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Langmuir 1997, 13, 3422-3426
1. Ultrathin Multilayer Polyelectrolyte Films on Gold: Construction and Thickness Determination Frank Caruso,†,§ Kenichi Niikura,‡ D. Neil Furlong,*,† and Yoshio Okahata‡ CSIRO, Division of Chemicals and Polymers, Private Bag 10, Clayton South MDC, Victoria 3169, Australia; and Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuda, Midori-ku, Yokohama 227, Japan Received August 16, 1996. In Final Form: November 25, 1996X Thin organic films fabricated by the successive deposition of the polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) have been successfully grown up to 24 layers on gold surfaces. These films are formed via electrostatic attraction between adjacent layers of opposite charge. Their construction has been examined using a quartz crystal microbalance (QCM), reflection spectroscopy (RS), surface plasmon resonance (SPR), and X-ray photoelectron spectroscopy (XPS). The thickness of the multilayer assemblies increases with the number of adsorbed layers, although a linear increase is observed only after the deposition of four polyelectrolyte layers ((PAH/PSS)2). The (PAH/PSS)2 film facilitates regular, stepwise deposition of subsequent PAH and PSS layers. The thickness of (PAH/ PSS)2 on gold was determined independently by QCM, SPR, and XPS to be 7.9 ( 0.6 nm. The PAH/PSS layer pair thickness after regular film growth was calculated to be 10.0 ( 0.8 nm. The formation of these thin films on gold surfaces opens the possibility of constructing supramolecular assemblies for use in the areas of biological and chemical sensing.
Introduction Ultrathin organic films are of considerable interest because of their potential technological applications in the fields of sensors, optoelectronics, and surface coatings.1,2 These films are commonly formed using Langmuir-Blodgett (LB) deposition or self-assembly techniques based on chemisorption. Recently, a new method of preparing ultrathin films by the successive deposition of oppositely charged polyelectrolytes was developed by Decher and co-workers.3-8 This method makes use of the electrostatic attraction between oppositely charged polyelectrolytes to produce multilayer thin films. By sequentially dipping a substrate into a solution containing polyions of opposite charge and allowing the polyelectrolytes to spontaneously adsorb, ordered multilayer assemblies are formed on a solid substrate. This method has been shown to produce films of high uniformity (surface roughness < 10 Å) and a well-defined (controllable) thickness. Ultrathin films prepared using the above method have been characterized using small-angle X-ray reflectivity and UV-vis spectroscopy,3-8 ellipsometry,9 a planar optical wave guide system,10 and quartz crystal micro* To whom correspondence should be addressed: Fax: 61-3 9542 2515. E-mail:
[email protected]. † CSIRO. ‡ Tokyo Institute of Technology. § Current address: Max-Planck-Institute for Colloids and Interfaces Rudower Chaussee 5, D-12489 Berlin, Germany. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Ulman, A. An Introduction to Ultrathin Films, from LangmuirBlodgett to Self-Assembly; Academic Press: Boston, New York, Toronto, 1991; p 440. (2) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (3) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (4) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (5) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (6) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (7) Lvov, Y.; Decher, G.; Mo¨hwald, H. Langmuir 1993, 9, 481. (8) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (9) Tronin, A.; Lvov, Y.; Nicolini, C. Colloid Polym. Sci. 1994, 272, 1317.
