Characterization of the Sensor Response of Gold Nanoparticle

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Characterization of the Sensor Response of Gold Nanoparticle Chemiresistors Edith Chow,* Karl-Heinz Mu¨ller, Erika Davies, Burkhard Raguse, Lech Wieczorek, James S. Cooper, and Lee J. Hubble CSIRO Materials Science and Engineering, Future Manufacturing Flagship, P.O. Box 218, Lindfield, NSW, 2070, Australia ReceiVed: July 1, 2010; ReVised Manuscript ReceiVed: August 1, 2010

The response time of chemiresistors consisting of gold nanoparticles coated with self-assembled monolayers (SAM) of 1-hexanethiol, exposed to different hydrocarbons dissolved in water, is investigated with respect to the analyte SAM/water partition coefficient, the analyte-water flow velocity, and the nanoparticle film thickness. We show experimentally that the response time is proportional to the analyte SAM/water partition coefficient and that for sufficiently large velocities the response time becomes flow velocity independent. The dependence of the response time on film thickness is found to be different in the two different flow velocity regimes examined. We outline in detail a theoretical model that explains our experimental data. Introduction Chemiresistors, comprising a thin sensing layer deposited across two electrodes, have been studied for gas- and vaporphase sensing1-4 and more recently liquid-phase sensing.5-9 These sensors are based on changes in the electrical resistivity of the film induced by exposure to chemical vapors or solutions. Thin films consisting of gold nanoparticles coated with alkanethiolate ligands10-13 are widely used due to their ease of synthesis, stability, and high surface area. The uptake of analytes into the organic layer of the nanoparticle film causes it to swell, increasing the distance between nanoparticles. As the conduction is exponentially dependent on the nanoparticle separation, small changes in the nanoparticle separation can lead to a large change in film resistance. The first demonstration of chemiresistors based on gold nanoparticles was by Wohltjen and Snow10 where it was shown that the octanethiol coating on the nanoparticles was highly sensitive to organic vapors. Vapor-induced electrical resistivity changes were fast and reversible with sensitivities high enough to allow detection of toluene and tetrachloroethylene at the parts per million (ppm) level. Since then, there have been multiple reports exploring film resistance changes upon analyte exposure and their dependence on the type of organic coating on the nanoparticle.12-16 Evans and co-workers14 demonstrated that selectivity can be imparted to the chemiresistor sensors by using gold nanoparticles coated with aromatic thiol derivatives of different functional groups. Nanoparticles coated with a methyl-terminated ligand responded strongly to nonpolar analytes whereas carboxyl-terminated gold nanoparticles showed only small changes to these analytes. Later on, Steinecker et al.17 derived a model relating the responses to the vapor concentration, the vapor/film partition coefficient, and the properties of the nanoparticle and the vapor. The model was able to accurately predict the chemiresistor response for a wide range of vapors. More recently, we also developed a simple theoretical model7 describing the maximum film resistance change in water and related this to the partition coefficient of the analyte between the aqueous solution and the organic coating on the nanoparticle. * To whom correspondence should be addressed. E-mail: edith.chow@ csiro.au. Phone: +61-2-9413-7062.

