Gold Nanoparticle Chemiresistor Sensors: Direct Sensing of Organics

Aug 28, 2007 - This was achieved by miniaturizing the chemiresistor device such that the double layer capacitance of the electrodes in contact with th...
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Anal. Chem. 2007, 79, 7333-7339

Gold Nanoparticle Chemiresistor Sensors: Direct Sensing of Organics in Aqueous Electrolyte Solution Burkhard Raguse,* Edith Chow, Christopher S. Barton, and Lech Wieczorek

CSIRO, Industrial Physics, P.O. Box 218, Lindfield, NSW, 2070, Australia

A novel chemiresistor sensor for detection of organic analytes in high-conductivity aqueous electrolyte solution is reported. The chemiresistor sensor is based on thin films of gold nanoparticles capped with a 1-hexanethiol monolayer that is inkjet printed onto a microelectrode. In order for a change in nanoparticle film resistance to be measured, the electronic conduction must preferentially occur through the nanoparticle film rather than through the high-conductivity electrolyte solution. This was achieved by miniaturizing the chemiresistor device such that the double layer capacitance of the electrodes in contact with the electrolyte solution gives rise to a significantly larger impedance compared to the nanoparticle film resistance. This system was shown to be sensitive to simple organics dissolved in an aqueous electrolyte solution. The organic analytes, dissolved in the aqueous solution, partition into the hydrophobic nanoparticle film causing the nanoparticle film to swell, resulting in an increase in the low-frequency impedance of the sensor. An increase in the impedance, at 1 Hz, of the gold nanoparticle chemiresistor on exposure to toluene, dichloromethane, and ethanol dissolved in 1 M KCl solution was demonstrated with detection limits of 0.1, 10, and 3000 ppm, respectively. Titration curves over 3 orders of magnitude could be obtained for analytes such as toluene. Over the past few years, research in the area of sensor development has evolved considerably owing to the constant desire for devices that offer higher sensitivity, greater analyte discrimination, and lower operating costs than current existing technologies.1 Chemiresistors are attractive for use as chemical sensors due to their simplicity, ease in fabrication, and ability to satisfy the above requirements.2-10 Such devices function by * Corresponding author. E-mail: [email protected]. Tel.: +61-2-94137549. (1) Katz, H. E. Electroanalysis 2004, 16, 1837-1842. (2) Franke, M. E.; Koplin, T. J.; Simon, U. Small 2006, 2, 36-50. (3) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856-2859. (4) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R. E.; Mullen, K.; Yasuda, A. Adv. Mater. 2002, 14, 238-242. (5) Evans, S. D.; Johnson, S. R.; Cheng, Y. L. L.; Shen, T. H. J. Mater. Chem. 2000, 10, 183-188. (6) Ahn, H.; Chandekar, A.; Kang, B.; Sung, C.; Whitten, J. E. Chem. Mater. 2004, 16, 3274-3278. (7) Zhang, H. L.; Evans, S. D.; Henderson, J. R.; Miles, R. E.; Shen, T. H. Nanotechnology 2002, 13, 439-444. (8) 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. 10.1021/ac070887i CCC: $37.00 Published 2007 Am. Chem. Soc. Published on Web 08/28/2007