S0743-7463(96)00821-9 CCC: $14.00
gravimetry.11 The substrates in these studies have been glass, quartz, and silanized quartz. Multilayer formation on these surfaces has been aided by the initial charge on the bare substrate. More recently, silver (oxide) substrates have also been used; film assembly was found to proceed after the spontaneous adsorption of poly(ethyleneimine). Since gold surfaces are widely employed in the areas of biochemical and chemical sensing (e.g. surface plasmon resonance, piezoelectric quartz crystals), as well as in numerous electrical techniques, it was of particular interest to us to examine the construction of multilayer polyelectrolyte thin films on gold. This paper reports on the assembly of thin multilayer films formed by the consecutive spontaneous adsorption of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) on gold. The benefit of modifying the gold surface with mercaptopropionic acid (MPA) prior to polyelectrolyte multilayer buildup is also examined. Multilayer thin film formation is monitored using a quartz crystal microbalance (QCM), reflection spectroscopy (RS), surface plasmon resonance (SPR), and X-ray photoelectron spectroscopy (XPS). The polyelectrolyte layer thicknesses on gold have been independently determined, and we are therefore able to compare the results obtained from the different techniques employed. Thin film buildup to 24 layers is demonstrated with the QCM, and the first 4 layers ((PAH/PSS)2) are investigated using QCM, RS, SPR, and XPS methods. This four-layer polyelectrolyte film is subsequently used for the immobilization of proteins and as a precursor film for the fabrication of protein/polyelectrolytre multilayer films (part 2). Experimental Section Materials. Poly(allylamine hydrochloride) (PAH), Mr 50 00065 000, and poly(sodium 4-styrenesulfonate) (PSS), Mr 70 000, were purchased from Aldrich Chemical Co. 3-Mercaptopropionic acid (MPA) was obtained from Sigma. Sulfuric acid, nitric acid, acetone, and propan-2-ol were all AR grade and supplied by Rhoˆne-Poulenc. Hydrogen peroxide (AR grade) and ethanol (AR grade) were purchased from BDH. Spectroscopic grade chloro(10) Ramsden, J. J.; Lvov, Y. M.; Decher, G. Thin Solid Films 1995, 254, 246. (11) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117.
© 1997 American Chemical Society
Ultrathin Multilayer Polyelectrolyte Films on Gold form was obtained from Merck. The water used in all experiments was prepared in a three-stage ‘Milli-Q’ purification system to a conductivity less than 1 µS cm-1. All adsorption experiments were performed at 22 ( 1 °C. Substrate Preparation. Details on the preparation of the QCM electrodes, and the equilateral glass prisms for SPR measurements, can be found elsewhere.12 Pyrex microscope slides were used as the substrates for RS and XPS measurements. These surfaces were cleaned by ultrasonication in acetone for 10 min, followed by rinsing with ethanol and drying with nitrogen. For the RS experiments, a 10 nm chromium adhesion layer was first deposited onto the microscope slides using thermal evaporation, and gold was then sputter-coated onto this layer to a thickness of 200 nm. (This thickness was chosen as it provides a highly reflective gold surface.) Slides for XPS experiments were metal-coated using the same procedures to give a 20 nm chromium layer, followed by a 80 nm gold layer. All substrates were used immediately after preparation. Polyelectrolyte Layer Formation. The gold substrates were first exposed to a 1 mM MPA-ethanol solution for 24 h, followed by rinsing with Milli-Q water and drying with nitrogen. (MPA self-assembles onto the gold, thereby introducing carboxyl groups, which are then able to interact with the polyelectrolyte under certain pH conditionsssee later.) Polyelectrolyte adsorption was then performed as follows. The substrates were immersed in ca. 5 mL of 3 mg mL-1 PAH solution (pH 8.0) for 5 min, followed by washing with water and nitrogen drying. (Drying of the polyelectrolyte film does not affect further film growth.8) The PAH-coated substrate was then exposed to ca. 5 mL of 3 mg mL-1 PSS solution (0.01 M HCl and 1 M MnCl2, pH ∼ 2) for 1.5 min. (The pH values used are required to achieve opposite charges on the polyelectrolyte for multilayer buildup. MnCl2 has previously been used for the successful addition of PSS layers onto positively charged polyelectrolytes.5 It should be noted that the conditions used for the polyelectrolyte layer buildup are slightly different from those used previously.3-8 For example, the adsorption time for PSS was carefully chosen, as the exposure of the gold (-coated) substrate to the PSS solution for longer times often led to deterioration of the gold surface.) This surface was then washed with pure water and dried with nitrogen. This procedure was repeated until 10 polyelectrolyte layers were deposited. Above 10 layers, the assembly process was stopped at 16 and 24 layers, with intermediate water washing and drying at each step. Quartz Crystal Microbalance (QCM) Measurements. AT-cut quartz crystals with a fundamental resonance frequency (F0) of 9 MHz were supplied by Kyushu Dentsu Co. (Omura-City, Nagasaki, Japan). These crystals (4.5 mm diameter) were supplied with 100 nm thick gold-coated electrodes. The QCM measurement system has been described in detail elsewhere.12-14 The MPA-modified crystals (as described above) had their in air frequencies (Fair) measured. The crystals were then sequentially immersed in polyelectrolyte solutions of alternating charge for the required time (see above). After adsorption of each layer, the crystals were removed from solution, washed thoroughly with pure water, and nitrogen dried, and the Fair was measured. The in air frequency changes (∆Fair) were used to determine the mass of polyelectrolyte adsorbed after each immersion step. The quartz crystal microbalance is an extremely sensitive mass sensor, capable of measuring sub-nanogram levels.15,16 The piezoelectric quartz crystal changes its fundamental oscillation frequency, F0, as mass is deposited onto (or depleted from) the crystal surface in accordance with the Sauerbrey equation:17 (12) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546. (13) Caruso, F.; Rodda, E.; Furlong, D. N. J. Colloid Interface Sci. 1996, 178, 104. (14) Caruso, F.; Rinia, H.; Furlong, D. N. Langmuir 1996, 12, 2145. (15) Guilbault, G. G. In Applications of Piezoelectric Quartz Crystal Microbalances; Lu, C., Czanderna, A. W., Eds.; Elsevier: Amsterdam/ New York, 1984; p 251. (16) Lucklum, R.; Henning, B.; Hauptmann, P.; Schierbaum, K. D.; Vaihinger, S.; Gopel, W. Sens. Actuators 1991, A25, 705. (17) Sauerbrey, G. Z. Phys. 1959, 155, 206.
