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Langmuir 2009, 25, 1224-1232
Layer-by-Layer Deposition of Polyelectrolytes. Dipping versus Spraying Marta Kolasinska,*,† Rumen Krastev,† Thomas Gutberlet,‡ and Piotr Warszynski§,| Max-Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany, Laboratory for Neutron Scattering, ETH Zu¨rich & Paul Scherrer Institute, 5232 Villigen PSI, Switzerland, and Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Cracow, Poland ReceiVed October 15, 2008. ReVised Manuscript ReceiVed NoVember 10, 2008 We studied the properties of polyelectrolyte multilayer films prepared using the technique of polyelectrolyte deposition from solution (dipping) or supplying the solutions to the surface by spraying. The quality of films obtained by those two techniques was compared to find out whether the well-established dipping procedure can be replaced with the spraying technique. Neutron and X-ray reflectometric studies were performed on the samples of interest. We found that multilayers prepared by dipping are thicker, denser and less rough than films having the same number of layers, i.e., having the same number of deposition cycles, obtained by spraying.
Introduction Polyelectrolytes (PEs) are macromolecules possessing ionic groups along the chain. Their structure is determined by the combination of long-range Coulombic forces and intrachain van der Waals interactions with the constraints of chain connectivity, resulting in many specific features, which makes PEs an interesting subject to study.1,2 A thorough understanding of properties of PEs has become increasingly important in biochemistry and molecular biology, as all proteins as well as DNA are PEs.3 Because of their specific features, PEs are used to modify surfaces of various materials. They can form multilayer nanostructures. It has a broad potential application (see refs 4-9 and references cited therein). The assembly of consecutive (oppositely charged) layers is possible as a result of charge reversal on the surface of the previous layer during the deposition process. Charge overcompensation was proved by ξ potential measurements.10,11 The procedure of PE thin film formation by sequential adsorption of polycation and polyanion layers, layer-by-layer (LbL) deposition, has progressed significantly being an efficient method for obtaining various materials of exactly defined properties.12-14 The versatility of multilayer formation process * Corresponding author. E-mail:
[email protected]. † Max-Planck Institute of Colloids and Interfaces. ‡ ETH Zu¨rich & Paul Scherrer Institute. § Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences. | Present address: Forschungszentrum Ju¨lich GmbH, Ju¨lich Centre for Neutron Science, Lichtenbergstrasse 1, 85747 Garching, Germany. (1) Hara, M. Polyelectrolytes. Science and Technology; Marcel Dekker, Inc.: New York, 1992. (2) von Solms, N.; Chiew, Y. C J. Stat. Phys. 2000, 100, 267–277. (3) Radeva, T. Physical Chemistry of Polyelectrolytes; Marcel Dekker, Inc.: New York, 2001. (4) Decher, G., Schlenoff, J. B. Multilayer Thin Films; Wiley-VCH: New York, 2003. (5) Scho¨nhoff, M. J. Phys.: Condens. Matter 2003, 15, R1781–R1808. (6) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319–348. (7) Shi, X.; Shen, M.; Mo¨wald, H. Prog. Polym. Sci. 2004, 29, 987–1019. (8) Decher, G. Science 1997, 277, 1232–1237. (9) Zhang, X.; Chen, H.; Zhang, H. Chem. Commun. 2007, 14, 1395–1405. (10) Castelnovo, M.; Joanny, J. F. Langmuir 2000, 16, 7524–7532. (11) Adamczyk, Z.; Zembala, M.; Kolasin´ska, M.; Warszyn´ski, P. Colloids Surf., A 2007, 302, 455–460. (12) Scho¨nhoff, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 86–95. (13) Riegler, H.; Essler, F. Langmuir 2002, 18, 6694–6698. (14) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430–442.