Rather than relying solely on the magnitude of the sensor response, Matsuno18 demonstrated experimentally for vaporphase measurements that there was also a correlation between the response time of quartz crystal microbalances coated with a polyvinylchloride-based membrane and the affinity of the analyte for the membrane. Higher boiling point vapors were able to partition more strongly into the membrane resulting in a greater decrease in frequency. The sensor response (change in magnitude and response time) was not directly related to the size of the analyte but rather to the extent of analyte partitioning into the membrane. The response time was also highly dependent upon diffusion in the gas phase and was faster upon increasing the flow velocity. Compared to other vapor sensors, the analytes chosen in this study had low volatility and had higher partition coefficients, which meant longer response times could be observed than otherwise possible. In this paper, we characterize the dynamic response of the chemiresistor sensor in the liquid phase by investigating the parameters that influence the response time. We extend our range of analytes used in previous studies (toluene and hexane)5,8 to larger hydrocarbons and to analytes that we predict would have a higher partition coefficient (heptane, octane, isooctane, and pyrene) as well as a chlorinated hydrocarbon (1,3dichlorobenzene) and a long-chained alcohol (1-octanol). We use 1-hexanethiol-coated gold nanoparticles and examine the influence of the analyte-water flow velocity, the analyte partition coefficient, and the film thickness on the dynamic response of the chemiresistor. A theoretical model is developed to describe the chemiresistor behavior. Understanding the dynamic response of the sensor may allow one to design sensors for a particular application or to screen nanoparticle coatings that may be suitable for an array system. Experimental Section Chemicals and Reagents. Gold(III) chloride trihydrate (HAuCl4. · H2O), tetraoctylammonium bromide (TOAB), 4-(dimethylamino)pyridine (DMAP), N-methyl-2-pyrrolidone (NMP), acetonitrile, pentane, hexane, octane, 2,2,4-trimethylpentane (isooctane), pyrene, sodium borohydride, sodium carbonate, potassium chloride, potassium iodide, and iodine were from Sigma-Aldrich, Australia; (3-mercaptopropyl)triethoxysilane

10.1021/jp106055p  2010 American Chemical Society Published on Web 09/17/2010

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Figure 1. Schematic cross-sectional diagram of the flow cell region showing the nanoparticles deposited across two electrodes and the direction of the analyte flow.

(MPTES) and 1-hexanethiol were from Fluka, Australia; sulfuric acid, hydrochloric acid, and nitric acid were from AJAX, Australia; toluene and acetone were from LabScan, Thailand; 1-octanol and 1,3-dichlorobenzene were from BDH, Australia; acetone and heptane were from Chem-Supply, Australia; ethanol was from CSR, Australia; and Deconex OP-120 glass cleaning solution was from Borer Chemie, Switzerland. All reagents were of analytical grade and were used as received. Solutions were prepared with a Milli-Q deionized water system (>18.0 MΩ cm from Millipore, Australia) unless otherwise stated. Gold Band Microelectrodes. An array of six gold band microelectrodes (nominal length of 3 mm and width of 5 µm) with an electrode spacing of 5 µm was prepared on a glass microscope slide (Borofloat 33, Schott, Australia) with use of a standard photolithographic technique as described previously.5 Synthesis of DMAP-Coated Gold Nanoparticles and Film Deposition. Synthesis, inkjet printing, and 1-hexanethiol functionalization of DMAP-coated gold nanoparticles were carried out as previously described.5-8 The thickness of the nanoparticle film was varied by adjusting the number of deposited droplets (1 drop for 70 nm thick films and 10 drops for 130 nm thick films). Analyte Preparation and Electrical Measurements. Saturated aqueous solutions of pentane, hexane, heptane, octane, isooctane, 1-octanol, and 1,3-dichlorobenzene were prepared by adding an aliquot of the analyte (typically 100-200 µL) into a separating funnel containing between 400 and 800 mL of water. The solution was shaken, the layers were allowed to separate, and the saturated solution of the analyte in water was collected. For pyrene, 50 mg was added to 500 mL of water, then the solution was shaken and then filtered through a funnel to collect the saturated pyrene solution. The concentrations of the analytes stated herein are nominal concentrations, obtained from the IUPAC-NIST solubility database.19 The analyte concentration is given as ppm where 1 ppm ) 1 mg/L. Dilute solutions of the analyte were prepared by first preparing the saturated solution of the analyte, and then diluting the analyte solution to the appropriate concentration. For delivery of solutions to the nanoparticle film chemiresistors, the chemiresistor array was placed into a flow cell with a channel 46.5 mm long, 1.6 mm wide, and 1.5 mm high. A schematic cross-sectional area diagram of the flow cell region is shown in Figure 1. Separate glass syringes (50 mL from Sigma-Aldrich) held the analyte and control sample (water). These were driven by syringe pumps (Extech Equipment Pty. Ltd., Australia) along polytetrafluoroethylene tubing to the flow cell at a flow velocity of 4.2 cm s-1. With this particular setup, the full concentration of the analyte does not reach the sensor abruptly but gradually due to laminar flow along the tubing (18 cm in length). The gradual change lasted ∼4 s, and thus the data presented include a 4 s correction. The response of the 1-hexanethiol-functionalized gold nanoparticles films to the

