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In Situ Investigations of Polymer Self-Assembly Solution Adsorption by Surface Plasmon Spectroscopy Rigoberto Advincula,*,†,‡ Emil Aust,† Wolfgang Meyer,† and Wolfgang Knoll*,†,§ Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55021, Mainz, Germany, and Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), 2-1, Hirosawa, Wako, Saitama, 351-01, Japan Received February 22, 1996. In Final Form: May 21, 1996X In this study, we report the use of surface plasmon spectroscopy (SPS) to investigate the self-assembly polymer solution adsorption process in situ. A sealed sample cell made of Teflon was assembled with the substrate initially coated with a positively charged surface using the Langmuir-Blodgett-Kuhn (LBK) deposition technique. The cationic and anionic polymer combinations were comprised mainly of a class of polyelectrolytes called ionenes and several anionic polymers. The solution adsorption process generally showed linear behavior except were the deviation is attributed to a poorly charged initial layer. The adsorption behavior is postulated to be due to conformational requirements and charge distribution of the individually adsorbed polymers. The time of adsorption for each polymer pair was also investigated. The technique promises to be an important tool in determining the mechanism for these types of adsorption processes.
Introduction The polymer self-assembly solution adsorption process is a relatively new technique for creating ultrathin films as initially reported by Decher et al.1 It promises to be a simple method for fabricating ultrathin films with various thicknesses and architecture incorporating not only polymers but various biological compounds and functionalities.2 The adsorption process involves a layer by layer deposition of oppositely charged polymers alternately from solution. The key interaction being the noncovalent electrostatic attraction (Coulombic interaction) between opposite charges of the polymer from solution and the surface of the previously adsorbed polymer. By control of the surface charge excess and the polymer species introduced in solution, superlattice or supramolecular structures of charged polymers have been created.3 Thus, it is important to determine the mechanism of this adsorption process in order to define and establish the parameters responsible for reproducible adsorption, control of thickness, surface coverage, etc. To this goal, we have initiated in situ investigations using the surface plasmon spectroscopy (SPS) technique. The polymer deposition process from solution was observed on a layer by layer adsorption basis. Previous work by Ramsden et al. involved in situ characterization of film optical constants using a waveguide flow-cell setup.4 With the concentration constant, the polyelectrolyte pairs and time of adsorption were varied. The resonance angle shift was observed at the solid-liquid interface. It has been shown that the technique is highly sensitive for characterizing ultrathin films at the nanometer scale.5 With the so-called Kretschman configuration and attenuated total reflection (ATR) conditions, the presence of a thin dielectric film such as the adsorbed polymers can be * Authors to whom all correspondence should be addressed. † Max-Planck-Institut fu ¨ r Polymerforschung. ‡ Present address: Department of Chemical Engineering (CPIMA), Stanford University, Stanford, CA 94305. § The Institute of Physical and Chemical Research (RIKEN). X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Decher, G.; Hong, J. D. Makroml. Chem., Macrol. Symp. 1991, 46, 321. (2) Decher, G.; Lvov, Y.; Lehr, B.; Lowack, K.; Schmitt, J. Biosens. Bioelectron. 1994, 9, 67. (3) Lvov, Y.; Essler, F.; Decher, G. J. Phys. Chem. 1993, 97, 13773. (4) Ramsden, J.; Lvov, Y.; Decher, G. Thin Solid Films 1995, 254, 246-251, and erratum: Thin Solid Films 1995, 261, 343-344. (5) Knoll, W. MRS Bull. 1991 16, 29.
