Dendrimer Films

In the present work, specular neutron reflectometry was used to study the sorption of D2O and perdeuterated toluene in self-assembled Au ...
0 downloads 0 Views 207KB Size
Download PDF
7754

Langmuir 2003, 19, 7754-7760

Vapor Sorption in Self-Assembled Gold Nanoparticle/ Dendrimer Films Studied by Specular Neutron Reflectometry† Nadejda Krasteva,‡ Rumen Krustev,*,§,| Akio Yasuda,‡ and Tobias Vossmeyer*,‡ Materials Science Laboratories, Sony International (Europe) GmbH, Hedelfingerstrasse 61, D-70327 Stuttgart, Germany, Hahn-Meitner Institute, Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany, and Max Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, D-14476 Golm, Germany Received October 24, 2002. In Final Form: January 29, 2003 In the present work, specular neutron reflectometry was used to study the sorption of D2O and perdeuterated toluene in self-assembled Au nanoparticle/poly(amidoamine) composite films. The method provides information about analyte-induced film swelling and analyte distribution profiles across the films. Our data suggest that film swelling upon exposure to the analytes is negligible within the resolution of the method. Strong differences are observed between the distribution profiles of D2O and toluene-d8. Water penetrates into the film with an exponentially decaying concentration profile. In contrast, toluened8 forms a thin “wetting” layer on top of the film, while the bulk of the film remains essentially analyte free. These results are explained by taking into account the chemical structure of the dendritic polymer used for film preparation.

Introduction In two recent publications we described the preparation of self-assembled Au nanoparticle/dendrimer composite films and their use as chemiresistor sensors for the detection of volatile organic compounds.1,2 These films respond sensitively, quickly, and reversibly with an increase of their resistance to exposure to solvent vapors. However, while such vapor sensitivity of Au nanoparticle/ organic films has been documented by several publications,1-5 the molecular mechanism of this property is poorly understood. Currently it is believed that the charge transport in these films occurs through an activated tunneling mechanism.7-9 Accordingly, vapor sorption may influence the charge transport by two main effects: (i) swelling of the organic film component, which increases the tunneling distances between the nanoparticles, and (ii) variations of the permittivity of the medium surrounding the Au nanoparticles. Both effects require the diffusion of vapor molecules into the films. Thus, for * Corresponding authors, e-mail [email protected] and vossmeyer@ sony.de. † Part of the Langmuir special issue dedicated to neutron reflectometry. ‡ Sony International (Europe) GmbH. § Hahn-Meitner Institute, Berlin. | Max Planck Institute of Colloids and Interfaces. (1) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R.; Mu¨llen, K.; Yasuda, A. Adv. Mater. 2002, 14, 238. (2) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Mu¨llen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551. (3) Evans, S. D.; Johnson, S. R.; Cheng, Y. L.; Shen, T. J. Mater. Chem. 2000, 10, 183. (4) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856. (5) Han, L.; Daniel, D. R.; Maye, M. M.; Zhong, C.-J. Anal. Chem. 2001, 73, 4441. (6) Han, L.; Maye, M.; Leibowitz, F.; Ly, N. K.; Zhong, C.-J. J. Mater. Chem. 2001, 11, 1258. (7) Abeles, B.; Sheng, P.; Coutts, M. D.; Arie, Y. Adv. Phys. 1975, 24, 407. (8) Neugebauer, C. A.; Webb, M. B. J. Appl. Phys. 1962, 33, 74. (9) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson Jr, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537.

