Chemiresistive Vapor Sensing with Microscale Films of Gold

Anal. Chem. 2006, 78, 753-761

Chemiresistive Vapor Sensing with Microscale Films of Gold Monolayer Protected Clusters Francisco J. Iban˜ez,† Usha Gowrishetty,‡ Mark M. Crain,‡ Kevin M. Walsh,‡ and Francis P. Zamborini*,†

Department of Chemistry and Department of Electrical and Computer Engineering, University of Louisville, Louisville, Kentucky 40292

Here we report the stability, conductivity, and vaporsensing properties of microcontact-printed films of 1.6nm average diameter hexanethiolate-coated gold monolayer protected clusters (C6 Au MPCs). The C6 Au MPCs were stamped into parallel lines (∼1.2 µm wide and 400 nm thick) across two Au electrodes separated by a 1-µm gap. The chemiresistive vapor-sensing properties were measured for saturated toluene and 2-propanol vapors. As-prepared patterned Au MPC films were unstable in the presence of saturated toluene vapor, and their current response was irreversible. Chemically linking the films with vapor-phase hexanedithiol greatly improves their stability and leads to reversible responses. The extent of Au MPC cross-linking and vapor response to organic vapors varies with different exposure times to dithiol vapor. The response to toluene changed from 61 to 8% for exposures of 1 and 60 min, respectively, which is likely due to greater film flexibility with less dithiol exposure. The current measured through the films varies from 10-11 to 10-3 A as a function of the temperature between 250 and 320 °C, which correlates with the loss of organic material as measured by FT-IR spectroscopy and the change in thickness and width of the film as measured by atomic force microscopy. The vapor-sensing properties vary with temperature, current, and organic content in the film, which are all interrelated. Response to toluene decreased with increasing temperature and conductivity, while the response to 2-propanol was less predictable. Reducing the size of vapor-sensing devices based on Au MPCs is important for creating highly portable devices that can simultaneously detect multiple analytes. This work demonstrates a simple method for reducing the size of such devices down to the microscale and describes methods for maximizing response, stability, and reversibility. Electronic noses1-6 have gained tremendous attention recently as vapor-phase or gas sensors because they provide a less * To whom correspondence should be addressed. E-mail: f.zamborini@ louisville.edu. Fax: 502-852-8149. † Department of Chemistry. ‡ Department of Electrical and Computer Engineering. (1) Albert, K. J.; Lewis, N. S.; Schauer, C. L.; Sotzing, G. A.; Stitzel, S. E.; Vaid, T. P.; Walt, D. R. Chem. Rev. 2000, 100, 2595-2626. (2) Lewis, N. S. Acc. Chem. Res. 2004, 37, 663-672. 10.1021/ac051347t CCC: $33.50 Published on Web 12/31/2005

© 2006 American Chemical Society

expensive, smaller, more portable alternative to the traditional techniques of gas chromatography and mass spectrometry. They are especially useful for applications that require portable field testing or on-line monitoring with applications ranging from medical diagnostics4,5 to bomb detection.3,6 Many different transduction methods have been utilized in the design of these sensors. Piezoelectric tranducers include surface acoustic wave devices,7,8 the quartz crystal microbalance (QCM),8 and the more recent cantilever sensors.9-11 Colorimetric12 and fluorescence-based13,14 sensing methods have also been explored. Chemiresistive sensors are another type based on materials whose conductivity changes in the presence of a vapor phase or gas analyte. A big advantage of conductivity-based sensors is that the instrumentation is simple and the electronic components of the sensor may be easily miniaturized using microfabrication techniques, making these types of sensors highly portable. Chemiresistive vapor or gas sensors based on metal oxides,1,15 nanowires,16 carbon nanotubes,17 carbon black/polymer composites,2 and Au nanoparticles18-36 have (3) Strike, D. J.; Meijerink, M. G. H.; Koudelka-Hep, M. Fresenius J. Anal. Chem. 1999, 364, 499-505. (4) Gopel, W. Sens. Actuators, B 1998, 52, 125-142. (5) Pavlou, A. K.; Turner, A. P. F. Clin. Chem. Lab. Med. 2000, 38, 99-112. (6) Yinon, J. Anal. Chem. 2003, 99A-105A. (7) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219-227. (8) Grate, J. W. Chem. Rev. 2000, 100, 2627-2648. (9) Datskos, P. G.; Sepaniak, M. J.; Tipple, C. A.; Lavrik, N. Sens. Actuators, B 2001, 76, 393-402. (10) Sepaniak, M.; Datskos, P.; Lavrik, N.; Tipple, C. Anal. Chem. 2002, 568A575A. (11) Yang, Y.; Ji, H.-F.; Thundat, T. J. Am. Chem. Soc. 2003, 125, 1124-1125. (12) Rakow, N. A.; Suslick, K. S. Nature 2000, 406, 710-713. (13) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321-5322. (14) Drew, S. M.; Janzen, D. E.; Buss, C. E.; MacEwan, D. I.; Dublin, K. M.; Mann, K. R. J. Am. Chem. Soc. 2001, 123, 8414-8415. (15) Dutta, R.; Hines, E. L.; Gardner, J. W.; Kashwan, K. R.; Bhuyan, M. Sens. Actuators, B 2003, 94, 228-237. (16) Walter, E. C.; Favier, F.; Penner, R. M. Anal. Chem. 2002, 74, 1546-1553. (17) Qi, P.; Vermesh, O.; Grecu, M.; Javey, A.; Wang, Q.; Dai, H.; Peng, S.; Cho, K. J. Nano Lett. 2003, 3, 347-351. (18) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856-2859. (19) Evans, S. D.; Johnson, S. R.; Cheng, Y. L.; Shen, T. J. Mater. Chem. 2000, 10, 183-188. (20) Zhang, H.-L.; Evans, S. D.; Henderson, J. R.; Miles, R. E.; Shen, T.-H. Nanotechnology 2002, 13, 439-444. (21) Ahn, H.; Chandekar, A.; Kang, B.; Sung, C.; Whitten, J. E. Chem. Mater. 2004, 16, 3274-3278. (22) Briglin, S. M.; Gao, T.; Lewis, N. S. Langmuir 2004, 20, 299-305. (23) Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. J.; Malik, M. A.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 8958-8964. (24) Leopold, M. C.; Donkers, R. L.; Georganopoulou, D.; Fisher, M.; Zamborini, F. P.; Murray, R. W. Faraday Discuss. 2004, 125, 63-76.