S0743-7463(96)00162-X CCC: $12.00
Figure 1. Schematic diagram of the sample cell and the arrangement of the prism in relation to the glass substrate. The solution is injected in and out of the cell using a syringe. The arrangement of the consecutive layers of adsorbents from the glass substrate is also shown.
detected.6 This is observed as a shift in the resonance angle, which can be evaluated with a thickness and refractive index component based on the Fresnel equations.7 The thicknesses can be confirmed by an independent method such as X-ray reflection measurements. Such measurements are also of interest for studying the adsorption properties not only of polyelectrolytes but also of biological proteins, emulsifiers, wetting agents, etc., in situ from solution and waste.8 They could yet be utilized as a powerful complement to absorbance measurements, ellipsometry, calorimetry, etc in studying the uptake of adsorbents from solution. (6) Kretschmann, E. Opt. Commun. 1972, 6, 185. (7) Gordon, J. G., II; Swalen, J. D. Opt Commun. 1977, 22, 374. (8) Elaissari, A.; Cros, P.; Pichot, C.; Laurent, V.; Mandrand, B. Colloids Surf. 1994, 83, 25.
© 1996 American Chemical Society
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Figure 2. Reflectivity-incidence angle scans are shown for each polymer pair: (a) Ionene-3/PSS; (b) Ionene-2/PAZO; (c) Ionene2/PES; (d) Ionene-2/PSS. The Au/glass/water curve is shown for Ionene-3/PSS to highlight relative thickness between deposited layers. All scans begin with the octadecyl mercaptan/Ionene-1 (4.0 nm) layer. The structure of the polymers is also shown.
Experimental Section The ionene polymers were synthesized using a modification of the Menschutkin reaction and details of which have been
reported elsewhere.9 The polymers and their structural abbreviations are as follows: Poly(N,N′-dimethyl-N,N′-dioctadecyl1,6-hexanediamine) is hereby abbreviated as Ionene-1 (I-6,6-
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18-X). The numbers indicate the spacer group and the alkyl group substituent on the quaternary nitrogen with X referring to the counterion (Cl-, Br-, BF4-, etc.). Thus the other polymers are abbreviated as: ionene-2 (I-6,6-Me-X) and ionene-3 (I-Do,Pip-Me-X) and are of the range of Mw ) 20000-30000 (see Figure 2). Poly(styrenesulfonate) or PSS has a Mw ) 20000, and poly(propyl-1,3-phenyldicarboxylate-5-sulfonate) ester sodium salt or PES has a Mw ) 5000. Both have been synthesized using conventional techniques and structurally characterized by NMR and other spectroscopic techniques. Poly(1-(4-(3-carboxy-4hydroxyphenylazo)benzenesulfonamido)-1,2-ethanediyl) sodium salt or PAZO was obtained commercially from Aldrich. The LBK deposition of Ionene-1 was done on a threecompartment alternate trough from KSV Instruments (alternate KSV LB5000) under class 100 clean room conditions. The temperature of deposition was at 21 °C, and Milli-Q quality water at 18 MΩ resistance and pH ) 5.6 was used. The high refractive index glass substrate (LASFN9) preparation involved sonicating with a 2% Helmanex solution, water, and ethanol, respectively. This was followed by putting in a Nochromix sulfuric acid bath overnight and then rinsing with water 15 times. For the preparation of thinly gold coated substrates, metal vapor deposition under vacuum was done on plasma-cleaned substrates up to 450 Å thicknesses. Hydrophobization by self-assembly of gold substrates was done by treating them in a 1 × 10-4 M solution of octadecanethiol in EtOH for a minimum of 4 h and then washing with EtOH.10 The solution cell was prepared as follows (Figure 1):11 Initially, the gold-coated substrate is rendered hydrophobic. A uniform positively charged surface is then created