Polyelectrolyte Multilayers as Nanocontainers for Functional

Jun 24, 2003 - ... Université catholique de Louvain, Place Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium (European Union), and Unité de Chimie d...
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Langmuir 2003, 19, 6178-6186

Polyelectrolyte Multilayers as Nanocontainers for Functional Hydrophilic Molecules Erwan Nicol,†,‡,§ Jean-Louis Habib-Jiwan,*,‡ and Alain M. Jonas*,† Unite´ de Physique et de Chimie des Hauts Polyme` res, Universite´ catholique de Louvain, Place Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium (European Union), and Unite´ de Chimie des Mate´ riaux Inorganiques et Organiques, Universite´ catholique de Louvain, Place L. Pasteur, 1, B-1348 Louvain-la-Neuve, Belgium (European Union) Received May 16, 2003 We report on the introduction of small organic hydrophilic molecules (fluorescein, rhodamine B, and two coumarin-based dyes) in multilayers of strong polyelectrolytes, studied by X-ray reflectometry, UV/visible spectroscopy, and fluorescence measurements. Very low diffusion coefficients (about 10-17 cm2‚s-1) were found for the inward diffusion of fluorescein in preformed multilayers. In addition, diffusion was accompanied by substantial variations of the thickness of the multilayer (up to 300%), ruling out the practical significance of inward diffusion as a tool to dope multilayers. We then attempted to coadsorb the fluorophores simultaneously with the polyions during the construction of the multilayer. However, displacement of small molecules by polyions of identical charge and outward diffusion of the fluorophores during the rinsing step resulted in very limited inclusion of the dye by this procedure. This issue was solved by introducing the fluorophore in all baths, including the rinsing ones. Then, the concentration of the multilayers in dye is directly related to the concentration of the dipping solutions and is dependent on the nature of the dye and of the multilayer. The outward diffusion of the fluorophores from these multilayers was studied, and very low diffusion coefficients were again determined, depending on the net charge of the dye. The ability to load rapidly polyelectrolyte multilayers with a variety of hydrophilic organic molecules of small molar mass, in tunable concentration, is a major outcome of the present study. It offers new opportunities to use these multilayers as templates for the confinement of active molecules in functional devices.

Introduction Electrostatically self-assembled multilayered films have been extensively studied over the past 10 years.1,2 The sequential adsorption of polycations and polyanions from dilute solutions is a simple, cheap, and robust method allowing a high degree of control over film thickness in the nanometer range. A large variety of charged macromolecules or nanoparticles have been successfully assembled by this method, including, for example, proteins,3,4 DNA,5 viruses,6 inorganic platelets,7,8 or dyes.9-16 Various * To whom correspondence should be addressed. E-mail: jonas@ poly.ucl.ac.be; [email protected]. † Unite ´ de Physique et de Chimie des Hauts Polyme`res. ‡ Unite ´ de Chimie des Mate´riaux Inorganiques et Organiques. § Present address: Polyme ` res, Colloı¨des, Interfaces, UMR CNRS 6120, Universite´ du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex, France. (1) Decher, G. Science 1997, 277, 1232. (2) Arys, X.; Jonas, A. M.; Laschewsky, A.; Legras, R. In Supramolecular Polymers; Ciferri, A., Ed.; Marcel Dekker: New York, 2000; p 505. (3) Hong, J. D.; Lowack, K.; Schmitt, J.; Decher, G. Prog. Colloid Polym. Sci. 1993, 93, 98. (4) Lvov, Y.; Ariga, K.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (5) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (6) Lvov, Y.; Haas, H.; Decher, G.; Mo¨hwald, H.; Michailov, A.; Mtchedlishvily, B.; Margunova, B.; Vainshtain, B. Langmuir 1994, 10, 4232. (7) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (8) Glinel, K.; Laschewsky, A.; Jonas, A. M. Macromolecules 2001, 34, 5267. (9) Yoo, D.; Wu, A.; Lee, J.; Rubner, M. F. Synth. Met. 1996, 85, 1425. (10) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (11) Kometani, N.; Nakajima, H.; Asami, K.; Yonezawa, Y.; Kajimoto, O. Chem. Phys. Lett. 1998, 294, 619. (12) Fukumoto, H.; Yonezawa, Y. Thin Solid Films 1998, 327-329, 748.