Langmuir, Vol. 13, No. 13, 1997 3423 ∆F ) -
2F02
∆m A(µqFq)1/2
(1)
where ∆F is the change in resonant frequency, resulting from a change in mass ∆m, µq is the shear modulus of the quartz (2.947 × 1013 g m-1 s-2), Fq is the density of the quartz (2.648 × 106 g m-3), and A is the electrode area (1.59 × 10-5 m2). Substituting these values in eq 1 with F0 ) 9 × 106 Hz (operating frequency of our crystals), we obtain the following relationship between adsorbed mass and frequency shift:
∆F ) -(1.83 × 104)∆mA
(2)
where mA is the mass change per quartz crystal unit area, in g m-2. Hence, the QCM can be used to monitor the formation of single and multilayers of polyelectrolytes via their adsorption onto the face of the crystal. Reflectance Measurements. Reflectance measurements were made using a Cary 5E UV-vis-NIR spectrophotometer with a specular reflectance accessory. The reflectance was measured using an integrating sphere installed in the sample compartment of the instrument which features an inbuilt photomultiplier tube. The reflectance spectrum of the MPAmodified gold plate was first measured. (This plate was used as the reference, and its reflectance spectrum, as the “background” spectrum.) The polyelectrolyte layers were then prepared by immersing the MPA-modified gold substrate in the polymer solutions of alternating charge (as described above). After one adsorption cycle of PAH + PSS, the polyelectrolyte layer pair was thoroughly rinsed with Milli-Q water and dried with nitrogen. The reflectance spectrum was then measured. This procedure was repeated, and reflectance spectra for the deposited layers were recorded. Surface Plasmon Resonance (SPR) Measurements. The SPR experimental system and measurement principle have been previously described in detail.12,18 A full plasmon resonance curve (reflectivity vs internal angle) for the MPA-modified gold/air system was first measured. The polyelectrolyte solution (PAH) was then injected into the vessel and allowed to adsorb for the required time (see above). The solution was removed and the surface rinsed thoroughly with pure water and dried using a gentle stream of nitrogen. The full SPR curve for this surface was then recorded. The above procedure was repeated for each polyelectrolyte layer. The SPR curves obtained were fitted to Fresnel theory by assuming the “idealized layer model”. In this model the layers are isotropic and the substrate is perfectly flat. In the fitting procedure, the real (r) and imaginary (i) components of the relative permittivity of the layer are kept fixed (and hence refractive index, since r ) n2, where n is the refractive index), and the thickness (d) is extracted. Details concerning the fitting procedure can be found in earlier work.12,19 X-ray Photoelectron Spectroscopy (XPS) Measurements. Details of the XPS system and analysis procedure are given elsewhere.12,20 Spectra were collected normal (θ ) 0°) to the sample surface. The elements present were identified from survey spectra. High-resolution spectra were recorded from individual peaks for every element detected. The elemental composition of the surface was determined by a first principles approach.21 The adsorbed layers on gold were prepared by immersing the MPA-modified gold substrates in polyelectrolyte solutions of alternating charge (as above). After adsorption, each polyelectrolyte layer was thoroughly rinsed with Milli-Q water and dried with nitrogen. (18) Caruso, F.; Vukusic, P. S.; Matsuura, K.; Urquhart, R. S.; Furlong, D. N.; Okahata, Y. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 103, 147. (19) Geddes, N. J.; Martin, A. S.; Caruso, F.; Urquhart, R. S.; Furlong, D. N.; Sambles, J. R.; Than, K. A.; Edgar, J. A. J. Immunol. Methods 1994, 175, 149. (20) Gengenbach, T. R.; Vasic, Z. R.; Chatelier, R. C.; Griesser, H. J. J. Polym. Sci., Polym. Chem. Ed. 1994, 32, 1399. (21) Grant, J. T. Surf. Interface Anal. 1989, 14, 271.