enabling a quantitative understanding of the films’ vapor sensitivity, it is necessary to know the distribution of vapor molecules within the film. Several studies on analyte sorption in composite films of surface-functionalized Au nanoparticles have been performed by using microgravimetry5,6,10 and ellipsometry.3,11 These methods give information about the total amount of vapor molecules interacting with the film but are not capable of providing information about the vapor distribution within the sensitive layer. Here we report on the use of specular neutron reflectivity for studying the swelling and vapor distribution profiles in thin selfassembled Au nanoparticle/dendrimer composite films. Our aim is to gain a quantitative understanding of the processes that affect the electrical conductivity of such films when using them as vapor-sensing chemiresistor devices. To the best of our knowledge, neutron reflectometry is the only technique that provides information about the analyte distribution within the film. Neutron reflectivity measurements are gaining increasing importance for investigating thin polymer layers deposited on solid substrates.12-15 The method has been used for analyzing the structure and swelling of polyelectrolyte layers,12,16,17 composition variations in thin polymer films caused by chemical reactions18 or solvent diffusion,19 and the component distribution in polymerbased composite films used for electronic nose applications.20 Neutron reflectivity experiments provide struc(10) Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. L.; Malik, M. A.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 8958. (11) Zhang, H.-L.; Evans, S. D.; Henderson, J. I.; Miles, R. E.; Shen, T. H. Nanotechnology 2002, 13, 439. (12) Plech, A.; Salditt, T.; Mu¨nster, C.; Peisl, J. J. Colloid Interface Sci. 2000, 223, 74. (13) Russell, T. P. Physica B 1996, 221, 267. (14) Penfold, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 139. (15) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171. (16) Decher, G. Science 1997, 277, 1232. (17) Steitz, R.; Leiner, V.; Siebrecht, R.; v. Klitzing, R. Colloids Surf., A 2000, 163, 63. (18) Glidle, A.; Bailey, L.; Hadyoon, C. S.; Hillman, A. R.; Jackson, A.; Ryder, K. S.; Saville, P. M.; Swann, M. J.; Webster, J. R. P.; Wilson, R. W.; Cooper, J. M. Anal. Chem. 2001, 73, 5596.

10.1021/la0267488 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/13/2003

Vapor Sorption in Dendrimer Films

Langmuir, Vol. 19, No. 19, 2003 7755

tural information in the direction z perpendicular to the film surface over a length scale of 1-500 nm with a resolution down to about 0.2 nm. Neutrons possess strong penetration power as a consequence of their weak interaction with almost any material. Their sensitivity to different isotopes allows measuring the composition profiles along the z direction. An important advantage of the methodology as compared to X-ray methods is the absence of sample damage, even upon prolonged exposure to the neutron beam. The specular neutron reflectivity method is based on the variation in the specular reflection at the interface between two phases with the wave vector transfer Q ) 4π sin θ/λ, where θ is the angle of incidence of the neutron beam and λ is the neutron wavelength. The variation of the reflectivity R with Q (the reflectivity profile) depends on the neutron refractive index profile. The neutron refractive index n is defined as n ) 1 - λ2F(z)/2π in cases where the absorption of neutrons is negligible and depends on the scattering length density (SLD) profile F(z). The SLD is given by F ) ∑ibiNi, where bi is the neutron scattering length and Ni is the number density of the ith sort of nuclei. The reflectivity can be approximated by13,21

R(Q) )

16π2 Q4

[∫

+∞

-∞

]

∂F(z) iQz e dz ∂z

2

(1)

The reflectivity from the sample decreases proportional to Q4. The integral in eq 1 gives the dependence of R on the Fourier transform of the F(z) profile perpendicular to the reflecting interface. When a layer of a certain material is deposited onto the bare interface, the reflectivity profile R(Q) becomes an oscillating function with a period of oscillations depending on the thickness of the layer. At large Q, the layer thickness d can be estimated from the ∆Q spacing of the minima of two neighboring interference fringes that arise from the superposition of the reflection from both interfaces of the layer by d ) 2π/∆Q. The amplitude of the fringes depends on the change of F(z) across the interface. Damping of these fringes indicates high roughness of the interface or smaller contrast between the film and the substrate.15 The information that can be extracted in a single experiment from a neutron reflectivity curve includes the film thickness, the surface roughness at the film/gas interface, and the scattering length density profile across the film.13,21 The latter can be easily converted into a concentration profile and thus gives information about the species distribution within the film. Experimental Section Film Fabrication. The Au nanoparticle/poly(amidoamine) dendrimer (Au/PAMAM) films were deposited via layer-by-layer self-assembly from nanoparticle and dendrimer solutions. As the nanoparticle component, dodecylamine-stabilized gold nanoparticles were used. The nanoparticles were synthesized similarly to the procedure used by Leff et al.22 Transmission electron microscopy (TEM) analysis revealed that this synthesis produced crystalline particles with an average diameter of about 4 nm and a standard deviation of about 30%. Third generation poly(amidoamine) dendrimers (Starburst; PAMAM, Aldrich) were used as the organic component, which served to interlink the nanoparticles. The structural formula of the PAMAM dendrimer is shown in Figure 1. (19) Glidle, A.; Swann, M. J.; Gadegaard, N.; Cooper, J. M. Physica B 2000, 276-278, 359. (20) Swann, M. J.; Glidle, A.; Gadegaard, N.; Cui, L.; Barker, J. R.; Cooper, J. M. Physica B 2000, 276-278, 357. (21) Zhou, X.-L.; Chen, S.-H. Phys. Rep. 1995, 257, 223. (22) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723.