monitoring the change in electrical resistance of the active material in the presence of chemical species. Thin-film composites of metal nanoparticles coated with selfassembled monolayers (SAMs) of organic molecules are of particular interest as chemiresistor materials.2-8 This is due to their unique electronic properties11-16 and the ability to readily manipulate the physicochemical properties of the thin film by varying the type, length, and functionality of the SAM.17,18 Such functionalized gold nanoparticle (AuNP) thin-film chemiresistors have been shown to exhibit high sensitivity and reversibility when exposed to chemical vapors.2-8 This concept was first demonstrated by Wohltjen and Snow3 using 2-nm-diameter gold clusters encapsulated by a monolayer of 1-octanethiol deposited on interdigitated electrodes. On exposure to organic vapors such as toluene in nitrogen carrier gas, reversible changes in the resistance of the thin film were observed. However, such chemiresistors are generally only used for the detection of analytes in the gas or vapor phase,2-8 or in pure, (nonelectrolyte containing) organic liquid,19 and to the best knowledge of the authors, no sensor based on resistance changes has been shown to function in high-conductivity aqueous electrolyte solution. In this paper, we demonstrate for the first time that highresistance (MΩ) nanoparticle-based chemiresistors can directly sense organic analytes in ionically conductive aqueous solutions. We demonstrate that this can be achieved by controlling the ratio of the chemiresistor film resistance to the impedance due to the double layer capacitance of the total electrode surface in contact with the electrolyte solution. Additionally, by providing a thin (9) Gao, T.; Woodka, M. D.; Brunschwig, B. S.; Lewis, N. S. Chem. Mater. 2006, 18, 5193-5202. (10) Holliday, B. J.; Stanford, T. B.; Swager, T. M. Chem. Mater. 2006, 18, 56495651. (11) Hill, R. M. Proc. R. Soc. A 1969, 309, 377-395. (12) Abeles, B.; Sheng, P.; Coutts, M. D.; Arie, Y. Adv. Phys. 1975, 24, 407461. (13) Terrill, R. H.; Postlethwaite, T. A.; Chen, C. H.; Poon, C. D.; Terzis, A.; Chen, A. D.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548. (14) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465-11472. (15) Mu ¨ ller, K. H.; Herrmann, J.; Raguse, B.; Baxter, G.; Reda, T. Phys. Rev. B 2002, 66, 075417. (16) Mu ¨ ller, K. H.; Wei, G.; Raguse, B.; Myers, J. Phys. Rev. B 2003, 68, 155407. (17) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103-1169. (18) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (19) 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.

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Figure 1. Schematic representation of hexanethiol-AuNP deposited between two gold microelectrodes. The presence of dissolved toluene in KCl solution affects the electrical conductivity of the film.

dielectric layer between the contacts and the chemiresistor thin film, the effective double layer capacitance may be further reduced, thus increasing its impedance, without significantly affecting the chemiresistor resistance measurement in response to an analyte. Gold nanoparticles encapsulated with 1-hexanethiol were used to demonstrate the feasibility of the chemiresistor operating in an aqueous solution environment for the determination of dissolved levels of toluene, dichloromethane, and ethanol (see Figure 1). EXPERIMENTAL SECTION Chemicals and Reagents. Gold(III) chloride trihydrate (HAuCl4·3H2O), tetraoctylammonium bromide (TOAB), 4-(dimethylamino)pyridine (DMAP), sodium borohydride, sodium carbonate, potassium chloride, potassium iodide, and iodine were purchased from Sigma-Aldrich. (3-Mercaptopropyl)triethoxysilane (MPTES) and 1-hexanethiol were obtained from Fluka. Sulfuric acid, hydrochloric acid and nitric acid were from AJAX. Toluene and acetone were purchased from LabScan. Dichloromethane was from BDH, acetone was from Chem-Supply, ethanol was from CSR, propan-2-ol was from Merck, and Deconex OP-120 glass cleaning solution was obtained from Borer Chemie. All reagents are of analytical grade and were used as received. Solutions were prepared using Nanopure deionized (DI) water (>17.4 MΩ cm from Barnstead) unless otherwise stated. Stock solutions of dichloromethane (10 and 5% v/v) and toluene (1 and 0.1% v/v) were prepared in ethanol. Dilute solutions for analytical measurement were prepared by serial dilution of the stock solution in 1 M KCl solution. DMAP-AuNP Synthesis. Gold nanoparticles were synthesized following the Brust method20 and transferred to the aqueous phase containing DMAP following the method by Gittins and Caruso.21 Briefly, 70 mL of 2% HAuCl4·3H2O was added to a stirred solution of 8.752 g of TOAB in 320 mL of toluene. Stirring was continued for 10 min and was followed by the addition of 1.892 g of sodium borohydride, which resulted in reduction of the gold. After 2 h, the lower aqueous phase was removed and the toluene phase was subsequently washed with 200 mL of 0.1 M sulfuric acid, followed by 200 mL of 1 M sodium carbonate, and twice with 200 mL of water. For the phase transfer of the nanoparticles (20) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (21) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001-3004.

