J. Phys. Chem. 1993,97, 13773-13777
13773
Combination of Polycation/Polyanion Self-Assembly and Langmuir-Blodgett Transfer for the Construction of Superlattice Films Yuri LVOV,~ Frank Essler, and Cero Decher' Institut f i r Physikalische Chemie, Johannes Gutenberg-Universitht, J. Welder- Weg 11, 0-55099 Mains, FRG Received: July 9, 1993'
The Langmuir-Blodgett transfer (LB) of amphiphile monolayers from the air/water interface onto solid supports is a powerful tool to create periodic lattices in many varieties, usually composed of bilayers. By self-assembly adsorption (SA) of polyelectrolytes, uniform molecular films with very exact thickness adjustment are obtained. The combination of Langmuir-Blodgett transfer and polycation/polyanion self-assemblyallows a precise extension of the LB lattice, and a superlattice is formed by combination of LB and S A units. The superlattice was realized in a multicomponent film, in which additional polyelectrolyte sheets are inserted between the amphiphile headgroups. At first, Langmuir-Blodgett Alms of the pure lipid/polyelectrolyte complex DODAB/PVS (dimethyldioctadecylamonium bromide/poly(vinyl sulfate)) were prepared and a bilayer spacing of 4.43 nm was found. For the construction of film superlattices, sequences of polyelectrolyte layers were adsorbed between the PVS headgroups of the DODAB bilayer by the following method: After deposition of one bilayer, the wet substrate was removed from the Langmuir-Blodgett trough and PAH (poly(allylaminehydrochloride)) and PVS were adsorbed from their solutions. The resulting superlattice repeat unit consists therefore of one DODAB/ PVS bilayer and the inserted assembly of polyelectrolyte layers. Samples of 1,3, and 5 additional polyelectrolyte layers per unit cell were prepared and investigated with small angle X-ray reflectivity. Variation of the repeat unit of superlattice results in a linear increase of the superlattice layer spacing of 0.4 nm per additionally inserted polycation/polyanion pair. Also correlation lengths and roughnesses were estimated, and a decrease of layer correlation and an increase of roughness with increasing number of polyelectrolyte layers were found. Heating the samples to 70 O C and slowly cooling result in a remarkable improvement of the multilayer ordering.
1. Introduction
Ultrathin organic films are currently gaining interest in many areas such as integrated optics, sensors,friction reducing coatings, or surface orientation layers.14 Most of these tasks require the preparation of well-defined films composed of molecules with appropriate properties in a unique geometrical arrangement with respect to each other and to the substrate. The construction of multilayer assemblies from molecularly thin layers offers the possibility to prepare quasi-two-dimensional layered aggregates in which the distance between two molecules along the layer normal can be controlled in the angstrom range. A common method for preparation of multilayered films is the now classic Langmuir-Blodgett (LB) technique, which shows promising results in designing films with predetermined alternationsof monoand bilayers of different amphiphilic molecules. A new method of creating ultrathin filmswas developed recently in our laboratory, which is based on the electrostatic attraction between opposite charge^.^^^ Two bipolar amphiphiles with identical charges at each end of the molecule, one positively the other negatively charged, were used in this technique. Consecutively alternating adsorption of the anion and the cation leads to the formation of multilayer assemblies. Instead of bipolar amphiphiles, polyelectrolytescan also be used for the preparation of multilayer fib" The increment of growth for one polyanion/ polycation layer pair is of the order 10-100 A and can precisely be adjusted by addition of standard electrolytes such as sodium chlorideto the polyelectrolyte solution^.^ This techniqueis named as 'self-assembly" adsorption in the following. Therefore it was a challenge to combine these two techniques in order to fabricate superlattice films with predetermined alternation of lipid and polyelectrolyte layers. The formation of insoluble monolayers of complexes of ionic Permanentaddress: InstituteofCrystallography,Moscow 117333, Russia. To whom correspondence should be addresed. * Abstract published in Aduance ACS Abstracts. November 15, 1993.
