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Gold Nanoparticle Chemiresistor Sensors in Aqueous Solution: Comparison of Hydrophobic and Hydrophilic Nanoparticle Films Burkhard Raguse,* Christopher S. Barton, Karl-Heinz Mu¨ller, Edith Chow, and Lech Wieczorek CSIRO, Materials Science and Engineering, Future Manufacturing Flagship, P.O. Box 218, Lindfield, New South Wales, 2070 Australia ReceiVed: April 15, 2009; ReVised Manuscript ReceiVed: June 18, 2009
Chemiresistor sensors based on thin films of gold nanoparticles capped with hydrophobic hexanethiol or hydrophilic 6-hydroxyhexanethiol self-assembled monolayers are shown to respond to organic analytes in aqueous solution. Although considerable swelling of the 6-hydroxyhexanethiol-capped nanoparticle film by water occurred, the 6-hydroxyhexanethiol-capped nanoparticle film still responded toward the presence of organic analytes in aqueous solution. The response toward nonpolar analytes (toluene, hexane, dichloromethane) was reduced for the 6-hydroxyhexanethiol-capped chemiresistor compared to the hexanethiol-capped chemiresistor. However, for polar analytes such as ethanol the response sensitivity was reversed. A simple theoretical model describing the chemiresistor response in water is presented and was used to determine the partition coefficients between the nanoparticle film and water for ethanol and toluene. Introduction Chemiresistors are an important form of electronic sensor technology.1 However, to date, chemiresistors have been applied principally as gas- or vapor-phase sensors. In particular chemiresistors involving gold nanoparticles capped with organic surface layers have been shown to be highly sensitive to volatile organic compounds, e.g., toluene, in the vapor phase.2-10 A feature of gold nanoparticle thin-film chemiresistors is that the sensing surface can be readily tailored through selective functionalization of the gold nanoparticles with different molecular capping agents through thiol-based self-assembled monolayer (SAM) formation.11-16 Arrayed sensor systems involving multiple electrode elements, each with different sensor surface functionalities, have been shown to provide specificity in the analysis of volatile organic compounds,17-20 particularly when combined with pattern recognition analysis techniques such as principal component analysis and artificial neural networks.21,22 Chemiresistors sense the presence of analytes through a change in resistance of the sensor element which is a consequence of a modification of its structural and/or electronic properties. For gold nanoparticle film chemiresistors the active element is the nanoparticle film, which is swelled through partitioning of analyte from the surrounding medium into the organic capping layer surrounding the nanoparticles. This swelling results in an increased separation between neighboring nanoparticles. Conduction between closely separated nanoparticles is due to electron tunnelling, which is exponentially dependent on the nanoparticle separation. Thus, small changes in nanoparticle separation can lead to a large change in film resistance. Recently we have shown that by miniaturizing the electrodes, chemiresistors based on gold nanoparticle films deposited on microelectrodes can be used to directly detect the presence of organic analytes in the aqueous phase.23 The gold nanoparticles were capped with hydrophobic hexanethiol SAMs. Small * To whom correspondence should be addressed.
[email protected]. Tel: +61-2-9413-7549.
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organic analytes such as toluene, dichloromethane, and ethanol were detected in an ionic solution of 1 M KCl down to 0.1, 10, and 3000 ppm, respectively. In the present work we compare the effect of varying the polarity of the headgroup of the SAM on the sensor response toward organic analytes in aqueous solution. In particular, the response of gold nanoparticle films capped with SAMs of hydrophobic hexanethiol are compared with gold nanoparticle films capped with the more hydrophilic 6-hydroxyhexanethiol SAM. The response of the two types of chemiresistors toward immersion in water and toward a range of organic analytes of widely varying octanol/water partition coefficients (i.e., toluene, hexane, dichloromethane, and ethanol) are investigated. A semiquantitative model for the change in film resistance based on film swelling and the partition coefficient of the analyte between aqueous solution and the organic phase of the SAM is presented. Experimental Section Chemicals and Reagents. Gold(III) chloride trihydrate (HAuCl4 · 3H2O), tetraoctylammonium bromide (TOAB), 4-(dimethylamino)pyridine (DMAP), sodium borohydride, N-methyl2-pyrrolidinone (NMP), 6-hydroxyhexanethiol, sodium carbonate, potassium chloride, potassium iodide, and iodine were purchased from Sigma-Aldrich. (3-Mercaptopropyl)triethoxysilane (MPTES) and 1-hexanethiol were obtained from Fluka. All reagents are of analytical grade and were used as received. Aqueous solutions were prepared using Nanopure deionized (DI) water (>17.4 MΩ cm Barnstead, IA). Stock solutions of dichloromethane (10% v/v and 5% v/v) and toluene (1% v/v, and 0.1% v/v) were prepared in ethanol. Dilute solutions for analytical measurement were prepared by serial dilution of the stock solution in DI water. DMAP-AuNP Synthesis. Gold nanoparticles were synthesized following the Brust method25 and transferred to the aqueous phase containing DMAP following the method by Gittins and Caruso.26 Briefly, a solution of HAuCl4 was added to a stirred solution of TOAB in toluene. Stirring was continued for 10 min
