Sorptive Behavior of Monolayer-Protected Gold Nanoparticle Films

Films of gold MPNs consist of gold particles separated from one another by the protective ... Nanoparticle Syntheses Using a Two-Phase System. ..... (...
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Anal. Chem. 2003, 75, 1868-1879

Sorptive Behavior of Monolayer-Protected Gold Nanoparticle Films: Implications for Chemical Vapor Sensing Jay W. Grate,* David A. Nelson, and Rhonda Skaggs

Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352

Monolayer-protected gold nanoparticle materials were synthesized and characterized for use as sorptive layers on chemical sensors. Thiols investigated as monolayerforming molecules included dodecanethiol, benzenethiol, 4-chlorobenzenethiol, 4-bromobenzenethiol, 4-(trifluoromethyl)benzenethiol, 4-hydroxybenzenethiol, and 4-aminobenzenethiol. Films of selected monolayer-protected nanoparticle (MPN) materials were deposited on thickness shear mode devices and vapor uptake properties were measured at 298 K. Many, but not all, MPN-based sensing layers demonstrated rapid and reversible uptake of vapors, and sorptive selectivity varies with the monolayer structure. The mass of vapor sorbed per mass of sorptive material was determined and compared with poly(isobutylene) and poly(epichlorohydrin) as examples of simple sorptive polymers that have been used on vapor sensors. The nanoparticle-based films considered here were less sorptive than the selected polymers on a permass basis. Partition coefficients, which measure the mass of vapor sorbed per volume of the sorptive phase, were estimated for these MPN materials and found to be comparable to or less than those of the polymer layers. Implications for the roles of sorption and transduction in determining the performance of chemical sensors coated with nanoparticle-based films are discussed. Nanoparticles and nanoparticle-based materials are attracting great interest for their unique properties and potential for application in diverse areas. Monolayer-protected nanoparticles (MPNs) are of particular interest because the surface monolayer stabilizes them relative to aggregation and their properties can be influenced by the structure of the monolayer-forming molecules.1-4 In addition, they can be taken up in solution, synthetically modified, or cast into thin films. * Corresponding author. E-mail: [email protected]. (1) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (3) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (4) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397-406.

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Initial investigations by a number of groups have demonstrated the potential of MPN-based films for chemical vapor sensing. Wohltjen and Snow first reported the use of an octanethiol-coated gold nanoparticle material as a thin film on a chemiresistor device.5,6 The chemiresistor microsensor transduces vapor sorption by the applied layer into a change in current across the sensor’s interdigital transducers. Vapors such as toluene and trichloroethylene caused large decreases in the conductance of the film with slightly nonlinear calibration curves. Materials for detection of polar vapors have also been developed.7 Snow and Wohltjen have observed that MPN-coated chemiresistors can offer detection limits that are significantly better than those of polymercoated surface acoustic wave (SAW) devices.8,9 Subsequently, Evans demonstrated that chemiresistor devices with four different arenethiol-protected gold nanoparticles yielded distinct patterns for each of eight vapors, although reproducibility was not ideal for all vapors on all sensors.10 Subsequent work by this group described chemiresistor sensor responses attributed to film swelling that increases core-core distances at high vapor concentrations and responses at low vapor concentrations related to film permittivity changes.11 Zhong and co-workers investigated two types of novel networked nanoparticle film materials as layers on both chemiresistor and thickness shear mode (TSM) devices.12 (The TSM device is also know as the quartz crystal microbalance, or QCM.) These studies correlated vapor uptake as measured on the TSM device with film resistance changes as measured by the chemiresistor for four diverse test vapors. Murray and co-workers also described both chemiresistor and TSM sensor measurements (5) Snow, A. W.; Wohltjen, H. Materials, Method and Apparatus for Detecting and Monitoring Chemical Species. Provisional filing date of November 25, 1997, U.S. Patent 6,221,673, April 24, 2001. (6) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856-2859. (7) Foos, E. E.; Snow, A. W.; Twigg, M. E.; Ancona, M. G. Chem. Mater. 2002, 14, 2401-2408. (8) Snow, A. W., Naval Research Laboratory, personal communication. (9) Comparisons between chemiresistor vapor sensors coated with monolayerprotected nanoparticles and surface acoustic wave vapor sensors coated with polymers have been presented by Snow and co-workers at the April 2001 ACS Meeting (Defense Applications of Nanomaterials) in San Diego and at the AVS Topical Conference on Understanding and Operating in Threat Environments, Monterey, CA, April 30-May 2, 2002. (10) Evans, S. D.; Johnson, S. R.; Cheng, Y. L.; Shen, T. J. Mater. Chem. 2000, 10, 183-188. (11) Zhang, H. L.; Evans, S. D.; Henderson, J. R.; Miles, R. E.; Shen, T. H. Nanotechnology 2002, 13, 439-444. (12) Han, L.; Daniel, D. R.; Maye, M. M.; Zhong, C.-J. Anal. Chem. 2001, 73, 4441-4449. 10.1021/ac0206364 CCC: $25.00

© 2003 American Chemical Society Published on Web 03/20/2003

on networked nanoparticle materials, concluding that vapor uptake leads to film swelling that reduces electron-hopping rates.13 Gold nanoparticle/dendrimer composites have also been used as layers on chemiresistor vapor sensors.14,15 Zellers described the vaporsensing performance of a pair of MPN-coated chemiresistors, one with octanethiol-protected gold nanoparticles and the other with 2-phenylethanethiol-protected gold nanoparticles.16 Rapid and reversible responses to most of 11 test vapors were observed, and detection limits were reported to be 10-90-fold (with 20 as a typical value) better than those of selected polymer-coated SAW sensors. Films of gold MPNs consist of gold particles separated from one another by the protective monolayers on their surfaces. These monolayers represent insulating layers of molecular dimensions. (The distance may be as small as one monolayer if the monolayers on adjacent nanoparticles interpenetrate one another.17,18) Electron travel through a film progresses from particle core to particle core, which can be modulated by the sorption of vapor molecules in the insulating regions. Snow and Wohltjen described films of gold MPNs as metal-insulator-metal ensembles (MIME) in recognition of the nanostructure of the films.6 The use of conducting particles in insulating matrix materials is a well-known basis for chemiresistor vapor sensors, and the insulating component can be used to influence the chemical selectivity. Conducting carbon particles in insulating polymer matrixes were reported for vapor sensing in the 1980s.19,20 Phthalocyanine nanoparticles in a polymer matrix were described by Grate and co-workers in 1990, using a fluoropolyol polymer component to promote sensitivity and selectivity for basic organophosphorus vapors.21 (The phthalocyanine component formed disklike structures on the order of 50-500 nm in diameter and 5 nm in height.21,22) It was noted that arrays could logically be made by varying the insulating polymer components as a means to vary selectivity. More recently, Lewis and co-workers have varied polymer matrixes containing carbon black particles to develop chemiresistor sensor arrays.23-26 The transduction of a change in gas-phase vapor concentration to an electronic signal by a coated microsensor entails a sorption (13) 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. (14) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Muellen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551-555. (15) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R. E.; Mullen, K.; Yasuda, A. Adv. Mater. 2002, 14, 238-242. (16) Cai, Q.-Y.; Zellers, E. T. Anal. Chem. 2002, 74, 3533-3539. (17) 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 1998, 14, 1730. (18) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785-8796. (19) Lundberg, B.; Sundqvist, B. J. Appl. Phys. 1986, 60, 1074-1079. (20) Talik, P.; Zabkowska-Waclawek, M.; Waclawek, W. J. Mater. Sci. 1992, 27, 6807-6810. (21) Grate, J. W.; Klusty, M.; Barger, W. R.; Snow, A. W. Anal. Chem. 1990, 62, 1927-1934. (22) Barger, W. R.; Dote, J.; Klusty, M.; Mowery, R.; Price, R.; Snow, A. Thin Solid Films 1988, 159, 369-378. (23) Doleman, B. J.; Lonergan, M. c.; Severin, E. J.; Vaid, T. P.; Lewis, N. S. Anal. Chem. 1998, 70, 4177-4190. (24) Lonergan, M. c.; Severin, E. J.; Doleman, B. J.; Beaber, S. A.; Grubbs, R. H.; Lewis, N. S. Chem. Mater. 1996, 8, 2298-2312. (25) Severin, E. J.; Lewis, N. S. Anal. Chem. 2000, 72, 2008-2015. (26) Severin, E. J.; Doleman, B. J.; Lewis, N. S. Anal. Chem. 2000, 72, 658668.