with respect to a variety of support materials, in combination with other assembly techniques and the possibility of incorporation of different functional species into multilayers, results in extreme interest in such PE structures.4-9,12,14 They are of special importance in the biomaterials area, and they can be used in the field of chemical and biochemical sensing. They are perfect cushions for studies of lipid bilayers or membrane systems in general, as they can provide neutral conditions, leading to undisturbed structure of the sample of interest.15,16 Polyelectrolyte multilayers (PEMs) are prepared by sequential adsorption of polyions (LbL) either by dipping the substrate in solutions of PEs8 or by spraying PE solutions onto a vertically held sample17 (see Figure 1a,b). The basis of the LbL deposition technique carried out by dipping is depicted in Figure 1a. Substrate (in our case, negatively charged) (1) is immersed into an aqueous solution of polycation (2). After a specific time needed for a layer to adsorb on the substrate, the sample is rinsed with rinsing solution (3) to remove weakly bounded polycation molecules in order to avoid their bulk reaction with polyanions, which could happen during the following adsorption step. After rinsing, the monolayer film is obtained (5). As the adsorption of polyions leads to the overcompensation of the original surface charge, in the following step, deposition of oppositely charged polyions (in this case, polyanions) occurs when positively charged film is dipped into polyanion solution (6). The procedure is repeated until the required number of layers is obtained, and rinsing is demanded after every single deposition step. Because of the long deposition time of single-layer deposition, the procedure is time-consuming and inconvenient for routine sample preparation. An alternative for the classical LbL technique is the consecutive spraying of polyion solutions on the sample instead of dipping of the sample into PE solutions.17 In this case, the polyion solutions and rinsing solutions are supplied by enforced spraying. The procedure requires two spraying episodes per single layer with waiting time in between. This waiting time is to allow for the solution drainage. The second supply of the same polyion refills (15) Delajon, C.; Gutberlet, T.; Steitz, R.; Mo¨hwald, H.; Krastev, R. Langmuir 2005, 21, 8509–8514. (16) Chen, J.; Dong, W.-F.; Mo¨hwald, H.; Krastev, R. Chem. Mater. 2008, 20, 1664–1666. (17) Schlenoff, J. B.; Dubas, S. T.; Farhat, T. Langmuir 2000, 16, 9968–9969.
10.1021/la803428f CCC: $40.75 2009 American Chemical Society Published on Web 12/10/2008
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Figure 2. Structural formulas of the polyions applied in the described studies.
Figure 1. Scheme of alternating adsorption cycle: (a) dipping; (b) spraying. (1) Initial negatively charged substrate, (2) polycation solution, (3) first adsorbed layer with weakly bounded chains, (4,8) rinsing solutions, (5) first irreversibly adsorbed layer, (6) polyanion solution, (7) two layer film with weakly bounded chains, (9) two-layer film after rinsing.
the loss of material removed by drainage.18 Spraying with rinsing solution is needed after a complete layer is formed. Then, after rinsing, there is drainage time after which the layer of oppositely charged polyion can be sprayed.18 The main advantage of spraying is the meaningful reduction of time needed for multilayer formation.17,18 Time needed for preparation using spraying is reduced by a factor of 64 comparing to dipping. Studies on PEM include their thickness, hydrophobicity, dependence of their properties on ionic strength19 or pH value during multilayer formation,20 dependence of film structure on the presence of the anchoring layer,21 swelling, and water uptake.22 (18) Izquierdo, A.; Ono, S. S.; Voegel, J.-C.; Schaaf, P.; Decher, G. Langmuir 2005, 21, 7558–7567. (19) Carriere, D.; Krastev, R.; Scho¨nhoff, M. Langmuir 2004, 20, 11465– 11472. (20) Elzbieciak, M.; Kolasinska, M.; Warszynski, P. Colloids Surf., A: Physicochem. Eng. Aspects 2008, 321, 258–261. (21) Kolasinska, M.; Krastev, R.; Warszynski, P. J. Colloid Interface Sci. 2007, 305, 46–56. (22) Kolasinska, M.; Krastev, R.; Gutberlet, T.; Warszynski, P. Prog. Colloid Polym. Sci. 2008, 134, 30–38.