3424 Langmuir, Vol. 13, No. 13, 1997
Caruso et al. Table 1. Polyelectrolyte Layer Thicknesses on MPA-Modified Gold, Obtained Using Various Techniquesa film thicknessb (nm) polyelectrolyte layers
QCM
SPRc
XPS
1. PAH 2. PAH + PSS 3. PAH + PSS + PAH 4. (PAH + PSS)2
0.5 2.9 4.7 8.3
0.6 2.9 4.3 7.2
7.5,d 8.5d
a Tabulated values are for dry polyelectrolyte layers. b The error in these values is (10%. c Thicknesses were determined using a refractive index of 1.63. d Values for two separate experiments.
Figure 1. QCM frequency changes due to the successive adsorption of polyelectrolyte layers of alternating charge on MPA-modified gold electrodes. The odd layer numbers correspond to PAH adsorption, and the even layer numbers to PSS adsorption. Each symbol is the average of five separate experiments. The inset shows QCM frequency changes for the buildup of two PAH/PSS layer pairs: layer 1, PAH; layer 2, PAH + PSS; layer 3, PAH + PSS + PAH; layer 4, (PAH + PSS)2. Each symbol represents a separate experiment.
Results and Discussion QCM, RS, SPR, and XPS Data. In order to render the initially hydrophobic gold surface12,22 sufficiently negative (i.e. anionic) to aid immobilization of cationic PAH via electrostatic interactions, the gold surface was first exposed to MPA. (Under the conditions used for first layer (PAH) adsorption (pH 8), the carboxyl groups of MPA are ionized.) The presence of MPA on the surface could not be detected by QCM, as short chain thiols are known to dissolve gold, hence concealing any frequency decrease due to MPA adsorption.23 However, reflection-absorption spectroscopy FTIR measurements confirm the presence of carboxyl groups (due to MPA) on the gold surface.24 Figure 1 shows the QCM frequency changes for the successive adsorption of PAH and PSS layers on gold pretreated with MPA (total of 12 PAH/PSS layer pairs). The odd layers correspond to PAH adsorption and the even layers to PSS. The average frequency change for one PAH/PSS layer pair is 498 Hz. Figure 1 shows that regular, stepwise growth is obtained after the first 2 PAH/ PSS layer pairs have been deposited. (The key to a regular, stepwise multilayer buildup is the complete reversal of surface charge on the surface with each adsorption step.) The inset in Figure 1 shows the QCM frequency changes (in more detail) for the first four layers formed. Each symbol represents a separate experiment (five in total). Reproducible frequency changes are observed below n ) 4 when gold was pretreated with MPA prior to polyelectrolyte adsorption (Figure 1 inset). PAH/PSS multilayers were also successfully constructed on bare gold surfaces (as assessed by QCM) (data not shown): after regular layer buildup (above n ) 4), the frequency changes were within ca. 10% of those reported for MPA-treated gold surfaces. Inconsistent frequency changes on bare gold were, however, observed for the formation of the first 3-4 layers, prior to a uniform surface charge excess being created for subsequent layer adsorption (see later). In particular, direct adsorption of PAH onto bare gold yielded the most inconsistent frequency changes, presumably due (22) Geddes, N. J.; Paschinger, E. M.; Furlong, D. N.; Caruso, F.; Hoffmann, C. L.; Rabolt, J. F. Thin Solid Films 1995, 260, 192. (23) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (24) Caruso, F.; Ariga, K.; Ichinose, I.; Furlong, D. N.; Kunitake, T. Manuscript in preparation.