recent reviews highlight the potentialities and versatility of electrostatic self-assembly, ESA (also known as layerby-layer assembly, LBL).1,2,17,18 There are different ways by which functional LBL multilayers can be obtained: grafting of polyelectrolytes by functional moieties,13,19 alternate deposition of polyelectrolytes and functional molecules,9-13,20 or postdiffusion of the molecules in the multilayers.21-25 These methods are not without drawbacks: Chemical grafting may prove to be difficult for some molecules, and the grafted polyelectrolyte may not give rise to regular growth; alternate deposition of molecules of limited charge often fails, due to rapid desorption of small molecules;14 and postdiffusion of molecules in multilayers of strong polyelectrolytes is a slow process of limited practical applicability as will be shown here. It is therefore highly (13) Wu, A.; Lee, J.; Rubner, M. F. Thin Solid Films 1998, 327-329, 663. (14) Linford, M. R.; Auch, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 178. (15) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (16) Dragan, S.; Schwarz, S. Macromol. Symp. 2002, 181, 155. (17) Bertrand, P.; Jonas, A. M.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (18) Multilayer Thin Films. Sequential Assembly of Nanocomposite Materials; Schlenoff, J. B., Decher, G., Eds.; Wiley-VCH: Weinheim, 2003. (19) Kaschak, D. M.; Lean, J. T.; Waraksa, C. C.; Saupe, G. B.; Usami, H.; Mallouk, T. E. J. Am. Chem. Soc. 1999, 121, 3435. (20) Rousseau, E.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2000, 16, 8865. (21) Fahrat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184. (22) Dai, J.; Balachandra, A. M.; Lee, J. I.; Bruening, M. L. Macromolecules 2002, 35, 3164. (23) Ibarz, G.; Da¨hne, L.; Donath, E.; Mo¨hwald, H. Macromol. Rapid Commun. 2002, 23, 474. (24) Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176. (25) Klitzing, R.; Mo¨hwald, H. Macromolecules 1996, 29, 6901.

10.1021/la034855b CCC: $25.00 © 2003 American Chemical Society Published on Web 06/24/2003

Polyelectrolyte Multilayers as Nanocontainers

Figure 1. Molecular structures of the compounds used in this study: (a) poly(vinylbenzyl chloride) quaternized with N,Ndimethylethanolamine (PVBAC), (b) poly(sodium 4-styrenesulfonate) (PSS), (c) rhodamine B, (d) 4-hydroxycoumarin, (e) fluorescein, and (f) coumarin 343.