Chow et al. different test analytes was investigated by using AC impedance measurements. Impedance measurements were performed with a PARSTAT 2273 electrochemical system (Princeton Applied Research, USA) at an excitation voltage amplitude of 50 mV and a fixed frequency of 1 Hz. It has been shown previously5 that at 1 Hz, the frequency is low enough that we are measuring the nanoparticle film DC resistance and not the electrode double layer capacitance. For investigation of the effect of flow rate on the analyte response, a different flow cell setup was utilized where the distance between the sensor surface and the point of switching between the water and analyte was minimized (2 cm away). In this case, the full concentration of the analyte was reached within ∼0.5 s. The analyte was held in 50 mL glass syringes and was delivered to the sensor by a multichannel peristaltic pump (Ismatec, Switzerland) at flow velocities between 0.7 and 21.6 cm s-1. An electrical measurement system built in-house was used instead of the PARSTAT 2273 system as it was able to record data with greater time resolution. The electronics biased the electrodes, converted the resultant current to a potential signal, and then amplified that signal. The data were recorded with an eDAQ e-corder at 1000 points/s with eChart 5.5.6 software (eDAQ, Australia). The sensors were biased at 50 mV DC and amplified 1000 times. It was verified that there was no difference in results obtained when using 50 mV DC measurements or 50 mV AC measurements at 1 Hz. Results and Discussion Gold nanoparticle films coated with organic SAMs can function effectively as chemiresistors for detecting small organic analytes in aqueous solution. The electrical resistance R of the chemiresistor depends on the separation gap L between the gold cores of adjacent nanoparticles and can be expressed as20,21

R ) γeβLeEC/kT

(1)

where β is the electron tunneling decay constant, EC is the Coulomb blockade energy, k is the Boltzmann constant, and γ is a constant. In the presence of an analyte, the separation gap L slightly increases with time t from L to L + ∆L(t), due to swelling as the analyte partitions into the SAM that surrounds each nanoparticle. Since22

EC )

L e2 2πε0εr D(D + 2L)

(2)

where D is the nanoparticle diameter, the partitioning analyte affects not only the tunneling term exp(βL) but also the Coulomb blockade term exp(EC/kT) because EC is dependent on L and the relative permittivity εr of the medium that surrounds the gold nanoparticles. Using eqs 1 and 2, we find that for our chemiresistors, when operated at room temperature in water, the contribution from the Coulomb blockade term can be neglected because

β.

1 ∂EC kT ∂L

(3)

where β for alkane SAMs is about 13 nm-1.23,24 Assuming β remains constant and denoting the initial resistance (before

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analyte exposure) by R0, the relative resistance change ∆R(t)/ R0 is given by

∆R(t) R(L + ∆L(t)) - R(L) ≡ ) eβ∆L(t) - 1 R0 R(L)

(4)

For small swelling (i.e. β∆L(t) , 1), one finds

∆R(t) ) β∆L(t) R0

(5)

Previously,7 we have shown that for large times (t f ∞) when the analyte has completely diffused into the film, the maximum gap change ∆L(∞) is given by

∆L(∞) ) ηPSAM/water

Fa,water Fa

(6)

where PSAM/water is the SAM/water partition coefficient of the analyte, Fa,water is the analyte mass density in water (referred to as the analyte concentration further on), and Fa is the mass density of the pure analyte. The quantity η is a geometrical length factor7 which, in the case of touching SAM coatings, can be estimated as

η)

1 (D + L)3 - D3 3 (D + L)2

(7)

and, in the case of fully penetrating SAM coatings, as

η)L

4D2 + 5DL + L2 /3 4D2 + 10DL + L2

(8)

