensitivity by means of carrier scattering as a result of swelling of the organic film upon exposure to nonpolar VOCs, while, at the same time, the sensitivity to water molecules is not enhanced. The results presented here may be considered a further step toward the development of a cost-effective, portable, and noninvasive diagnostic tool for the widespread screening of cancer via breath analysis.

2. Experimental Section The sensors were prepared on device quality, degeneratively doped p-type Si(100) wafers capped with a 2 μm thick thermally grown SiO2 insulating layer. Ten pairs of 4.5 mm wide, interdigitated (ID) electrodes with an interelectrode spacing of 100 μm were formed on the substrates by evaporation of 5 nm/40 nm Ti/Pd layer through a shadow mask (see Figure 1). Singlewall CNTs (from ARRY International LTD, Germany; ∼30% metallic, ∼70% semiconducting, average diameter: 1.5 nm, length: 7 mm, purity >90 wt %) were dispersed in dimethylformamide (DMF, from Sigma Aldrich Ltd., >98% purity), using sonication followed by ultracentrifugation. Electrically continuous, submonolayer thick (i.e., quasi-2D) RN-CNTs, which show overall p-type semiconducting characteristics in ambient air (see Results and Discussion section), were formed by drop-casting the CNT solution onto the preprepared ID electrodes, as described elsewhere.26,27 The devices were slowly dried under ambient conditions for 10 min to enhance the self-assembly of the RNCNTs. The devices were then heated to 150 °C for 0.5 h on a hot plate to evaporate the solvent. This process was repeated, if necessary, until the RN-CNT layer had a resistance of about 30-40 kΩ. The RN-CNTs were then functionalized with two types of discotic hexa-peri-hexabenzocoronene (HBC) derivatives having different side groups. The synthesis of the HBC derivatives used in the current study has been described elsewhere.28 These HBCs contain hydrophobic mesogens that are terminated with hydrophobic functional groups. These molecules are able to self-assemble into long molecular stacks with a large, electron rich, semiconducting center, which guarantees good charge carrier transport along the molecular stacking direction, and a relatively insulating periphery28 (cf. also ref 29). Furthermore, the nanometer thick HBC columns can easily form 3D, micrometer size, spongelike structures with a high surface-to-volume ratio. In this work, we did not exploit the conductive nature of the HBC columns, but rather we examined the affinity of large spongelike HBC structures to nonpolar VOCs (of cancer breath). Discrete, statistically distributed HBC microstructures were formed on top of the RN-CNTs by drop casting 40 μL of either 10-4 M HBC-C12 solution in toluene or 10-3 M HBC-C6,2 solution in xylene (see Figure 2). The devices were slowly dried under ambient conditions for 0.5 h to enhance the self-assembly of the HBC molecules. The devices were then annealed at 100 °C for 1 h on a hot plate to evaporate the residual solvent. We used DI water, methanol, octane, and decane as analytes. Methanol, octane, and decane were obtained from Sigma Aldrich Ltd. (>98% purity). DI water (18.2 MΩ cm) was supplied by a commercial water purification system (Easypure II). Octane and :: (28) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Mullen, K. J. Am. Chem. Soc. 2005, 127, 4286. (29) Aebischer, O. F.; Aebischer, A.; Donnio, B.; Alameddine, B.; Dadras, M.; Guedel, H.-U.; Guillon, D.; Jenny, T. A. J. Mater. Chem. 2007, 17, 1262.