to the hydrophobic adsorption of PAH onto bare gold and the poorly charged (i.e. hydrophobic) surface for adsorption. Thus, since we were interested in reproducibly producing a thin film on gold with a uniformly charged surface for protein adsorption and immunosensing studies (part 2), we examined PAH/PSS multilayer buildup on MPA-treated gold surfaces using RS, SPR, and XPS (see below) up to n ) 4. The thickness of the polyelectrolyte layers on MPAmodified gold can be calculated by substituting Fd for ∆mA in eq 2 (see Experimental Section), where F is the polyelectrolyte film density (in g m-3) and d is the polyelectrolyte film thickness (in m). By taking into account that the effective surface area is approximately 20% larger than that calculated directly from the diameter of the electrode (due to surface roughness),11 eq 2 becomes
∆F F
d ) -(2.18 × 10-5)
(3)
for two sides of the QCM crystal. Thus, assuming (1.2 ( 0.1) × 106 g m-3 for the density of polyion layers25 and using the average frequency shifts in Figure 1, the polyelectrolyte layer thicknesses can be calculated. These are given in Table 1 for layers n ) 1-4. For layers 5 (n ) 5) to 10 (n ) 10) the corresponding thicknesses are 12.8, 16.5, 20.9, 27.6, 31.8, and 37.3 nm, respectively. The average thickness increments between n ) 4-10 for PAH and PSS are 4.4 and 5.3 nm, respectively. For (PAH/ PSS)8 (n ) 16), the film thickness is 68.2 nm, and for (PAH/PSS)12 (n ) 24), it is 108.5 nm. The assumption of F = 1.2 × 106 g m-3 for the polyion layers has recently been validated by scanning electron microscopy.24 The polyelectrolyte assembly process was also monitored by UV reflection spectroscopy. Reflectance (instead of direct absorption) measurements were performed because gold is highly absorbing. Since the relative spectral reflectance (transmittance) is defined as the ratio of the flux reflected (transmitted) by the sample to that of a standard, the reflectance can be converted to absorbance using
A ) -log
( ) R R0
(4)
where A is the absorbance, R is the reflectance from the sample (gold plate with adsorbed polyelectrolyte), and R0 is the reflectance from the standard (MPA-modified gold plate). Figure 2 shows the absorbance spectra for one to five PAH/PSS layer pairs on MPA-modified gold (from bottom to top). The absorbance peak at around 230 nm is due to the benzene chromophores in PSS, and its intensity increases with adsorption of each PAH/PSS layer pair. (PAH does not absorb above 200 nm, and therefore its deposition is not seen in the reflectance spectra.) The measured absorbance at 230 nm ranges from 0.06 to 1.1 (25) Polymer Handbook; Brandrup, J., Immergut, E., Eds.; John Wiley and Sons: New York, Chichester, Brisbane, Toronto, 1975; Part 5.
Ultrathin Multilayer Polyelectrolyte Films on Gold
Figure 2. Absorbance spectra (determined from reflectance spectra, see text for details) for PAH/PSS layers on gold pretreated with MPA. The curves, from bottom to top, correspond to 1, 2, 3, 4, and 5 PAH/PSS layer pairs, respectively. The MPA-modified gold substrate was used as reference. The absorbance values correspond to absorption by the layers both before and after light reflection.