In the case that D . L, eqs 7 and 8 both give η ) L, where L is equal to twice the thickness of the SAM in the case of touching SAM coatings and just equal to the thickness of the SAM in the case of fully penetrating SAM coatings. The measured response of our chemiresistor sensors is characterized by two parameters, the maximum relative resistance change ∆R(∞)/R0 and the response time τ. Shown in Figure 2 is a typical measured response of a 1-hexanethiolcoated gold nanoparticle film in the presence of a 0.5 ppm aqueous octane solution. Initially, water is delivered to the film to establish a stable baseline, before delivering the analyte at the same flow rate V (V ) 4.2 cm s-1). This results in a rise in electrical resistance due to analyte-induced swelling of the nanoparticle film. The flow solution is switched from the analyte back to water after saturation is achieved and a return to the original resistance is observed. The maximum relative resistance change, ∆R(∞)/R0, was measured after saturation of the sensor with the analyte (after 480 s of octane exposure) and was found to be 0.31 for the 0.5 ppm octane solution. For these sensors, we find that although the baseline resistance may drift upon long-term exposure to water, there is only a slight and insignificant decrease in the maximum relative resistance change. In addition to the maximum resistance change ∆R(∞)/R0, the sensor performance is characterized by its response time τ. The response time is determined by fitting the experimental response curves, R(t), to

Figure 2. Response curve, R versus t, of a 1-hexanethiol-coated gold nanoparticle chemiresistor in the presence of 0.5 ppm octane dissolved in water. Measurements were first performed in water, before switching to 0.5 ppm octane at a time of 90 s, and switching back to water at a time of 570 s. The response time τ, initial resistance R0, the resistance upon saturation with the analyte R(∞), and the maximum resistance change ∆R(∞) are indicated in the figure. The solid curve is an exponential fit to the data with use of eq 9. The flow rate across the sensor was 4.2 cm s-1 and the film thickness was 70 nm.

R(t) ) ∆R(∞)(1 - exp(-t/τ)) + R0

(9)

Here, t ) 0 is taken as the time when the flow solution is switched from deionized water to the analyte dissolved in water. From the response curve shown in Figure 2 one extracts a response time τ ) 74 s for the 0.5 ppm octane solution. In the following we investigate how the flow velocity V of the solution through the flow cell and the analyte partition coefficient PSAM/water influence the response time τ and the maximum relative resistance change ∆R(∞)/R0. Shown in Figure 3a is the response time τ versus the flow velocity V for a 1-hexanethiol-coated gold nanoparticle film in the presence of three analytes (hexane, heptane, and octane) with quite different partition coefficients PSAM/water. The data reveal that the response time τ strongly depends on the type of analyte with τ(octane) > τ(heptane) > τ(hexane). The analyte concentrations (2, 0.4, and 0.15 ppm for hexane, heptane, and octane, respectively) were chosen such that ∆R(∞)/R0 values were similar across all analytes and below 0.6. The PSAM/water values were extracted by determining ∆R(∞)/R0 from the response curves using eqs 4 and 6 in combination with eq 7 which assumes touching SAM coatings, as well as eq 8 which assumes fully penetrating SAM coatings. The two PSAM/water values differ by a factor of about 2 in our case. Given the probability that the nanoparticle film is most likely made of partially penetrating SAMs,20,25,26 the values of partition coefficients PSAM/water quoted in this paper were taken as the mean of these two limiting cases. The log PSAM/water values were calculated to be 3.34, 4.00, and 4.42 for hexane, heptane, and octane, respectively. The experimental data in Figure 3a combined with Figure 3b reveal that

τ ∝ PSAM/water

(10)

PSAM/water V

(11)

for V > Vc and that

τ∝

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Chow et al.