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from the organic to the aqueous phase, an aqueous solution (50 mL) of 0.6446 g of DMAP was added to the as-prepared nanoparticles. The solution containing the DMAP-AuNP was then separated and evaporated under a gentle stream of nitrogen to leave a gold nanoparticle powder. The dried DMAP-AuNP powder was stored at -20 °C, and DMAP-AuNP ink solution was prepared as required (1% w/v in water). All ink solutions were filtered with a 0.22-µm Millex GS filter unit (Millipore) prior to use. The diameter of the DMAP-AuNP was determined using dynamic light scattering (high-performance particle sizer, Malvern Instruments, Worcestershire, UK) to be 6 ( 2 nm. The exchange of the DMAP ligand with hexanethiol was performed after ink deposition on the gold band microelectrodes. Fabrication of Gold Band Microelectrodes. Gold band microelectrodes (nominal dimensions of 3-mm length and 5-µm width) with an electrode gap of 5 µm were prepared on a glass slide. Borofloat 33 glass microscope slides (75.6 mm × 25.0 mm × 1.0 mm) were purchased from Schott and were cleaned prior to use in Deconex OP120 (1% v/v in DI water for 10 min), and washed with DI water. The glass slides were mounted in a vacuum chamber for sputter deposition of chromium (10 nm) and then gold (200 nm). Positive photoresist (Microposit 1813, Shipley) was spin-coated at 4 krpm for 1 min. After a soft bake (115 °C for 1 min), the slides were aligned with a custom-made photolithography mask (5-µm minimum feature size, Photronics Inc.) to pattern the electrode structures, exposed to UV (13 s). The photoresist was developed for 20 s (AZ 300 MIF, Clariant Corp.), and washed with DI water. After a hard bake (125 °C for 1 min), the slides were incubated with gold etchant (2 KI/1 I2) for ∼1 min and then chrome etchant (Extran, Merck KGaA) for ∼3 min. The remaining photoresist was removed by sonication of the slides in acetone for 5 min. This resulted in an array of six gold band microelectrodes per slide as shown in Figure 2 with widths measured by optical microscopy (Nikon MM-40) to be 5.1 ( 0.1 µm with a separation of 5.0 ( 0.1 µm. A second photoresist layer was added to the glass slides in order to passivate the gold electrode areas not required for flow cell operation and to provide a gasket between the poly(tetrafluoroethylene) (PTFE) flow cell and the glass slides. The regions on the glass slides not covered with the photoresist are the gold contact pads and a rectangular region (1.5 mm wide × 50 mm length) along the center of the slides where the gold band microelectrodes are. Positive photoresist (Microposit 1813) was spin-coated at 4 krpm for 1 min, soft baked at 115 °C for 1 min, aligned to a second custom-made mask (5-µm minimum feature size, Photronics Inc.), and exposed to UV for 13 s. After developing the photoresist (AZ 300 MIF for 20 s), the slides were washed in DI water, hard baked at 125 °C for 1 min, and cleaned by oxygen plasma reactive ion etching (2 min at 15-W rf power). The slides were then baked in a laboratory oven for 10 min at 200 °C to thoroughly cross-link the second photoresist layer. Nanoparticle Film Preparation. The glass microscope slides patterned with the six gold band microelectrodes were pretreated with a silanizing agent prior to gold nanoparticle deposition for better adhesion of the nanoparticle to the surface.22-24 Thus, the glass slides were immersed in a solution containing 2% v/v (22) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85-88. (23) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353-2361. (24) Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 41094117.

Figure 2. Illustrative drawing of an array of six chemiresistor devices photolithographically patterned onto a glass microscope slide. The region within the dashed rectangle represents the area that is exposed to the electrolyte solution. Inset shows a photograph of the gold band microelectrodes (3 mm length × 5 µm width, 5 µm separation) with a gold nanoparticle film deposited using 10 180-pL drops of a 1% w/v DMAP-AuNP solution.