Figure 1. Structural formula of the third generation poly(amidoamine) (PAMAM) dendrimer used to interlink the Au nanoparticles. The films were deposited onto glass microscope slides (76 × 25 × 1 mm). Prior to the deposition the plasma-cleaned substrates were immersed into a solution of aminopropyldimethylethoxysilane in toluene (1% v/v) and kept at 60 °C for 30 min. The films were assembled via repetitive exposure of the substrates to Au nanoparticle and dendrimer solutions. This was done by first dipping the substrates for 15 min into a solution of Au nanoparticles in toluene,23 washing with toluene, and then dipping the substrates for another 15 min into the PAMAM solution in methanol (ca. 10 mg of dendrimer in 5 mL of solvent). The treatment with particle and dendrimer solutions was performed 14 times. Accordingly, the film deposition was finished by treating the sample with the PAMAM solution. Atomic Force Microscopy (AFM). A Digital Instruments atomic force microscope Dimension 3100 with a Nanoscope IV controller was used to investigate the thickness and the morphology of the Au/PAMAM films. The film thickness was determined by imaging the edges of several scratches. All images were acquired using the “Tapping Mode” function. Neutron Reflectivity. Neutron reflectivity measurements were performed at the V6 reflectometer at the Hahn-Meitner Institute, Berlin. A detailed description of the instrument is given elsewhere.24 The reflectometer had a vertical scattering plane. The neutron wavelength was selected by a graphite monochromator in the incident white beam and fixed at λ ) 4.66 Å. The incoming beam geometry was defined by two adjustable slits between the monochromator and the sample. The scattered neutrons were recorded with a 3He detector. The incoming intensity was measured simultaneously using a monitor placed directly in the incident beam path. The reflectivity, which is the ratio between the detector counts and the monitor counts, was measured as a function of the reflection angle θ. The measurements were performed in a θ/2θ geometry (detector angle) from Q ) 0.0047 to 0.12 Å-1. Each scan lasted typically 5-7 h of beam time. The resolution was set by the slit system on the incident side to ∆Q ) 0.001 Å-1 for Q e 0.0518 Å-1 and ∆Q ) 0.002 Å-1 otherwise. A beam of rectangular cross section 0.5 × 40 mm2 incident on the sample was set by the slit system on the sample side and was not changed during the data acquisition. The background signal was collected simultaneously with a 3He counter offset from the specular position by 0.44° toward larger scattering angle. It was directly subtracted from the specular signal to obtain the background-corrected intensity. The samples used were comparatively small and thus overillu(23) The Au nanoparticle solution was prepared by diluting an aliquot of a stock particle dispersion in toluene until the UV-vis absorbance of the solution reached 0.4 at λ ) 514 nm and 2 mm path length. (24) Mezei, F.; Goloub, R.; Klose, F.; Toews, H. Physica B 1995, 213/ 214, 898.

