Swelling of Composite Films at Interfaces - The Journal of Physical

On the basis of Stoney's equation, the surface stress changes were calculated to be 8.2 ± 0.2 (μN m1−)/ppm. After having exposed the MCS for 20 mi...
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2012

J. Phys. Chem. C 2010, 114, 2012–2017

Swelling of Composite Films at Interfaces Masaya Toda,† Yvonne Joseph,‡ and Ru¨diger Berger*,† Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany, and Materials Science Laboratory, Sony Deutschland GmbH, Hedelfinger Strasse 61, D-70327 Stuttgart, Germany ReceiVed: September 10, 2009; ReVised Manuscript ReceiVed: NoVember 30, 2009

Composite materials made of metallic nanoparticles embedded in an organic matrix are highly promising candidates as coatings for novel chemical sensors. Micromechanical cantilever measurements revealed that the Au-nanoparticle terphenyldithiol composite material swells upon dosing with toluene vapor. The mass increase was found to be linear with the toluene vapor concentration, ≈ 40 fg/ppm. Furthermore, significant differences in the mechanical transduction of ≈100 nm thick Au-nanoparticle terphenyldithiol composite material that was prepared on a 3-aminopropyldimethylmonoethoxysilane (APDMES) surface and on a Au surface were observed. The transduction of swelling of the composite film into a mechanical deflection was found to be more efficient for the composite film prepared on the Au surface attributed to covalent binding of the terphenyldithiol molecules with the Au surface. In contrast, the interface of the APDMES layer and the Au-terphenyldithiol composite material is based on electrostatic interaction between the Au nanoparticles and the amino interface. The analysis of the micromechanical cantilever sensor measurements lead to the conclusion that the composite film at the APDMES interface is more mobile compared to a similar film that was prepared on Au. Introduction Composite materials made of metallic nanoparticles in an organic matrix are highly promising candidates as coatings for novel chemical sensors due to a fast response time, high sensitivity, and selectivity upon exposure to solvents.1–3 In particular, composite films have been used as sensing layers for specific vapor detection on interdigitated array electrodes, which are potentially applicable in portable hand-held devices. The mass uptake and the structure of composite films were studied intensively by quartz crystal microbalance (QCM)4,5 and X-ray photoelectron spectroscopy (XPS).6 The adsorption of analyte molecules from the vapor phase into composite films leads mainly to swelling of the composite film in the direction of the surface normal. This swelling increases the tunneling distance between the metal nanoparticles7,8 which increase the electrical resistivity of the composite film, e.g., measured by interdigitated array electrodes. Furthermore, changes in the permittivity of the composite films due to the adsorption of the analyte molecules can result in a decrease in the electrical resistance.6 Recently, Joseph and co-workers reported that the measured electrical resistance of the composite film, made from dodecanedithiol linker molecules and Au nanoparticles, depends significantly on the thickness and structure when exposed to solvent vapors. In particular, a significant decrease in the electrical resistivity was measured at a film thickness close to the percolation threshold upon exposure to analyte molecules, such as toluene, 4-methyl-2-pentanone, and 1-propanol.9 Two possible reasons were proposed by Joseph and co-workers to explain the decrease in the electrical resistivity: (i) The Au nanoparticles within the composite film are pinned to the surface in such a way that the film is hindered from swelling when exposed to analyte molecules. Then the adsorption of analyte * To whom correspondence should be addressed. Tel.: +49-6131-379114. Fax: +49-6131-379-100. E-mail: [email protected]. † Max Planck Institute for Polymer Research. ‡ Sony Deutschland GmbH.