0022-3654/93/2097- 13773$04.00/0
amphiphiles with polyelectrolytes dissolved in the subphase is well s t ~ d i e d . ~ JOur ~ l ~approach requires a LB system, which forms stable hydrophilic top layers on solid substrates in order to further adsorb polyelectrolytes and construct the superlattice structures. Normally LB films of low molecular weight amphiphiles are not stable, when their headgroup points to the air. The top layer will not survive a treatment with aqueous solutions, which is necessary for the self-assembly adsorption steps. So complexation of the amphiphile with oppositely charged polyelectrolyte will protect the top amphiphile layer from dissolving and also provide a charged surface for adsorption. We have chosen dimethyldioctadecylammonium bromide (DODAB) as amphiphile, complexed by the macroion poly(viny1 sulfate) (PVS) for LB deposition and the polyelectrolytes: poly(vinyl sulfate)/poly(allylamine) (PVS/PAH) for the self-assembly (SA) adsorption. DODAB itself cannot be transferred to multilayer films, but the polyelectrolyte complex with a polymericcounterion builds excellent multilayers. The structure of such LB films is normally Y-type, with the polyion layered between the amphiphile headgroups. The layer structure of this LB film can be expressed as {DODAB/PVS/DODAB),, where mis thenumberofrepeatunits. Thecombinationofbothultrathin film techniques should lead to a layer structure of (DODAB/ PVS/(PAH/PVS),/DODAB},, where n is the number of selfassembly layer pairs and m is the number of the superlattice repeat unit. We have prepared films with n = 0-3 and m = 6-10 and controlled the realization of this concept with small-angle X-ray reflectivity (SAXR). Figure 1 shows the chemical structures of the used materials and their graphic representations. As example for our new method of film construction the built up of the superlattice film (DODAB/ PVS/(PAH/PVS)l/DODAB), is schematicallyshown in Figure 2. A detailed description of the preparation steps is given in section 2. (0
1993 American Chemical Society
Lvov et al.
13774 The Journal of Physical Chemistry, Vol. 97, No. 51, I993 60
30 3 L
Q
10
0
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0.4
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area per molecule [nm2]
u
Figure3. Pressurearea per molecule(PA) isotherm at 20 OC of DODAB monolayer on a water subphase (trace 1) and on the subphase containing polyanion PVS (trace 2).
PVS
PAH Figure 1. Chemical structures of the materials and their corresponding schematic representations.
. . LB
SA
_____,
SA, LB ...
I
unit cell
Figure 2. Side view schematic depicting of the preparation steps for the superlatticesamplewith n = 1. Langmuir-Blodgett transfer of the polyion complex DODAB/PVS is marked as LB, self-assemblyadsorption steps of PAH as SA.