MPTES in toluene for 2 h, followed by rinsing with copious amounts of toluene, and drying under a gentle stream of nitrogen. The treated glass slides were then baked in an oven at 110 °C for 1 h. Inkjet printing of gold nanoparticles was carried out using an Autodrop printing system (from Microdrop Technologies). The printhead used was an AD-K-501 micropipet (25-µL holding volume, 70-µm-diameter nozzle), and a typical droplet volume of 180 pL was used. The drop frequency was set at 200 Hz, and the tip of the micropipet was positioned 1 mm above the substrate during the printing process. Subsequently, 1, 10, or 100 drops of 1% w/v DMAP-AuNP solution were inkjet-printed over each of the six band microelectrodes on the glass slide, and the solvent was allowed to evaporate. This process resulted in ringlike deposits with ring diameters and widths determined using an optical microscope and ring thicknesses determined using a Sloan Dektak 3030 stylus profilometer or ThermoMicroscopes Autoprobe M5 AFM. All dried films resulted in a circular ring formation, which is a result of pinning of the original contact line as the solvent evaporated, which carried virtually all the solute from the center outward.25 Measurements of the film formed by the 10-drop deposition process showed a circular ring with a diameter of 330 ( 12 µm, a ring width of 20.0 ( 0.7 µm, and a ring thickness of 600 ( 100 nm. The central region was found to contain only a few layers (4-6) of nanoparticles. The film from the 10-drop deposition process was found to exhibit higher reproducibility than a single drop and was less prone to cracking than films produced from 100 drops. Consequently, results obtained by depositing films from 10 drops of 1% w/v DMAPAuNP are reported herein. After 2 h, the DMAP-AuNP deposited electrodes were exchanged with 1-hexanethiol by exposing the microelectrodes to 1-hexanethiol vapor for 30 min. Subsequently, the microelectrode (25) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827-829.

array was removed from the 1-hexanethiol vapor, allowed to stand for a further 30 min, and then rinsed with water. Flow Cell Design. For delivery of solutions to the nanoparticle film chemiresistors, the glass slide was placed into a flow cell. A channel (46.5 mm long, 1.6 mm wide, and 2.0 mm high) was cut into a PTFE block, with an injection/extraction port at each end of the channel. A lip (3.0 mm wide, 0.2 mm high) surrounding the flow channel was incorporated into the flow cell to improve the seal between the PTFE block and the glass slide. Perspex backing and fronting plates were used to press the PTFE flow cell against the glass slide to ensure a leak-free seal. Separate glass syringes (20 mL from Sigma-Aldrich) held the analyte and control samples. These were driven by syringe pumps (Extech Equipment Pty. Ltd.) at a flow rate of 0.66 mL/min. A two-way solenoid selector valve (Extech Equipment Pty. Ltd.) allowed switching between the analyte and control samples during operation. PTFE tubing with stainless steel interconnects was used to provide an inert flow path throughout the injection system. Analyte Determination. The response of the nanoparticledeposited electrodes to different concentrations of dichloromethane, toluene, and ethanol in 1 M KCl solution was investigated using ac impedance measurements. Impedance measurements were performed using the gold band microelectrodes (bare or with nanoparticle film) with a Parstat 2273 electrochemical system (from Princeton Applied Research) at frequencies between 0.1 Hz and 1 MHz at an excitation voltage amplitude of 50 mV. For measurements where the response of the sensor was recorded as a function of time, a fixed frequency of 1 Hz was applied and the absolute value of the impedance was recorded. In a typical experiment, 1 M KCl solution was pumped over the sensor surface for at least 1 min prior to switching the flow solution to the analyte dissolved in 1 M KCl solution for 4 min. After this time, the analyte was removed by switching back to 1 M KCl solution. Analytical Chemistry, Vol. 79, No. 19, October 1, 2007