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Figure 1. Schematic (not to scale) representation and dimensions of the ID electrodes used to fabricate the two terminal chemiresistors. The sensors were formed by successively drop casting the CNT and HBC solutions onto 10 pairs of preprepared Ti/Pd ID electrodes. decane, two important breath biomarkers of cancer,12 are alkane hydrocarbons, which are chemically stable and nonpolar (dipole moment: 0 D). For the sake of comparison, methanol was used as an example for a polar organic molecule.12 The sensors were electrically tested during exposure to the analytes in a homemade setup consisting of an automated, computer-controlled flow system, capable of regulating the analytes’ concentration via their vapor pressure, and an exposure chamber which can accommodate up to 10 devices. For this study, the sensors were investigated under a wide range of concentrations, ranging between pa/po = 0.05 to pa/po = 1, where pa stands for the partial pressure of the analytes and po for the saturated vapor pressure. Note that the purpose of the current experiment is not to simulate a patient’s breath, but rather to show the feasibility to use HBC derivates for sensing nonpolar VOCs. An Agilent multifunction switch 34980 was used to subsequently address the devices in the exposure chamber. A Stanford Research System SR830 DSP lock-in amplifier controlled by an IEEE 488 bus was used to supply the AC voltage signal (0.2 V at 1 kHz) and measure the corresponding current, which was for our devices below 10 μA. This setup allows us to measure normalized changes in conductance as small as 0.01%. Changes in thickness of the HBC layers during exposure were monitored by spectroscopic ellipsometery (SE). For this purpose, we deposited discontinuous HBC layers directly on the Si/SiO2 substrates, without the underlying 2D RN-CNT layer and the electrodes. The spectra were recorded over a range from 250-1700 nm at an incidence angle of 75°, using a spectroscopic phase modulated ellipsomer30 equipped with a specially designed triangular exposure cell. The analytes were supplied via a flow system similar to the one connected to the exposure chamber for electrical testing. The exact thickness of the SiO2 layers was determined experimentally for every substrate prior to the deposition and functionalization of the RN-CNT layers, using tabulated values for the refractive indices of Si and SiO2.31,32 We used za three-phase overlayer/SiO2/Si model to extract an average thickness of the HBC layers. SE probes an area of about 1  5 mm2, and the thickness might vary locally. However, the increase of the average thickness upon exposure to the analyte gases, measured in the same place, can be used as a measure for the amount of analytes adsorbed to the HBC structures, for which we assumed a Cauchy dispersion of the refractive index. We used Bruggeman’s effective medium approximation to account for the inclusion of voids.

3. Results and Discussion Figure 1 represents schematically our test device, a twoterminal chemiresistor. Note that the device is not drawn to scale. A thin quasi 2D random network composed of intersecting (30) M-2000U Automated Angle; J. A. Woollam Co., Inc.. (31) Aspnes, D. E.; Theeten, J. B. J. Electrochem. Soc. 1980, 127, 1359. (32) Philipp, H. R. Hand book of optical constants of solids; Palik, E. D., Ed.; Academic Press, Inc.: Orlando, FL, 1985.