7756

Langmuir, Vol. 19, No. 19, 2003

Figure 2. Schematic drawing of the experimental chamber showing the mounting of the sample and the geometry of the measurement setup. minated at small θ angles. The reflectivity data were footprint corrected for the varying flux on the sample as θ increased and normalized to the measured incident intensity to obtain the reflectivity R(Q). A home-built gastight experimental chamber (Figure 2) for neutron reflectivity studies on solid/gas interfaces was used. The chamber consisted of a cylindrical aluminum body of about 500 cm3 volume. The neutron beam passed through two windows of flat, 1 mm thick aluminum, fixed opposite to one another on the chamber walls. The chamber cover was equipped with an inlet and an outlet connection for flushing the chamber with dry nitrogen or with test vapors. Sample mounting was done on a PTFE support with a geometry that allowed samples with different sizes and shapes to be fixed in the chamber. The temperature in the chamber was kept constant (precision (0.5 °C) with a water-cooled meander at the bottom of the cell connected to a thermostat. The temperature was controlled using a thermocouple placed in the wall of the chamber on the height of the sample. Deuterated water D2O (99.0% isotope enrichment, Aldrich) and perdeuterated (-d8) toluene (98.5% isotope enrichment, Deutero GmbH, Kastellaun, Germany) were used to produce deuterated solvent vapors. The vapors were created by dropping ca. 0.5 mL of the solvent into a small glass pan placed in the experimental chamber. The experiments started after an equilibration time of at least 30 min after closing the cell. All experiments were performed at 24 °C. The specular reflectivity from each sample was measured first under dry nitrogen (purity 5.0) in order to yield reference data. Then the sample reflectivity was measured in a D2O atmosphere, and after that in toluene-d8 atmosphere. Between the exposures to the different analytes, the sample was purged with dry nitrogen for at least 12 h in order to ensure complete desorption of the vapor. At least three consecutive scans over the entire Q-range were carried out for each analyte to prove whether the film had reached equilibrium with its surroundings. The reflectivity from the bare substrate was measured in a separate experiment. The substrate reflectivity curve is shown in Figure 3. The scattering length density of the substrate was determined by fitting the experimental reflectivity data with a Fresnel model. The obtained SLD value (Fsubstrate ) 3.45 × 10-6 Å-2) is close to the calculated SLD of SiO2 (FSiO2 ) 3.42 × 10-6 Å-2).25 The reflectivity spectra were analyzed by applying the standard fitting routine Parratt 32.26 This program calculates the optical reflectivity of neutrons or X-rays from flat surfaces. The calculation is based on Parratt’s recursion scheme for stratified media.13,27 The film was modeled as consisting of layers of specific (25) The SLD value of the SiO2 was calculated taking a mass density of 2.16 g/cm3 (Handbook of Chemistry and Physics, 54th ed.; Weast, R. C., Ed.; Chemical Rubber Co.: Cleveland, OH, 1973) and scattering lengths from Sears, V. F. Neutron News 1992, 3, 26. (26) Braun, C. Parratt32 Fitting routine for reflectivity data; HMI: Berlin, 1997-1999. (27) Parratt, L. G. Phys. Rev. 1954, 95, 359.

Krasteva et al.

Figure 3. Reflectivity profile of the glass substrate. The solid line shows the Fresnel fit of the data.

Figure 4. Specular reflectivity curve of the Au/PAMAM film in dry nitrogen. The best fit of the experimental data obtained by using the single layer model (see Table 1) is plotted as a solid line. thickness and scattering length density. The reflectivities were calculated using the dynamic iterative model.13 The model reflectivity profile was calculated and compared to the measured one. Then the model was adjusted to the best chi-square fit to the data (see Appendix). From the fits the thickness, scattering length density F, and the roughness σ of each layer were obtained. The errors in the parameter values were set in accordance to a level of 10% increase in chi-square.