process and signal transduction related to the amount of vapor absorbed in the coating. Although the vapor sensor’s response, R, is typically expressed as a function of the gas-phase concentration of the test vapor, Cv, as in eq 1, it is more directly a function

R ) f(Cv)

(1)

R ) f(Cs)

(2)

of the concentration of the vapor in the sorptive coating on the sensor’s surface, Cs, as indicated in eq 2. The concentrations Cv and Cs are related by the thermodynamic partition coefficient, K (eq 3), and hence, the sensor’s

K ) Cs/Cv

(3)

R ) f(K Cv)

(4)

response to the gas-phase vapor concentration can be expressed to explicitly include the partition coefficient as a measure of absorption (eq 4). Hence, understanding sensor behavior in detail requires an underlying understanding of the vapor sorption properties of sensing film material,27,28 and this knowledge can help to elucidate the relative roles of sorption and transduction in observed sensitivities and detection limits. Accordingly, the sorptive properties of films of MPNs are of interest in understanding and developing nanoparticle-based sensors and sensor arrays for chemical vapor detection. Are these materials containing large mass percentages of gold and much smaller mass percentages of organic thiol effective at sorbing organic vapors, and how do they compare with sorptive polymers? In this paper, we describe the investigation of several readily prepared gold MPNs using the TSM device to transduce vapor sorption into a frequency change related to the mass of vapor sorbed.29-34 It is demonstrated that many, but not all, MPN-based sensing layers provide rapid and reversible uptake of vapors, as is desirable for vapor sensors. Sorptive selectivity varies with the monolayer structure, which provides a basis for sensing material design and use in sensor arrays. Accordingly, MPN-based materials are promising for applications as the sorptive layer on chemical microsensors and in sensor arrays. It was also found that the MPN films considered in this paper were somewhat less sorptive than polymers in terms of mass of vapor sorbed per mass of sorptive film. Estimates of partition coefficients indicate that vapor sorption by the MPN films was comparable to or less than the polymers on a per volume of sorptive film basis. By either measure, these materials are not unusually sorptive compared to polymeric materials in the concentration range studied. Therefore, it can (27) Grate, J. W.; Abraham, M. H. Sens. Actuators, B 1991, B3, 85-111. (28) Grate, J. W.; Abraham, M. H.; McGill, R. A. Handb. Biosens. Electron. Noses 1997, 593-612. (29) Alder, J. F.; McCallum, J. J. Analyst (London) 1983, 108, 1169-1189. (30) Guilbault, G. G.; Jordan, J. M. CRC Crit. Rev. Anal. Chem. 1988, 19, 1-28. (31) McCallum, J. J. Analyst (London) 1989, 114, 1173-1189. (32) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 940A. (33) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 987A996A. (34) Ballantine, D. S.; White, R. M.; Martin, S. J.; Ricco, A. J.; Zellers, E. T.; Frye, G. C.; Wohltjen, H. Acoustic Wave Sensors. Theory, Design, and PhysicoChemical Applications; Academic Press: New York, 1997.

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be argued that if chemiresistors coated with similar MPN materials have significantly better detection limits than polymercoated SAW devices, then this performance is likely the result of better signal-to-noise ratio per sorbed vapor molecule. EXPERIMENTAL SECTIONS Materials. Solvents and reagents obtained from commercial sources were used as received. Hydrogen tetrachloroaurate trihydrate was obtained from Aldrich Chemical Co., as were all thiols except 4-(trifluoromethyl)thiophenol (Lancaster) and 4-aminothiophenol (Fluka). Thin-layer chromatography was performed on Whatman thin-layer chromatography plates (K6 Silica Gel 60A) and were typically developed with dichloromethane/methanol (9:1 v/v). Instrumentation. Thermogravimetric Analysis (TGA). TGA measurements were performed on a Netzsch STA-409. Transmission electron microscope (TEM) images were obtained on a JEOL 2010 HR TEM. Images for particle size measurements were collected at 400000× magnification, and the imaging program Desktop Microscopist (Gatan) was used for particle diameter measurements. Samples were prepared by spray coating a dilution solution of the nanoparticles in dichloromethane on 200-mesh copper grids (Electron Microscopy Sciences) coated with Formvar and carbon. The solvent was readily removed under a nitrogen stream. Nanoparticle Syntheses Using a Two-Phase System. A number of nanoparticle materials were synthesized using the twophase approach first described by Brust2 and used subsequently by Wohltjen and Snow6 to make an octanethiol-protected gold nanoparticle material for chemiresistor sensors. For example, dodecanethiol-protected gold nanoparticles (sample 63A, Au/thiol mole ratio 1:1) were prepared as follows. Triply distilled water and HPLC grade toluene were used as solvents, and solutions were prepared in acid-cleaned glassware. Solutions were as follows: tetraoctylammonium bromide (2.34 g, 4.28 mmol) in 85 mL of toluene; HAuCl4‚3H2O (0.42 g, 1.07 mmol) in 35 mL of water; dodecanethiol (0.20 g, 1.07 mmol) in 1 mL of toluene; and sodium borohydride (0.41 g, 10.9 mmol) in 25 mL of water. The HAuCl4/water solution was added to the tetraoctylammonium bromide/toluene solution with vigorous stirring. After 2 min of continued stirring, the dodecanethiol/toluene solution was added, followed by the NaBH4/water solution. This reaction mixture was stirred for 3 h. The toluene phase was separated, and the volume was reduced by rotary evaporation to ∼10 mL. The product was precipitated by the dropwise addition of the toluene solution into ∼800 mL of stirred ethanol. After 24 h at 10 °C, the clear solvent was decanted and the settled product was centrifuged. The crude product was redissolved in 4 mL of toluene and precipitated in 200 mL of stirring ethanol. After standing at 10 °C, the clear solvent was decanted and centrifuged. It was washed with ethanol and vacuum-dried. Similarly, gold nanoparticles protected with 4-(trifluoromethyl)benzenethiol (sample 387) were prepared using the two-phase process and a gold/thiol mole ratio of 1:1 as follows: A solution of 1.07 g (0.0027 mol) of hydrogen tetrachloroaurate hydrate in 65 mL of water was added to 4.56 g (0.083 mol) of tetraoctylammonium bromide in 17 mL of toluene. The addition of the aurate solution provided a light brown coloration. To this was added 0.48 g (0.0027 mol) of 4-(trifluoromethyl)thiolphenol in 2 mL of toluene. 1870