10.1021/jp9034453 CCC: $40.75 Published 2009 by the American Chemical Society Published on Web 07/31/2009
Gold Nanoparticle Chemiresistor Sensors in Aqueous Solution and was followed by the addition 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 twice with sulfuric acid, twice with sodium carbonate, and twice with water. For the phase transfer of the nanoparticles from the organic to the aqueous phase, an aqueous solution 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). NMP was added to the DMAP-AuNP solution to give a 4% v/v solution. All ink solutions were filtered with a 0.22 µm Millex GS filter unit (Millipore, Australia) prior to use. The diameter of the DMAP-AuNP was determined using dynamic light scattering (High Performance Particle Sizer, Malvern Instruments, Worcestershire, UK) and found to be 6 ( 2 nm. The exchange of the DMAP ligand with hexanethiol or 6-hydroxyhexanethiol was performed after ink deposition on the gold band microelectrodes. Fabrication of Gold Band Microelectrodes. Gold band microelectrodes were fabricated as described in detail previously.23 Briefly, electrodes with nominal dimensions of 3 mm length and 5 µm width, with an electrode gap of 5 µm, were prepared on Borofloat 33 glass microscope slides (purchased from Schott, Australia). Vacuum chamber deposition was used to coat the slides with chromium (10 nm) and then gold (200 nm), and photolithography and wet-etching with gold (2:1 KI/ I2) and chrome etchant (Extran, Merck KGaA, Germany) were used to pattern the slides. A final 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 polytetrafluoroethylene (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 long) along the center of the slides where the gold band microelectrodes are situated. The slides were then baked in a laboratory oven for 10 min at 200 °C to thoroughly cross-link the final 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. Thus, the glass slides were immersed in a solution containing 2% v/v 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, Germany). 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, 10 drops of 1% w/v DMAP-AuNP solution containing 4% v/v NMP were inkjet-printed over each of the six band microelectrodes on the glass slide and the solvent was allowed to evaporate. The 10-drop deposition process resulted in flat circular disks of nanoparticle film of diameter 280 ( 10 µm as measured with optical microscopy and AFM. After 2 h, the DMAP-AuNP deposited electrodes were exchanged with 1-hexanethiol or 6-hydroxyhexanethiol by exposing the microelectrodes to 1-hexanethiol or 6-hydroxy-
J. Phys. Chem. C, Vol. 113, No. 34, 2009 15391 hexanethiol vapor for 60 min. Subsequently, the microelectrode array was removed from the thiol vapor, allowed to stand for a further 30 min, and then rinsed with water. Analyte Determination. For delivery of solutions to the nanoparticle film chemiresistors, the glass slide was placed into a flow cell.23 The flow channel was 46.5 mm long, 1.6 mm wide, and 2.0 mm high. The response of the 1-hexanethiol- and 6-hydroxyhexanethiol-functionalized nanoparticle thin-film chemiresistors to different concentrations of toluene, hexane, dichloromethane, and ethanol in aqueous solution was investigated using AC impedance measurements. Impedance measurements were performed using a PARSTAT 2273 electrochemical system (from Princeton Applied Research, Oak Ride, TN) at frequencies between 0.1 Hz and 10 kHz 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 0.8 Hz was applied and the absolute value of the impedance was recorded. In a typical experiment, DI water was pumped over the sensor surface for at least 1 min prior to switching the flow solution to the analyte dissolved in DI water for 2 min. After this time, the analyte was removed by switching back to DI water. Results and Discussion We have previously demonstrated that gold nanoparticle thinfilms, where the gold nanoparticles had been functionalized with hydrophobic hexanethiol monolayers, function effectively as chemiresistors in aqueous solution to detect small organic molecules.23,24 The mechanism of the chemiresistor function is believed to be due to partitioning of the organic molecules from the aqueous phase into the hexanethiol layer, causing an overall swelling of the nanoparticle film, thereby decreasing the electrical conductivity of the film. The hydrophobic hexanethiol coating meant that the nanoparticle film was insoluble in the aqueous solution, and indeed there was only a small change in