∂cWC(z, t) ξSAM ∂cSAM(z, t) ∂2cWC(z, t) ) Ddiff 2 ∂t ξWC ∂t ∂z

(12) where cWC(z,t) is the x-y-averaged analyte concentration in the water channels at depth z of the film, i.e. cWC(z,t) ≡ Fa,water(z,t)/ Fa, and cSAM(z,t) is the concentration of the analyte in the SAMs. The symbols ξSAM and ξWC denote the film volume fractions of the SAMs and of the water channels, respectively, and Ddiff is the diffusion coefficient of the analyte through the water channel network. The second term in eq 12 is a concentration loss term that takes into account the partitioning of the analyte into the SAMs. In the case of rapid partitioning one can assume

∂cSAM ∂cWC ) PSAM/water ∂t ∂t

(13)

and thus the diffusion eq 12 becomes

∂cWC(z, t) Ddiff ∂2cWC(z, t) ) ∂t 1 + ξSAMPSAM/water /ξWC ∂z2

(14) Solving eq 14 for a slab geometry (thin film of thickness d) one finds, using the method of separation of variables,27 that the time τ for the analyte to diffuse into the film is

τ)

Figure 3. (a) Response time τ versus flow velocity V. (b) Log τ versus log PSAM/water at flow velocities of 0.7 and 21.6 cm s-1. The data can be fitted by τ ∝ (PSAM/water)δ where δ ) 0.93 ( 0.04 for V < Vc and δ ) 0.89 ( 0.02 for V > Vc. (c) Maximum relative resistance change ∆R(∞)/R0 versus flow velocity V. In all cases 1-hexanethiol-coated gold nanoparticle films with a thickness of 130 nm were used and tested in the presence of 2 ppm hexane, 0.4 ppm heptane, and 0.15 ppm octane. The dotted line is used as a guide to the eye.

for V < Vc where the critical velocity Vc is 7.2 cm s-1. This response behavior of the sensor can be understood as follows: For V > Vc, the concentration of analyte molecules delivered to the film surface does not get depleted due to the diffusion of analyte into the film. The experimentally observed dependence, i.e. τ ∝ PSAM/water, can be understood by assuming that the analyte molecules diffuse into the film via a network of interconnected water-filled channels of voids and pores that are present between the SAM-coated nanoparticles, and that analyte diffusion along the network of interconnected SAMs is much slower. In this case the diffusion equation for the analyte is

)

(

ξSAM d2 4 P 1 + ξWC SAM/water Ddiff π2

(15)

Thus, for PSAM/water . 1 and since ζSAM /ζWC ≈ 1, one obtains from eq 15 τ ∝ PSAM/water, as seen experimentally in panels a and b of Figure 3. Also, τ in eq 15 is independent of the flow velocity V. For V < Vc, the delivery of analyte molecules to the film surface is too slow to keep up with the speed of diffusion of the analyte into the film. One can understand the experimentally observed τ ∝ PSAM/water/V behavior seen in panels a and b of Figure 3 as follows: The maximum number of analyte molecules, Na,max, that can be taken up by the film is

Na,max ) (ξWC + ξSAMPSAM/water)Fa,watermaAd

(16)

where A is the film surface area and ma is the mass of the analyte molecule. The number of analyte molecules Na that can be delivered per unit time to the film surface is

dNa ∝ VAFa,waterma dt

(17)

From eqs 16 and 17 one can estimate that for PSAM/water . 1 the response time is

τ∝

PSAM/waterd V

(18)

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Figure 5. Measured chemiresistor response time τ versus the thickness d of a film of 1-hexanethiol-coated gold nanoparticles in the presence of 2 ppm heptane dissolved in water at a flow velocity V ) 4.2 cm s-1. Above and below dc (arrow) different dependencies of τ on d are expected. The dotted line on the left is a fit to the data with a slope of one and the dotted line on the right is a fit to the data with a slope of two.