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This provided a deep claret coloration. The two-phase solution was stirred vigorously, and a solution of 0.79 g (0.021 mol) of sodium borohydride in 50 mL of water was added over 4 min. The two-phase solution turned deep purple. The reaction mixture was stirred at 25 °C for 3.5 h. The phases separated readily, and the toluene phase was retained. The aqueous phase was extracted with 2 × 25 mL of toluene, and these extracts were combined with the toluene phase. The toluene was removed under nitrogen, the residue was extracted with hexane (5 × 100 mL), and these extracts were discarded. A black product, soluble in dichloromethane, remained. Gold nanoparticles protected with 4-chlorobenzenethiol (sample 389) were also prepared using the two-phase process and a gold/ thiol mole ratio of 5:1. A solution of 1.01 g (0.0026 mol) of hydrogen tetrachloroaurate hydrate in 65 mL of water was added to 4.56 g (0.083 mol) of tetraoctylammonium bromide in 17 mL of toluene. The addition of the aurate solution provided a brown coloration. To this was added 0.07 g (0.0005 mol) of 4-chlorothiolphenol in 2 mL of toluene. This provided a very wine-red coloration. The two-phase solution was stirred vigorously, and a solution of 0.79 g (0.021 mol) of sodium borohydride in 50 mL of water was added over 4 min. The two-phase solution turned black. The reaction mixture was stirred at 25 °C for 3 h. The phases separated readily, and the toluene phase was retained. The aqueous phase was extracted with 2 × 25 mL of toluene, and these extracts were combined with the toluene phase. The toluene volume was removed under nitrogen. The residue was extracted with hexane (5 × 100 mL), which was discarded, leaving a black product soluble in dichloromethane. Nanoparticle Syntheses in a Single Phase. Nanoparticle materials were also synthesized by the single-phase methanol/ water synthesis developed previously by Brust1 and used by others.10 Benzenethiol-protected gold nanoparticles (sample 352) were prepared with a Au/S mole ratio of 0.42. To 40 mL of methanol was added 0.3 g (0.000 76 mol) of hydrogen tetrachloroaurate hydrate and 0.2 g (0.0018 mol) of benzenethiol. A yellow flocculant formed upon addition of the benzenethiol. To this mixture was added 10 mL of glacial acetic acid. Over 5 min, dropwise addition of 22.3 mL of aqueous 0.4 M sodium borohydride was made to the methanol mixture. The color change was yellow to black over this period. The mixture was stirred for 3 h. The methanol was removed under vacuum and with a final nitrogen stream. The mixture was dissolved in dichloromethane and washed with 2 × 25 mL of water. The dichloromethane solution was dried over anhydrous sodium sulfate. The solvent was removed under nitrogen, and a dark brown solid was recovered. The residue was extracted with diethyl ether, which was discarded. The remaining solid was dried under nitrogen to yield the product. For this material, the solvent used to develop the silica gel plate was dichloromethane/hexane (1:1 v/v). Gold nanoparticles protected with 4-hydroxybenzenethiol were also prepared using a similar single-phase synthesis procedure. Thickness Shear Mode Devices and Instrumentation. The 10-MHz TSM devices of polished AT-cut quartz with gold electrodes (obtained already mounted from International Crystal Manufacturing) were operated with an oscillator card originally built at the University of Washington. The frequency signal was

measured using a Hewlett-Packard 53131A high-performance universal counter with a medium-stability time base, with data transferred to a microcomputer by GPIB. The frequency counter was controlled and data were logged using LabVIEW software (National Instruments, Austin, TX). A Hewlett-Packard 4194A impedance analyzer was used to measure the motional resistances before and after coating crystals and to determine the effect of coating amount on motional resistance. The motional resistance was taken as the reciprocal of the peak in the admittance spectrum, and these are the values reported. We routinely checked the resistance value determined by this method by using the equivalent circuit model fitting capability of the HP 4194A to determine equivalent circuit parameters. The standard Butterworth van Dyke equivalent circuit model for the TSM was selected.34,35 Nanoparticle materials were applied to one surface of the TSM device by spray coating with solutions in either dichloromethane or methanol, depending on their solubility. Vapor sensors were always cleaned in a UVO cleaner (model 342, Jelight Co., Inc, Irvine, CA) unit prior to film application. Vapor Response Measurements. The standard cap for a mounted TSM sensor was modified with inlet and outlet gas delivery tubes, and this flow cell was placed in a brass box suspended in a refrigerated circulating water bath (Neslab) to maintain sensor temperatures at 298 K. Good thermal contact was maintained between the flow cell and the brass box walls. Test vapors were generated from bubbler sources with dry nitrogen carrier gas. Bubblers were maintained at 288 K in a machined aluminum block with water circulating from another water bath. Carrier gas flows for the bubbler and additional dilution gas mixed downstream were controlled with electronic mass flow controllers. The total diluted gas flow was split, with 100 mL/min delivered to the sensor via a needle valve and rotameter and the rest was vented to the hood via a back pressure regulator. The entire vapor blending system was automated. The vapor concentrations for the four test vapors were generated by a consistent protocol of nine dilutions. As a result, each vapor was generated at the same partial pressures relative to their saturated vapor pressures at 288 K. Graphs of response against relative vapor pressure P/Psat, are referenced to the saturated vapor pressure at the sensor temperature of 298 K, the conversion factor being the ratio of the saturated vapor pressures at the two temperatures (∼0.62).35 The test vapor concentrations are given in Table 1. The sensors were exposed to 10-min intervals of each vapor concentration followed by 15 min of carrier gas for recovery before the next concentration of the same vapor. QCM data were typically collected every 2 s, but longer intervals were sometimes used. Multivariate Data Analysis. Principal components analysis and hierarchical clustering were carried out in MATLAB using the PLS_Toolbox, Eigenvector Research, Inc. The data set for multivariate analysis included the five highest test concentrations of each vapor. The lowest concentrations were left out since some vapor-MPN pairs gave responses at these concentrations that were not much greater than that of a bare TSM device and, hence, less representative of sorbent material properties. (35) Grate, J. W.; Kaganove, S. N.; Bhethanabotla, V. R. Faraday Discuss. 1997, 107, 259-283.