on the gold substrate by depositing the Ionene-1 polymer layer on the downstroke, withdrawing through the clean water surface. A Teflon cuvette (1.5 mL volume) was immediately placed on the adsorbed side of the substrate and sealed with a rubber gasket. A 45° prism is then coupled to the other side with a refractive index matching fluid and mounted to the rotating sample stage. A tight seal is thus created by clamping the whole setup in the sample holder. Water is immediately introduced to the cell to keep the substrate surface hydrated. The solutions are introduced and withdrawn from the cell by means of a syringe. In between depositions, the cell was washed 3 times with water. All the polymer solutions used consist of 1 × 10-2 M concentrations in Milli-Q quality water. The surface plasmon spectroscopy (SPS) setup is based on the Kretschmann configuration which is essentially an ATR technique. The details of the setup have been described previously.12 In summary, the goniometer is accurate up to 0.1°. p-Polarized light from a HeNe laser, λ ) 632.8 nm, illuminates the prism and is mechanically chopped in conjunction with a lock-in amplifier. The light signals were then reflected to a phase sensitive photodiode array detector. The dc signal is then stored and graphed in a computer after analogue to digital conversion to produce reflectivity angle of incidence scans. A total of three scans were taken for every polymer that is adsorbed to the surface: before the polymer is injected to the cell, after 8 min, and then after 15 min. Scans also were taken as a function of adsorption time or kinetics experiments. The incident critical angle increased with adsorption. Data were fitted using the Fresnel theory by assuming an idealized layer model.13 That is the layers are isotropic and the substrate is flat. These fits fix the real and imaginary components of the relative permitivity (and thus refractive index) and yield the film thickness.
Data and Results In order to create a uniformly charged first layer, Ionene-1 with a long alkyl chain was initially deposited by LBK transfer. The Ionene-1 forms good monolayers, because the ammonium groups are oriented and can be (9) Wang, J.; Meyer, W.; Wegner, G. Acta. Polym. 1995, 46, 233. (10) Bain, C.; Troughton, E.; Tao, Y.; Evall, J.; Whitesides, G.; Nuzzo, R. J. Am. Chem. Soc. 1989, 111, 321. (11) Miller, C. E.; Meyer, W. H.; Knoll, W.; Wegner, G. Ber. Bunsenges. Phys. Chem. 1992, 96 (7), 869. (12) Aust, E. F.; Ito, S.; Sawodny, M.; Knoll, W. Trends Polym. Sci. 1994, 2, 9. (13) Cowen, S.; Sambles, J. Opt. Commun. 1990, 79, 427.
Letters Table 1. Average Layer Thickness for Each Polymer with Specific Pairs Are Shown Including the Corresponding Standard Deviation for the Negative Polymersa charged thickness slope of adsorption polymer in nm standard curve: nm pair pair (0.01 M) (average) deviation layer number 1 (-) (+) 2 (-) (+) 3 (-) (+) 4 (-) (+) a
PAZO Ionene-2 PES Ionene-2 PSS Ionene-2 PSS Ionene-3
1.55 0.63 0.88 0.46 0.70 0.78 1.62 3.00
0.27
1.09
0.15
0.63
0.20
0.82
0.26
polynomial
The slopes of the curve in reference to Figure 3 are also included.
Table 2. Thickness Measurements on the Alternate Deposition of PAZO and Ionene-2a polymer adsorbed 0.01 M PAZO (-), Ionene-2 (+)
thickness in nm n ) 1.5, λ ) 633 nm
PAZO Ionene-2 PAZO Ionene-2 PAZO Ionene-2 PAZO Ionene-2 PAZO Ionene-2 PAZO Ionene-2 PAZO Ionene-2 PAZO
1.5 1.0 1.6 0.7 1.2 0.4 1.0 0.4 1.7 0.8 1.9 1.0 1.6 0.8 2.0
a Notice the alternating high and low values of the thickness with PAZO and Ionene-2 adsorption, respectively. An index of refraction, n ) 1.5 was assumed for the calculations at λ ) 633 nm. Curve fitting and thickness data extraction were done using the SPALL 4 software.