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single-wall CNTs creates multiple paths over micrometric lateral dimensions (see Figure 2a).27 The single-wall CNTs used in this study were a mixture of ∼30% metallic and ∼70% semiconducting CNTs. Semiconducting CNTs show p-type conduction in ambient air due to oxygen adsorption. It is well-known that good electrical contacts with an electrical conductance ∼0.1e2/h (where e stands for electron charge and h stands for Planck’s constant) form at the intersection of two metallic CNTs and Schottky barriers with a barrier height of ∼Eg/2 form at the intersections of metallic and p-type semiconducting CNTs33 (Eg stands for the band gap of the semiconducting CNTs).26,27 The resistance of the test devices used in the current study was typically 30-40 KΩ in ambient air. Device fabrication may be improved in the future, using a method for controlling the density and alignment of the CNTs during random network formation34 and/or by deliberate dispersion of the CNTs into the organic phase, as was reviewed recently by Grossiord et al.35 The morphology of the discontinuous HBC layers critically affects the sensing properties of the HBC-functionalized RN-CNT chemiresistors. A combination of several parameters (e.g., the nature of side groups, concentration of HBC molecules in solution, type of solvent, etc.) affect the self-aggregation of the HBC molecules.28 Note that the two HBC derivatives used in this study, HBC-C12 and HBC-C6,2, differ greatly in their aggregation behavior when casted from solution. Comparable structures cannot be obtained using solutions with the same solvent and the same concentration. In order to achieve approximately comparable surface coverage of ∼40% and ∼20% for HBC-C12 and HBC-C6,2 layers, respectively, we used a 10-4 M HBC-C12 solution in toluene and a 10-3 M HBC-C6,2 solution in xylene. Figure 2a shows typical scanning electron micrography (SEM) pictures of the 3D, self-assembled, microscopic HBC-C6,2 structures cast from 10-3 M solution in xylene. It can be clearly seen that the conducting HBC wires do not form a continuous network, so that no conducting paths are formed within this layer and electric conduction occurs solely within the underlying CNT random network. Instead, the stacks of HBC molecules selfassemble into well-separated, micrometer size, spongelike structures with large surface-to-volume ratios. The HBC-C6,2 structures are on average 20 μm long, 4 μm thick and cover ∼20% of the surface. HBC-C12 layers cast from 10-4 M solution in toluene are also discontinuous and consist of discrete, spongelike microstructures (see Figure 2b). However, the HBC-C12 structures are on average 50 μm long, 1 μm thick and cover ∼40% of the surface. These marked differences in the structures formed from the HBCC12 and HBC-C6,2 molecules can be explained as follows. The aromatic core of the HBC-C12 molecules is surrounded by a corona of long linear hydrocarbon chains, whereas the side groups in the periphery of the HBC-C6,2 molecules are shorter and branched (see Figure 3). Due to π-π and van der Waals interaction between adjacent molecules, HBC molecules form 1D stacks having an electron rich, semiconductive center and a relatively insulating periphery. HBC-C6,2 molecules, which carry larger space-filling alkyl, dove-tailed substituents, self-associate in solution only at much higher concentration than HBC-C12 molecules. The steric hindrance induced by the side chains prevents the disks from (33) Fuhrer, M. S.; Nygard, J.; Shih, L.; Forero, M.; Yoon, Y. G.; Mazzoni, M. S. C.; Choi, H. J.; Ihm, J.; Louie, S. G.; Zettl, A.; McEuen, P. L. Science 2000, 288, 494. (34) LeMieux, M. C.; Roberts, M.; Barman, S.; Jin, Y. W.; Kim, J. M.; Bao, Z. Science 2008, 321, 101. (35) Grossiord, N.; Loos, J.; Regev, O.; Koning, C. E. Chem. Mater. 2006, 18, 1089.

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Figure 2. Scanning electron micrograph of (a) RN-CNT cast from DMF solution; (b) HBC-C6,2 structures cast from 10-3 M solution in xylene; and (c) HBC-C12 structures cast from 10-4 M solution in toluene.

Figure 3. Schematic (not to scale) representation of the HBC-C6,2 and HBC-C12 molecules.

approaching one another. Therefore, we had to cast HBC-C6,2 from solutions with a higher concentration (10-3 M) than HBCC12 solution (10-4 M) in order to obtain comparable surface coverage. The self-assembly process is also known to strongly depend on the nature of the solvent and processing conditions. Aromatic solvents such as toluene and xylene are chemically similar to the HBC core and, hence, enhance solubility and hinder π-stacking. However, their relatively high boiling points of 138.5 and 110.6 °C for xylene and toluene, respectively, minimize the role of dewetting and maximization of the contribution of intermolecular interactions and thus favor the self-assembly of larger structures upon casting.36 When cast on a solid surface, the HBC aggregates in the solution self-assemble into larger structures. The 10-4 M HBC-C12 solution in toluene produced relatively homogeneous, long, thin microwires upon casting. In contrast, the 10-3 M HBCC6,2 solution in xylene produced rather anisotropic, shorter, and thicker structures with a more pronounced spongelike morphology and with a higher surface-to-volume-ratio than the more regular HBC-C12 structures. The cast HBC-C6,2 structures contain a lot of irregular spaces between the aggregated nanowires due to their nonideal aggregation and stacking. Nonpolar VOCs, such as decane and octane, can penetrate the spongelike HBC-C12 and HBC-C6,2 structures, adsorb to the side chains, and cause swelling. The HBC-C6,2 with its more pronounced spongelike structure can be expected to have better sensing potential. Pristine and functionalized RN-CNT chemiresistors were electrically tested during exposure to the analytes. A supply of purified, dry air was split into two streams, one of which was used as carrier gas and the other one was directed through a bubbler containing the liquid analyte, to generate saturated analyte vapor with pa/po = 1 and directed to the exposure chamber for a time interval of 10 min, followed by 10 min of carrier gas flow. The :: (36) Palermo, V.; Morelli, S.; Simpson, C.; Mullen, K.; Samori, K. J. Mater. Chem. 2006, 16, 266.