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been reported recently. As with all sensors, sensitivity, selectivity, reversibility, speed, cost, and multianalyte detection are important issues when designing and considering the merits of a particular sensor. Compared to GC and MS, or GC/MS, selectivity, multianalyte detection, or detection of a particular analyte in the presence of interferences is more challenging with electronic noses. The development of sensor arrays1,7,8,34 combined with principle component analysis has alleviated many of these problems, making electronic noses very useful for gas and vaporsensing applications. This paper describes the use of Au monolayer-protected clusters (MPCs) for chemiresistive vapor sensing of volatile organic compounds (VOCs). Au MPCs are composed of a roughly spherical Au core surrounded by an organic monolayer shell, which is usually an alkanethiolate, ω-functionalized alkanethiolate, or aromatic thiolate. Following the report of their synthesis in 199437 and their electronic conductivity as films in the solid state,38 Wohltjen and Snow reported their use in sensing VOCs.18 This first report showed that the conductivity of drop-cast films of Au MPCs changes in the presence of various VOCs. Later reports examined drop-cast films of MPCs functionalized with aromatic thiols containing different functionalities19,20 and films of MPCs chemically linked by metal ion-carboxylate bridges using layerby-layer deposition23,24 or chemically linked by hydrogen bonding or dithiol bridges using a precipitation cross-linking method.33,34 Others have used ethylene oxide thiol-36 and thienyl-functionalized21 MPCs for vapor sensing. Au MPCs stabilized with alkylamines have been utilized for preparing layer-by-layer films with dithiols25-27 or various different types and generations of dendrimers27-31 as the linker and tested for vapor-sensing properties. The conductivity of alkylamine-stabilized Au MPCs has been shown to respond to sulfur-containing analytes.22 The following generalities are true for VOC sensing with Au MPCs for most of the above examples. The conductivity of the films occurs by an electron-hopping mechanism,23,38-40 which (25) Joseph, Y.; Guse, B.; Yasuda, A.; Vossmeyer, T. Sens. Actuators, B 2004, 98, 188-195. (26) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H.-G.; Wessels, J. M.; Wild, U.; Knop-Gericke, A.; Su, D.; Schlogl, R.; Yasuda, A.; Vossmeyer, T. J. Phys. Chem. B 2003, 107, 7406-7413. (27) Joseph, Y.; Krasteva, N.; Besnard, I.; Guse, B.; Rosenberger, M.; Wild, U.; Knop-Gericke, A.; Schlogl, R.; Krustev, R.; Yasuda, A.; Vossmeyer, T. Faraday Discuss. 2004, 125, 77-97. (28) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R. E.; Mullen, K.; Yasuda, A. Adv. Mater. 2002, 14, 238-242. (29) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Mullen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551-555. (30) Krasteva, N.; Krustev, R.; Yasuda, A.; Vossmeyer, T. Langmuir 2003, 19, 7754-7760. (31) Krasteva, N.; Guse, B.; Besnard, I.; Yasuda, A.; Vossmeyer, T. Sens. Actuators, B 2003, 92, 137-143. (32) Cai, Q.-Y.; Zellers, E. T. Anal. Chem. 2002, 74, 3533-3539. (33) Han, L.; Daniel, D. R.; Maye, M. M.; Zhong, C.-J. Anal. Chem. 2001, 73, 4441-4449. (34) Han, L.; Shi, X.; Wu, W.; Kirk, F. L.; Luo, J.; Wang, L.; Mott, D.; Cousineau, L.; Lim, S. I.-I.; Lu, S.; Zhong, C.-J. Sens. Actuators, B 2005, 106, 431-441. (35) Pang, P.; Guo, Z.; Cai, Q. Talanta 2005, 65, 1343-1348. (36) Foos, E. E.; Snow, A. W.; Twigg, M. E.; Ancona, M. G. Chem. Mater. 2002, 14, 2401-2408. (37) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 7, 801-802. (38) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-h.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson Jr., C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548.