Results and Discussion The reflectivity curve of the Au/PAMAM film measured under dry nitrogen is shown in Figure 4. The characteristic Kiessig fringes, caused by the interference of the beams reflected from the gas phase/film and from the film/glass substrate interface are somewhat smeared, indicating a relatively high roughness of the sample. The total film thickness estimated from the width of the Kiessig fringes is around 36 nm. A model describing the sample as a single layer (Au/ PAMAM film) with a constant scattering length density sandwiched between two semi-infinite phases (gas phase and substrate) was used to fit the reflectivity curve. This model (referred to hereafter as the single-layer model) gave the best fit of the experimental data. The thickness of the film, its scattering length density, and the surface roughness of both gas phase/film and film/glass substrate interfaces were used as fitting parameters. The scattering

Vapor Sorption in Dendrimer Films

Langmuir, Vol. 19, No. 19, 2003 7757

Table 1. Thickness, Scattering Length Density, G, and Interfacial Roughness, σ, Obtained from the Fits of the Experimental Reflectivity Data of the Au/PAMAM Film under Dry Nitrogen and Exposed to Deuterated Analyte Vapors model

thickness, nm

F, Å-2

σfilm/gas phase, nm

σfilm/substrate, nm

Au/PAMAM in dry N2 Au/PAMAM in D2O vapors

single layer three layer

1.5

two layer

1.56 × 10-6 3.02 × 10-6 3.97 × 10-6 (exp. decay) 3.22 × 10-6 3.38 × 10-6 1.80 × 10-6 3.94 × 10-6 1.94 × 10-6

5.9 1.9

Au/PAMAM in toluene-d8 vapors (1st scan) Au/PAMAM in toluene-d8 vapors (2nd scan)

36.6 ( 0.8 4.3 ( 0.2 24.1 ( 0.5 8.0 ( 0.7 4.9 ( 0.2 35.6 ( 0.5 6.8 ( 0.1 36.0 ( 0.2

sample

two layer

length density of the substrate was kept constant during the fitting procedure. The best fit of the experimental data is shown in Figure 4 with a solid line. The selected model describes with good precision the experimentally measured reflectivity at small and intermediate Q values. Deviations between the experiment and the model are only observed at high Q, where also the scatter of the acquired data becomes larger. The values of the fitting parameters are listed in Table 1. The morphology and the thickness of the Au/PAMAM film were investigated also by AFM. The film appeared as granular material with relatively high surface roughness (root mean square (rms) ≈ 1 nm28) as seen in Figure 5a. The thickness of the films was determined to be 36 ( 2 nm by measuring the height of the edges of several scratches (Figure 5b). This result is in good agreement with the film thickness determined by the fit to the reflectivity data (Table 1), indicating that the suggested model of the film is reasonable. We note that the measured thickness of the film is less than the expected thickness of 14 complete layers of 4 nm gold nanoparticles as reported previously.2 The scattering length density of the film, Ffilm, obtained from the best fit was used to estimate the volume fraction of gold, φAu, in the Au/PAMAM composite film. Considering the film as a homogeneous two-component layer in which the Au nanoparticles are embedded in the PAMAM matrix (i.e., neglecting the existence of empty voids in the film), Ffilm can be expressed as

Ffilm ) φAuFAu + (1 - φAu)FPAMAM

(2)

where FAu and FPAMAM are the scattering length densities of pure gold and PAMAM dendrimers, respectively. The latter two can be obtained from

FPAMAM ) NA

dPAMAM

FAu ) NA

Mw

∑i nibi

dAu b Aw Au

(3)

(4)

where dPAMAM and dAu are the mass densities of the PAMAM dendrimer and gold, Mw is the molecular mass of PAMAM, Aw is the Au atomic weight, ni and bi represent the stoichiometry and the scattering length of the elements in the PAMAM molecule (i.e., carbon, hydrogen, oxygen, and nitrogen), and bAu is the scattering length of Au. The element scattering lengths bi were taken from the literature.29 The calculated scattering length densities of pure gold and the PAMAM dendrimers are FAu ) 4.5 × 10-6 Å-2 and FPAMAM ) 9.52 × 10-7 Å-2. Thus, the film SLD (28) The surface roughness root mean square is defined as the standard deviation with respect to the mean surface height within a given area. (29) Sears, F. V. Neutron News 1992, 3, 26.