molecules at unoccupied areas on the surface (called bottleneck junctions) is possible, which results in an increase in permittivity and thus a decrease in electrical resistance. (ii) In an alternative model, swelling of the Au-nanoparticle composite film was proposed to happen in all directions, in particular also laterally. Swelling of the nanocomposite material, which is considered to have a structure close to the percolation threshold, decreases the distance of individual nanoparticle islands and result in a decrease in electrical resistance. In contrast to the first model, here the nanoparticle network film is mobile at the interface to the substrate. Up to now, it is not known whether the composite film is pinned to the surface or may swell laterally upon exposure to analyte molecules. The knowledge about the interaction of composite films with surfaces is crucial for applications where swelling of films play a major role, e.g., swelling-induced resistance changes in films situated between electrodes.6 Swelling effects in thin films can be studied by a micromechanical cantilever sensing method10–16 and ellipsometry.17 Ellipsometry is a widely used method to study thickness changes of films under different environmental conditions, even of films made from a nanoparticle network.18 Ellipsometry is sensitive to the thickness and refractive index of the film and requires a suitable model for data analysis. Because the interface of the nanoparticle network film to its substrate is not known, ellipsometry data are difficult to interpret in this respect. Thus, the use of micromechanical cantilevers offers an alternative method for studying swelling effects. Volume expansion of thin films situated on micromechanical cantilever sensors (MCSs) due to swelling is mediated by its interface and is one of the main transduction effects causing a bending of MCSs.19–21 As a model system to study the response of composite materials by means of MCS, we used a film composed of Au nanoparticles in a terphenyldithiol (TPT) organic matrix, wherein TPT molecules act as linker molecules between the Au nanoparticles forming a network. In addition, the binding of analyte molecules

10.1021/jp9087578  2010 American Chemical Society Published on Web 01/12/2010

Swelling of Composite Films at Interfaces

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2013 Experimental Section

Figure 1. Outline of preparation process of Au-nanoparticle TPT networks on MCS. The MCS having an Au film on one side and having a SAM of APDMES on the opposite surface are labeled with A. The reference MCSs having APDMES surfaces on both sides are labeled with B. The deposition cycle to fabricate Au-nanoparticle TPT network films were performed with chips carrying both types of MCS. The fabrication process started by alternatively exposing a MCS chip treated by 3-aminopropyldimethylethoxysilane to the solutions of nanoparticles stabilized by dodecylamine and a linker of terphenyldithiol. At the end of the preparation process, nanocomposites were obtained (from A to C and from B to D). Both types of interfaces were schematically outlined in the insets interface 1 (Au surface) and interface 2 (APDMES on Si).

in the films lead to a change in mass loading which can be detected via a change of MCS’s resonance frequency. Mass loadings down to the femtogram level have been reported.22 Since MCS bending and resonance frequency changes can be recorded in parallel, isotropic volume and mass changes can be determined independently for the same system under identical environmental conditions. The Au-nanoparticle TPT (Au-TPT) network can be immobilized on substrates in different ways: A self-assembled monolayer (SAM) can be formed on a silicon oxide surface having amino end groups, e.g., 3-aminopropyldimethylmonoethoxysilane (APDMES). The surface of a monolayer of APDMES results in electrostatic interaction between the Au nanoparticles and the amino interface (Figure 1, interface 2).23,24 Alternatively, films made from the same composite system can be made on Au surfaces directly. Here, the organic TPT linker molecules can bind covalently to the Au nanoparticles and to the Au surface of the substrate (Figure 1, interface 1). Here, we report on significant differences in the mechanical transduction of a Au-nanoparticle TPT network that was prepared on a APDMES surface and on a Au surface. On the basis of our results, we were able to clarify that the model where the Au-nanoparticle composite film can swell in all directions upon exposure to analyte molecules is more appropriate.