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depends on the cluster-to-cluster distance in the film and the dielectric of the surrounding medium. The presence of VOCs may affect either variable, but in most cases, the change in conductivity is dominated by a change in cluster-to-cluster distance. The sensing operation is usually explained by the fact that the vaporphase analyte partitions into the Au MPC film, causes it to swell, and increases the average cluster-to-cluster distance, which lowers the conductivity of the film. The extent of vapor-phase partitioning and film swelling depends on the functionality of the MPCs and the characteristics of the analyte. Films containing clusters with nonpolar organic shells respond more to nonpolar analytes and vice versa. The selectivity for different vapors has been altered by using various types of organic shells or linkers between the clusters, including simple alkanethiolates,18 functionalized aromatic thiols,19,20 COOH-terminated thiols,23,24,33,34 dithiols,25-27,33,34 dendrimers27-31 (different types and generations), ethylene oxide,36 and thienyl-containing21 thiols. Most often a current decrease is observed, but there are examples where the conductivity of the films increased in the presence of alcohols or water.19,33 Another generality that has not been extensively studied in the literature is that film flexibility is important for high sensitivity to organic vapors. A close comparison of reported sensitivities shows that drop-cast films18 or films chemically linked with a very small number of linkers between clusters23,24,33,34 exhibit higher responses to organic vapors compared to films of clusters rigidly linked in a layer-by-layer fashion,25-31 suggesting that film flexibility is an important issue in sensor design based on Au MPCs. A fair amount of work has been reported on the vapor-sensing properties of nonlinked and chemically linked films of MPCs with different functionalities and different methods of film formation showing a wide range of sensitivities to various types of organic vapors. There are also examples of MPCs used for QCM-based sensing23,33,41 and as chemiresistive detectors for GC.32 However, most of the films prepared are on a larger scale having dimensions of millimeters or centimeters. There is one recent report of patterned nonanedithiol-linked Au nanoparticle films exhibiting vapor response similar to the larger, nonpatterned films.42 Along these lines, our aim is to study the conductivity and vapor-sensing properties of MPC films patterned and assembled on the microand nanoscale to determine their properties and explore the optimal sensing conditions. Miniaturization is sought to increase portability, perform sensing in highly confined spaces, reduce manufacturing costs, and allow for multianalyte detection by placing many sensing elements within one device. Here we describe microscale patterned films of 1.6-nm average diameter hexanethiolate Au MPCs that bridge a pair of electrodes separated by a 1-µm gap. The stability of as-prepared films is compromised when placed in high vapor concentrations of solvent the clusters readily dissolve in, such as toluene. Cross-linking the Au MPCs in the film with dithiols from the vapor phase leads to improved stability and sensor reversibility. The extent of crosslinking depends on the time exposed to dithiol vapor. Short exposures lead to more flexible films and larger sensor response (39) Zamborini, F. P.; Smart, L. E.; Leopold, M. C.; Murray, R. W. Anal. Chim. Acta 2003, 496, 3-16. (40) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465-11472. (41) Grate, J. W.; Nelson, D. A.; Skaggs, R. Anal. Chem. 2003, 75, 1868-1879. (42) Harnack, O.; Raible, I.; Yasuda, A.; Vossmeyer, T. Appl. Phys. Lett. 2005, 86, 034108-034101-034108-034103.

to toluene and 2-propanol (IPA). The response also depends on the conductivity and organic content in the film, which are varied with temperature. This work is important because the sensing characteristics of microscale films of Au MPCs have not been closely explored, and we show not only that they differ from larger films but also that the stability, flexibilty, and sensor response can be controlled using very simple vapor-phase dithol exposure and heat treatment. Combining microcontact printing with vaporphase cross-linking allows the fabrications of highly sensitive, stable microscale sensors based on Au MPCs that would not be possible or easily achieved by drop-casting or layer-by-layer deposition methods. The sensor properties as a function of the different exposure times to dithiol and different temperature reveal the optimal conditions in terms of response and signal-to-noise ratio (S/N) for these microscale sensing devices. EXPERIMENTAL SECTION Chemicals. Hexanethiol (96%), 1,6 hexanedithiol (96%), sodium borohydride (99%), tetraoctylamonium bromide (TOABr, 99%), toluene (99.9%), and 2-propanol (IPA, 99.9%) were purchased from commercial sources and used as received. Hydrogen tetrachloroaurate (HAuCl4‚3H2O) was synthesized according to a literature procedure.43 Barnstead Nanopure water (R g 17.8 MΩ‚ cm) was employed for all aqueous solutions. Synthesis. Hexanethiolate-coated gold monolayer-protected clusters (C6 Au MPCs) were synthesized according to the Brust reaction.37 A 2.40-g sample of HAuCl4 was dissolved in 25 mL of water, and 4.89 g of TOABr was dissolved in 150 mL of toluene. The two solutions were combined and stirred until all of the AuCl4transferred into the toluene phase. The toluene phase was separated and 2.60 mL of hexanethiol, corresponding to a 3:1 thiol/Au ratio, was added to the toluene and stirred until the solution became colorless. The solution was cooled to ∼0 °C using an ice bath, and a 10-fold excess of NaBH4 (2.30 g in 10 mL of water) was added to the toluene solution with stirring. The solution turned black within a few seconds, indicating the formation of metallic Au MPCs. An additional 10 mL of water was added, and the solution was stirred overnight. The toluene layer was separated and removed by rotary evaporation. The remaining black sludge was suspended in 200 mL of acetonitrile and collected by filtration on a glass fritted Bu¨chner funnel. The black solid product was washed with an additional 250 mL of acetonitrile and thoroughly dried before collecting. Au MPCs prepared this way are 1.6 ( 0.4 nm in diameter according to the literature.44 Microcontact Printing (µCP) Au MPCs. C6 Au MPCs were used as ink for microcontact printing onto solid substrates following the technique of Xia and Whitesides.45 A patterned poly(dimethylsiloxane) (PDMS) stamp with featured lines (∼1.2 µm wide, ∼0.5 cm long, ∼2.2 µm separation) was inked with a solution of 0.09 g/mL C6 Au MPCs, allowed to dry for 3-5 min, and brought into conformal contact with a solid substrate for ∼10 s. Removal of the PDMS leaves behind a patterned structure of nanoparticle lines with shapes corresponding to the stamp (43) Block, B. P. Inorg. Synth. 1953, 4, 14-17. (44) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1999, 14, 1730. (45) Xia, Y.; Whitesides, G. M. Polym. Mater. Sci. Eng. 1997, 77, 596.