An important property is the stability upon exposure to solutions of electrolyte. According to our previous studies, multilayers prepared by dipping turned out to be sensitive to exposure to electrolyte solution. After treatment with electrolyte solutions, the water uptake by specific multilayers decreased, comparing with water uptake by nontreated samples.22 We aimed to find out whether the dipping process can be replaced by a much faster spraying procedure without waning the quality of the obtained multilayers. For that reason, structures and properties of PEMs prepared by dipping or spraying were determined and compared. The samples were characterized using neutron (NR) and X-ray (XRR) reflectometry. The experiments were performed in liquid water, water vapor, and dry nitrogen. The techniques used allow one to obtain information on thickness, density, roughness, swelling, and water uptake by PEMs. We aimed to compare the “pH” response of the studied samples. For that purpose, multilayers were checked for stability at various pH conditions. Films were exposed to solutions of different pH value and the same ionic strength. After such treatment, the structure of the multilayers was investigated. Different termination of PE films was also taken into account and films with polycation or polyanion as the top layer were studied.
Materials and Methods Materials. The PEs used were poly(allylamine hydrochloride) (PAH) with a mean molecular weight of 70 kDa and branched poly(ethyleneimine) (PEI) of molecular weight 750 kDa as polycations, and poly(sodium 4-styrenesulfonate) (h-PSS) of 70 kDa and perdeuteurated PSS (d-PSS) of molecular weight ca. 80 kDa were used as polyanions. PAH, PEI, and PSS were purchased from Sigma-Aldrich (Germany), and d-PSS was obtained from Polymer Standards Service (Mainz, Germany). The molecular structures of the polyions used are depicted in Figure 2. NaCl (99.5%) and HCl were obtained from Fluka (Germany). H2SO4 96% and H2O2 32% used for the silicon cleaning procedure were from Aldrich (Germany). Deuterium oxide (D2O; 99.9%) used in some of the NR experiments was from Aldrich (Germany). Aqueous solutions were prepared using a Mili-Q Plus 185 water generation system with a resistance of over 18 MΩ (Milipore). Silicon blocks were used as support materials for PE deposition studies by NR. They were of dimensions 8 cm × 5 cm × 1.5 cm and orientation 〈100〉 (Siliciumbearbeitung Andrea Holm (Tann/Ndb., Germany)). Silicon wafers were applied for XRR. They were of orientation 〈100〉 ( 0.5° and diameter 12 cm from On Semiconductor (Roznov, Czech Republic).
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Figure 3. NR curves (left) (points: experimental results; lines: best fits to the experimental points) and SLD profiles (right) for all the studied samples exposed to liquid D2O.