Table 1. Test Vapors and Concentrations concentration (mg/m3)

dilution factora at 288 K

P/Psab at 298 K

hexane

toluene

2-butanone

1-butanol

0.0027 0.0055 0.0106 0.0329 0.0507 0.1003 0.1502 0.2001 0.2500

0.0017 0.0034 0.0066 0.0204 0.0314 0.0622 0.0931 0.1240 0.1550

1210 2420 4690 14500 22300 44200 66200 88100 110000

237 474 921 2850 4380 8680 13000 17300 21600

613 123 2380 7350 11300 22400 33600 44700 55900

38 76 148 459 706 1400 2090 2790 3490

a Dilution factor relative to saturated vapor generated at 288 K. Relative partial pressure at 298K, obtained by multiplying the dilution factor by 0.62.

b

Figure 1. Thiols used for synthesis of diverse monolayer-protected gold nanoparticle materials.

RESULTS AND DISCUSSION Nanoparticle Materials. Dodecanethiol-protected and several arenethiol-protected gold nanoparticle materials were prepared for investigation by the reduction of AuCl4- in the presence of thiol. The thiols used are shown in Figure 1. Dodecanethiol-, chlorobenzenethiol-, bromobenzenethiol-, and trifluorobenzenethiolprotected nanoparticles were synthesized using the two-phase water/toluene approach first reported by Brust2 and used by others as well.6,17,36 Benzenethiol-, hydroxybenzenethiol-, and aminobenzenethiol-protected nanoparticles were prepared in a single-phase methanol/water synthesis also developed previously by Brust1 and used by others.10 Some types were prepared by both methods. The nanoparticle materials were not fractionated and were used as isolated. Specific synthesis procedures for the materials listed in Table 2 are provided in the Experimental Section. Nanoparticle formation was confirmed by high-resolution TEM. Typical nanoparticle core sizes were in the range of 1.6-6 nm with ∼3 nm most common. TEM images of selected nanoparticles are shown in Figure 2. While TEM reveals nanoparticles, it is blind to the presence of excess thiol or disulfide in the product. Thin-layer chromatography was used to confirm the absence of detectable product contamination by unbound thiol or disulfide. The organic contents of satisfactory samples were determined quantitatively by TGA. A summary of the materials used for vapor sorption measurements is given in Table 2. These materials had particle core sizes of ∼3 nm average, except the benzenethiolprotected material, where the average particle size was closer to 2 nm. Histograms of the core sizes of the arenethiol-protected gold nanoparticles are provided as Supporting Information. (36) Chen, S.; Murray, R. W. Langmuir 1999, 15, 682-689.

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Table 2. Monolayer-Protected Gold Nanoparticle Materials synthesis conditions thiol

sample

phases

dodecanethiol benzenethiol 4-chlorobenzenethiol 4-(trifluoromethyl)benzenethiol

63A DN352 DN389 DN387

2 1 2 2

Figure 2. TEM images of monolayer-protected gold nanoparticles: Top: dodecanethiol-protected gold nanoparticles (sample 63A). Bottom: chlorobenzenethiol-protected nanoparticles (sample 387).

We were particularly concerned about product purity since we were interested in the sorptive properties of the MPNs, not those of the free thiol, its corresponding disulfide, or mixtures of these species. The mass percentage of bound thiol in the MPN is small to begin with (see Table 2) so the presence of free thiol (or its disulfide) as a contaminant might significantly bias the measurements. Samples with these impurities were either further purified or disqualified from vapor sorption measurements. The aminobenzenethiol-protected nanoparticles were not readily redissolved after isolation, as has been observed previously;10 this material was not further investigated. Nanoparticle Films on TSM Devices. Before investigating the sorptive properties of the nanoparticle-based materials, we used impedance analysis of coated TSM devices to examine the motional resistance as a function of the amount of material deposited on TSM devices. The first objective of these measurements was to determine whether films deposited to a frequency change of 10 kHz, as is typical for TSM-based vapor sensors, yielded acoustically thin films where the device would act as a gravimetric sensor. If the film is not acoustically thin, viscoelastic effects will influence the device response and it will not act as a valid mass balance.32-34,37,38 The viscoelastic effects arise if the film material does not move synchronously with the TSM device surface across the entire thickness of the film, and this leads to high motional resistance values. Second, we were curious to know whether the hybrid organic/metal materials would behave as solid (37) Martin, S. J.; Frye, G. C. Proc. IEEE Ultrason. Symp. 1991, 393-398. (38) Grate, J. W.; Frye, G. C. Sens. Update 1996, 2, 37-83.

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Au/thiol mole ratio 1:1 0.42:1 5:1 1

TGA % organic

film on TSM (kHz)

Ω

26 20 14 20

10.27 9.97 11.25 9.65

15 7.2 7 12

Figure 3. Motional resistance as a function of increasing film thickness on the TSM device measured using an impedance analyzer. Results for several coatings are plotted; the legend indicates the thiol, the sample number, and the gold-to-thiol ratio of the synthesis.

rigid materials at larger thicknesses or acoustic losses would occur as if they were viscoelastic materials. Impedance measurements were made as material was applied by spray coating as described in the Experimental Section. For all materials and all samples, we found that motional resistance values remain low at 10 kHz of film application. Therefore, monolayer-protected nanoparticle layers on the TSM devices yield gravimetric sensors and valid vapor uptake measurements can be made. Results for several materials are shown in Figure 3. It is interesting to see that some, but not all, of the materials display significantly larger motional resistances as the amount of materials applied increases. Viscoelastic polymers also exhibit large motional resistance increases with increasing film thickness on the TSM. It would be advantageous to be able to coat TSM devices with large film thicknesses to obtain higher vapor sensitivities, as has been described by Porter using micrometersized carbon particles (from a formulation containing a polymer binder) as the sorptive sensing material.39 The results in Figure 3 indicate that this may be possible for some nanoparticle films but it cannot be assumed to be a general feature of these nanoparticle materials. As we shall describe below, small nanoparticles are largely organic material by volume, so it is not surprising that they may display lossy behavior as is typical of organic materials and polymers. The results in Figure 3 represent the range of behavior (39) Shinar, R.; Liu, G.; Porter, M. D. Anal. Chem. 2000, 72, 5981-5987.