1.5 2.5 3.5 2.8 3.1

derived from the fit (Ffilm ) 1.56 × 10-6 Å-2) would correspond to a gold volume fraction of about 0.24 in the film. This number is in accordance with the composition data obtained by X-ray photoelectron spectroscopy analysis of Au/PAMAM films.30 D2O-Dosed Films. The reflectivity curves of two successive scans of the Au/PAMAM sample exposed to saturated D2O vapor are presented in Figure 6a. The reflected intensity of the D2O-dosed film (curves 1 and 2) is higher compared to that of the dry film (curve 3). This is caused by an increase in the scattering length density of the film material, which is due to D2O absorption in the film. The observed flattening of the Kiessig fringes, which suggest that the SLD of the film material approaches that of the glass substrate, supports this assumption. No significant difference was detected between the reflectivities measured during the first and the second scans, revealing that the film was saturated with D2O when the experiment was started. The width of the Kiessig fringes remains almost the same as in the case of the dry film, indicating that the total thickness of the composite layer did not change significantly. Finding the best fits of the reflectivity data from the Au/PAMAM film exposed to D2O vapors was difficult, because of the poorly resolved fringes in the experimental curves, which suggest small difference between the SLD of the film and the glass substrate. As a starting model for fitting the experimental reflectivity curves in the presence of deuterated solvents, the single layer model was applied as in the case of the dry film. This model did not give a satisfactory fit of the experimental data, suggesting that the scattering length density distribution in the analyte-dosed film is more complex. Therefore, several different models with increasing number of layers and varying scattering length density across the film were tested. A more detailed description of the procedure applied to find the best physically meaningful model for the sample is presented in the Appendix. We were able to model the experimental data by using the following three-layer model. The first layer, located at the film/gas phase interface, was assumed to represent the outermost interfacial region of the composite film. This layer had a constant SLD and a thickness of only few nanometers. The second layer was the thickest one with high SLD, which was found to decay exponentially with increasing distance from the film/gas interface. This layer represents the bulk of the film. The third layer, with a constant lower scattering length density and a thickness of few nanometers was located at the film/substrate interface and thus represents the innermost interfacial region of the film. The best fits of the experimental reflectivity data obtained by this model are shown in Figure 6a (curves 1 and 2, solid lines). The model provides good fits of the data, suggesting a reasonable description (30) Detailed studies presenting results of the XPS analysis will be reported elsewhere.

7758

Langmuir, Vol. 19, No. 19, 2003

Krasteva et al.

Figure 6. (a) Specular reflectivity curves of two consecutive θ/2θ scans of the Au/PAMAM film under saturated D2O vapors (curves 1 and 2). The solid lines show the best fits of the data calculated by using the three-layer model (see Table 1). The reflectivity data and the fit of the single-layer model used for the same sample in dry N2 are shown for comparison (curve 3). All curves are offset for clarity. (b) Scattering length density profiles across the sample in D2O vapors (1 and 2) and in dry nitrogen (3) obtained from the fits of the reflectivity data.

Figure 5. Tapping Mode AFM images of the Au/PAMAM film showing (a) the surface morphology and (b) the thickness determination under ambient conditions.

of the film structure. The thickness, SLD, and the interfacial roughness of each layer are summarized in Table 1. The scattering length density profiles from the two successive scans are shown in Figure 6b together with the profile of the dry film. The SLD profiles of the D2Odosed film derived from the two reflectivity scans coincide within the limits of the experimental error. This indicates

that the film reached equilibrium shortly after starting D2O exposure. The analyte-dosed film is characterized by a higher scattering length density than that of the dry one. The total thickness of the D2O-dosed film derived from the fits is 36.4 ( 1.4 nm. Toluene-Dosed Films. The behavior of the film exposed to toluene-d8 is different, as seen in Figure 7a. In this case a strong increase of the reflectivity from the sample was observed during the first θ/2θ scan (curve 1). The Kiessig fringes become more pronounced and their width decreases, indicating an increase of both the scattering length density and the total thickness of the sample. Further increase in the amplitude and decrease in the width of the fringes were observed during the second scan taken about 4 h after the end of the first scan (curve 2). This suggests that the scattering length density and the total thickness of the sample were still changing even after longer exposure to toluene vapors. The data were fitted with a two-layer model, in which each layer had a constant scattering length density. The best fits of the data obtained by this model are shown in Figure 7a (solid lines), and the corresponding fit parameters are listed in Table 1. As seen, there is a good agreement between the experimental data and the model