Figure 4. (a) Log τ of 1-hexanethiol-coated gold nanoparticle chemiresistors versus log PSAM/water for various analytes. The analyte concentration was at its saturated concentration level: toluene, 450 ppm; pentane, 48 ppm; hexane, 12 ppm; heptane, 3.4 ppm; octane, 2 ppm, isooctane, 2 ppm; 1-octanol, 75 ppm; 1,3-dichlorobenzene, 100 ppm; and pyrene, 0.13 ppm. Log PSAM/water values were determined from eqs 4, 6, 7, and 8 and the average PSAM/water values calculated from touching and fully penetrating SAM caps were used. Error bars represent (1 standard deviation of 4 or more repeat measurements with different chemiresistors. The dotted line is used as a guide to the eye. (b) Log Poctanol/water values versus log PSAM/water values for various analytes. The dotted line is a linear fit to the data (r2 ) 0.95). The flow rate across the sensor was 4.2 cm s-1 and the film thickness was 130 nm.

which is in agreement with the experimental data of panels a and b of Figure 3. The dependence of τ on the film thickness d, predicted by eqs 15 and 18, will be discussed below. Although varying the flow velocity affects the response time τ for a particular analyte, it does not affect the maximum relative resistance change ∆R(∞)/R0 of the 1-hexanethiol-coated gold nanoparticle film as illustrated in Figure 3c. This is expected because in eq 6, the maximum separation gap (maximum swelling) depends only on the analyte concentration and the product ηPSAM/water. In other words, operating at a slower velocity may result in a slower uptake of the analyte into the film, but this does not affect the maximum amount of analyte that can be taken up by the film. To further investigate the response time τ, a range of other analytes with widely varying log PSAM/water values were examined. Figure 4a shows the response time τ determined in the analyte transport limiting regime (i.e. V < Vc) for a number of alkane and aromatic analytes. The log PSAM/water values were calculated as described previously using eqs 4, 6, 7, and 8. In accordance with eq 11, an approximately linear relationship between τ and PSAM/water was observed. The fact that one observes very different response times for similar-sized analytes

such as 1-octanol and octane further confirms that it is the partition coefficient of the analyte that plays a dominant role in determining the response time and not the analyte size. We also compared the PSAM/water values with Poctanol/water values from the literature28 with the exception of 1-octanol and isooctane. The partitioning of an analyte between octanol and water is typically used as a standard as it is especially useful, for example, for estimating the distribution of drugs within the body.29 There was a strong linear correlation (r2 ) 0.95) between the PSAM/water values and the Poctanol/water values with a slope of 0.96 as illustrated in Figure 4b. Though the PSAM/water values are lower than the Poctanol/water values by a factor of ∼3, it suggests that the observed response time τ of the sensor may be used to predict Poctanol/water for other analytes. Figure 5 illustrates the measured chemiresistor response time τ versus the thickness d of a film of 1-hexanethiol-coated gold nanoparticles in the presence of 2 ppm heptane dissolved in water at a flow velocity V ) 4.2 cm s-1. According to eqs 15 and 18, the response time is τ ∝ d for V < Vc and τ ∝ d2 for V > Vc. Equating eqs 15 and 18 reveals that the critical velocity, Vc, changes with film thickness like V ∝ d-1. The arrow in Figure 5 marks the thickness dc where one expects τ ∝ d for d < dc and τ ∝ d2 for d > dc. The experimental data points in Figure 5 indeed reveal that for small film thickness (d < 100 nm) the data can be fitted well by a linear curve while for a larger thickness (d > 100 nm) a quadratic curve fit is needed. The value dc ) 218 nm (arrow in Figure 5) was determined by using the fact that Vc ) 7.2 cm s-1 for a film of thickness d ) 130 nm (Figure 3a) and that the flow rate in Figure 5 is 4.2 cm s-1. Conclusions The experimental electrical resistance change due to analyte exposure, ∆R(t), can be fitted well by using a 1 - exp(-t/τ) law. At flow velocities, V, below the critical velocity, Vc, the chemiresistor response time τ is analyte transport limited, i.e. τ ∝ V-1, while at flow velocities above Vc the response time τ is found to be velocity independent. In both flow velocity regimes we observe that τ is proportional to the partition coefficient PSAM/water, where we determine the partition coefficient experimentally from ∆R(∞)/R0 measurements. Using a theoretical model we show that for V > Vc, τ is determined by the rate of