the conductivity of the hexanethiol coated gold nanoparticle film on immersion in the aqueous solution, indicating that the film did not swell appreciably with water. In order to extend potential utility of the chemiresistors for sensing applications, for example as part of an array of chemiresistors (i.e., the development of “electronic tongue” type sensor arrays analogous to the vapor-phase sensor arrays),21,22 it will be important to be able to functionalize the gold nanoparticles with ligands containing a variety of functional groups, including hydrophilic moieties. It is therefore desirable to determine whether it is possible to form nanoparticle films with hydrophilic ligands that are stable in aqueous solution, whether such films swell in the presence of water and whether they are subsequently still able to function as chemiresistors toward small organic molecules. To this end, in the current work, we compare the electrical and sensing properties of hexanethioland 6-hydroxyhexanethiol-capped gold nanoparticle thin-film chemiresistors. The thin-film chemiresistors were produced as previously described using 6 nm diameter DMAP-stabilized gold nanoparticles. DMAP-stabilized gold nanoparticles are versatile precursors for thiol-functionalized gold nanoparticles as the DMAP ligand is readily displaced by a variety of thiols under mild conditions.27 Another advantage is that the direct synthesis of each desired thiol-functionalized gold nanoparticle, which is time-consuming, is not necessary. The gold nanoparticle films were formed by inkjet printing a 1% w/v solution of 6 nm diameter DMAP-stabilized gold nanoparticle solution onto a band microelectrode array
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Figure 1. (a) Photograph of a dried circular inkjet printed nanoparticle film (10 drops of 1% w/v DMAP-AuNP solution containing 4% v/v NMP). The MPTES-treated gold band microelectrodes (3 mm length, 5 µm width, and 5 µm separation) are visible as the two vertical bands in the middle of the photograph. The microelectrodes extend above and below the image into large area gold contact pads. The dashed rectangle represents the flow channel region. (b) Schematic diagram showing the functionalization of DMAP-stabilized gold nanoparticles with hexanethiol or 6-hydroxyhexanethiol. The DMAP-stabilized gold nanoparticles were exposed to thiol vapor for 1 h.
(Figure 1). In order to minimize the formation of the “coffeering” phenomenon,28 where the majority of the nonvolatile component is left as a ringlike deposit at the circumference of the drop on drying of the solvent, 4% v/v NMP was added as a cosolvent. In the absence of NMP, pinning of the contact line and a faster evaporative loss from the edge of a drop than from the center leads to an outward flow of the solute and a “coffeering”-like deposit. The use of a mixture of solvents,29,30 where one has a higher boiling point and a lower surface tension than the other, prevents the outward flow of solute. As the evapora-
tion proceeds, a Marangoni flow is generated from regions of low surface tension to high surface tension (from the edge to the center).31 This results in almost complete suppression of the “coffee ring” (Figure 1). In order to functionalize the gold nanoparticles, the films were exposed to either of the two thiol vapors (hexanethiol or 6-hydroxyhexanethiol) in a sealed container for 1 h. The DMAP coating on the gold nanoparticles is readily displaced by thiol ligands and exposure of the DMAP-Au nanoparticles to the two different thiols in the vapor phase yielded the thiol-capped
Gold Nanoparticle Chemiresistor Sensors in Aqueous Solution
Figure 2. Circuit model for the sensor including the nanoparticle film resistance RNPF, and the electrode double-layer capacitance Cdl in series with the solution resistance RS.
nanoparticle films. The films were subsequently washed with DI water to remove excess DMAP and NMP. Sensors were used within 2 days of preparation to minimize any changes in the nanoparticle film response over time; however, the functionalized nanoparticle films were found to be functional after more then 5 days with only a slightly diminished sensor response. Substitution of the DMAP with hexanethiol or 6-hydroxyhexanethiol resulted in the nanoparticle film resistance changing from ∼30 kΩ to 1.91 and 1.25 MΩ for hexanethiol and 6-hydroxyhexanethiol, respectively. The resistance change is due primarily to an increase in core-core separation due to the longer hexane capping layer compared to the shorter DMAP molecules (vide infra). Additionally, some rearrangement of the nanoparticle film structure might also affect the electrical properties.23 In order for the gold nanoparticle film to function as a chemiresistor, it must be possible to measure small changes in the resistance of the nanoparticle film due to incorporation of analyte molecules. The typical resistance of the nanoparticle films investigated here are ∼1-2 MΩ, whereas the electrolyte solution resistance (RS in Figure 2) into which the nanoparticle film is immersed may be