2. Materials and Methods Dimethyldioctadecylammonium bromide (DODAB) was obtained from SIGMA, poly(viny1 sulfate) potassium salt (PVS, molecular weight 250 000) from SERVA, poly(ally1amine hydrochloride) (PAH, from Aldrich, molecular weight 50 000). The chemicals were used without further purification. Water was purified by reverse osmosis (Milli-RO 35, Millipore GmbH) followed by ion exchangeand filtration steps (Milli-Q, Millipore GmbH). The amphiphile-polyion complex was prepared at the air-water interface by spreading a solution of DODAB onto a subphase containing 100 mg/L polyelectrolyte. LB films were prepared on a commercial LAUDA FW-1 film balance (LAUDA Konigshofen, FRG). The self-assembly films were adsorbed from acidic solutions (0.01mol/L HC1) of 0.02 M/L polyelectrolyte. Adsorption time was 16 min/per layer. The samples were prepared on glass slides, which were treated as follows. The first step was cleaning the surface ultrasonically in a hot H~S04/H202(7:3) mixture for 1 h and then washing with Milli-Q-water. Further purification was carried out with the RCA cleaning procedure in a H20/H202/NH3 (5:l:l)
mixture.l4 On the hydrophilic surface a first layer of bola dication was adsorbed, and additional layers of PVS/PAH completed the precursor film.15 The top layer in each case was PAH. On this positively charged surface the superlattices were built up with the following preparation steps: (1) The substrate with precursor film was submerged into the subphase. A monolayer of DODAB was spread and the film compressed until the transfer pressure of 35 mN/m is reached. (2)By the following upstroke and downstroke cycle one bilayer of DODAB/PVS complex was transferred (dipping speed down 7 mm/min, up 3 mm/min). The now hydrophilic substrate has to be removed from the film balance without any further LB deposition,which was realized with a floating ring of PTFE tubing. The film in the inner ring area (containing the substrate) was sucked off, and then the wet substrate was quickly removed trough the monolayer free compartment. (3) The substrate was washed three times with water for 1 min each. (4)Now the layers of PAH and PVS polyelectrolytes were consecutively adsorbed from their solutions. ( 5 ) The substrate was resubmerged into the subphase of the film balance again through the monolayer free area within the ring. After the ring was removed, a delay of 5 min was used in order to reestablish a constant transfer pressure by compression and before LB transfer was resumed. Steps 2-5 were repeated until the desired number of superlattice repeat units was reached. We have prepared four different superlattices containing 0, 1, 2, and 3 (n = 0, 1, 2, and 3) polyelectrolyte layer pairs inserted between the lipid bilayers. Each sampleconsists of 6-10 repeat units, in order to obtain good X-ray diffraction patterns. Small-angle X-ray reflectivity (SAXR) was measured in the scattering angle range 29 = 0.5-4O with 0.01' steps and 5-s sampling time on a Siemens D 500 diffractometer, using Nifiltered Cu Ka radiation of X = 0.154nm wavelength. Temperature-dependent measurements were made in the range 20180 f 1.5 OC. 3. Results and Discussion (1) DODAB/PVS LB Film. After spreading the amphiphile on the subphase, the polyion complex is spontaneously formed, because of the strong interaction of the positive charged lipids and negatively charged PVS polyelectrolyte. Figure 3 shows the pressure-area isotherm (T-A) of DODAB on pure water and on PVS subphaseat 20 "C. The complex formationis due to reduced headgroup repulsion and increased headgroup coupling by the polymeric counterions, which results in a lower requirement of area and a higher surface pressure stability for the DODAB/ PVS complex monolayer in contrast to pure DODAB. The transfer ratios of the deposited monolayer are close to unity. With light microscopy a homogeneous interference color of the multilayer films is observed. In Figure 4 a SAXR
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13775
Polycation/Polyanion Self-Assembly
n=3
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.* . . 0.0
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scattering angle 2 8 [deg.]
n=l
Figure4. X-ray reflectivity curveof a Langmuir-Blodgett film DODAB/ PVS composed of 10 bilayers. Intensity 14" is plotted versus scattering angle 28. 10"p
.. . .. .. 1
1
1
. .
. .. . ....d 1
r
[
n=O Figure6. Schematic sideview ofthe unit cellsof the prepared superlattice films (DODAB/PVS/(PAH/PVS)n/DODABJ. From bottom to top: n = 0 (pure LB film), n = 1, n = 2, n = 3.
5 0.5
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-
scattering angle 2 0 (deg.]