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diffusion of the analyte into the chemiresistor film along a network of water channels where the analyte is locally, rapidly partitioning into the SAMs that surround the gold nanoparticles. The dependence of the response time τ on the film thickness agrees with our model predictions. Using τ together with the resistance change ∆R(∞)/R0 should allow one to better identify unknown analytes. References and Notes (1) Franke, M. E.; Koplin, T. J.; Simon, U. Small 2006, 2, 36. (2) Maldonado, S.; Garcia-Berrios, E.; Woodka, M. D.; Brunschwig, B. S.; Lewis, N. S. Sens. Actuators, B 2008, 134, 521. (3) Wang, F.; Yang, Y.; Swager, T. M. Angew. Chem., Int. Ed. 2008, 47, 8394. (4) Ibanez, F. J.; Zamborini, F. P. ACS Nano 2008, 2, 1543. (5) Raguse, B.; Chow, E.; Barton, C. S.; Wieczorek, L. Anal. Chem. 2007, 79, 7333. (6) Chow, E.; Herrmann, J.; Barton, C. S.; Raguse, B.; Wieczorek, L. Anal. Chim. Acta 2009, 632, 135. (7) Raguse, B.; Barton, C. S.; Mu¨ller, K.-H.; Chow, E.; Wieczorek, L. J. Phys. Chem. C 2009, 113, 15390. (8) Chow, E.; Gengenbach, T. R.; Wieczorek, L.; Raguse, B. Sens. Actuators, B 2010, 143, 704. (9) Cooper, J. S.; Raguse, B.; Chow, E.; Hubble, L.; Muller, K. H.; Wieczorek, L. Anal. Chem. 2010, 82, 3788. (10) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856. (11) Joseph, Y.; Guse, B.; Vossmeyer, T.; Yasuda, A. J. Phys. Chem. C 2008, 112, 12507. (12) Ahn, H. J.; Chandekar, A.; Kang, B. W.; Sung, C. M.; Whitten, J. E. J. Macromol. Sci., Part A: Pure Appl. Chem. 2005, A42, 1477. (13) Zhang, H. L.; Evans, S. D.; Henderson, J. R.; Miles, R. E.; Shen, T. H. Nanotechnology 2002, 13, 439.

Chow et al. (14) Evans, S. D.; Johnson, S. R.; Cheng, Y. L. L.; Shen, T. H. J. Mater. Chem. 2000, 10, 183. (15) Kim, Y. J.; Yang, Y. S.; Ha, S. C.; Cho, S. M.; Kim, Y. S.; Kim, H. Y.; Yang, H.; Kim, Y. T. Sens. Actuators, B 2005, 106, 189. (16) Krasteva, N.; Fogel, Y.; Bauer, R. E.; Mullen, K.; Joseph, Y.; Matsuzawa, N.; Yasuda, A.; Vossmeyer, T. AdV. Funct. Mater. 2007, 17, 881. (17) Steinecker, W. H.; Rowe, M. P.; Zellers, E. T. Anal. Chem. 2007, 79, 4977. (18) Matsuno, G. Sens. Mater. 1999, 11, 401. (19) IUPAC-NIST Solubility Database, 2007. (20) 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. (21) Mu¨ller, K.-H.; Herrmann, J.; Wei, G.; Raguse, B.; Wieczorek, L. J. Phys. Chem. C 2009, 113, 18027. (22) Abeles, B.; Sheng, P.; Coutts, M. D.; Arie, Y. AdV. Phys. 1975, 24, 407. (23) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2000, 122, 2970. (24) Mu¨ller, K.-H.; Herrmann, J.; Raguse, B.; Baxter, G.; Reda, T. Phys. ReV. B 2002, 66, 075417. (25) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem.sEur. J. 1996, 2, 359. (26) 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, 17. (27) Holman, J. P. Heat Transfer; McGraw-Hill, Inc.: New York, 1982. (28) Ruelle, P. Chemosphere 2000, 40, 457. (29) Moriguchi, I.; Hirono, S.; Liu, Q.; Nakagome, I.; Matsushita, Y. Chem. Pharm. Bull. 1992, 40, 127.

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