transferred to a substrate with the cationic charge exposed to the surface. Contact angle measurements verified the presence of the hydrophilic layer after the substrate was removed through the clean water compartment.14 The solution cells were assembled and reflectivity-angle scans were made. All fittings of the plasmon curves assume a refractive index value of 1.5.15 This assumption should be validated by measuring in a range of conditions such as air, water, and different electrolyte concentrations against its fit in the Fresnel equations. Using this value, the mercaptan and the initially adsorbed Ionene-1 gave a reasonable thickness of 4.0 nm against water. The results of the intitial studies are shown in Figure 2. For various combinations, it is evident that the degree of shift in the plasmon curve is unique. Some important observations are as follows: The Ionene-3 and PSS (positive and negative) polymer combination showed the biggest shift in the plasmon curve with an average thickness of 3.0 and 1.62 nm for Ionene-3 and PSS, respectively. PES and Ionene-2 and PSS and Ionene-2 showed the smallest thickness changes with every adsorption layer. The average thickness values for each polymer are shown on Table 1. To some extent, these values seem to be unique for each pair. For example, a large difference is observed in comparing the PSS thick(14) Advincula, R. C.; Aust, E.; Steffen, W.; Meyer, W.; Knoll, W. Submitted to Polym. Adv. Technol., PAT 95 Conference proceedings. (15) Knoll, W.; Hickel, W.; Sawodny, M.; Stumpe, J. Makromol. Chem. Macromol. Symp. 1991, 48/49, 363-379.
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Figure 3. Plots of the thickness (nm) vs layer number for the combinations: (a) Ionene-3/PSS; (b) Ionene-2/PAZO; (c) Ionene2/PES; (d) Ionene-2/PSS. The fitting was done using linear regression analysis. Ionene-3/PSS did not follow a linear behavior but a polynomial fit.
ness between the Ionene-2/PSS and the Ionene-3/PSS pairs. Also, Ionene-2 gave different average thickness values for each pair. The large standard deviation may be due to the technique used in introducing the solution, which could be improved by using a continuous flow system. These values are also thicker than that observed in literature, in particular for PSS and PAZO. Previous evaluation of the films was usually done on dried samples, or drying is introduced between every adsorption.16 In this case, it could correspond to a swelled layer structure due to continuous exposure to the aqueous subphase. The same effect has been observed by Ramsden et al. in the characterization of adsorbed polyelectrolyte multilayers with and without the solvent.4 Thus, more “loops” and “tails” are exposed to the aqueous subphase.17 To verify this, the surface coverage (Γ) should be calculated to determine the extent of these loops and tails.18 The surface coverages can be examined even by SPS through calibration with a Langmuir adsorption isotherm, i.e., determining the thickness and refractive index as a function of concentration. X-ray reflectivity measurements will be useful for distinguishing the thickness values from the refractive index component. The factors affecting this behavior may also be attributed on the adsorption mechanism being influenced (16) Decher, G.; Hong, J.; Schmitt, J. Thin Solid Films 1992, 210, 211, 831. (17) Singer, S. J. J. Chem. Phys. 1948, 16, 872. (18) de Feijter, J.; Benjamins, J.; Veer, F. Biopolymers 1978, 17, 1759.
by the conformational requirements of the polymer chains in solution.19 The possible conformations and degree of hydration for each polymer may be influenced by the the oppositely charged polymer it is paired with, e.g., in the case of PSS. Between the positive polymers, Ionene-3 has the most conformational requirements owing to the conformations available to the piperidine ring, e.g., halfchair and boat, and the isomerizations of the double bond, e.g., cis and trans. On the other hand, Ionene-2 is simply governed by the motions of the methylene spacers.20 Although, PAZO has an ethylenic main chain, it has a large chromophoric side group. It is interesting to note that Ionene-3 and PAZO gave the larger average thickness values between pairs as verified by time dependent adsorption measurements (Figure 4). This influence was also observed as alternating steps in thickness increase, as in the case of Ionene-2/PAZO combination (Table 2). However, it is not clear how much the PAZO chromophore group actually contributes to the absorption (′′) component of the refractive index. In principle, differentiation experiments can be made by substituting another polymer of a similar size but without the absorbing group. The hydration of the polymer chains and the charge distribution per monomer unit affect the dx∆n values from SPS. Swelling results in a significant inclusion of water or increase in the external excluded volume. Contraction (19) Elias, H. G. Macromolecules. 1 Structure and Properties; Plenum Press: New York, 1984. (20) Rietz, R., Meyer, W. Polym. Adv. Technol. 1993, 4, 164.