We used MCSs made from silicon as flexible substrates having a thickness h of 1.64 ( 0.02 µm, a length L of 750 ( 20 µm, a width of 90 µm, and a spring constant of 40.1 ( 1.2 mN/m (Octosensis, Micromotive GmbH). A single chip supports eight identical MCSs arranged in an array. The pitch between adjacent MCSs is 250 µm. Always a few out of the eight MCSs on one chip were covered with a shadow mask. Then 2 nm of Cr and 10 nm of Au were thermally evaporated (MCS-010, Baltec). Afterward the entire MCSs chip was immersed into a solution of 5 mL of toluene containing 50 µL of 3-aminopropyldimethylethoxysilane (97%, bought from ABCR GmbH & Co. KG). The solution was heated to 60 °C for 30 min. This process resulted in chemisorptions of APDMES onto the SiOx surfaces only (Figure 1, schematic images A and B). Au-TPT films were deposited on the above-prepared MCS chip using a layer-by-layer self-assembly method (Figure 1) based on a procedure described in detail by Bethell and coworkers:25 The MCS chip was immersed for 15 min into a solution containing dodecylamine-stabilized Au nanoparticles having a diameter of 4.0 ( 0.8 nm.1,26,27 After dipping the MCS chip into pure toluene to wash off weakly adsorbed Au nanoparticles, the MCSs were immersed into a solution of 4,4′terphenyldithiol (25 µmol of TPT in 50 mL of toluene) for 15 min. Details on the synthesis of TPT are given in the literature.6 The latter process step exchanged the dodecylamine stabilizer against the dithiols which linked neighboring Au nanoparticles via Au-S binding. In the vicinity of the Au surface, Au nanoparticles were also linked to the surface. Then the MCS chip was rinsed by dipping it into pure toluene to remove excessive, unbound TPT molecules. The preparation cycle was repeated until the desired film thickness around 100 nm was obtained (here always approximately 25 times). Finally, the MCS chip was gently dried under a nitrogen gas stream. The surface of the composite film was studied by a confocal surface profilometer (µSurf, Nanofocus GmbH). The film thickness was recorded with a surface profiler (Alpha-Step 200, Tencor Instruments, Mountain View, CA) at a scratch in the film which was made by a needle. Such prepared MCSs were mounted into a fluid cell of SCENTRIS (VEECO Instruments) and were sealed by a glass window. Then the cell was filled with dry N2 at a flow of 100 µL/min for a few hours for drying the MCS chip. The deflection of the MCS was read-out via the beam deflection principle. Light from eight individual superluminescence light-emitting diodes (SLEDs) was focused at the end of each of the eight MCSs, respectively. Multiplexing of the light sources allowed sequentially recording the static (bending) and dynamic (resonance frequency) response of each MCS. The fluid cell was filled with toluene vapor at concentrations ranging from 0 ppm to saturated vapor of 28630 ppm at 1 atm (29 hPa, the pressure of saturated vapor at 20 °C) by using a home-built gas mass flow setup (flow controller, Model 80s, McMillan Co.). Different concentrations were obtained by mixing dry N2 gas and N2 gas saturated by toluene vapor at 20 °C. The setup was operated at a constant flow speed of 1.0 L/min. After buffering the vapor, a small amount of the vapor was sucked into the fluid cell by using an electric pump (NMP30, Neuberger Inc.). For the measurements, we adjusted a constant flow of 100 µL/min through the fluid cell if not otherwise stated (see the Supporting Information for a schematic drawing of the gas dosing setup and the corresponding calculation of the vapor concentration). Thus, this flow resulted in an

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Toda et al.