for one to five PAH/PSS layer pairs. Because these absorption values correspond to the absorption of light by the layers before and after light reflection by the gold substrate, the values should be halved for single-absorption by the PAH/PSS films. The absorption of light through the films depends on the path length (l) of the light that travels through the film (which is dependent on incident angle (θ) and film thickness (d)), as well as the inherent properties of the polyelectrolytes. The measured absorbance values obtained cannot be readily compared with reported values for other systems unless θ and d are known. They can, however, be compared with previous values quoted for the relative increase in absorbance with the number of PSS layers. Decher et al.5 obtained an increase in absorbance of 180% for a five PAH/PSS layer pair film on fused quartz compared to the absorbance for a three PAH/PSS layer pair film on the same substrate. This increase is in close agreement with the results obtained in this work: 190% increase for five PAH/PSS layer pairs compared to three PAH/PSS layer pairs on MPA-modified gold. This result demonstrates the selfconsistency of the self-assembly process and suggests that the substrate has little influence on the layer buildup after six polyelectrolyte layers (n ) 6, three PAH/PSS layer pairs). The absorbance at 230 nm (for each spectrum in Figure 2) is plotted against the number of PSS layers in Figure 3, along with the PSS QCM frequency changes for the PAH/PSS assembly process. The data from the two techniques are in good agreement, confirming that the reflectance measurements are a measure of the total amount of polyelectrolyte (PSS in this case) in the film. Further, the absorbance characteristics of the chromophore do not vary from one layer to the next. Suffice it to say, measurement of the absorbance of the multilayers (via reflectance) allows the assembly process to be readily monitored and confirms the growth of the thin film with successive depositions of polyelectrolytes of alternating charge. The technique of SPR is based on the optical changes that occur at interfaces between a thin metal film and a dielectric fluid due to, for example, adsorption. SPR has been widely used to determine the thickness of adsorbed layers on metal surfaces.12,18,19 Figure 4 displays the reflectivity as a function of internal angle (full angle plasmon resonance curves) obtained for a MPA-modified gold film exposed to air (curve a) and the same gold film
Langmuir, Vol. 13, No. 13, 1997 3425
Figure 3. Dependence of PSS absorbance at 230 nm and PSS QCM frequency on the PSS layer number. The film is assembled from alternating layers of PAH and PSS on MPA-treated gold.
Figure 4. Full plasmon resonance curves for PAH/PSS multilayer formation on MPA-modified gold: (a) MPA-modified gold; (b) gold + PAH; (c) gold + PAH + PSS; (d), gold + PAH + PSS + PAH; (e) gold + (PAH + PSS)2. The solid lines represent the fitted theory, and the symbols are experimental data points. (The MPA-modified gold film was treated as a “gold only” surface in the fittingsssee text for details.)
after exposure to PAH and PSS polyelectrolyte solutions, rinsing, and drying (the SPR curves b, c, d, and e correspond to gold + PAH, gold + PAH + PSS, gold + PAH + PSS + PAH, and gold + (PAH + PSS)2, respectively). In Figure 4 it is seen that there is a sequential shift in the plasmon resonance angle with the addition of each polyelectrolyte layer. The magnitude of this shift is dependent on the thickness of the layer adsorbed on the gold surface and the optical refractive index of the polyelectrolyte layer at 632.8 nm (the wavelength of the incident light).26 The solid lines in Figure 4 are generated by fitting the Fresnel equations to the experimental full angle plasmon data. (The MPA-modified gold film was treated as a “gold only” surface in the fittings, as MPA had a negligible effect on the thickness of the gold layer.) These fits yield the optical parameters and/or the thickness of the gold film and subsequently the polyelectrolyte layers.27 The thicknesses of the polyelectrolyte layers can be calculated if the refractive index of the adsorbed layer is known. Ramsden et al.10 determined the optical parameters of PAH and PSS thin films using monomode optical waveguides as substrates and by measuring the mode of indices of waveguide modes. They reported a mean refractive index value (n) of 1.63 for PSS/PAH polyelec(26) Kretschmann, E.; Raether, H. Z. Naturforsch. A 1968, 23, 2135. (27) Cowen, S.; Sambles, J. R. Opt. Commun. 1990, 79, 427.
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Caruso et al.