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exposure chamber was continuously flushed with the carrier gas. The saturated vapor can be mixed with the carrier gas to yield lower concentrations. The sensors were alternatively exposed to the analyte and to dry air between three and six times during each test sequence. The response of the RN-CNT devices to all four analytes was tested before functionalizing them with the HBC layers. Thereafter, the device was flushed for 0.5 h with dry air to clean the surface of any residual analytes. Quartz crystal microbalance (QCM) measurements of similar HBC/RN-CNT composites confirmed that the surface of the RN-CNTs after the gas exposure is cleaned by flushing the device with dry air. Later, the response of the HBC-functionalized RN-CNT sensor to the same analytes was tested and the results compared. Figure 4 shows the time dependence of the response, ΔR/Rb = (R - Rb)/Rb, of a RN-CNT chemiresistor to pulses of decane and octane at pa/po = 1, and to water37 and methanol at pa/po = 1, before and after functionalization with an HBC-C12 layer. R is the steady state resistance of the sensor when exposed to the analyte, and Rb is its baseline resistance when flushed with dry air, in the absence of the analyte. Rb was typically between 30 and 40 kΩ for our RN-CNT devices and did not change after the functionalization. Note that it is customary to normalize the resistance change, R - Rb, with respect to Rb, because it allows comparing sensors with different baseline resistance. To be consistent with the literature, we also present the response of the two sensors, despite their almost identical baseline resistance. Rb(t) showed a small, linear drift when measuring over long periods of time (∼1 kΩ/h). This is most probably due to one or a combination of the following reasons: (i) a gradual depletion of water molecules and contaminants present in the ambient air to which the sensors were exposed before they were introduced into the exposure chamber or (ii) a gradual accumulation of analyte molecules during cyclical exposure. The time dependence of Rb was interpolated by a linear fit and used to calculate the response shown in Figure 4. The response upon exposure to the analyte vapors was rapid and fully reversible when flushed with dry air. The pristine RN-CNT sensors showed a large increase of the resistance of about 25% upon exposure to the polar analytes water and methanol (see Figure 4c and d).38 Water and methanol molecules, which are adsorbed on the p-type RN-CNTs, act as electron donors. They deplete the number of charge carriers and, hence, increase the resistance of the CNT mesh.26 The effect of the HBC functionalization on the conductance response is almost negligible. The response to water of the pristine and functionalized sensor is identical within the margin of error. This is to be expected, since, owing to the hydrophobic nature of the HBC molecules and their side chains, the discrete, widely spaced HBC structures neither affect nor inhibit the interaction of the CNT (37) Exhaled breath of people contains ∼80% RH, and therefore, it is important to investigate the effect of water on the sensing characteristics of the developed sensors. (38) No systematic increase or decrease of the signals was observed with time. Any apparent changes in the displayed graphs are due to statistically distributed experimental scattering.