exchange of sample volume twice per minute (volume of the fluidic cell ∼ 30 µL and volume of the connecting tubes ∼ 20 µL). It is expected that a complete exchange of sample volume was obtained after the sample and tube volume was filled 10 times by a volume of 50 µL. On the basis of this assumption the time until stable new environmental conditions were reached is approximately 5 min at a flow of 100 µL/min. Results and Discussion The use of a chip having eight cantilevers allowed the use of selected MCSs within one chip for reference measurements. Two out of the eight MCSs on one chip were protected completely by a shadow mask during the Au-deposition process. Thus, after preparation of the Au-nanoparticle (Au-TPT) network by dip coating, six MCS were obtained having two different interfaces at the top and bottom sides of the MCS (Figure 1C). One side of the MCS had an interface between the Au film and the Au-TPT network and the other side an interface between APDMES self-assembled monolayer and the Au-TPT network. The remaining two MCSs had on both sides the same interface, i.e., the one between the APDMES self-assembled monolayer and the Au-TPT network. These MCSs were used for reference (Figure 1D). Deflection Response of MCS. The measurement cell was filled with a vapor of toluene (20880 ppm) at a flow rate of 100 µL/min (Figure 2A). MCSs having on one side an Au-TPT/ Au interface were found to bend significantly away from the Au surface. The deflection of these MCSs saturated after approximately 15 min. This is most likely due to the gas dosing setup, as Au/organic composite coated chemiresistor devices usually saturate after a few seconds. In the case of MCSs where both sides consist of an Au-TPT/APDMES interface no significant deflection was recorded (Figure 2A). Thus, the swelling of the composite films on both APDMES sides and the transduction of stress to the MCS was similar. These reference measurements showed that the preparation process of the Au-TPT network was independent of the respective MCS sides. Then the MCS chip was exposed three times subsequently to dry N2 flow and saturated toluene vapor at a 5 times higher flow speed of 500 µL/min to test reversibility of the MCS deflection (Figure 2B). All deflections showed similar responses upon exposure to toluene vapor and dry N2. In the second and third cycles of exposure to dry N2 the deflections returned close to the initial positions, i.e., 0 nm, as measured in the beginning of the experiment. Then, the toluene exposure sequence was repeated several times at different toluene vapor concentrations: 715, 1430, 5720, 11440, and 20880 ppm. Between each toluene exposure the fluid cell was exposed to dry N2 flow for 20 min (the measured responses to the individual toluene vapor exposures are provided in the Supporting Information). The differential deflection signal that was calculated from MCSs having identical (reference cantilever) and nonidentical interfaces always saturated within 15 min of exposure to toluene. We found a linear dependence between the saturated differential deflection magnitude and the toluene vapor concentration (Figure 2C). A line fitted to the data revealed a slope of 23.3 ( 0.6 pm/ppm. Typically, deflection changes of MCS can be read-out with an accuracy of 1 nm, resulting in a limit of detection on the order of 40-50 ppm. In general, the measured differential deflections of MCS in different toluene vapors depend on the geometry of the used MCS. In our case, the ratio of the layer thicknesses between the Au-TPT network film (≈100 nm) and the MCS (≈1.64 ( 0.02 µm) is about 0.06.28 Therefore, Stoney’s equation can be

Figure 2. (A) Measured deflection responses of the MCS coated by a composite film of Au/APDMES and the APDMES/APDMES reference MCS. At 2 min the environment of the measurement cell was changed from dry nitrogen to a flow of 20880 ppm toluene vapor in nitrogen. The APDMES/Au MCS bent significantly upon exposure to toluene vapor (>500 nm). In contrast the APDMES/APDMES reference MCS did not bend (5 ( 6 nm). (B) Deflection responses of MCS in three subsequent exposure steps to toluene vapor at an increased vapor flow of 0.5 mL/min (28630 ppm). (C) Exposure of the MCS to various concentrations of toluene vapor at a flow of 0.1 mL/min, resulting in a saturated deflections change of 509 ( 17 nm for 20880 ppm, 273 ( 17 nm for 11440 ppm, 141 ( 17 nm for 5720 ppm, 53 ( 17 nm for 1430 ppm, and 38 ( 14 nm for 715 ppm.

used as an acceptable approximation in order to estimate a geometry-independent value29

∆σ )

Eh2 |δ| 3L2(1 - ν)

where E is the Young’s modulus (ESi ) 170 GPa) and ν is the Poisson’s ratio (ν ) 0.23) of the MCS material, i.e., silicon. L is the effective length (L ) 730 µm), h is the thickness (h ) 1.64 µm), and |δ| is the deflection of the MCS. On the basis of Stoney’s equation, the surface stress changes were calculated to be 8.2 ( 0.2 (µN m1-)/ppm. After having exposed the MCS for 20 min at the highest concentration of 28630 ppm, the surface stress changes were calculated to be 0.24 N/m. These surface stress values are in agreement with the ones reported by Battiston and co-workers for the swelling of polymer materials upon exposure to different concentrations of toluene (0.1-0.6 N/m).30 [Here we want to note that the calculation of

Swelling of Composite Films at Interfaces

Figure 3. (A) Confocal microscopy picture displays the surface of a Au-TPT network film on a MCS that was partially coated by Au film before Au-TPT preparation process. (B) Scratched part on the MCS supporting chip, measured by surface profiler. (C) Measured surface profile across the scratch.