Figure 1. (A) Optical image of microscale lines of C6 Au MPCs stamped across two Au electrodes separated by a 1-µm gap at the shortest point. AFM images of the electrodes (B) before and (C) after stamping the C6 Au MPCs across them. (D) I-V curves of the (b, blue) bare electrodes, (c, red) electrodes bridged with stamped C6 Au MPCs, and (“dithiol”, green) electrodes bridged with C6 Au MPCs and exposed to hexanedithiol for 30 min. The currents in the I-V curves are averaged data to avoid the hysteresis observed at these small currents and show the increase in current better.

features. To link the Au MPCs within the stamped lines, the patterned substrate was placed above 5 mL of pure hexanedithiol for 1, 30, and 60 min as noted. The stamped lines were then placed in IPA for 5 s to remove the excess dithiol in the film. In separate experiments to prove that Au MPCs were linked in the patterned films, stamped lines were removed from the substrate and transferred to another substrate by exposing to hexanedithiol for 30 min and immersing the substrate in IPA for ∼12 h. This resulted in the lines becoming detached from the original substrate. They were transferred to a new substrate by drop-casting them from the IPA solution. Electronic Conductivity of Microstamped Lines of C6 Au MPCs. Solid-state electronic conductivity measurements were made on the C6 Au MPC films stamped across electrodes having a 1-µm gap (Figure 1). The electrodes were fabricated in a clean room by photolithography on an Si/SiOx substrate by sputtercoating Au with a Cr adhesion layer onto a patterned photoresist and using liftoff to generate the electrode gap. Wire leads were attached to the electrode contact pads with Ag epoxy (cured 12 h, 80 °C), which was further insulated with an overlayer of Torrseal epoxy (cured 12 h, 80 °C). The electrode was cleaned by immersion in acetonitrile and dichloromethane followed by rinsing in acetone, ethanol, and IPA and drying under N2. The electrode Analytical Chemistry, Vol. 78, No. 3, February 1, 2006

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was placed in a UVO ozone cleaner (Jelight Co. Inc., Irvine, CA) for 10 min before microcontact printing the Au MPC films across the electrodes. I-V curves were obtained on a CH Instruments 660A electrochemical work station in cyclic voltammetry mode by scanning between +1.0 and -1.0 V or +0.3 and -0.3 V. One electrode was connected to the reference and counter electrode leads and the other electrode was connected to the working electrode lead. Vapor-Sensing Experiments. Vapor-sensing experiments were obtained with a CH Instruments 660A electrochemical workstation operating in chronoamperometry (CA) mode. The current was monitored with time while a -0.3-V potential was applied between the two electrodes and the sample was exposed to alternating flow of pure nitrogen or pure nitrogen bubbling through toluene or IPA. This was achieved by separating the nitrogen gas into two lines with a T-junction. One line was pure N2, and the other bubbled through the solvent. The two lines were recombined into one line with a T-junction before flowing over the sample. On/off valves were used to expose the sample to pure N2 or saturated vapor. All CA plots were treated with the smoothing function in the software. Heating Clusters. Microstamped lines crossing the electrode gap were heat treated (Lindberg-Bluem Tube Furnace) for 2 min at temperatures of 250-320 °C in air at intervals of 25 °C for vaporsensing experiments and at intervals of 10 °C for conductivity, Fourier transform infrared (FT-IR) spectroscopy and atomic force microscopy (AFM) measurements. I-V curves and vapor-sensing measurements were obtained immediately following each heat treatment. Characterization. FT-IR data were acquired using a Digilab FTS 7000 spectrometer (Varian, Cambridge, MA) in reflectance mode. A film of C6 Au MPCs was drop-cast onto a silicon substrate sputter-coated with 250 Å Ti/W and 2000 Å of Au (Lance Goddard Assoc., Foster City, CA). An FT-IR spectrum was measured following heat treatments of the film between 250 and 320 °C for 2 min at 10 °C intervals. AFM images of similarly heated films were acquired on a Veeco Digital Instruments Nanoscope 3A (Santa Barbara, CA) using a Si tip operating in tapping mode. Scanning Electron Microscopy (SEM) images were obtained with a Carl Zeiss SMT AG SUPRA 35VP field emission scanning electron microscope operating at an accelerating voltage of 20 kV and using an in-lens ion annular secondary electron detector. RESULTS AND DISCUSSION Our goal in this paper was to characterize the vapor-sensing properties of microscale films of C6 Au MPCs. The general setup is shown in Figure 1. Frame A shows an optical image of the two tapered Au electrodes (E1 and E2) fabricated on an Si/SiOx substrate with a 100-µm gap at the farthest point, 1-µm-gap separation at the closest point, and the microscale lines of C6 Au MPCs deposited across the electrodes by microcontact printing using an approach similar to Santhanam and Andres.46 Frames B and C are AFM images showing a closeup of the electrode gap before and after the deposition of Au MPC patterns, respectively. The bright stripes in Figure 1C contain the 1.6-nm-diameter C6 Au MPCs. Normally, a well-performed stamp produces ∼30 lines (46) Santhanam, V.; Andres, R. P. Nano Lett. 2004, 4, 41-44.