Both silicon blocks and wafers were cleaned before use with piranha solution, which is a mixture of equivalent volumes of concentrated sulfuric acid and perhydrol. (Precaution! This solution is a Very strong oxidizing agent and should be handled carefully.) Silicon was dipped into piranha for 30 min and then carefully rinsed with hot Milli-Q water followed by 30 min of dipping into hot water (ca. 70 °C). Samples and Experimental Procedures. Samples were prepared on Si supports. They had the following structures: (i) 13 layer sample with a structure Si/PEI(PSS/PAH)6 (terminated with the polycation layer) and (ii) 14 layer sample with the structure Si/PEI(PSS/PAH)6/PSS (terminated with the polyanion layer). The samples were prepared either by dipping or by the spraying technique. Deposition of the PEs was performed for both techniques from 0.15 M NaCl solutions at PAH and PSS concentrations of 0.5 g/L. PEI solution was prepared in water, without any addition of salt. Dipping Technique. Deposition was carried out as follows: each adsorption step took 20 min, and rinsing in between was done three times for 2 min in water. Spraying Technique. The polyion solutions and water for rinsing were supplied by enforced spraying using an Air-Boy container from Carl Roth. The sequence of spraying was as follows: each polyion was sprayed twice for 2 s with 2 s pauses in between; rinsing step: 10 s of constant spraying with rinsing
solution. The duration of every spraying step was in the same range as described in the literature.18 The number of PE layers in the samples was chosen for reflectometric studies to obtain reliable data in the ranges accessible by the reflectometry techniques.23 Choosing positively and negatively terminated samples was devoted to investigate the response of the sample to the charge of the last PE layer, the so-called odd-even effect.24 To ensure high scattering contrast to polymer layers, D2O having a scattering length density (SLD) of 6.4 × 10-6 Å-2 was used instead of H2O (SLD ) -0.56 × 10-6 Å-2) as a subphase. Thus, the uptake of D2O into PE films and reversibility to swelling in respect to dry N2 atmosphere were studied with high sensitivity.21,24 A lack of scattering contrast would be expected between fully protonated PE films and the Si support for measurements against N2 because of the similar values of SLDs of these two phases.15 Similar problem could have existed for experiments carried out in liquid D2O as the subphase using fully deuterated polymers. Therefore, to obtain the highest possible contrast for all experiments and to study the very same samples in all mentioned conditions, films were prepared with moderately deuterated PEs using an equimolar mixture of h-PSS and d-PSS as the polyanion. (23) Tolan, M. X-Ray Scattering from Soft-Matter Thin Films; Springer: New York, 1999. (24) Scho¨nhoff, M.; Ball, V.; Bausch, A. R.; Dejugnat, C.; Delorme, N.; Glinel, K.; von Klitzing, R.; Steitz, R. Colloids Surf., A: Physicochem. Eng. Aspects 2007, 303, 14–29.
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Figure 4. NR curves (left) (points: experimental results; lines: best fits to the experimental points) and SLD profiles (right) for all the studied samples exposed to N2. Table 1. Parameters (Thickness, SLD, and Roughness) Giving the Best Fit to the NR Experimental Results Presented in Figures 3-5 dipping 13 layers 14 layers
D2Oliq N2 D2Ovap D2Oliq N2 D2Ovap
spraying
thickness /nm
F × 104/nm-2
roughness/nm
thickness/nm
F × 104/nm-2
roughness/nm
20.7 ( 1.2 14.6 ( 1.7 19.4 ( 1.4 21.7 ( 1.4 16.4 ( 0.9 20.2 ( 1.1
5.1 ( 0.2 2.5 ( 0.1 3.8 ( 0.2 5.0 ( 0.1 2.6 ( 0.1 4.2 ( 0.1
1.0 ( 0.3 1.5 ( 0.7 2.6 ( 0.6 0.5 ( 0.1 1.1 ( 0.3 2.2 ( 0.4
14.1 ( 1.0
5.1 ( 0.1
2.4 ( 0.4
12.2 ( 0.7 15.5 ( 0.5
3.6 ( 0.2 5.1 ( 0.3
2.2 ( 0.8 1.5 ( 0.4
13.9 ( 0.8
4.3 ( 0.3
2.0 ( 0.6
The stability of the multilayers upon exposure to electrolyte solutions was determined after the samples’ immersion into HCl or NaOH of ionic strength 10-3 M and pH values 3 or 11, respectively. Duration of that post-treatment was 48 h. Afterward, films were thoroughly rinsed with H2O and then with D2O. NR experiments were carried out against D2O. Experimental Techniques. Neutron Reflectometry. NR experiments were performed at the neutron reflectometer AMOR at the Paul Scherrer Institute, Villigen, Switzerland,25 in timeof-flight (ToF) mode at three angles of incidence (0.4, 0.9, and 1.5°), covering the whole necessary Q range. Every single experiment took ca. 8 h. The samples were studied against liquid D2O directly after their preparation and without any drying in between. These experiments were performed using a solid/liquid experimental cell, which was described in detail in ref 15. The experiment against liquid D2O was followed by drying the sample and measuring it in dry nitrogen atmosphere in a gastight cell.