a surface stress change was performed only to derive a geometry-independent value. Stoney’s formula is only valid for a one-side coating which cannot slip laterally relative to the MCS. As already mentioned the Au-nanocomposite film might slip slightly relative to the APDMES interface. Therefore the estimated surface stress change can only be regarded as a coarse value for orientation.] In general, the bending of MCS is influenced by (i) homogeneity, (ii) thickness, and (iii) density differences of the nanocomposite films situated on opposing sides of the MCS. Thus, the above film properties need to be studied and discussed to draw a conclusion regarding the appropriate swelling model of the nanocomposite film. Morphology and Film Thickness. Confocal microscopy of the MCS was performed to study the morphology of the Au-TPT films (Figure 3A). This measurement showed no significant differences in the morphology of the films that was deposited on Au and on APDMES surfaces. Thus, we conclude that there is no significant difference in Au-nanoparticle TPT network homogeneity. To measure the thickness of the nanocomposite films, we removed locally the nanocomposite film by scratching with a needle tip across the Au and APDMES intersecting areas on the MCS support chip (Figure 3B). The thicknesses of composite films on both surfaces were determined at the edges of several scratches using a mechanical profilometer. From the measured surface profiles (Figure 3C) average thicknesses of 97.5 ( 8.2 nm for the Au-TPT films on APDMES and 83.1 ( 12.9 nm for films on Au were determined. Thus, the thickness of composite film on the Au surface is on average 15% thinner compared to the one on the APDMES surface. Mass Loading of the MCS. The mass loading ∆m on the MCS was calculated using

∆m )

(

K 1 1 - 2 2 2 4π n f2 f1

)

where K (40.1 ( 1.2 mN/m) is the spring constant, f1 is the initial resonance frequency of the MCS, f2 is the resonance

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2015 frequency after coating/dosing, and n is an effective function due to the shape of the cantilever (n ) 0.2427 for rectangular cantilevers excited at the first harmonic resonance frequency; n ) 0.006179 for rectangular cantilevers excited at the second harmonic resonance frequency). Composite Film Deposition. The mass loading of MCSs having a one-side Au layer can be compared to the ones having two APDMES surfaces (reference cantilever). We detected the resonance frequencies of the first vibrational mode to be 3883 ( 29 Hz for the Au-coated MCSs (A in Figure 1) and 4050 ( 15 Hz for the reference MCSs (B in Figure 1) in dry N2 flow. After deposition of composite films, the resonance frequencies were shifted to 3775 ( 9 Hz for the Au-coated MCSs (A to C) and 3895 ( 11 Hz for the reference MCSs (B to D). Masses of 17.5 ( 0.3 ng in the case of the one side Au-coated MCSs and of 21.0 ( 0.7 ng for the reference MCS were calculated for the Au-TPT films. The error is given by the standard deviation over the similar MCSs. On the basis of this measurement, we can now separate the mass loading for the individual surfaces. In the case of a MCS having two APDMES surfaces, the mass loading on one side is given by (21.0 ( 0.7 ng)/2 ) 10.5 ( 0.4 ng. Accordingly the mass loading on MCS having a APDMES surface and a Au surface can be calculated by subtracting the mass loading of the a single APDMES surface, resulting in (17.5 ( 0.3) - (10.5 ( 0.4) ) 7.0 ( 0.7 ng. In conclusion, the mass loading of the Au-TPT film on a Au surface can be calculated to be ≈33% less compared to that on the APDMES surface. This observation is consistent with the profilometer data which indicated a thinner Au-TPT film on the Au surface. On the basis of both, the measured mass loading and the measured film thicknesses, we have calculated a Au-nanoparticle TPT film density of 1.25 ( 0.33 (on Au) and 1.60 ( 0.20 g/cm3 (on APDMES), respectively. The two values are comparable considering the estimated error of the measurements. These calculated density values are rather low, indicating a highly porous structure of the Au-nanoparticle TPT film. Toluene Vapor Dosing. After drying the MCS chip for 5 h in dry N2 gas flow, the environment was switched from N2 gas flow to toluene vapor flow of concentrations at 2860, 5720, 11440, and 20880 ppm. Hereby the resonance frequencies of all MCSs were measured at intervals of 15 min at times where equilibrium conditions in the measurement cell are established. At these measurement intervals the resonance frequency was observed to be constant. To further increase the sensitivity of mass detection, we excited the second resonance frequency of the MCS which corresponded to 24.5 ( 0.4 kHz in N2 gas. We found that the resonance frequencies of all coated MCSs by composite films were shifted toward lower frequencies, indicating the mass loading with toluene.31 On the basis of the equilibrium condition, mass changes induced by the toluene uptake in the composite films were calculated (Figure 4). Our data show that the mass increase is linear proportional to the vapor concentration. A line fitted to the data revealed a slope of 39 ( 1 fg/ppm in the case of the one side Au-coated MCSs (C in Figure 1) and 42 ( 2 fg/ppm for the reference MCS (D in Figure 1). Subtracting the response from the identical APDMES side (21 ( 1 fg/ppm) from the one of the Au-coated MCSs results in 18 ( 2 fg/ppm. This difference in toluene loading (approximately 15%) is in agreement with the above finding that the Au film is thinner upon preparation on the Au surface. Discussion From the MCS mass measurement and profilometry we conclude that the composite film of Au-TPT had a slightly