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from electrode-to-electrode with about 2 or 3 lines crossing the shortest electrode path. Frame D shows I-V curves of the electrodes before deposition of Au MPCs (b, blue curve), after deposition of Au MPC patterned lines (c, red curve), and after exposing the Au MPC lines to hexanedithiol for 30 min (“dithiol”, green curve). The plot before stamping indicates that the background current of the instrument is ∼0 pA. After stamping, the slope of the I-V curve increases and the current at 0.3 V is ∼50 pA, showing that current is flowing through the stamped MPCs. We are uncertain about the number of lines and the exact location (shortest gap or wider gaps) of the lines contributing to the current. Based on previous conductivity measurements of drop-cast films of C6 Au MPCs (1.8 × 10-4 Ω-1cm-1),40 we calculate that two 1.2-µm-wide C6 Au MPC lines stamped across a 0.100-µm tall pair of electrodes separated by a 1-µm gap should exhibit a current of ∼1300 pA (R ) 231 MΩ) at 0.3 V. Α ∼1900 pA (R ) 158 MΩ) current is expected if all 30 lines crossing the electrode from 1- to 100-µm separation are considered. Figure 1D (plot c) shows an increase in current of only ∼50 pA (R ) 6 GΩ). There are three possible reasons the lines do not exhibit the expected current based on the conductivity of drop-cast films. First, the lines are probably not packed as well and contain more defects compared to evaporated drop-cast films, resulting in higher resistance. Second, higher resistance contacts between microntact printed MPCs and the electrodes likely form compared to MPCs drop-cast over the electrodes from solution. Third, many of the stamped lines may not be well-connected, and the measured current could represent connected lines crossing larger regions of the electrode gap. We believe the current is dominated by stamped lines crossing the 1-10-µm spaced gaps (1-5 lines) of the electrodes because the current drops off linearly with distance and the lines are more likely to have defects or poor connections as the distance between the electrodes increases. As evidence, we rarely observe a current increase for Au MPC lines stamped across 20-µm-gap electrodes. The lower current (higher R) than expected is therefore likely due to the first two reasons discussed above. This also demonstrates the importance of using small electrode gaps (1 µm or less) in these experiments. Conductivity and Vapor Sensing as a Function of Dithiol Cross-Linking. Conductivity in drop-cast or chemically linked films of Au MPCs has been shown to occur by an electron-hopping process, where the measured current depends exponentially on the distance between Au MPCs and on the dielectric constant of the material between the MPCs.23,38-40 In the absence of a major change in the dielectric constant, the distance between Au MPCs dominates the electrical response. Exposure to dithiol chemically links the clusters in the film by forming two Au-thiolate bonds on adjacent clusters per dithiol molecule. No noticeable increase in current occurs upon exposure to dithiol (Figure 1D, “dithiol”), indicating that the dithiol linkages did not significantly alter the distance between the Au MPCs carrying the current. This is expected since the dithiol chain length is the same as the hexanethiolates surrounding the MPCs and the dielectric constant is similar. Others have observed changes in conductivity upon successive layer-by-layer assembly of Au nanoparticles and dithiol linkers at nanoscale electrode gaps.47 Early on in this work, we observed that the as-prepared microscale lines of MPCs were not stable in saturated vapors of

Figure 2. Chronoamperometry plot (current versus time) for (A) asprepared (nonlinked) C6 Au MPC lines and (B) C6 Au MPC lines exposed to dithiol (linked) in the presence of toluene (“on”) and nitrogen (“off”). AFM image of nonlinked lines (C) before and (D) after exposure to the headspace of pure toluene for 40 s. AFM image of linked lines (E) before and (F) after exposure to the headspace of pure toluene for 350 s.

solvents that the MPCs readily dissolve in, such as toluene. Taking advantage of the fact that thiols can assemble onto Au surfaces from the vapor phase22,48 and undergo vapor-phase placeexchange reactions, we exposed the stamped lines to hexanedithiol vapors in order to impart chemical stability to the microscale lines via dithiol cross-linkage. Figure 2 shows the benefit of the hexanedithiol exposure. Figure 2A shows the current as a function of time (CA) for the electrodes containing asprepared (nonlinked) C6 Au MPC lines stamped across them. The current is stable in N2 and decreases upon exposure to saturated toluene vapor (toluene “on”). When exposed to pure N2 again (toluene “off”), the current does not return to its original value. This is one important difference between microscale lines of Au MPCs and larger, drop-cast films, which behave reversibly. The same experiment was performed on stamped Au MPC films exposed first to hexanedithiol for 1 min and rinsed with IPA (Figure 2B). In this case, the current decreases in the presence of toluene (toluene on) and returns to baseline when exposed to pure N2 (toluene off). Linking the Au MPCs with vapor-phase dithiols clearly imparts reversibility into the sensor. The current decrease occurs as the Au MPC film swells upon toluene sorption. Swelling causes the average Au cluster-to-cluster distance to increase, leading to a decrease in conductivity. The AFM images in Figure 2 explain the irreversibility observed in the as-prepared films. Figure 2C is an AFM image of stamped lines (∼1.2 µm wide and 400 nm thick) containing 1.6(47) Snow, A. W.; Ancona, M. G.; Kruppa, W.; Jernigan, G. G.; Foos, E. E.; Park, D. J. Mater. Chem. 2002, 12, 1222-1230. (48) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459-12467.

Figure 3. (A, B) AFM images of stamped lines exposed to hexanedithiol for 1 min and placed in pure toluene liquid for 5 s. (C) SEM and (D) optical image of C6 Au MPC stamped lines exposed to hexanedithiol for 30 min, placed in pure IPA overnight, and dropcast onto a new Si/SiOx or glass substrate.

nm average diameter C6 Au MPCs on a clean Si/SiOx substrate, and Figure 2D shows the same area of the sample and the same lines after a 40-s exposure to the headspace of a vial containing toluene. The Au MPCs are weakly connected by van der Waals interactions between the alkyl chains and are readily soluble in toluene. The toluene vapor causes an irreversible distortion and disconnection of the stamped lines, which led to irreversibility in the sensing experiment (Figure 2A). At saturation, the toluene may be condensed on the surface in liquid form to cause such changes in the patterns. Figure 2E shows an AFM image of stamped C6 Au MPC lines exposed to hexanedithiol for 1 min, rinsed briefly with IPA, and dried under N2, and Figure 2F shows the same lines after a 350-s exposure to the headspace of toluene. In this case, no distortion or disconnection of the lines occurs, consistent with the linking of the clusters with dithiol and reversibility in the sensing experiment (Figure 2B). It is important to note that saturated IPA vapor did not cause any noticeable changes in the nonlinked patterned Au MPC lines due to the insolubility of the clusters in more polar solvents. Therefore, the instability of the nonlinked Au MPC patterns is limited to solvents that the clusters are soluble in and at high vapor concentrations. Films of Au MPCs linked with dithiols, dendrimers, hydrogen bonding, or metal ion-carboxylate bridges formed using layer-by-layer deposition are also more stable compared to drop-cast or as-prepared stamped films. Patterning these films would require more sophisticated approaches, however, and longer times using the layer-by-layer deposition technique. Combining microcontact printing and vapor-phase crosslinking of the films, as described here, is very fast and convenient. To further prove that cluster cross-linking occurred upon dithiol exposure, we intentionally dislodged the dithiol-linked cluster patterns by placing them directly in pure IPA or toluene liquid. AFM images of various stamped lines that were linked with Analytical Chemistry, Vol. 78, No. 3, February 1, 2006