Figure 5. TSM sensor responses of hydroxybenzenethiol-protected gold nanoparticles to 2-butanone in a test protocol with extended exposure and recovery periods. Data points were collected and plotted every 12 s. The responses to lower concentrations early in the experiment are magnified by a factor of 10 and offset for clarity.

Figure 4. TSM sensor responses of (trifluoromethyl)benzenethiolprotected nanoparticles (a) and chlorobenzenethiol-protected nanoparticles (b) to selected test vapors, with data points plotted every 2 s.

we have observed and provide specific results for materials in Table 2 as well as a couple of others. Other preparations of materials with the same thiols sometimes yielded results different from those in Figure 3. For example, films of (trifluoromethyl)benzenethiol-protected nanoparticle prepared by the single-phase synthesis showed much less increase in motional resistance with increasing film thickness than the (trifluoromethyl)benzenethiolprotected nanoparticle material shown in Figure 3. Two different samples of hydroxybenzenethiol-protected nanoparticles yielded films with the behavior shown in Figure 3. Similarly, bromobenzenethiol-protected nanoparticle materials prepared in gold-to-thiol mole ratios ranging from 0.55 to 5 all yielded similar results. However, various samples of chlorobenzenethiol- and benzenethiolprotected nanoparticles gave quite variable results. It should be noted that these impedance analysis results relate to both the intrinsic properties of the material and the film characteristics on the device. Vapor Sorption Behavior. Vapor sorption measurements were made on TSM devices coated with an amount yielding

∼10-kHz frequency shifts (an indication of film “thickness”). Exact coating frequency shift values are given in Table 2 along with the motional resistance values after coating. The test sensors were evaluated against nine concentrations of each of four vapors: n-hexane, toluene, 2-butanone, and 1-butanol (see Table 1). The vapors were selected to represent a diverse set with nonpolar hexane, more polarizable toluene, basic and dipolar 2-butanone, and hydrogen bond acidic 1-butanol. These test vapors provide a preliminary probe of differences in behavior and chemical selectivity among the nanoparticle materials. The concentration range spanned nearly 2 orders of magnitude, P/Psat ) 0.16-0.0017, with emphasis on determining sorptive properties at trace concentrations. Sensor responses to the test vapors were generally rapid and reversible, with exposures leading to steady-state or near-steady-state responses within the 6-min exposure period. Typical rapid and reversible responses are shown in Figure 4 with data points plotted every 2 s. The steady-state responses of (trifluoromethyl)benzenethiol-protected nanoparticles (Figure 4a) are reached rapidly, while those of chlorobenzenethiol-protected nanoparticles (Figure 4b) are rapid to near 90% response but slower to reach steady state and slower final recoveries. Responses on dodecanethiol-protected nanoparticles were also quite rapid to steady state, whereas some vapors on benezenethiol-protected nanoparticles yielded responses that were a little slower than those shown in Figure 4b for toluene on chlorobenzenethiol-protected nanoparticles. The sorptive behavior of bromobenzenethiolprotected nanoparticles was highly variable from sample to sample in our experiments (cause unknown), and so we report no quantitative results for these. Hydroxybenzenethiol-protected nanoparticles were judged unsatisfactory as sorbent sensing layers where rapid responses to steady state are required. This material yielded responses indistinguishable from bare TSM devices for hexane and toluene, while the responses to hydrogen bonders 2-butanone and 1-butanol were not rapid or rapidly reversible. Figure 5 shows Analytical Chemistry, Vol. 75, No. 8, April 15, 2003

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responses to 2-butanone in a test protocol with extended exposure and recovery periods. The sensor does not recover to the original baseline in the time periods shown but does recover on extended purging. Calibration curves based on the observed responses were highly nonlinear. We also measured the motional resistance of the sensor while under the highest 2-butanone concentration, and it was not significantly higher than that of the device under dry air. (Motional resistance increased from 7 Ω just before the exposure to 8.5 Ω during the exposure.) The slow response and recovery behavior of this material was observed for two independently synthesized samples. It is noteworthy, however, that these responses at high concentrations for a basic vapor on a hydrogen bond acidic material are much larger than any others observed in our data set. The results suggest that hydrogen bonding can be useful for enhancing sensitivity to basic vapors, but the response behavior requires improvement. The unresponsiveness of this material to hexane and toluene at the test concentrations also indicates selectivity. We speculate that the unusual response behavior is due to strong interparticle hydrogen-bonding interactions, leading to film rigidity and low sorptivity until a hydrogenbonding vapor at sufficient concentration can compete for those interactions, which then results in large uptake. Calibration curves for the four vapors on selected coatings are shown in Figure 6. They are plotted against the vapor concentrations as P/Psat, which puts the vapors all on the same scale on the x-axis and is a useful way for comparing chemical selectivity. For gravimetric sensors at a fixed temperature, these represent sorption isotherms. These calibration curves range from mostly linear to varying degrees of downward concavity. The calibration curves for the benzenethiol-protected nanoparticle TSM sensor (Figure 6b) were the most curved in our data set. Quantitative data on the response sensitivities relative to vapor concentrations (in mg/m3) are reported in Table 3. These were determined from the slopes of linear regressions on the response data from the coated sensors (Table 2). The intercepts and correlation coefficients are also reported. Except for the sensor coated with benzenethiol-protected nanoparticles, the data show small intercepts (typically 1 Hz or less) with high correlation coefficients (typically 0.998 or higher for R2), confirming linearity over the concentration range examined. Control experiments on a bare TSM device were also run. Absorption of hexane and toluene are nearly linear over the test concentration range with maximum response magnitudes of 5 Hz. 2-Butanone and 1-butanol yielded curved (downward) sorption isotherms with maximum response magnitudes of 6 and 7.4 Hz, respectively. These curves are approximately linear above P/Psat ) 0.025 with extrapolated intercepts of 2 Hz. The TSM devices with nanoparticles coated on one side will have a small contribution to the signal due to adsorption on the uncoated surface. This adsorptive effect contributes to the typically larger nonzero intercepts seen in calibration curves for 2-butanone and 1-butanol (see Table 3). However, bare surface adsorption does not appear to be sufficient to explain the nonlinearities observed for the vapors on the benzenethiol-protected nanoparticle-coated devices. Comparisons with Sorptive Polymers. Polymers are widely used in chemical sensing, and their sorptive properties have been examined in detail. We have previous data for the sorption of hexane, toluene, and 2-butanone by poly(isobutylene) (PIB) and 1874 Analytical Chemistry, Vol. 75, No. 8, April 15, 2003

Figure 6. Calibration curves for the magnitudes of the responses to the four test vapors on selected coatings plotted against the vapor concentrations as P/Psat: (a) dodecanethiol-protected gold nanoparticle TSM sensor, (b) benzenethiol-protected gold nanoparticle TSM sensor, and (c) trifluorobenzenethiol-protected gold nanoparticle TSM sensor.