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RESULTS AND DISCUSSION Although gold nanoparticle chemiresistors have been widely reported for use as gas- or vapor-phase sensors, their use in aqueous ionic solutions has not been reported to date. This may be due to the, initially intuitive, idea that it is not possible to observe small changes in the resistance of gold nanoparticle films that have resistances of the order of 100 kΩ to MΩ when placed in highly conductive ionic solutions, where the ionic solution creates a bypass resistance of only a few Ω. As shown in the present work, by using microscale electrodes and low-frequency ac impedance spectroscopy, it is possible to produce chemiresistor sensors that function even in highly concentrated aqueous salt solutions. The gold nanoparticle thin-film chemiresistors were formed by inkjet printing aqueous solutions of 6-nm-diameter, DMAPstabilized gold nanoparticles onto the gold band microelectrode arrays (Figure 2). The DMAP-stabilized nanoparticles are convenient as the aqueous solutions are stable for several weeks, and the DMAP may be readily displaced by thiol-containing ligands. Prior to printing, the surface of the glass electrodes was functionalized with MPTES. The nanoparticle films printed on the MPTES surface were more reproducible and stable than those prepared on untreated glass. We believe that this is due to the exposed thiol groups of the MPTES binding to the printed gold nanoparticle film, thus anchoring the bottom layer of the nanoparticle film to the glass slide.22-24 As the printed films of DMAP-AuNP remain highly soluble in water, even after drying, and the hexanethiol-functionalizedAuNP thin films are soluble in organic solvents such as hexane, a vaporphase ligand-exchange reaction was used to prepare the hexanethiol-coated nanoparticle films. Thus, the DMAP-AuNP film was exposed to hexanethiol vapor in a sealed container for 30 min followed by rinsing with water to remove excess DMAP. The resulting films after hexanethiol functionalization were no longer water soluble but could be solubilized in hexane solvent. Although the chemiresistor film can be formed by drop-coating a solution of hexanethiol-functionalized gold nanoparticles in hexane, the two-step functionalization process was used in the current work due to the ease of printing aqueous solutions of DMAP-stabilized nanoparticles and the potential versatility in functionalization of the nanoparticles with a variety of different alkanethiol compounds after printing, as well as the additional stabilization afforded to the nanoparticle film by the interaction with the MPTES surface. Alternating current impedance spectroscopy was used to provide information on the frequency dependence of the conductivity of the film. Obtaining the double layer capacitance of the gold band microelectrodes from impedance spectra provided an insight into the parameters that need to be controlled for the chemiresistor to be able to operate in a conductive aqueous solution. Additionally, ac impedance measurements can potentially eliminate small resistance drifts observed during dc measurement, which is a result of migration of ions into the film.3 The resistance of the nanoparticle film is influenced strongly by the capping agent on the nanoparticle. The most notable effect of substituting the DMAP for hexanethiol in the nanoparticle film is that the resistance increased by almost 3 orders of magnitude (from approximately 15.4 kΩ to 5.8 MΩ). The resistivity of the nanoparticle film is exponentially dependent on the separation between the nanoparticles as shown in eq 1. Although the electrical resistivity, F, of a nanoparticle film is influenced by a 7336

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number of parameters,15,16 including particle size, homogeneity, and porosity, the basic electron conduction properties can be described in terms of an electron tunneling mechanism13-15 where

F ) F0 exp[βδ]exp[EA/kT]

(1)

Here β is determined by the effective tunnel barrier of the capping molecules, δ is the barrier width, EA is the activations energy of conductivity, T is the temperature, and k is the Boltzmann constant. Although it is usual to include in eq 1 the factor (exp[EA/kT]), which includes the temperature-dependent activation energy for electron transfer between nanoparticles, it should be noted that for our nanoparticle films we do not observe Arrhenius behavior at temperatures above 100 K. By substituting DMAP for hexanethiol, there is a change in the nanoparticle separation, δ, from 1.22 to 1.76 nm26 (twice the length of the respective functionalizing molecules). Assuming β remains constant upon exchange of ligands, eq 1 would indicate that the resistivity would increase by a factor of 220 due to the increased interparticle separation. This accounts for ∼57% of the experimentally observed change in film resistance. However, the substitution of DMAP with hexanethiol would change the parameter β due to the aromatic ring in DMAP being substituted for an alkane chain in the hexanethiol. As such, the discrepancy between experimentally observed and calculated resistances is likely due to a modification in β27,28 during film functionalization, in addition to the increased separation between the nanoparticles. The dry films gave linear current-potential responses at low potentials, with no hysteresis effects, indicating that ionic conduction is unlikely to be a cause of the low film resistance of the DMAPfunctionalized nanoparticle film compared to the hexanethiolfunctionalized nanoparticle film. Reorganization in the film due to substitution of DMAP with hexanethiol may also change F0 due to differences in film homogeneity, porosity, or both. Typical impedance data obtained for the gold band microelectrodes in 1 M KCl solution with and without the hexanethiolfunctionalized nanoparticle film are shown as Bode plots in Figure 3a (absolute value of the impedance, |Z|) and Figure 3b (phase component). The impedance for a capacitor (ZC) and a resistor (ZR) is given by

ZC ) 1/iωC

(2)

ZR ) R

(3)

where i is the imaginary unit, ω is the angular frequency (rad s-1), and C is the capacitance (F).29 If the nanoparticle film chemiresistor in the presence of electrolyte solution is modeled as a simple resistor-capacitor circuit (RC circuit, Figure 3c), where the double layer capacitance (Cdl) is in series with the electrolyte resistance (RE) and in parallel with the nanoparticle film resistance (RNPF), then the overall impedance of the circuit is given by (26) CS ChemPro 3d. CambridgeSoft Corp., Cambridge MA. (27) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J. Y.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059-1064. (28) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075-5085. (29) Katz, E.; Willner, I. Electroanalysis 2003, 15, 913-947.