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Figure 4. Chronoamperometry plots showing the response of C6 Au MPC films to (A) toluene and (B) IPA as a function of exposure time to hexanedithiol during cross-linking. Times in hexanedithiol are 1, 30, and 60 min.

hexanedithiol for 1-min vapor exposures (Figure 3A,B) and immersed in toluene for a few seconds show that pure solutions of toluene can dislodge and cause bending of the lines but that the lines move as a connected unit. Figure 3A is actually the same lines observed in Figure 2E and F after immersion in pure toluene. The lines were bent and moved from their original position, but retain their wire shape, confirming that the clusters in the film are chemically linked together. The patterned lines could also be completely removed from the substrate by exposing them to toluene or IPA for extended times. The microscale lines of connected C6 Au MPCs could then be suspended in solvent and drop-cast deposited onto a new substrate. Figure 3C shows a SEM image, and Figure 3D shows an optical image of patterned lines that were exposed to dithiol, removed from the surface, suspended in IPA, and redeposited on a new Si/SiOx (frame C) or glass substrate (frame D) by dropcasting. The Au MPC lines retain their original width (1-1.5 µm) and length (several hundred micrometers) and are clearly flexible. The SEM image in Figure 3C shows that the wires are ribbonlike, which is expected since the stamped lines were ∼1.2 µm wide and 400 nm thick. The optical image in Figure 3D shows two entangled lines. The chemiresistive sensing of organic vapors with Au MPCs relies on swelling of the film, requiring some flexibility in the cross-linked film. Previous experiments on vapor sensors based on Au clusters show that films rigidly linked25-31 are less sensitive than drop-cast films.18 Films chemically linked with a small number of linker ligands in mixed monolayers also have a large degree of flexibility and sensitivities comparable to drop-cast films.23,33 Sensitivity is therefore related to film flexibility. We attempted to vary the extent of dithiol cross-linking by altering the exposure time to dithiol. Our hypothesis was that film flexibility and response to organic vapors would decrease as exposure time to dithiol increased. The vapor response to toluene and IPA was tested for films exposed to hexanedithiol for 1, 30, and 60 min as shown in Figure 4. The current was measured as a function of time in the presence of alternating flow of nitrogen and nitrogen bubbling through the organic solvent. The percent response to 758


Figure 5. Response of C6 Au MPC films linked with hexanedithiol for 1 min to toluene vapor (A) before heating and (B) after heating at 250 °C for 2 min. Response decreases slightly but S/N increases 1 order of magnitude.

the organic vapor is measured using the following equation (Figure 4):

% response ) (ir - ib)/ib × 100% ) ∆i/ib × 100% where ib is the initial baseline current, ir is the current in response to the vapor, and ∆i ) (ir - ib). A negative value is equal to a decrease in the current upon vapor exposure. As shown in Figure 4, the initial current observed in the presence of N2 drops when the sample is exposed to toluene or IPA due to the swelling mechanism. The current then reversibly returns close to the baseline in the presence of N2 again. Importantly, the negative percent response in current increases as the film is cross-linked using shorter dithiol exposures, which is consistent with less cross-linker and higher film flexibility. For example, 1-, 30-, and 60-min dithiol exposures led to -61, -33, and -8% response in current for toluene and -23, -13, and -7% for IPA, respectively. The response to toluene is larger than IPA due to the more hydrophobic nature of the Au MPCs in the film. Conductivity and Vapor Sensing as a Function of Temperature. The conductivity of stamped C6 Au MPC films in Figures 2 and 4 are low, and the measured current has significant noise fairly close to the baseline level. To increase the measured current and improve the S/N, we heated the films at various temperatures and then measured their current and vapor responses. Figure 5 shows the current response to toluene for a selected C6 Au MPC stamped film linked with dithiol for 1 min with no heating (frame A) and after heating at 250 °C for 2 min (frame B). The plot for the sensor not heated shows a large response (-71%) to toluene, but the S/N is low (∼2) because of the low current associated with microscale lines of C6 Au MPCs (5 × 10-12 A). Heating at 250 °C (Figure 5B) increased the current to 1 × 10-10 A and the S/N to ∼22 and still has a very good response to toluene of -51%. This shows that heating the

Figure 6. Response of C6 Au MPC films linked with hexanedithiol for 30 min to (A) toluene and (B) IPA as a function of temperature the Au MPCs lines were exposed to (room temperature, 300 and 315 °C).

microscale lines of Au MPCs can greatly improve the response characteristics. Figure 6 shows the vapor response to toluene (frame A) and IPA (frame B) over a wider range of temperatures for a selected stamped C6 Au MPC film linked with hexanedithiol for 30 min. The temperature is the temperature that the stamped film was exposed to before testing the vapor response at room temperature. Sensitivity to toluene and IPA was observed at all temperatures, but the response and noise clearly decreases with increasing temperature. Before heating, the baseline current is ∼3.0 × 10-10 A and the response is -55 and -26% for toluene and IPA, respectively. At 300 °C, the current increases to 5.4 × 10-8 A and the sensitivity drops to -20 and -15% for toluene and IPA, respectively. Finally, at 315 °C the current increases to 3.0 × 10-6 A and the sensitivity drops to -13% for both analytes. As temperature increases, the conductivity of the Au C6 MPC