Table 3. Response Sensitivity Dataa for the Sorption of Vapors by Films of Monolayer-Protected Gold Nanoparticles on TSM Devices

dodecanethiol hexane toluene 2-butanone 1-butanol benzenethiol hexane toluene 2-butanone 1-butanol chlorobenzenethiol hexane toluene 2-butanone 1-butanol (trifluromethyl)benzenethiol hexane toluene 2-butanone 1-butanol

sensitivity -(Hz per mg/m3)

intercept -(Hz)

corr coeff R2

0.0008 0.0059 0.0006 0.0046

0.09 -1.7 0.46 1

0.999 0.9973 0.9993 0.991

0.0005 0.0034 0.0013 0.0061

4.7 5.9 4.8 1.6

0.9842 0.9835 0.9837 0.9876

0.0002 0.0028 0.0012 0.0148

0.65 0.65 1.6 2.5

0.9983 0.9993 0.9974 0.9916

0.0002 0.0027 0.0017 0.0146

0.47 0.47 2.3 1

0.9989 0.9996 0.9981 0.9984

a Responses were frequency decreases in the normal direction for mass loading responses, reported for the amount of film applied to the sensor as given in Table 2. Reporting the negative of these values corresponds to the way the data are plotted in the figures.

poly(epichlorohydrin) (PECH) on 10-MHz TSM devices.35 These polymers have been used in a variety of vapor-sensing studies and represent simple prototypical sorptive polymers whose structures and sorptive properties have been described in detail elsewhere.28,35,40,41 PIB is an alkane-based low-polarity polymer interacting by dispersion forces. In this regard, it can be considered similar in interactive properties to the alkane chains of dodecanethiol. PECH is more dipolar due to the chloro substitution. The arenethiol-based nanoparticle materials are expected to be polarizable and, if halogenated, dipolar. The sorptive data for the polymers were compared with the corresponding data for the nanoparticle films, as shown in Figure 7. The data for the polymers were normalized to 10-kHz films to match the film amounts applied to the MPN-coated sensors. Thus, this analysis compares the sorptive properties on a mass of vapor sorbed per mass of sorbent material. This is a common way of normalizing data from different gravimetric sensors for comparison, and it is intrinsic to the relationship in eq 5 that governs

∆fv/∆fs ) ∆mv/∆ms

(5)

gravimetric sensor response. In this equation, ∆fs and ∆fv are the frequency shifts due to the application of the film to the bare device and the sorption of vapor by the applied film, respectively, while ∆ms and ∆mv are the mass-per-unit areas of the film and the sorbed vapor. It is evident from Figure 7 that the best (40) Abraham, M. H.; Andonian-Haftvan, J.; Du, C. M.; Diart, V.; Whiting, G. S.; Grate, J. W.; McGill, R. A. J. Chem. Soc., Perkin Trans. 2 1995, 369-78. (41) Grate, J. W.; Kaganove, S. N.; Bhethanabotla, V. R. Anal. Chem. 1998, 70, 199-203.

Figure 7. Calibration curves comparing the sensitivities of the nanoparticle-coated TSM sensors with polymer-coated TSM sensors, plotting the magnitudes of the responses against the vapor concentrations. The responses of a bare TSM device are included as a control. Test vapors are (a) hexane, (b) toluene, and (c) 2-butanone.

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Table 4. Mass and Volume Considerations for Alkanethiol and Arenethiol Monolayer-Protected Nanoparticle Materials densities calculated example thiola dodecanethiol benzenethiol chlorobenzenethiol (trifluoromethyl)benzenethiol

fractional massb gold thiol

fractional vol calcdc gold thiol

0.74 0.80 0.84 0.80

0.11 0.18 0.28 0.21

0.26 0.20 0.14 0.20

0.89 0.82 0.72 0.79

density of occupied vold (g/cm3)

film density assuming a free vol of 25%e

2.9 4.4 6.4 5.1

2.2 3.3 4.8 3.8

a Densities for thiols used were 0.845, 1.073, 1.25, and 1.3 g/mL respectively, where the value of 1.25 is used for 4-chlorothiophenol based on the value for 3-chlorothiophenol, and the value of 1.3 for (trifluoromethyl)benzenethiol is estimated from 3-(trifluoromethyl)phenol (1.33). b Fractional masses are based on experimental TGA measurements (see Table 2). c Fractional volumes based on bulk densities of gold and thiols where the densities of the thiols are in footnote a and the density of gold is 19.3 g/mL. d Density of the occupied volume, assuming no free volume. e Assumed fractional free volume of thin-film material of 0.25.

nanoparticle films for a given vapor are less sorptive on a permass basis than the best sorptive polymer considered within the concentration range studied, the polymer being better by a factor of 2-2.5. The nanoparticle-coated sensors, however, were significantly more sorptive than a bare TSM sensor, whose data are included in the graphs for comparison. Based on these data, the MPN films examined in the present study are not superior for chemical vapor sensing on acoustic wave devices if polymer and MPN films are applied in the same amounts based on frequency shift. However, it should be noted that films of equal mass but with different material densities will have different thicknesses. Volume Considerations. Vapor sorption is typically quantified on the basis of the partition coefficient defined in eq 3, where the concentration of vapor in the sorbent phase is in grams per liter. Thus, the partition coefficient describes sorption on a mass-pervolume basis. The partition coefficient is related to gravimetric sensor responses according to eq 6. The F parameter is the

∆fv ) ∆fsCvK/F

(6)

sorptive material density. This relationship assumes the observed frequency shifts are due to bulk absorption of vapor in the film. Unfortunately we do not know the densities of our polydisperse MPN materials as thin films. However, estimates can be made and the calculation of these estimates is illuminating with regard to the volume fractions of the thiol component of these materials. Table 4 presents the mass and volume fractions of the constituents of the MPN materials used in our sorption measurements, as well as estimated density values. These results are calculated using the mass compositions from the TGA data in Table 2. From our own data as well as data in the literature,17,36,42 it is apparent that MPN materials are typically 75-90% metal by mass and thus only 10-25% organic by mass. Volume fractions can be estimated if one assumes that the organic portion has the same density as the bulk thiol and the gold cores have the same density as bulk gold. Now the proportions are reversed, with 70-90% organic material by volume and only 1030% metal by volume. Thus, despite the low mass percentage of sorptive organic insulating material, these materials are actually primarily sorptive organic material by volume. Low volume fractions of gold are also indicated by structural studies.4 In structural (42) Snow, A. W.; Wohltjen, H. Chem. Mater. 1998, 10, 947-949.