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Figure 4. Normalized resistance, ΔR/Rb, of a RN-CNT sensor before and after the functionalization with a discontinuous HBCC12 layer drop cast from 10-4 M solution in toluene upon exposure to (a) decane and (b) octane in the vapor phase at pa/po = 1, and to (c) water and (d) methanol in the vapor phase at pa/po = 1.

mesh with the water molecules. The response to methanol increased by less than 10% after the functionalization. The response of the RN-CNTs to the nonpolar VOCs is more than 1 order of magnitude smaller than that of the polar analytes. Exposure to decane and octane causes only very slight and almost identical responses, increasing the resistance by 1.3% and 1.2%, respectively.38 This increase is due to the creation of scattering centers through absorption of the nonpolar VOCs along the sidewalls of the CNTs and at their intersections.26 The presence of the widely spaced chemisensitive HBC structures affects the response to nonpolar VOCs significantly. The response increases by a factor of 1.8 and of 2.7 for decane and for octane, respectively. Hence, the HBC-C12 functionalization does not only lead to a larger response with improved signal-to-noise ratio but also to improved selectivity, allowing us to distinguish between different nonpolar VOCs. Functionalization with the bulkier HBC-C6,2 structures leads to a further improvement of sensitivity and selectivity for nonpolar VOCs, increasing the response to decane and octane by a factor of 2.0 and 3.4, respectively. Figure 5 displays the calibration curves, ΔR/Rb versus analyte concentration (pa/po raging from 0.05 to 1) for a RN-CNT sensor before and after the functionalization with a discontinuous HBC-C12 or HBC-C6,2 layer. Figure 5 shows that the described enhancement can be observed at all concentrations. Note that the surface coverage of the HBC-C6,2 layer, which caused a stronger enhancement of the sensor’s sensitivity and selectivity, was actually much smaller than that of the HBC-C12 layer. Figure 6 shows the response of the HBC-functionalized sensors, normalized to the response of the corresponding pristine sensor, to water, methanol, decane, and octane. The improved sensitivity and selectivity to nonpolar VOCs can clearly be observed. The question of how the presence of discrete, micrometer-size HBC spongelike agglomerates, which do not contribute to the electrical conduction and cover only 20-40% of the surface area, doubles or triples the normalized resistance change of the underlying CNT mesh when exposed to nonpolar VOCs remains. The response of the RN-CNT chemiresistor to these analytes is solely due to scattering. No charge transfer to the CNTs or the nonpolar HBC structures can take place during exposure to decane and octane, since these molecules have no dipole moment.26 Polymers, on the other hand, are known to swell upon Langmuir 2009, 25(9), 5411–5416

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Figure 5. Calibration curves: ΔR/Rb versus analyte concentration (pa/po=0.05-1). Normalized resistance, ΔR/Rb, of a RN-CNT sensor before and after the functionalization with a discontinuous HBC-C12 or HBC-C6,2 layer upon exposure to (a) decane, (b) octane, (c) water, and (d) methanol in the vapor phase. All presented values of ΔR/Rb have a signal-to-noise ratio greater than 3.

Figure 6. Response of two kinds of HBC-functionalized RN-CNT sensors, normalized to the response of the corresponding pristine sensor, to water, methanol, decane, and octane at pa/po = 1.

exposure to certain analytes. For example, Severin and Lewis reported the swelling of polymer/carbon-black composite layers upon exposure to organic vapors and connected it to a decrease in transconductance of their devices.39 The chemical composition of organic molecules such as HBCs is similar to that of the VOCs and allows them to dissolve easily into their side groups. However, the HBC structures in our study do not form a conductive network. Therefore, their swelling may affect the sensor’s response only indirectly, by temporarily interrupting the conductive paths in the underlying RN-CNT network. In order to confirm that the exposure to the nonpolar VOCs indeed causes swelling of the HBC-structures, we have monitored the changes in average thickness of a discontinuous HBC-C12 layer deposited directly on a Si/SiO2 substrate after a 10 min exposure to water, decane, and octane by SE. Note that SE probes a macroscopic surface area of about 1  5 mm2 on which the HBC structures are statistically distributed. We therefore fitted the spectra using a three-phase 500 nm overlayer/SiO2/Si model, assuming a Cauchy dispersion of the refractive index. Keeping the refractive index constant, we extracted thickness changes of 2 ( 1, 15 ( 1, and 85 ( 4 A˚ upon exposure to the vapors of water, (39) Severin, E. J.; Lewis, N. S. Anal. Chem. 2000, 72, 2008.