stamped lines increases and the response to organic vapors decreases. The change in response for toluene is more pronounced than for IPA. At the final temperature measured, the response was similar for both vapors where it was originally much higher for toluene. Films that exhibit current in the 10-9-10-10 A range seem to be ideal (Figure 5), because although their response is slightly smaller compared to nonheated films, the overall signal, and S/N is 1-2 orders of magnitude higher. Gentle heating of the sample can improve the overall performance of the sensor, while too much heating destroys its sensing ability. To better understand what is occurring with the films as a function of temperature, the current was measured at 0.3 V on a typical stamped film linked with dithiol for 30 min and subjected to temperatures of 250-320 °C for 2 min at 10 °C intervals (Figure 7A). The film was subjected to the specified temperature for 2 min and cooled back to room temperature for the current measurements. The intensity of the asymmetric CH stretch of the alkanethiolates surrounding the Au MPCs from FT-IR measurements (see Experimental Section) and the thickness and width of the stamped lines as measured by AFM were also measured as a function of the same heat treatment as shown in Figure 7B-D, respectively. Heating at various temperatures for 2 min led to a controllable measured current between 10-11 and 10-6 A for this particular sample. Other films showed currents as high as 10-3 A. Initial heating from room temperature to 250 and 260 °C caused an increase in current from 10-11 to 10-9 A. The current remained fairly constant from 260 to 280 °C and then increased to 10-8 A at 290 to 300 °C. A more dramatic increase in current to 10-6 A occurred at 310 °C. The intensity of the CH stretch remains fairly constant from room temperature to 290 °C, drops slightly from 0.16 to 0.14 in the 300-310 °C range, drops more dramatically to 0.10 at 320 °C, and all alkanethiolates are completely desorbed at 330 °C. The thickness and width of the lines remain mostly constant at 1.4 µm and 440 nm, respectively, from room temperature to 310 °C and then drops dramatically to 800 and 200 nm, respectively at 320 °C, consistent with complete loss of alkanethiolates observed in the IR around this temperature. The

Figure 7. Effect of temperature on the (A) current flowing through a sample of C6 Au MPCs stamped across electrode, (B) intensity of asymmetric CH stretch from alkanethiolates surrounding the Au MPCs as measured with FT-IR, (C) width of stamped C6 Au MPC lines, and (D) height of stamped C6 Au MPC lines. Stamped lines were exposed to hexanedithiol for 30 min.


759

Table 1. Vapor Reponse to IPA and Toluene as a Function of Temperature and Time in Dithiola sample trials

heat trials

1 1 2 3 2

Figure 8. AFM images of stamped C6 Au MPCs exposed to hexanedithiol for 30 min (A) before heat treatment and (B) after the series of heat treatments from 250 to 320 °C for 2 min at each temperature.

AFM images in Figure 8A and B show a stamped film before heating and after undergoing the heat treatment, respectively. The line scans below the images show the dimensions of a typical stamped line before and after the heat treatment. The current, IR, and AFM data taken together interestingly show that the current passing through the films increases from 10-11 to 10-8 A with a minimal decrease in alkanethiolates on the Au MPCs or change in thickness or width of the film. This initial increase in current may be due to (1) thermal annealing of the clusters into larger clusters or (2) improvement in the contact to the electrodes upon heating. Thermal annealing of clusters in the solid state has been reported,49 and larger clusters are known to exhibit higher electron-hopping conductivities.38 There is still a significant response to organic vapors in the 10-10-10-8-A range because most of the alkanethiolates remain on the clusters, which can adsorb the vapors and cause a change in the conductivity. Also, the higher current leads to greater S/N, especially in the 10-10-10-9-A range. Total loss of alkanethiolates from the film is not observed until ∼320 °C, as evidenced by the largest increase in current, a complete loss of the asymmetric CH stretch in the FT-IR, and largest drop in film thickness and width. The response to vapors beyond 320 °C is usually less than 1%, consistent with complete loss of alkanethiolates. There is a small response to vapors at 315 °C in Figure 6 due to a small amount of organic content remaining in the film. It is interesting to note that, after heat treatment at 320 °C, complete desorption of alkanethiolates from the clusters occurs, but the stamped, now metallic, Au lines do not exhibit bulk Au conductivity. Based on bulk Au conductivity (4.0 × 105 Ω-1 m-1),19 currents in the 2-3-A range would be expected for a bulk Au wire 800 nm wide crossing two electrodes 0.1 µm tall and spaced by 1 µm as opposed to the measured 10-6 A from Figure 7. Therefore, there are still high-resistance junctions along the heated stamped lines, similar to granular metal films.16,38 It is also important to note that the temperature observed for complete alkanethiolate desorption (320 °C) is much larger than thermal desorption temperatures reported in the literature for hexanethiolates on Au (∼220 °C).50,51 We believe this is due to (49) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719-2724. (50) Lu, J.; Jones, V. W.; Maye, M. M.; Han, L.; Kariuki, N. N.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 13988-13989.

760 Analytical Chemistry, Vol. 78, No. 3, February 1, 2006

1 2 3 4 5 3 1 2 3 4 5 1 1 2 3 4 5 2 1 2 3 4 5 6 7 8 9 1 1 2 3 4 5 2 1 2 3 4

temp (°C)

current (A)

IPA

Tol

1 min Dithiol rt 3.7 × 10-11 250 1.1 × 10-10 270 3.7 × 10-10 275 1.0 × 10-11b rt 1.0 × 10-11 250 1.1 × 10-10 275 5.7 × 10-10 275 3.5 × 10-9 300 1.2 × 10-6 300 5.3 × 10-7e rt 2.3 × 10-10 250 7.7 × 10-10 275 2.7 × 10-9 275 8.0 × 10-8 300 6.6 × 10-6 315 2.1 × 10-3

-21d -3d -2d no CA -26d -6d -9 -13 -9 -7 -23 -12 no CA -25 -33 0

-72 -51 -40 no CA -72d -51d -49 -47 -8 -8 -61 -54 no CA -42 -29 0

30 min Dithiol rt 1.3 × 10-10 250 6.9 × 10-10 300 7.5 × 10-8 300 2.1 × 10-6 315 2.9 × 10-6 315 3.0 × 10-6 rt 2.7 × 10-10 250 7.1 × 10-9 275 6.3 × 10-9 275 9.5 × 10-9 300 2.9 × 10-8 300 5.3 × 10-8 315 1.0 × 10-7 315 2.9 × 10-6 315 2.5 × 10-5 315 2.5 × 10-5