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studies of crystalline samples of monodisperse alkanethiolprotected gold nanoparticles, where the flexibility of the alkane chains permits very efficient packing, the gold cores have been assumed to have the density of bulk gold metal, and the volume not occupied by gold has been found to have a density very close to that of the bulk thiol.18 The mass and volume fraction numbers can be further used to estimate net densities for the “occupied” volume of the nanoparticle materials. (Because they are based on densities for bulk thiol, the “occupied” volume in this case includes the free volume of the bulk thiol condensed phase.) These are presented in Table 4, with values from about 3 to 6 g/mL depending on the thiol and the mass percentages of gold and thiol.43-45 Densities estimated in this way were used to calculate MPN/vapor partition coefficients that we have designated “higher” estimates. These occupied-volume densities are the highest reasonable density values, and the partition coefficients calculated using them will then be the highest estimated values for comparisons with sorptive polymers. However, it is possible that films of these MPN materials, especially those without flexible alkyl chains, may contain additional free volume. In addition, disorder in the packing of the particles may result in lower density films. The resulting lower densities would result in correspondingly lower partition coefficients. Liquids typically have a fractional free volume of ∼0.2,46-48 and close-packed spheres of uniform size have an unoccupied (43) Arthur Snow has measured the densities for alkanethiol-protected gold nanoparticle materials and finds that they vary systematically with core size and alkanethiol length. Values from 2.7 to 3.7 g/mL were determined with a value of 3 g/mL for dodecanethiol-protected gold nanoparticles. Personal communication. (44) Zellers has reported estimated effective densities of 1.2 g/mL for as-deposited films of his monolayer-protected gold nanoparticle materials. This value was estimated from the thicknesses and masses of spray-coated films on glass cover slips as determined by profilometry and microbalance measurements, respectively. (45) Monodisperse dodecanethiol-protected gold nanoparticles with 1.6-nm gold cores crystallized in a body-centered cubic lattice with a lattice constant of 3.64 nm and a unit cell volume of 48.2 nm3. The unit cell contains two molecular units, each with a molecular weight of 40 000 given the approximate formula of Au145(SR)56. The calculated density from these values is 2.8 g/cm2. This material has a metal mass fraction of 0.72 and a metal volume fraction of only 0.10. (46) Ferry, J. D. Viscoelastic Properties of Polymers, 1st ed.; John Wiley and Sons: New York, 1961. (47) Marcus, Y. The Properties of Solvents; John Wiley and Sons: New York, 1998. (48) Grate, J. W.; Zellers, E. T. Anal. Chem. 2000, 72, 2861-2868.

Table 5. Comparisons of Estimated Partition Coefficientsa monolayer-protected gold nanoparticle materials: estimated partition coefficients (conservative estimate/“higher” estimate)

polymers: partition coefficients

vaporb

dodecanethiol

benzenethiol

chlorobenzenethiol

(trifluoromethyl)benzenethiol

PIB

PECH

hexane toluene 2-butanone

180/240 1100/1500 150/190

260/350 1900/2500 670/890

110/140 1300/1800 650/860

110/150 1200/1500 830/1100

190 1000 110

43 1700 720

a Values for nanoparticle films are preliminary estimates based on occupied volumes to obtain “higher” estimates and assuming a fractional free volume of 0.25 to obtain more conservative estimates. See last two columns of Table 3. b Values calculated using data from vapors at P/Psa ) 0.031.

volume of 0.26. Body-centered cubic packing results in an unoccupied volume of 0.32. Randomly packed spheres of uniform size have unoccupied volumes of 0.36, although conditions have been considered where higher packing factors could lead to unoccupied volumes of 0.26-0.32.49,50 We selected a fractional free volume of 0.25 in order to estimate lower densities of our materials as films, giving values from about 2 to 5 g/mL as shown in Table 4. These lower densities result in more conservative estimates for the partition coefficients for vapors in the films. Using these estimated film densities, we calculated preliminary estimates for partition coefficients. The results are presented in Table 5 and compared with partition coefficients for vapors in polymers PIB and PECH. The values given in Table 5 were calculated from sensor responses at P/Psat ) 0.031 for both the nanoparticle materials and the polymers. On a per-volume basis, it appears that the sorption of organic vapors by these MPN materials is of the same order of magnitude as the polymers at this concentration. This conclusion arises from either the more conservative or the “higher” values of the estimated partition coefficients for our MPN materials. Thus, it appears unlikely that the nanoparticle-based materials considered here are more sorptive than conventional polymers. If the actual densities of the film materials were lower than the estimated values used in the calculations, then the actual partition coefficients would therefore be proportionatly lower, and it would still be true that these nanoparticle-based materials are not more sorptive than the polymers used for comparison. Alternatively, if polymers were selected for comparison that are more sorptive than the simple prototypical polymers considered here, it would again be the case that the nanoparticle materials considered (in the vapor concentration range studied) are not more sorptive than sensing polymers. These results provide a basis for considering the origins (sorption or transduction) of the superior detection limits reported by others for MPN-coated chemiresistors (see Discussion). The estimated partition coefficients suggest that MPN materials may be suitable as sorbent layers on acoustic wave sensors affording sensitivities similar to polymer-coated acoustic wave vapor sensors if they are coated to equivalent thicknesses. If acoustic wave sensors can be coated to thicknesses greater than those of viscoelastic polymers (and remain acoustically thin), as is implied for at least some of the materials in Figure 3, then MPN-coated SAW sensors could be more sensitive than polymer-coated SAW sensors. (49) Jaeger, H. M.; Nagel, S. R. Science 1992, 255, 1524. (50) Torquato, S.; Truskett, T. M.; Debenedetti, P. G. Phys. Rev. Lett. 2000, 84, 2064-2067.

Figure 8. Response patterns for the four test vapors on the four nanoparticle-coated sensors as an array. Response data for the bare TSM device are included for comparison.

Monolayers were initially investigated as sensing layers on acoustic wave devices as a single film on a planar gold coating.51-53 The MPN materials create three-dimensional monolayers17 significantly enhancing the amount of sorptive material on the surface compared to two-dimensional monolayers and, hence, enhancing the sensor sensitivity. Multivariate Analysis. The set of four well-characterized and well-behaved nanoparticle materials yielded distinct patterns for the four test vapors in the data set, as shown in Figure 8. (Patterns are for each vapor at the same relative partial pressure P/Psa ) 0.031.) The responses of the bare TSM device are included for comparison. For each vapor in this data set, a different nanoparticle material yielded the highest response: hexane/dodecanethiol; toluene/benzenethiol; 2-butanone/trifluoromethanethiol; and butanol/chlorobenzenethiol. Principal components analysis indicates that there are two or at most three principal components in the data set. The variance captured by each principal component is given in Table 6. (The data were pattern normalized and autoscaled for this analysis, using data for the five highest concentrations of each of the four (51) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 31913193. (52) Thomas, R. C.; Yang, H. C.; DiRubio, C. R.; Ricco, A. J.; Crooks, R. M. Langmuir 1996, 12, 2239-46. (53) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219-227.