We
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Q)
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A A
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scattering angle 2 8 (deg.]
x
-
Figure 7. X-ray reflectivity curves of the superlattices (DODAB/PVS/ (PAH/PVS)n/DODABJ with n = 1,2,3. Intensity 1q" is plotted versus
5 -E
scattering angle 28.
ii 15
20
25
30
35
40
45
Number of layers n
Figure 5. (A) X-ray reflectivity curves of PAH/PVS self-assembly films with different numbersof layers. Intensity 14"is plotted versus scattering angle 28: 20 layers (1). 26 layer (2), 32 layers (3). 39 layers (4) and 41 layers (5). (B) Dependence of the PVS/PAH film thickness on number of deposited polymeric layers L(n).
measurement of a film consisting of 10 bilayers DODAB/PVS on an OTS (octadecyltrichlorosilane)coated silicon substrate is shown. The diffractogram shows one Bragg peak and so-called Kiessig fringes, which result from interferenceof X-rays reflected at the film/substrate and film/air interfaces, from which the film thickness dis calculated as 49.5 f 0.3 nm. The Bragg spacing D is 4.43 f 0.03 nm, which is approximately twice the lipid length and shows, that thedeposited LB film has Y-typestructure (headto-head and tail-to-tail or bilayer structure). The difference of the total films thickness of 49.5 and 44.3 nm for 10 bilayers is equal to 5.2 nm, which we assign to the OTS coating and to the native silicon oxide layer on the silicon substrate. (2) Self-Assembly Films of PAH and PVS. The self-assembly system PAH/PVS was investigated recently by Lvov et al.I5and for clearness some results are repeated here. Figure 5A shows the SAXR curves for different number of layers, which were assembled layer by layer from PVS and PAH polyelectrolyte solutions on glass substrate. In Figure 5A well-marked Kiessig fringes, whose periods becoming smaller for thicker films are shown. The film thicknesses was calculated from these periods, taking intoaccount a refraction corre1ation.l6 We found that the thickness of the film is growing linearly with the number of deposited layers in the range 16 to at least 50 polyelectrolyte
layers (for films thinner than 10 nm it is difficult to calculate the thickness from X-ray reflectivity data with the proper precision) and the growth step for a layer pair of PVS/PAH was equal to D1 = 1.35 f 0.05 nm (Figure 5B). The established knowledge about PVS/PAH film preparation is now used for the design of LB SA superlattices. (3) Superlattice Films, by Combination of LB Transfer and Polyelectrolyte Self-Assembly. We prepared the samples of superlattice films on glass supports, as described in section 2. In Figure 6 a scheme of the repeat units of the prepared films is given to illustrate the principle of our film architecture. From pure self-assembly films of PVS and PAH we known that an equilibrium of adsorption os reached a few minutes after immersion into the polymer solution. The films are stable while washing with water, so we are quite sure to exclude a desorption of the top layer during submersionof the substrate in the subphase of the film balance. The DODAB/PVS complex without additionally self-assembled interlayers (i.e., the poor LB film as described in section 3.1) has n = 0. Every next sample has one layer pair of polyelectrolyte more, i.e., in the sueprlattice expression: (DODAB/PVS/(PAH/PVS)n/DODABJ, n = 1,2, and 3. The X-ray reflectivity curves of all superlatticesare shown in Figure 7,and a pronounced Bragg reflection is observed for each sample, which proves the realization of the planned film architecture. In Table I are shown the layer spacingsD(calculated from the positions of Bragg reflections) of superlattice repeat units. In addition we give the correlation length R,, which characterizes the quality of stacking of the layers in the film (R, is calculated from the half-width of the Bragg peaks A(28) (in radian) as R,= 0.88 X/cos Om,, A(28), i.e., the Scherrer formula).
Lvov et al.
13776 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
Ik
TABLE I: Structural Parameters of Superlattice Films {DoDAB/PVS/(PAH/PVS),,/DoDAB n (Number of Inserted Layer (FBm Thickness, Excluding Pairs {PVS/PAH), m (Number of Repeat Units), D (Layer Spacing), Re (Correlation Length), the Precursor Film), J~SL(Superlattice Thickness, Calculated from 0,) 1 0 2 n 3 10 10 8 m 6 4.72 5.16 D t 0.03 nm 4.43 5.51 30.0 20.0 & t 2.0 nm 47.0 16.0 47.1 hh t 1.0 nm 47.3 42.5 33.0 &L t 0.3 nm 44.3 (4.43 x 10) 47.2 (4.72 X 10) 41.3 (5.16X 8) 33.1 (55.1 X 6) 5.6
I
I
+D
I
5 6 . 0 7
I
t 0
f
0 .
i
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0 0
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J I I I 110 0 1 2 3 4 number of lnwrted layer pairs PAHlPVS
4.41
Figure 8. Dependenceof layer spacings D and correlation length & of superlatticefilms (DODAB/PVS/(PAH/PVS),/DODAB) on the number of polymeric interlayer pairs n.