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Figure 4. Time dependence adsorption measurements for the five polymers. Initial surface for adsorption consisted of five previously deposited layers. The solution (0.01 M) of the polymer was injected and immediately the scan was taken at various time intervals up to 7000 s.
or high electrostatic interaction results in the reduction of the internal excluded volume.19 The extent by which each polymer is affected by these properties may be tested by changing the ionic strength and pH of the solution. The presence of salt in the adsorption solutions and its effect on the layer thickness have previously been observed only by X-ray reflectivity measurements after drying of the polyelectrolyte films.21 Thus, in various combinations the average thickness value may change according to the pair and the salt concentration. Assuming that the films are near solution equilibria within the time frame of measurement, a generally linear behavior is observed for all systems as shown in Figure 3.22 A clear case of nonlinearity is shown for the Ionene-3 and PSS combination. The curve appears linear only at higher layer numbers. This may be attributed to a poorly charged initial adsorption surface or possible “induction period” before a uniformly charged surface excess is created for subsequent layers.23 It has been shown since then that a linear plot can be obtained from the lower regime when adsorption begins on a uniformly charged surface. (21) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1944, 244, 772 (22) Cheung, J.; Fou, A.; Rubner, M. Thin Solid Films 1944, 244, 985. (23) Decher, G., private communication.
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The adsorption behavior monitored as a function of time for each polymer gave a measure of the adsorption equilibrium. The measurements were done after the addition of five layers to preclude the possibility of having an initial poorly charged surface. It is observed that at the first 15 min, almost 90% of adsorption takes place.24 For the next 3 h, there is no significant change of the thickness. This result seems to verify the assumption of near solution equilbria within the time frame of the adsorption experiments. Measurements were done even up to 24 h for Ionene-3 and PSS and no significant uptake is observed. It seems that once the surface excess charge is created after the first 15 min the attraction is replaced by repulsion. Thus the measurements are done within the adsorption equilibrium regime. Fluctuations in thickness with longer time may be due to rearrangements within a previously adsorbed layer or solution equilibria.25 Accurate changes in the attractive and repulsive forces for these films may be determined by surface force investigations.26-28 Conclusion Our initial studies have shown the possibilities of doing in situ surface plasmon spectroscopy during the adsorption of polymers on oppositely charged surfaces. Other parameters that can be studied further for a particular combination are the Mw of the polymers, concentration of the solution, temperature of adsorption, salt content, pH, etc. Careful combination of these parameters may give valuable information on some of the solution properties of polyelectrolytes, in particular, their swelling behavior, adsorption kinetics, charge density, conformational behavior, hydrophobicity, etc. These data should be complementary to other techniques such as X-ray reflectivity, quartz crystal microbalance, and ellipsometry. Further investigations will be made by our group. Acknowledgment. Rigoberto C. Advincula acknowledges the Alexander von Humboldt Stiftung for the fellowship. LA9601622 (24) Ferreira, M.; Cheung, J.; Rubner, M. F. Thin Solid Films 1994, 244, 806. (25) Okahata, Y.; Kimura, K.; Ariga, K. J. Am. Chem. Soc. 1989, 111, 9190. (26) Israelichvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans 1 1978, 74, 975. (27) Lowack, K.; Helm, C. Macromolecules 1995, 28, 2912. (28) Hartley, P.; Bailey, A.; Luckham, P.; Batts, G. Colloids Surf., A 1993, 77, 191.