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Table 6. Principal Components Analysisa % variance

a

principal component no.

captured this PC

captured total

1 2 3 4

80.95 17.19 1.47 0.40

80.95 98.14 99.60 100.00

Response data were pattern normalized and autoscaled.

vapors, such that the every response in the data set is 5 Hz or more.) It is apparent that the chemical diversity in this data set is rather limited, since a diverse set of sorptive polymers can yield more significant principal components.54 The sorptive MPN films in the present data set vary primarily in their polarizability and dipolarity, while none has significant hydrogen-bonding potential. More research is required to obtain materials with more diverse chemical selectivity while simultaneously offering rapid and reversible sorption (unlike the hydoxybenzenethiol-protected nanoparticles whose responses are shown in Figure 5). Hierarchical cluster analysis showed that the chloro- and (trifluoromethyl)benzenethiol-derivatized nanoparticles are most similar to one another while the dodecanethiol- and benzenethiolprotected nanoparticles are more dissimilar from the first two and from each other, as might be expected from chemical structures. These results are observed using KNN and K-means clustering, with and without autoscaling on the data and regardless of whether the data were first pattern normalized or not. Comparison with Other Monolayer-Protected Gold Nanoparticle Materials. There exist limited data on the vapor uptake properties of other MPN materials. Zhong et al. tabulated the slopes of the calibration curves, i.e., the sensitivities in frequency change per concentration, for two types of networked MPN films, each at two particle sizes.12 One of these was based on the displacement of surface thiols from decanethiol-protected gold nanoparticles by 1,9-nonanedithiol to precipitate networked films on TSM devices. (Film growth times from 3 to 60 h were reported.) These films yielded frequency shifts near 10 kHz, which is similar to the film amounts in the present study. The toluene sensitivites of these networked alkanedithiolprotected nanoparticle films were reported as -0.18 and -0.24, apparently in hertz per ppm, for films with 5- and 2-nm gold cores, respectively. In vapor concentrations of milligrams per cubic meter, these sensitivities become -0.048 and -0.064 Hz per mg/ m3. Zhong et al. did not report any vapor sensitivities for the starting decanethiol-protected nanoparticle films or any other films with simple alkanethiol (as opposed to dithiol) monolayers on the gold cores.12 The toluene sensitivity of the dodecanethiol-protected nanoparticle film in the present study was -0.006 Hz per mg/m3. (Table 3) These comparisons from separate laboratories indicate that the nanostructured networked films may yield greater vapor sorption than films of the simpler alkanethiol-protected gold nanoparticles, although rigorous side-by-side comparisons using monodisperse samples with the same core sizes remain to be (54) Grate, J. W.; Patrash, S. J.; Kaganove, S. N.; Abraham, M. H.; Wise, B. M.; Gallagher, N. B. Anal. Chem. 2001, 73, 5247-5259.

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done. This comparison suggests that the detailed nanostructure may influence the sorptive strength of the nanoparticle materials. Discussion. Vapor sorption by the sensing film is a fundamental influence on sensor response, and we report TSM device gravimetric measurements of vapor uptake by MPN films for comparison with vapor uptake by sorbent polymers. The TSM sensor results provide information on the amount of vapor uptake and the rate of vapor uptake for these film materials. Regardless of the tranduction mechanism ultimately used for sensing, this information is useful in predicting and interpreting sensor response behavior. Slow uptake will lead to slow response times regardless of the transduction mechanism. Gravimetric calibration curves relate directly to sorption isotherms and directly to the calibration curves of other types of sensors if the transduction mechanisms of those sensors are linear with sorbed concentration. If studied with chemiresistor sensors alone, nanoparticle response behavior may be difficult to understand, since different response mechanisms can contribute to the observed responses at various concentration levels,11 and response direction can even reverse itself over the course of a vapor exposure.12 TSM measurements can yield simpler response behavior12,13 and add more fundamental knowledge, although rigorous interpretation as gravimetric vapor uptake would require using measurements of motional resistance or other impedance analysis methods, which is often not done. Our measurements indicate that the nanoparticle-based materials examined sorb vapors with partition coefficients that are of the same order of magnitude as the organic polymers used for comparison, assuming material densities estimated as described above. Strictly from the standpoint of sorption, these materials do not appear to have advantages over polymers as sensing layers. On the other hand, the gold nanoparticle-based films reported by Zhong et al. that use dithiols to produce networked nanostructures appear to be substantially more sorptive. If so, they may compare more favorably with polymers.12 Transduction is also important in sensor performance, and these materials can be used as conducting films on chemiresistor sensors. It has been reported that chemiresistor sensors coated with MPN films can offer lower detection limits than polymercoated vapor sensors,8,9,16 such as those based on surface acoustic wave devices or those based on chemiresistors coated with carbon black-containing polymers. If MPN-coated chemiresistors do offer better detection limits than polymer-coated SAW vapor sensors, and the MPN layers and polymer layers absorb similar amounts of vapor, then the lower detection limits for MPN-coated chemiresistors must be due to more signal to noise per sorbed vapor molecule. This conditional argument stimulates thinking about signal transduction and nanostructure as a focus for improved sensors. The implications are that nanostructure matters and suitable nanostructures can lead to better signal-to-noise ratios per sorbed molecular unit. Tailoring nanostructure for more sensitive transduction represents an area of opportunity for nanoscience in chemical sensing. Several areas are indicated for further study. The variable space in MPN-coated sensors entails the thiol structure, the ratio of gold to thiol in the synthesis, the nanoparticle size, the polydispersity in size, the ratio of organic thiol to gold in the final product, and the characteristics of the material as a thin film on a sensor. MPN

structures giving better chemical diversity while simultaneously yielding rapid response and recovery times are needed for sensor arrays. The degree of sorption on a per-mass basis has been accurately measured in this study, but accurate and precise partition coefficients (i.e., sorption on a per-volume basis) from TSM sensor responses will require accurate density values for the thin films. The polydispersity of the nanoparticle materials will influence the nanostructure of the thin films and, hence, the transduction sensitivity. Methods to narrow the particle size distribution by synthesis conditions, postsynthesis treatments,18,55,56 or separations may influence sensor performance if it leads to changes in the nanostructure of the films. ACKNOWLEDGMENT The authors thank Dr. Scott Elder for the initial syntheses of some of the nanoparticle materials and Alice Dohnalkova for TEM (55) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515-7520. (56) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J. J. Nanoparticle Res. 2000, 2, 157-164.

images. J.W.G. thanks Dr. Arthur Snow for informative and motivating discussion of his work on nanoparticle-coated sensors at the Naval Research Laboratory in March 2000 as well as helpful discussions subsequently. This work was funded by the U.S. Department of Energy via Laboratory Directed Research and Development funds administered by the Pacific Northwest National Laboratory. The Pacific Northwest National Laboratory is operated for the U.S. DOE by Battelle Memorial Institute SUPPORTING INFORMATION AVAILABLE Histograms of the cores sizes of the arenethiol-protected gold nanoparticle materials. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review October 14, 2002. Accepted February 7, 2003. AC0206364

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