trolyte multilayers at a wavelength of 632.8 nm. Hence, in the fitting of the polyelectrolyte SPR full plasmon curves of Figure 4, r ()n2) was fixed at 2.66 and d and i were varied to obtain minimization of the least squares errors. (Setting i to zero and allowing only d to vary gave the same results as varying both i and d.) It is important to note that, in the fitting for the polyelectrolyte layers adsorbed on gold, it is assumed that the parameters previously deduced for the gold film remain unchanged. The calculated thicknesses for the polyelectrolyte layers are given in Table 1. The thickness (d) of polyelectrolyte films (in the dry state) adsorbed onto metal substrates can be calculated from XPS data using the equation28,29
()
d ) -ln
IS λ cos θ IR
(5)
where IS and IR are the percentages of detected gold in the presence and absence of the polyelectrolyte layer, respectively, λ is the mean free path of gold photoelectrons in the organic medium (3.5 nm),29 and θ is the photoelectron emission angle measured relative to the normal surface (θ ) 0°). At normal emission the percentage of gold detected from two (PAH/PSS)2 samples was found to be 11.8% and 8.8%, compared to 100% for the reference gold substrate. Equation 5 yields thicknesses of 7.5 and 8.5 nm (Table 1). (These thicknesses were determined by assuming the polyelectrolyte adsorbs to the surface as a uniform overlayer.30) Film Thickness Summary. Table 1 gives the film thicknesses determined from QCM, SPR, and XPS measurements for the polyelectrolyte layers n ) 1-4 for the multilayer thin films prepared from PAH and PSS on MPA-modified gold. It is clear from Table 1 that the three techniques give similar thicknesses for the layers. For these four layers, the step size thickness due to PSS is greater than that due to PAH. A similar observation has also been made by Ramsden et al.10 for PAH/PSS multilayer formation on (negatively charged) Si(Ti)O2 surfaces. Although the first adsorption step with PAH on MPAmodified gold yields an average thickness of only 0.5 nm, this amount of PAH on the surface is clearly sufficient to facilitate the subsequent adsorption of PSS, thereby allowing multilayer thin film formation. The formation of a very thin first layer (ca. 0.5 nm) (compared to subsequent layers) is also reported for PAH on Si(Ti)O2.10 The close agreement between the thicknesses (for the first four layers) calculated from QCM data and those obtained from the other methods employed suggests that (28) Andrade, J. D. In Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1985; Vol. 2, Chapter 7. (29) Chatelier, R. C.; Gengenbach, T. R.; Griesser, H. J.; BrighamBurke, M.; O’Shannessy, D. J. Anal. Biochem. 1995, 229, 112. (30) Paynter, R. W.; Ratner, B. D.; Horbett, T. A.; Thomas, H. R. J. Colloid Interface Sci. 1984, 101, 233.
the QCM frequency changes are a reliable measure of polyelectrolyte thickness. Above n ) 4, regular layer growth is seen (Figure 1). The reason for the irregular growth seen below n ) 4 is most probably due to the effect of the underlying gold substrate and/or to a poorly charged initial layer. Figure 1 clearly shows that stepwise, regular (linear) polyelectrolyte growth is obtained after the deposition of two PAH/PSS layer pairs, suggesting that above n ) 4 the gold surface has no effect on film buildup and/or that an even charge distribution of the polyelectrolytes on the surface is established. A linear increase in the film thickness is usually observed after deposition of the first few layers.6-10 The number of layers required before regular growth is achieved depends on the nature of the surface (e.g. charged or hydrophobic), the polyelectrolytes, and the adsorption conditions (e.g. electrolyte type and amount). The measured average thickness increments obtained in this work between n ) 4 and 10 are 4.4 nm for PAH and 5.3 nm for PSS, yielding an average thickness of 9.7 nm per PAH/PSS layer pair. This value compares favorably with the 10.0 ( 0.8 nm calculated for each PAH/PSS layer pair using the QCM data between n ) 4 and 24. The layer pair and total film thicknesses are also dependent on the nature and amount of electrolyte used for adsorption of the polyelectrolytes. A twofold increase in PSS layer thickness has been observed with increasing NaCl concentrations for films composed of consecutively alternating layers of PSS and PAH on quartz.6 In part 2 of this work the effect of electrolyte concentration on the formation of alternating layers of PAH and PSS layers on gold is also investigated. Conclusions We have shown that it is possible to reproducibly construct multilayer thin films of the polyelectrolytes PAH and PSS via sequential adsorption on gold surfaces pretreated with MPA. The multilayer buildup requires the deposition of four polyelectrolyte layers ((PAH/PSS)2) prior to regular, stepwise growth being established. Using QCM, RS, SPR, and XPS techniques, a layer thickness of 7.9 ( 0.6 nm was obtained for the multilayer (PAH/PSS)2 on MPA-modified gold, and a layer pair thickness of 10.0 ( 0.8 nm for PAH/PSS after regular growth. In part 2, thin films prepared via alternate polycation and polyanion adsorption will be used for immobilizing proteins, as will films prepared by the alternate adsorption of polyions and proteins. The potential of these thin films as immunosensors is also assessed. Acknowledgment. We are grateful to Dr. Yuri Lvov and Dr. Izumi Ichinose for valuable discussions concerning the modification of surfaces for polyelectrolyte layer buildup. We thank Anton Launikonis for help with the reflectance measurements and Peter Kingshott for the XPS measurements. LA960821A