The cell was described in ref 26. Final measurements were performed when the samples were exposed to D2O vapors. The NR experiments in dry atmosphere or water vapor medium were complemented with XRR experiments using the same gastight experimental cell. X-ray Reflectometry. XRR experiments were performed using a triple axis diffractometer built at the Helmholtz Centre Berlin for Materials and Energies. The instrument has a horizontal scattering plane. A detailed description of the instrument is given elsewhere.21 The background signal was directly subtracted from the specular signal to obtain the corrected intensity for the both techniques. The reflectivity data were footprint-corrected for the varying flux on the sample as Q increased. The information that can be extracted from a single reflectivity experiment includes the film thickness d, the SLD (in the case (25) http://kur.web.psi.ch/amor/. (26) Krasteva, N.; Krustev, R.; Yasuda, A.; Vossmeyer, T. Langmuir 2003, 19, 7754–7760.
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Figure 5. NR curves (left) (points: experimental results; lines: best fits to the experimental points) and SLD profiles (right) for all the studied samples exposed to D2O vapor. Table 2. Swelling of PEMs Prepared by Dipping or by Spraying 13-Layer Positively Charged and 14-Layer Negatively Charged Samples dipping 13 layers 14 layers
D2Oliq D2Ovap N2 (XRR) D2Oliq D2Ovap N2 (XRR)
spraying
thickness/nm
swelling dswollen/ddry
thickness/nm
swelling dswollen/ddry
20.7 ( 1.2 19.4 ( 1.4 14.1 ( 0.9 21.7 ( 1.4 20.2 ( 1.1 15.2 ( 0.9
1.47 1.38
14.1 ( 0.6 12.2 ( 0.7 10.4 ( 0.9 15.5 ( 0.5 13.9 ( 0.8 11.5 ( 1.1
1.36 1.17
of neutrons) or electron density (in the case of X-rays) profile F(z) across the film, and the surface roughness σ between the different layers. The experimentally obtained reflectivity curves were analyzed by applying the standard fitting routine Parratt 32.27 It determines the optical reflectivity of neutrons (X-rays) from planar surfaces with a calculation based on Parratt’s recursion scheme for stratified media.23,28 The film is modeled as consisting of layers of specific thickness, SLD (or electron density in the case of X-rays), and roughness, which are the fitting parameters. The model reflectivity profile is calculated and compared to the measured one. Then the model is adjusted by changing the fitting parameters to best fit the data. In our case, a single box model was sufficient. It means that a PEM is assumed to be one stratum of constant density. Introducing more complex models (consisting (27) Braun, C. Fitting Routine Paratt32, version 1.5.; HMI: Berlin, 1999.
1.43 1.33
1.35 1.21
of more independent layers with specific densities) did not improve the quality of calculated fits.
Results and Discussion The aim of our work was to study PEMs prepared from the same polyion couple (PAH/PSS) but formed with different experimental procedures: by alternating dipping in solutions of PEs4,8 or by supplying PEs to the surface by spraying.17,18 Experimental data obtained for all samples of interest are presented in Figures 3-5. They were quantitatively analyzed, and the results of the fittings are collected in Table 1. One can find there detailed information on the thickness, SLD, and roughness of the studied samples. (28) Parratt, L. G. Phys. ReV. 1954, 95, 359–369.
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Figure 6. XRR curves (points: experimental results; lines: best fits to the experimental data) for all the types of studied samples; experiments carried out in ambient conditions (dry N2).