Also two independently calculated values of the total film thickness-Lfil, and hL-are shown. The films differ slightly in the precursors, which we have taken into account for the evaluation of comparable L values. In one case Lfil, was determined from the Kiessig fringes after subtraction of the precursor film thickness and thus represents the total thickness of the superlattice film excluding the precursor film. The other &L was obtained by multiplication of measured layer spacings (from the Bragg peak) with the number of repeat units for every sample. These two determinations of film thickness are independent and show a good agreement. In Figure 8 the layer spacings D and the correlation lengths R, are plotted against the number of polymeric layer pairs n inserted between the lipid bilayers. A linear dependence D(n) is observed. The increment of spacings for each additionally inserted PVS/PAH layer pair between the DODAB bilayers is 0.4 nm. The correlation length R, of the pure DODAB/PVS LB-film (n = 0) is of the order of the total film thickness, which shows a good stacking of the layers. The value R, decreases with increasing of the number of polymeric layers inserted between the lipid bilayers, so the extension of the LB layer spacing via self-assembly adsorption of polymeric sheets is therefore connected with a loss of layer correlation. In the case n = 3 the R, value shows a correlation of only three layers. This loss can partially be compensated by annealing of the films which is discussed in section 3.4. Because of the lower concentration of PVS in the subphase (6.16 X 10-4 mol/dm3 as compared to 2.0 X lo-* mol/dm3 for the buildup of the polyelectrolyte layers) and the mobility of the lipids within the monolayer, the first adsorbed PVS polyanion layer may adsorb as an almost charge-compensated layer, yielding only a small negative excess charge. This in turn might lead to the adsorption of very thin layers in the polycation/polyanion deposition step. The adsorption of very thin layers would also account for significant interpenetration of adjacent layers which could explain the observed loss of superlattice correlation with increasing number of polyelectrolyte layer pairs n. (4) TemperatureInducedChangesin Film Structure. To gather information about the temperature behavior, the superlattice structures were studied with SAXR during a heating cycle from room temperature up to 160 OC. In Figure 9 the typical temperature dependence of film thickness, spacings, correlation length of the n = 1 sample is shown, which is representative for
4.0
0
!
100 1 temperature ["C]
Figure 9. Temperature dependenceof thickness, spacingand correlation length of the film superlattice with the repeat unit: (DODAB/PVS/ (PAH/PVS)I/DODAB)lo, Le., n = 1.
the other samples too. We observe a remarkable improvement of multilayer ordering at a temperature of about 70-80 OC, probably due to the increased pretransitional mobility of the alkyl chains. The loss of ordering a t temperatures above 80 OC is connected with the phase transition, which DODAB shows at 85 O C in bulk. The melting point is known from DSC measurements'' and also found by heating the compound in a capillary. Heating to higher temperatures leads to a decrease of spacing, ordering, and total thickness of the film. We remark that all three values show a similar temperature evolution. If the films are heated to only 70 OC and then slowly cooled down, this treatment results in a sharp increase of the Bragg peak intensity. In Figure 10 this is demonstrated on then = 2 sample. The Bragg peak remains sharp after cooling to room temperature too. The correlation length & increases from 20 to 35 nm, which means in terms of layers, that & extends from four to seven layers. A new measurement after 1 month, keeping the sample at room temperature, reveals some loss of intensity, but the width of the reflection remains the same, so no loss of multilayer ordering occurs. 4. Conclusions
We have combined two techniques for the preparation of ultrathin organic films: Langmuir-Blodgett transfer and polycation/polyanion adsorption and thereby constructed organic superlattices with alternationsof lipid bilayers and polyelectrolyte
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13771
Polycation/Polyanion Self-Assembly
---40°C cooling after 1 month
1.5
2
Scatterlng angle 2 8 Ideg.1 Figure 10. X-ray reflectivity curves for film superlattice: Intensity Id is plotted versus scattering angle 20. {DODAB/PVS/(PAH/PVS)z/ DODABI8, n = 2 sample: initial; after annealing at 73 O C ; after slow cooling till 40 OC and after 1 month.