-13 -13 -18 -23 no CA no CA -26 -10 no CA -13 no CA -15 -5 -13 -6 no CA

-33 -33 -28 -24 no CA no CA -55 -19 no CA -19 no CA -20 -17 -13 -7 no CA

60 min Dithiol rt 2.2 × 10-10c rt 5.7 × 10-10c 250 1.6 × 10-9 275 3.8 × 10-9 300 8.1 × 10-9 315 2.4 × 10-9 e 315 2.6 × 10-9 rt 1.2 × 10-11 250 1.8 × 10-10 275 4.7 × 10-9 275 6.0 × 10-9 300 1.6 × 10-8

+5 -2 -7 -27 no CA no CA -15d -7d -7 no CA -3

-8d -7 -15 -29 no CA no CA -20d -17 -5 no CA -5

a (-) Decrease in current; (+) increase in currrent; rt, room temperature; no CA, chronoamperometry not taken. b Wires disconnected during heating trial (current dropped back to 10-10-10-11 A). c Baseline currents are different for toluene (2.2 × 10-10 A) and IPA (5.7 × 10-10 A). d Estimated percent response due to high noise level. e Current decreased from previous heating.

inaccuracies in our tube furnace. While the temperature of desorption is not consistent with literature, it is consistent with the three techniques used (current, IR, AFM) because we used the same furnace for all experiments. Table 1 presents the percent response toward saturated toluene and IPA vapor as a function of the hexanedithiol exposure time and baseline current passing through the films (controlled by heating the films) for all of the samples studied in this work. (51) Lu, J.; Maye, M. M.; Han, L.; Kariuki, N. N.; Jones, V. W.; Lin, Y.; Engelhard, M. H.; Zhong, C.-J. Langmuir 2004, 20, 4254-4260.

The graph in Figure 9 summarizes the response data to toluene (A) and IPA (B) as a function of dithiol exposure time and film current. The responses shown are averages from all measurements where the current passing through the films was rounded to the lowest order of magnitude. Toluene follows the trend of increased response with lower dithiol exposure time and lower current, although very low currents also have low S/N. The response to IPA is more sporadic, and there is less dependence on dithiol vapor exposure or current passing through the film.

Figure 9. Summary of percent response, ∆i/ib × 100%, to toluene and IPA as a function of hexanedithiol exposure time and current passing through the stamped lines. The currents shown are rounded to the closest order of magnitude, and the responses are the averages for all the trials at a particular dithiol exposure time, current, and vapor tested.

Negative values correspond to decreases in current, and positive values correspond to increases in current. The experiment was performed at least twice for each time exposed to dithiol. In most cases, all responses were negative values (decrease in current). There is one exception for the response of 60-min linked films to IPA at low current. Others have also observed positive increases in conductivity for MPC films in the presence of alcohol vapors for samples having very low conductivity.33 The 1-µm-gap electrodes with no Au MPCs in the gap also show a positive increase in current in the presence of saturated IPA vapor. The results show that the films are consistently more sensitive to toluene compared to IPA. The nonpolar toluene is more compatible with the hydrophobic MPC film compared to the more polar IPA. Table 1 shows that the response to toluene and IPA increases as the exposure time to hexanedithiol decreases. This is due to greater film flexibility at low dithiol exposure times. The negative percent response decreases as the baseline current of the film increases in almost all cases for toluene (60 min, Tol, trial 1 is an exception) since the current increase is associated with increased cluster size and loss of alkanethiolates from the film that likely occurs upon heating, which leads to reduced film flexibility and less affinity for toluene. The response to IPA is more sporadic with increasing baseline current. The source of this irreproducibility is unknown at this time. It is not clear why some samples increased in response for IPA with increased current.

CONCLUSIONS Microscale patterned films of Au C6 MPCs were prepared and their conductivity and chemiresistive vapor-sensing properties measured for toluene and IPA vapor by microcontact printing MPCs across a 1-µm-gap electrode. The stability of the patterned films increases by exposure to dithiol vapor, which chemically links the MPCs in the patterned films to the extent that the patterns can be removed from the surface as fully connected, flexible wires and redeposited onto another substrate. The response to toluene is greater than IPA because the film is nonpolar. The response in general increases with decreased exposure to dithiols during cross-linking because of greater film flexibility. Heating of the films provides control over the measured current passing through the films in the 10-10-10-3-A range, depending on the temperature and time of heating. The increase in current correlates with the loss of organic content as measured by FT-IR and the change in width and thickness of the film as measured by AFM. The general trend is that the response decreases as the current passing through the film increases, which is related to the heat-induced increase in cluster size and loss of alkanethiolates surrounding the clusters. The trend holds much better for toluene compared to IPA, and more work is needed to better understand this. However, our main finding is that the optimal conditions for sensing are short dithiol exposures (small degree of cross-linking) and gentle heating to the 10-10-10-9-A range where the response is fairly high and the S/N is improved. This is important because the benefits associated with size reduction of Au MPC sensing devices, such as greater portability, multianalyte detection, and ability to make measurements in highly confined regions, are counteracted by reduced S/N. Our work describes two ways for improving S/N and film stability of microscale Au MPC vapor sensors while keeping the size of the device small. Future experiments will explore nanoscale films and other factors such as cluster size and chemical functionality. ACKNOWLEDGMENT This research was partially funded through the University of Louisville by an Intramural Research Incentive Grant from the Office of the Senior Vice President of Research.

Received for review July 28, 2005. Accepted November 4, 2005. AC051347T


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Chemiresistive Vapor Sensing with Microscale Films of Gold

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