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Figure 4. Measured mass changes of Au-TPT films prepared on Au/ APDMES and APDMES/APDMES MCS in dependence of toluene vapor concentrations.

reduced thickness on the Au-covered side of the MCS compared to the APDMES sides. Consequently, less toluene can be adsorbed on MCS that have a one-side Au coating. This finding agrees with our observation of mass change upon toluene exposure (Figure 4). Thus, we further conclude that the swelling behavior of composite films was almost independent of the two different interfaces that were investigated. Furthermore, the finding of a linear dependence of the MCS deflection in toluene vapor indicated that the film did not exhibit a significant change in mechanical properties of the Au-TPT film at the concentrations studied here. It is known that the content of solvent in polymers can lead to a significant reduction in Young’s modulus leading to a change in the slope of MCS deflection.21,32 More importantly, the measured thickness difference of approximately 15% between the Au-TPT films prepared on an Au and an APDMES surface does not cause the measured MCS deflection upon exposure to toluene vapor. If the thickness difference would be the dominating effect in the measured deflection, the MCS would bend toward the side of the thinner coating, i.e., the interface between the Au and the Au-TPT film. However, our measurements showed a significant deflection toward the opposite side, i.e., the APDMES self-assembled monolayer Au-TPT network side. Thus, we conclude that the type of interface on which the Au-TPT network was prepared played a major role in transduction of swelling into a nanomechanical deflection. The interface of the APDMES layer and the Au-TPT network films is based on electrostatic interaction between the Au nanoparticles and the amino interface (see again Figure 1D). Upon swelling of the nanocomposite film, individual Au nanoparticles can overcome this physical interaction and slide sideways along the interface. In contrast, the interface of the Au layer and the Au-TPT network film is given by covalent binding of the TPT molecules with the Au surface (see again Figure 1C). We assume that this covalent binding prevents or reduces a movement or sliding of the Au-nanoparticle TPT network film prepared on Au surface upon swelling. Consequently the transduction of swelling of the Au-TPT films is more efficient for films prepared on a Au surface. The results of the above measurements of the Au-TPT films on the MCS upon exposure to toluene vapor can now be used to clarify which model is appropriate to explain the decrease in resistance changes of Au-composite films that were made by organic linker molecules and Au nanoparticles. Our interpretation that the Au-nanoparticle film at the APDMES interface is mobile sideways upon swelling is consistent with the picture that the composite network film can swell in all directions upon exposure to analyte molecules.9 Consequently the model where the Au nanoparticles