tiers of variable (0.4-1.1 nm or even more, depending on the number of interlayers) and well-defined thickness. The increment of growth for one PVS/PAH layer pair in the gap between charged lipid headgroups is about 3 times less than the one in pure polyelectrolyte films. This is not unusual, as we know from other experiments the interface at which the polyelectrolytes are adsorbed strongly influences the layer thickness. Depending on experimental conditions, this effect might carry out to layer numbers 1&15. These nanocompositesuperlattices have a unique temperature behavior: the annealing process at the temperature close to the phase transition of the lipid component (but much lower than the destruction of the films) results in a remarkable improvement of multilayer ordering in the film. The possibility of separating lipid bilayers by well-defined hydrophilic polymeric interlayers opens a wide field of experiments, where one needs to separate different molecular layers in the range 0.5-5 nm. The construction of layered hydrophilic/ hydrophobiccompartments in which both regions can be adjusted easily and without synthesizing appropriate molecules is very difficult to achieve otherwise. It should be possible to replace
simple polyelectrolytes by more functional ones and thus create more interesting nanocomposites. We especially believe that lipid bilayers on polyelectrolytefilms that are highly swollen by buffer can be used as supported membranes for the incorporation and study of membrane proteins. Acknowledgment. We thank Professor H. MBhwald for stimulating discussions; the Bundesministerium fiir Forschung und Technolgie and the Fond der Chemischen Industrie for financial support. The bola dication solutionwas kindly donated from J.-D. Hong. Y.L thanks Alexander von Humboldt Foundation (FRG) for supportinghis workat the Johanna GutenbergUniversitit in Mainz. Also many thanks are due to A. Leuthe and Dr. H. Riegler for the discussion of the X-ray data and the improvements on the reflectometer.
References a d Notes (1) 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. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (3) Special Issue: Organic Thin Films, Adu. Mater. 1991,3, 3. (4) Roberts,G. G. Langmuir-BlodgetfFilms; PlenumPress: New York, 1990. (5) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol.Symp. 1991, 46, 321. (6) Decher. G.; Hong, J.-D. European Patent No. 0,472,990.A2. (7) Decher, G.; Hong, J.-D. Eer. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (8) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992,2lO/ 211, 831. (9) Decher, G.; Schmitt, J. Prog. Colloid. Polym. Sci. 1992,89, 160. (10) Shimomura, M.; Kunitake, T. Thin Solid Films 1985,132,243. (1 1) Higashi, N.; Kunitake, T. Chem. Lett. 1986,105. (12) Miyano, K.; Asano, K.; Shimomura, M. Langmuir 1991,7, 444. (13) Royappa, A. T.; Rubner, M. F. Langmuir 1992,8,3168. (14) Kern, W. Semiconduct. Int. 1%. 94. (15) Lvov, Y.; Decher, G.; MBhwald, H. Langmuir 1993,9,481. (16) Rieutord, F.; Benattar, J. J.; Bosio, L.; Robin, P.; Blot, C.; Kouchkovskv. R. J . Phvs. 1987. 48. 619. (17) Okdyama, K.;Hoso, K:; Maki, N.; Hamatsu, H. Thin Solid Films 1991, 203, 161.