Freshly prepared multilayers (either 13-layer-terminated with a polycation layer or 14-layer samples with polyanion as the outermost layer) were exposed to liquid D2O, and NR curves were collected (presented in Figure 3, left side). The best fits to the data are shown as solid lines. The respective SLD profiles normal to the film surface obtained for those 13- and 14-layer PEMs formed by dipping or spraying are also presented in Figure 3 (right side). The fitting parameters are summarized in Table 1. Films obtained by dipping are thicker than those prepared by spraying. There is also a difference in the roughness depending on the procedure technique. Films after dipping are smoother than those after spraying. From density profiles one can see that samples of both types have similar SLD. One can speculate that water uptake is similar for both types of samples, as D2O has a great contribution in the SLD of samples in liquid D2O because of its high SLD. After experiments in liquid water, samples were dried and measured against nitrogen. Reflectometric curves and density profiles for all samples are depicted in Figure 4. Reflectometric curves include fits (black solid lines). Pronounced fringes are needed to determine the sample’s thickness, roughness, and SLD with high accuracy. In the case of sprayed samples in dry conditions, one observes no fringes. Furthermore, reflectometric
curves for sprayed films follow a Fresnel curve for the Si/N2 interface. This fact could suggest that either there is no PE film at the interface or the contrast between the film and the support (Si block) does not exist. From previous experiments carried out in liquid water (see Figure 3), we are sure that there is PE film deposited by spraying. Also from the following experiment done after drying in water vapor (see Figure 5), one can observe a well-indicated fringe. This is a proof that the sprayed sample is irreversibly adsorbed at the interface. Thus, the only explanation of smearing out the fringes in the dry state is a lack of contrast between the film and the substrate. As our samples were prepared from partially deuterated PEs, they should be visible for neutrons at all studied interfaces. In our previous studies we did not observe any problems with lack of contrast for moderately deuterated dipped PE films.22 One can draw a conclusion that sprayed samples are much less dense compared to dipped ones. Thus their SLD matches that of Si, and the sample is not visible by neutrons. Then all samples were measured against D2O vapor. Results obtained for samples in D2O vapor are depicted in Figure 5. Similarly to the results described above, one can observe that films prepared by spraying are thinner and rougher than similar multilayers obtained by dipping.
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Figure 7. Electron density profiles for the four types of studied samples shown in Figure 6. Table 3. Mass Densities Calculated from XRR Resultsa
13 layers 14 layers a
dipping density, g/cm3
spraying density, g/cm3
1.22 1.22
1.11 1.11
Calculations based on eqs 1 and 2.
The film thicknesses (d) were obtained from the best fits to the experimental data shown in Figures 3, 4, and 6. The mass density of samples studied was estimated on the basis of XRR results obtained for dry samples. The estimation was based on the relation of electron scattering density (ESD) with the molecular volume (Vm).23 n
As the quality of sprayed dry films measured by NR is too low to use these results in quantitative analysis, we carried out XRR experiments in dry state and used the obtained data as the reference for swelling estimation (see Table 2) and for the density of dry samples. These XRR curves are depicted in Figure 6, and electron density profiles are presented in Figure 7. The swelling of the samples is defined as the film thickness in the swollen state (dswollen) (either in liquid water or in water vapor) normalized to the thickness of the dry sample (ddry) taken from XRR studies. The swelling between films obtained by dipping and spraying does not differ much. Dipped samples swell slightly more than sprayed ones. When swelling in liquid water is considered, the dipped samples swell ca. 9% more than sprayed ones for both samples (terminated with polycation or polyanion). There is a difference in the swelling of the samples in water vapor. When 13-layer films are compared, then multilayers prepared by dipping swell ca. 20% more than those having the same number of layers but obtained by spraying. For films with a polyanionic outermost layer, the difference in swelling is ca. 8% with higher swelling of dipped film as well.