Vapor Sorption in Dendrimer Films

Figure 7. Specular reflectivity curves of two consecutive θ/2θ scans of the Au/PAMAM film under saturated toluene-d8 vapors (curves 1 and 2). The solid lines show the best fits of the data calculated by using the two-layer model (see Table 1). The reflectivity data and the fit of the single layer model used for the same sample in dry N2 is shown for comparison (curve 3). All curves are offset for clarity. (b) Scattering length density profiles across the sample in toluene-d8 vapors (1 and 2) and in dry nitrogen (3) obtained from the fits of the reflectivity data.

predictions, which indicates that the film structure is reasonably described by this model. The SLD profiles across the sample obtained from fitting the data of two consecutive scans are shown in Figure 7b together with the SLD profile of the dry film. The exposure of the film to toluene leads to the formation of two welldistinguishable regions. The first region comprises a thin layer at the gas phase/film interface with a relatively high scattering length density. The thickness and the SLD of this layer increase with increasing the duration of the exposure to toluene vapors. The second region comprises a layer of larger thickness and a lower, constant scattering length density, which increases only slightly when increasing the duration of the vapor exposure. Both the thickness and the scattering length density of this second layer are close to those of the dry film. A comparison of the thickness data presented in Table 1 shows that the total thickness of the toluene-d8-dosed film is about 4 nm larger than the thickness of the dry film. The data presented in Figure 7 show that the reflectivity of the sample was still changing even after several hours of exposure to toluene-d8 vapor. However, when the three reflectivity scans are compared, it is seen that the most

Langmuir, Vol. 19, No. 19, 2003 7759

significant changes happened already about 30 min after starting the exposure to toluene (curves 1 and 3). Taking into account the relatively small differences between curves 1 and 2, we assume that the system was close to equilibrium during the reflectivity scans, so that only negligible changes of the reflectivity (due to further vapor sorption) occurred. On the basis of the obtained experimental results, the following picture of the interaction between the film and the analyte can be drawn. As can be deduced from the profile of the D2O-dosed film, a significant amount of D2O is absorbed in the bulk of the film. The D2O concentration varies across the film thickness, showing that the analyte is not evenly distributed. After the first relatively thin adsorption layer (film/air interface), a region with higher content of D2O is formed. Then the concentration slightly decreases with an exponential decay and reaches its lowest value at the interface between the film and the substrate. In contrast to this, the profile of the toluene-d8-dosed film shows that the diffusion of this analyte is limited to a thin region near to the film/gas-phase interface. The formation of a layer with a high scattering length density is attributed to a thin wetting layer of toluene condensing on the top of the film. This layer becomes thicker upon prolonged exposure to analyte vapors. Outside the interfacial region, no significant diffusion of toluene molecules occurs. The difference between the D2O and toluene distribution across the composite film can be explained by taking into account the chemical structure of the PAMAM dendrimers. In addition to its surface amine groups, which serve to cross-link the Au nanoparticles, the PAMAM molecule comprises a great number of tertiary amine and polar amide groups interconnected via short ethylene segments. These groups can act as exo- and endo-receptor sites, which are capable of forming hydrogen bonds with polar or proton-donating molecules such as water. The fraction of the aliphatic segments in the molecule seems to be insufficient to enable strong interactions with nonpolar molecules such as toluene. This molecular architecture accounts for the hydrophilic nature of PAMAM dendrimers and, as a consequence, for the affinity of the Au/PAMAM composite film to polar analytes. The latter is also well illustrated by the high sensitivity of Au/PAMAM-based chemiresistors for water and the very low sensitivity for toluene vapors.2 Our results presented above reveal that the composite Au/PAMAM film is a hydrophilic and porous material. These properties account for the observed diffusion of polar D2O molecules into the film interior. However, it seems that the film porosity changes across the film thickness. As could be deduced from the D2O distribution profile, fewer voids accessible for the diffusing analyte molecules may exist near to the film/substrate interface, which would account for the observed lower SLD of the innermost region of the Au/PAMAM film. Additionally, a very thin (few nanometers) layer with a slightly lower hydrophilicity than that of the bulk may exist near to the film/gas phase interface. This layer interacts more weakly than the bulk with water but may serve as a suitable support for the formation of a wetting layer of toluene on the film surface. Summary and Conclusions In this study we used neutron reflectometry for investigating the sorption of vapor molecules in a gold nanoparticle/PAMAM film. Reflectivity data were collected from the film under saturated atmospheres of D2O and toluene-d8 vapors, as well as under pure nitrogen as reference. The data were analyzed using single or multiple