(ZRC)-1 )

(

2 + RE iωCdl

)

-1

+ (RNPF)-1

(4)

Given the geometry of the gold band microelectrodes, we estimate that in a 1 M KCl solution, RE < 0.1 Ω. However, from eq 4 it becomes apparent that at low frequencies ZRC is approximately equal to RNPF.

Fitting of eq 4 to the impedance spectra (Figures 3a and b) shows a reasonable fit of the RC circuit model (Figure 3c) to the data, indicating the overall validity of applying the simple model to our sensor.30 Deviation from a perfect fit is expected to be due to electrode inhomogeneity and nanoparticle film porosity. From the fitted curves, the Cdl for the gold band microelectrodes in the absence of the nanoparticle film is 8.9 µF/cm2. The Cdl in the presence of the nanoparticle film is 15.6 µF/cm2. This apparent larger value of the Cdl is qualitatively consistent with the presence of a porous gold nanoparticle film close to the gold band microelectrode effectively increasing the surface area of the electrode. In order for the chemiresistor to function, RNPF must be significantly less than the absolute impedance of Cdl at the operating frequency chosen. In the present case, RNPF is ∼5.8 MΩ, which is less than the absolute impedance of Cdl for frequencies less than ∼10 Hz. The above analysis suggests that the chemiresistor will also function with an applied dc potential. Preliminary experiments with an applied dc potential of 100 mV indicate that this is indeed the case and may be relevant for the development of future sensor devices. The frequencies between 10 and 100 Hz correspond to a range where the Cdl starts to bypass the nanoparticle film resistance. This is seen as a roll off in the impedance spectra in this frequency range. At frequencies above ∼100 Hz, the absolute impedance of the Cdl and the RE is much less than the RNPF, and hence, the capacitive behavior of the sensor dominates. The above model also allows limitations to be placed on the geometry of the sensor and the nanoparticle film resistance. The capacitance of an electrode is given by eq 5, where 0 is the dielectric constant of vacuum,  is the effective dielectric constant of the layer separating the ionic charge and the electrode surface, d is the thickness of the layer separating the ionic charge and the electrode surface, and A is the electrode area.

Cdl ) 0A/d

Figure 3. (a) Bode (|Z|) and (b) Bode (phase) plots obtained using MPTES-coated gold band microelectrodes without (closed squares) and with hexanethiol-AuNP films (open circles) immersed in a 1 M KCl solution. Small discontinuities in the phase plot (b) are an instrument artifact. The solid lines correspond to data fitted to the equivalent circuit model (c) as described in the text.

(5)

Thus, the area of the gold band microelectrodes should be minimized in order to reduce the Cdl to increase the absolute impedance, |ZC|, at the operating frequency. It is also possible to reduce the capacitance by increasing the thickness of the layer separating the ionic charge and the electrode surface, d. For instance, in the present case, the functionalization of the surface of the electrode by an MPTES layer reduces the double layer capacitance, Cdl, from 28 µF/cm2 for the bare gold electrode prior to functionalization to 8.9 µF/cm2 after MPTES functionalization (Figure 4). This is expected to be due to the presence of the selfassembled monolayer of MPTES on the gold electrode. Additionally, the distance between the gold band microelectrodes should be made as small as possible, in order to decrease the nanoparticle film resistance RNPF, below that of the impedance of the Cdl at low frequencies. For the present geometry (5-µm electrode spacing, 5-µm gold electrode width, and 3-mm gold electrode length), this means that, for practical purposes (measurement frequency between 0.1 and 1 Hz), the length of the alkanethiol used should not be longer than octanethiol. However, by further reducing the electrode area and interelectrode distance, it would be possible to use nanoparticle films formed using longer alkanethiol molecules. Figure 5 shows the Bode (|Z| and phase) plots of the sensor in the presence and absence of 1000 parts per million (ppm) (30) ZSimpWin, EChem Software.