ESD )
∑ Zire i)1
Vm
(1)
where Zi is the atomic number of the ith atom in the molecular volume Vm, re ) 2.814 × 10-13 cm is the classical radius of the electron, and ESD is electron scattering density, known from reflectometric experiments. As we assume uniform polymer distribution across the film, monomers of PAH and PSS were taken for the calculations of one molecule of simple formula: C11H15O3NS. Mass density calculated out of the estimated volume was based on the relation
Fm )
m Vm
(2)
where m refers to the mass of the molecule of interest and is equal to 4.001 × 10-22 g. The mass densities calculated from XRR experiments based on the present assumptions are collected in Table 3. Densities of dipped samples are 10% higher than densities of sprayed samples, which is in agreement with our expectations
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Table 4. Studied Samples against N2 after Acidic/Basic Treatmenta dipping thickness/nm 13 layers 14 layers
a
untreated pH ) 3 pH ) 11 untreated pH ) 3p pH ) 11
14.1 ( 0.9 15.4 ( 0.7 14.8 ( 1.1 15.2 ( 0.9 15.5 ( 0.7 15.2 ( 1.1
spraying -2
F × 10 /nm 3
1.1 ( 0.1 1.0 ( 0.5 1.0 ( 0.2 1.1 ( 0.1 1.2 ( 0.4 1.3 ( 0.2
roughness/nm
thickness/nm
F × 103/nm-2
roughness/nm
1.0 ( 0.4 1.0 ( 0.3 1.1 ( 0.8 0.9 ( 0.4 1.0 ( 0.5 1.0 ( 0.8
10.4 ( 0.9 10.5 ( 0.7 10.7 ( 1.0 11.5 ( 1.1 13.9 ( 0.8 12.3 ( 15
1.0 ( 0.1 1.0 ( 0.3 1.1 ( 0.4 1.0 ( 0.3 1.0 ( 0.3 1.9 ( 0.3
1.0 ( 0.6 1.1 ( 0.5 2.3 ( 0.8 1.0 ( 0.5 0.8 ( 0.6 3.9 ( 0.4
Parameters that give the best fits to the experimental data in Figure 6 are presented as electron density profiles in Figure 7.
Table 5. Comparison of Some Properties of Selected 14-Layer Films (Terminated with PSS Layer) Prepared by Dipping or Sprayinga properties
dipping
spraying
time of deposition
26 min per layer (with necessary rinsing steps) 15.2 ( 0.9 1.22 0.9 ( 0.4 1.43 stable to variable external conditions
ca. 15 s per layer (with necessary rinsing steps) 11.5 ( 1.1 1.11 1.0 ( 0.5 1.35 unstable in basic solution
thickness, nm density, g/cm3 roughness, nm swelling pH response
a Time of deposition refers to single layer preparation, and it was based on cited papers; thickness and roughness (scattering roughness) in dry state were obtained from the best fits to the experimental data shown in Figure 6. Mass density was calculated from XRR results. The swelling of the samples is defined as the film thickness in the swollen state (in liquid water) normalized to the thickness of the dry sample.
from nonfitable NR data for sprayed samples. From those results we would even expect a higher difference in density. In many applications, PEMs can be exposed to conditions different than those encountered during their formation. Therefore, full characterization of PEM systems requires determining their susceptibility to various environments. In order to extend information to the behavior of multilayers in various conditions, the same series of samples were exposed to acid/base solutions with pHs of 3/11 and respective ionic strengths of 0.001 M. After this treatment, the samples were thoroughly washed with H2O, dried, and XRR experiments were performed. The samples consisted of 13 or 14 layers built up using dipping or spraying technique. The results of these experiments are presented as reflectometry curves in Figure 6, and the SLD profiles are summarized in Figure 7. The parameters for the best fits to the data are collected in Table 4. When films prepared by dipping are considered (Figure 6, left side, and Table 4), one can observe that acidic treatment causes a slight increase in film thickness compared to untreated samples. PAH is a weak electrolyte with a pH-dependent degree of dissociation (pK ≈ 8).29 In acidic conditions, it is fully charged, and that leads to electrostatic repulsion between PAH molecules in the consecutive layers, stronger than in neutral conditions (pH ≈ 5.5), in which multilayer films were formed. As a consequence, a little swelling can be observed. However, this increase is of the level of experimental error. On the other hand, no effect of basic treatment on the thickness of given films is observed for films prepared by dipping. Practically no changes in roughness are observed for dipped films, independently of the top PE layer. Slight differences in electron scattering densities mean that pH treatment may cause some internal changes in PEM structures. However, those differences are within experimental error. One can conclude that these multilayers are stable in studied conditions. On the contrary, films prepared by spraying are less stable upon changing the external conditions. When reflectometric curves of sprayed films (29) Buron, C. C.; Filiaˆtre, C.; Membrey, F.; Perrot, H.; Foissy, A. J. Colloid Interface Sci. 2006, 296, 409–418.