7760

Langmuir, Vol. 19, No. 19, 2003

layer models, which provided the scattering length density profiles across the film material. These profiles indicate that the diffusion of vapor molecules into the film is mainly controlled by the solubility properties of the dendrimer component. Since the dendrimer is hydrophilic, D2O easily diffuses into the film. Toluene-d8 does not penetrate into the film but forms a thin wetting layer at the film/gas interface. These results are in general accordance with previously reported in situ resistance measurements, which revealed that the films’ resistivity is sensitive to water vapor but rather insensitive to toluene vapor.2 The thickness of the film in the absence of vapors was determined by fitting the neutron reflectivity data and by using atomic force microscopy. Both methods gave very similar results (36.6 and 36 nm, respectively). The thickness of the film under D2O atmosphere was very similar to that of the film under dry nitrogen (36.4 nm). This result suggests that swelling of the film material is not very pronounced and, thus, not detectable with the method described here. We conclude that film swelling is limited to a few percent of the dry film thickness. We note that also this result is in accordance with previously published data from in situ resistivity measurements. According to the electron tunneling model,7-9 which has been employed to describe the charge transport in metal nanoparticle/organic materials, film swelling by just 3% would roughly double the film resistivity. The vaporinduced resistance variations of Au/PAMAM films, which we reported previously,2 are below such strong changes and, therefore, in accordance with film swelling of less than 3%. In conclusion the results presented in this paper suggest that neutron reflectometry is a powerful tool for studying sorption of solvent molecules in metal nanoparticle/organic films. The results reported here are in reasonable accordance with results obtained from other methods used for probing the thickness of such films and their vapor sensitivity. Moreover, neutron reflectometry provides valuable information about the analyte distribution profile across the film. We expect that this information together with results obtained through other methods will enable a more quantitative understanding of the films’ vaporsensing properties. Currently we are working on a detailed comparative study, in which we use in situ resistivity measurements, neutron reflectometry, and microgravimetry to characterize the interactions of the films with various vapors. Acknowledgment. We thank Berit Guse and Miriam Rosenberger for the skillful preparation of the samples. R.K. is thankful to Dr. Roland Steitz for the precise and fruitful discussions concerning the neutron reflectivity

Krasteva et al.

Figure 8. Experimental data from Figure 7a fitted with different models of increasing complexity (number of layers): one layer (curve 1); two layers (curve 2); three layers (curve 3); four layers (curve 4). The inset shows the decrease of the chisquare (χ2) value as a function of the number of layers increasing the complexity of the model.

experiment and data processing. Dr. William E. Ford is acknowledged for proof reading the manuscript. Appendix Our fitting strategy was to use the simplest physically reasonable model to describe the experimental data. For illustrating this approach, we use the fitting routine, which was employed to analyze the data obtained from the film under a toluene-d8 atmosphere. We began with one-layer models for the film and then added additional layers to improve the fit. If the simplest model gave a large chisquare value, we began to increase the complexity of the model by adding layers to describe the various film regions. The results shown in this work represent the best fits of the data using simple, insightful models. Large variations in parameter space were allowed, but we restricted our models to those that generated reasonable results based on the known number of deposited layers and scattering length densities of the ingredients. A series of fit functions of increasing complexity applied to one set of neutron reflectivity data is shown in Figure 8. The insets represent the change of chi-square as a function of increasing number of layers. The models that we used for describing the data correspond to that model for which chi-square appeared to reach a plateau. LA0267488