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Figure 4. Bode (|Z|) plot obtained using gold band microelectrodes in 1 M KCl solution before pretreatment with MPTES (closed squares) and after pretreatment with MPTES (open circles).

there is a significant increase in the absolute impedance of the sensor in the presence of 1000 ppm dichloromethane. The increase in the impedance at these frequencies is attributed to an increase in electrical resistivity of the nanoparticle film. Nanoparticle film swelling is known to occur, which increases the separation between gold nanoparticles and thus thickness of the film. This was demonstrated by ellipsometry7 and neutron reflectrometry8,31 studies in the presence of organic vapors and by UV-vis spectroscopy19 in the presence of organic solvents. According to the exponential term of eq 1, a small increase in the interparticle distance, δ, will significantly increase the resistivity of the film; thus, it is proposed that film swelling is the dominant mechanism in our chemiresistor sensors. However, this interpretation does need to be viewed cautiously as recent results for chemiresistors in the vapor or gas phase32 indicate that for very small “Au38” monolayer protected clusters, changes to the conduction properties of the nanoparticle films are more complex to interpret and significant effects due to local thermal motion of the monolayer protected clusters may be observed. The relative response of the chemiresistor to the analyte can be represented by

|Z| - |Z0| ∆ |Z| × 100% ) × 100% |Z0| |Z0|

Figure 5. (a) Bode (|Z|) and (b) Bode (phase) plots obtained in 1 M KCl using gold band microelectrodes with hexanethiol-AuNP film in the absence of dichloromethane (closed squares) and in the presence of 1000 ppm dichloromethane (open circles). Inset in (a) shows the magnified region between 0.1 and 10 Hz.

dichloromethane in 1 M KCl solution. As discussed above, at higher frequencies (>100 Hz), the response is largely capacitive and there is no apparent difference in the absolute impedance between these spectra. However, at frequencies below ∼10 Hz, 7338

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(6)

where |Z0| is the absolute impedance of the sensor in the absence of the analyte and |Z| is the absolute impedance in the presence of the analyte. For instance, at a frequency of 1 Hz, the presence of 100 ppm toluene, 1000 ppm dichloromethane, and 100 000 ppm ethanol resulted in a relative impedance change of 50.4 ( 6.7, 18.2 ( 1.4, and 12.1 ( 2.4%, respectively. As an alternative to recording impedance spectra over a wide range of frequencies, the absolute impedance of the sensor can be recorded as a function of time at a fixed frequency so as to monitor the uptake and desorption of the analyte in to and out of the hexanethiol-AuNP film. A frequency of 1 Hz was chosen for these purposes owing to a good balance between time resolution and low capacitive contribution (typically dichloromethane > ethanol. (33) World Health Organization, Guidelines for Drinking-Water Quality, incorporating first addendum to 3rd ed.: Geneva, Switzerland, 2006.

Figure 8. Relative change in impedance of hexanethiol-AuNP films as a function of concentration of toluene (closed squares), dichloromethane (open circles), and ethanol (closed triangles). Error bars represent (1 standard deviation for measurements on four or more electrodes.

The varying response indicates that the present hexanethiol-AuNP film sensor exhibits higher chemical selectivity for nonpolar analytes (toluene) than polar analytes (ethanol). The relative sensitivity of the chemiresistor to the three organic analytes approximately follows the 1-octanol/water partition coefficients (toluene, log P ) 2.28; dichloromethane, log P ) 1.25; ethanol, log P ) -0.28),26 suggesting that the response of the chemiresistor mirrors the partition of organic analyte from aqueous solution into the disordered, hydrophobic hexanethiol phase surrounding the gold nanoparticles. CONCLUSIONS From the data presented above, it is apparent that the three main criteria for producing a gold nanoparticle-based chemiresistor that functions in electrolyte solution are as follows: (1) The electrode area exposed to the electrolyte should be small, such that the double layer capacitance Cdl is suitably small, giving rise to a high impedance at low frequency compared to the overall impedance of the nanoparticle film. Additionally, a thin dielectric layer on the gold band electrode surface can be used to further decrease Cdl. (2) The electrode gap should be small in order to reduce the impedance of the nanoparticle film. (3) The measurements should be performed at a low enough frequency such that the impedance of the double layer capacitor formed by the electrodes is greater than the impedance of the nanoparticle film chemiresistor. By applying these criteria, we have been able to show that it is possible to produce a chemiresistor sensor capable of detecting small organic molecules in high-conductivity aqueous solution. Such nanoparticle-based chemiresistors are attractive due to the ease with which it is possible to produce arrays of nanoparticle thin films using inkjet printing techniques and due to the fact that it is possible to alter the properties of the nanoparticle film by varying the type of alkanethiol capping agent. Our initial work indicates that these chemiresistors have a fast response time (