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Zilberman et al.

decane, and octane, respectively. While the absolute values of the thickness changes of this assumed layer have no physical meaning, they nevertheless reveal that swelling of the discrete HBC structures takes place. We have observed by repeated exposure to alternating flows of analyte vapor and carrier gas that these changes are in principle reversible, though the baseline thickness increases slightly after each exposure to decane and octane, indicating that some residual swelling occurs. Water exposure has very little effect on the hydrophobic HBC structures, as expected. Exposure to decane causes an increase in the thickness of the assumed layer, which indicates swelling of the HBC agglomerates. Exposure to octane vapors of the same concentration result in a more pronounced increase in the thickness, indicating stronger swelling. We therefore propose that the swellability of the HBC structures in response to the exposure to different analytes governs the conductance response of HBC functionalized RN-CNT sensors by creating scattering centers for charge carriers at the intersections of the tubes. Based on the above-mentioned observations, the sensitivity and chemical selectivity of RN-CNT based sensors can be controlled by the swellability of spongelike, discrete HBC agglomerates on top of the RN-CNTs. Since the latter is independent of the analytes’ polarity, HBC functionalized RN-CNT sensors are feasible for detecting and differentiating between chemical and biological agents with small or negligible dipole moments, which are generally difficult to trace.16 For water vapor or polar organic molecules such as methanol, on the other hand, the conductance response of the RN-CNT device is dominated by charge transfer, which is not affected by the presence of the HBC structures. It should be noted that the enhancement of the response to nonpolar VOCs, which could be achieved by covering only a small part of the RN-CNT, is comparable to the enhancement which Peng et al. have achieved by completely embedding the RN-CNT into a swellable monomer layer: For example, coverage with a thin C23H45 layer led to a doubling of the sensor’s response.26 Whereas Peng et al. have already exploited the full potential of their functionalization with nonpolymeric layers, the beneficial effect of the HBC functionalization can be further enhanced by increasing the surface coverage. Therefore, HBC structures, whose microstructure, surface coverage, distribution, and chemi-

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DOI: 10.1021/la8042928

cal selectivity can be easily controlled by suitable choice of side groups, concentration in solution, and type of solvent, hold great promise for the development of robust RN-CNT based sensors for detecting nonpolar VOCs.

4. Summary and Conclusions In summary, we were able to detect and distinguish between different nonpolar VOCs of cancer breath by functionalizing RN-CNT chemiresistors with self-assembled, discrete, spongelike, micrometer-size HBC structures. The microstructure of the HBC structures, which critically affects the sensing properties, can be controlled by suitable choice of the HBC side groups, the concentration in solution, and the type of solvent. Functionalization with incomplete films of HBC-C6,2 structures resulted in better sensing properties than functionalization with HBC-C12 structures, even at much lower surface coverage. Sensitivity and chemical selectivity of the sensors were governed by the swellability of the HBC agglomerates, while carrier transport was restricted to the RN-CNTs. Since the former is independent of the analytes’ polarity, HBC functionalized RN-CNTs sensors are feasible for detecting and differentiating between chemical and biological agents with small or negligible dipole moments, among them many biomarkers for cancer, which have hitherto been difficult to trace.16 An immediate implication of these findings is that many more types of organic materials are feasible for incorporation as active components into sensors for nonpolar VOCs of cancer breath than has been considered hitherto. Ultimately, the results presented here could lead to the development of cost-effective, lightweight, low-power, noninvasive diagnostic tools for the widespread screening of cancer via breath analysis. Acknowledgment. The research was funded by the Marie Curie Excellence Grant of the FP6 European Commissions, the Technion’s Russell Berrie Nanotechnology Institute, the Max Planck Society through the program ENERCHEM, and the DFG Priority Program SPP 1355. We thank Carmelina Atallah and Irena Dvorkind for support with the SE measurements and the data analysis. H.H. holds the Horev Chair for Leaders in Science and Technology.

Langmuir 2009, 25(9), 5411–5416