Toda et al. are assumed to be pinned to the APDMES surface in such a way that the film cannot slide sideways upon exposure to analyte molecules seems to be not appropriate. In summary, our study showed that the interface between a film and a flexible substrate (e.g., a nanomechanical cantilever sensor) plays a major role in transducing a volume change into a mechanical bending. Thus, special attention has to be given to the film preparation of sensor coating materials. The knowledge about the interaction of films with surfaces is crucial for applications where swelling of films play a major role; e.g., the transduction of forces upon swelling of a film into a substrate bending or swelling induced resistance changes in films situated between electrodes. Acknowledgment. We thank Berit Guse for technical support as well as Dr. Tobias Vossmeyer, Dr. Akio Yasuda (SONY), Dr. Elmar Bonaccurso, and Prof. Dr. Butt (MPI-P) for helpful discussions and Dr. W. Ford for correction of the paper. Supporting Information Available: Text describing the gas dosing setup and a figure showing the differential signals between the averaged deflections of AuTPT coated cantilevers and reference cantilevers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H. G.; Wessels, J. M.; Wild, U.; Knop-Gericke, A.; Su, D. S.; Schlogl, R.; Yasuda, A.; Vossmeyer, T. Self-assembled gold nanoparticle/alkanedithiol films: Preparation, electron microscopy, XPS-analysis, charge transport, and vaporsensing properties. J. Phys. Chem. B 2003, 107, 7406–7413. (2) Vossmeyer, T.; Joseph, Y.; Besnard, I.; Harnack, O.; Krasteva, N.; Guse, B.; Nothofer, H. G.; Yasda, A. Gold-nanoparticle/dithiol films as chemical sensors and first steps towards their integration on chip. Proc. SPIE 2004, 5513, 202. (3) Haick, H. Chemical sensors based on molecularly modified metallic nanoparticles. J. Phys. D 2007, 40, 7173–7186. (4) Grate, J. W.; Nelson, D. A.; Skaggs, R. Sorptive behavior of monolayer-protected gold nanoparticle films: Implications for chemical vapor sensing. Anal. Chem. 2003, 75, 1868–1879. (5) Krasteva, N.; Fogel, Y.; Bauer, R. E.; Mullen, K.; Joseph, Y.; Matsuzawa, N.; Yasuda, A.; Vossmeyer, T. Vapor sorption and electrical response of Au-nanoparticle-dendrimer composites. AdV. Func. Mater. 2007, 17, 881–888. (6) Joseph, Y.; Peic, A.; Chen, X. D.; Michl, J.; Vossmeyer, T.; Yasuda, A. Vapor sensitivity of networked gold nanoparticle chemiresistors: Importance of flexibility and resistivity of the interlinkage. J. Phys. Chem. C 2007, 111, 12855–12859. (7) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Novel golddithiol nano-networks with nonmetallic electronic-properties. AdV. Mater. 1995, 7, 795. (8) 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. Monolayers in three dimensions: NMR, SAXS, thermal, and electron hopping studies of alkanethiol stabilized gold clusters. J. Am. Chem. Soc. 1995, 117, 12537–12548. (9) Joseph, Y.; Guse, B.; Vossmeyer, T.; Yasuda, A. Gold nanoparticle/ organic networks as chemiresistor coatings: The effect of film morphology on vapor sensitivity. J. Phys. Chem. C 2008, 112, 12507–12514. (10) Gimzewski, J. K.; Gerber, C.; Meyer, E.; Schlittler, R. R. Observation of a chemical-reaction using a micromechanical sensor. Chem. Phys. Lett. 1994, 217, 589–594. (11) Butt, H. J. A sensitive method to measure changes in the surface stress of solids. J. Colloid Interface Sci. 1996, 180, 251–260. (12) Lang, H. P.; Berger, R.; Battiston, F.; Ramseyer, J. P.; Meyer, E.; Andreoli, C.; Brugger, J.; Vettiger, P.; Despont, M.; Mezzacasa, T.; Scandella, L.; Guntherodt, H. J.; Gerber, C.; Gimzewski, J. K. A chemical sensor based on a micromechanical cantilever array for the identification of gases and vapors. Appl. Phys. A-Mater. Sci. Proc. 1998, 66, S61–S64. (13) Baller, M. K.; Lang, H. P.; Fritz, J.; Gerber, C.; Gimzewski, J. K.; Drechsler, U.; Rothuizen, H.; Despont, M.; Vettiger, P.; Battiston, F. M.; Ramseyer, J. P.; Fornaro, P.; Meyer, E.; Guntherodt, H. J. A cantilever array-based artificial nose. Ultramicroscopy 2000, 82, 1–9.

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