before and after exposure to basic solutions are compared (see Figure 6, right side), one can observe that fringes are not as pronounced (13-layer film) or almost smeared out (14-layer film) for samples after basic treatment. It means that the internal structure of samples prepared by spraying changes upon basic treatment. The comparison of multilayers prepared by dipping or spraying is summarized in Table 5. Preparation of PEMs by alternating dipping in solutions of PEs is much more timeconsuming than by supplying PEs to the surface by spraying.17,18 The formation of one layer lasts 26 min when it is dipped and only 15 s by spraying, which means that the time needed for preparation using spraying procedure is reduced by a factor of 64. This is extremely important when multilayer films have to be prepared. As far as thickness of studied systems is concerned, films prepared by dipping are thicker than those obtained by spraying, regardless of the number of layers, termination, or interface at which they are studied. Without kinetic studies of mass adsorbed via spraying, we cannot claim whether the sprayed layer is complete. Such experiments are impossible to carry out in situ because of technical reasons. We can only speculate that spraying for such a short period of time does not cover the surface completely or that solution drainage from the sprayed film is faster than deposition of polyions on a surface, which leads to the formation of incomplete layers. A difference in mass density between dipped and sprayed samples is observed. Films prepared by dipping are denser than sprayed ones. This difference is around 10%. PEMs are used as cushions, e.g., for lipid membranes to create conditions close to natural for those membranes. Such procedure is to bind, for example, lipid bilayer to the surface to enable further experiments.15 Reflectometric studies require the cushion to be as smooth as possible in order not to perturb the signal from the membrane and, more importantly, to be able to tune the PEM’s SLD to obtain proper contrast between a substrate/cushion and a lipid membrane. For such experiments one cannot use sprayed cushions, as they are far too rough and less predictable for tuning their SLD values. Swelling of those samples is similar. When stability to external conditions is concerned, one can observe that, for the studied pair of polyions, films prepared by dipping are more stable to exposure to electrolyte solutions.
Conclusions The aim of the presented work was to compare the properties of PEM films prepared using the technique of PE deposition from solution (dipping) or supplying the solutions to the surface by spraying. We studied the quality of films obtained by those two techniques to find out whether well established LbL deposition can be replaced with spraying technique without diminishing the quality of obtained multilayers. Films obtained using spraying are thinner and rougher than multilayers having the same number of layers, i.e., having the same number of deposition cycles, prepared by dipping.
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Sprayed samples are also less stable to variable external conditions. Having the great advantage of a really short time of preparation, they cannot provide some features common for dipped PEMs such as stability or uniformity. We do not know which films are of better quality. It depends on the point of view. We claim only a difference in the films’ structure and properties. Acknowledgment. Neutron reflectometric studies were performed at the Swiss spallation neutron source SINQ, Paul Scherrer
Kolasinska et al.
Institute, Villigen, Switzerland and at The Berlin Neutron Scattering Center, Hahn Meitner Institute, Berlin, Germany.The financial support by the European Commission under the sixth Framework Programme through the KeyAction: Strengthening the European Research Area, Research Infrastructures, Contract No. RII3-CT-2003-505925 (NMI3) is highly acknowledged. M.K. acknowledges the Alexander von Humboldt Foundation for the fellowship. LA803428F