desirable to devise more versatile ways to fabricate functional multilayers. In this paper, we report on our attempts to introduce functional hydrophilic molecules in polyelectrolyte multilayers and present a new, easy, and rapid way for so doing. Multilayers may thus be considered as reservoirs for virtually any water-soluble molecule or, equivalently, as templates for the confinement of such active molecules in films, opening new possibilities for these systems. To allow for a facile characterization of the systems and for the quantitative estimation of the amount of molecules introduced in the multilayers, we have selected four small hydrophilic dyes as typical molecules (Figure 1): rhodamine B, fluorescein disodium, coumarin 343, and 4-hydroxycoumarin. To grow multilayers, we selected strong polyelectrolytes of the polystyrene family since aromatic cycles could lead to enhanced interactions between the polymers and the polycyclic dyes, thus enhancing the capability of the multilayer to embed fluorophores. We first present the inward diffusion of the dyes into preformed films. Then, we investigate multilayer growth from solutions simultaneously containing the polyelectrolytes and the dyes and study dye inclusion and release. The thickness and structure of the films are assessed by X-ray reflectometry, while the dye concentration, aggregation, and diffusion are analyzed by spectroscopic methods. Experimental Section Materials. Poly(sodium 4-styrenesulfonate) (PSS), Mw ) 70 000 g‚mol-1, was obtained from Aldrich. The dyes, fluorescein disodium salt dihydrate 90%, rhodamine B 99+%, and coumarin 343 laser grade were purchased from Acros Organics and were used without any further purification. 4-Hydroxycoumarin was obtained from Aldrich. Hydrochloric acid (HCl) 37% and sodium hydroxyde (NaOH) were used to adjust the pH of the solutions. As polycations, we used either poly(diallyldimethylammonium chloride) (PDADMAC), Mw ) 100 000-200 000 g‚mol-1, from Aldrich or a poly(vinylbenzyl chloride) (PVBC) quaternized with

Langmuir, Vol. 19, No. 15, 2003 6179 N,N-dimethylethanolamine (redistilled, 99.5+%, Aldrich), hereafter abbreviated as PVBAC (Figure 1a). The synthesis of this polymer is described below. 4-Vinylbenzyl chloride (VBC, 98% of para-isomer, Aldrich, 90%, inhibited with tert-butylcatechol and nitroparaffin) was purified prior to polymerization by column filtration (aluminum oxide, activated, basic, 50-200 mesh, Acros Organics) to remove the inhibitors and stored at +4 °C in the dark. The solvents were analytical grade and used as received. Azoisobutyronitrile (AIBN) was recrystallized from ethanol just before use. Benzyl dithiobenzoate (BDTB) was synthesized according to a general procedure.26 Synthesis of the Polycation (PVBAC). PVBC was obtained by the reversible addition fragmentation chain transfer (RAFT) polymerization technique.27 VBC (4 mL), AIBN (4 mg, 2.4 × 10-5 mol), and BDTB (40 mg, 1.6 × 10-4 mol) are placed in a Schlenk tube. After degassing by several freeze-thaw cycles, the tube is placed into an oil bath at 60 °C. The polymerization is stopped by cooling the mixture quickly to room temperature after a defined time. Reaction mixtures were diluted with THF, and polymers were precipitated into methanol, isolated, and dried. PVBC was characterized by size exclusion chromatography (Mw ) 46 300 g‚mol-1, Mw/Mn ) 5.2, calibration with polystyrene standards). To prepare PVBAC from PVBC, 3 g of PVBC (1.96 × 10-2 mol of monomer units) is dissolved in 30 mL of DMF, 5.26 g of N,Ndimethylethanolamine (5.9 × 10-2 mol) is added, and the solution is stirred at 65 °C for 48 h. The precipitate is filtered and washed with DMF. The polymer is dissolved in a small amount of deionized water, precipitated in THF, washed with THF, redissolved in water, and freeze-dried. The rate of quaternization was estimated by elemental analysis to be 97%, as confirmed by 1H NMR. The complete characterization of the polymer is reported elsewhere.27 Substrate Preparation. Fused silica SUPRASIL slides and (100) one-side-polished silicon wafers were purchased from Hellma and ACF, respectively. Silicon and fused silica substrates were dipped for 30 min in a “piranha” acid bath (50 vol % of concentrated H2SO4/50 vol % of H2O2 (30%); caution: piranha solutions are strongly oxidizing solutions and should not be stored in closed containers) and then rinsed five times with deionized water and dried with warm air before use. Assembly of the Films. Films were built by alternated deposition of PVBAC and PSS onto the substrates. The concentrations of polyelectrolyte solutions were 10-2 M (concentration in monomer units) in deionized water (pH ) 6). If not specified, the substrates were first immersed for 8 min in the PVBAC solution, rinsed with a continuous flow of deionized water for 30 s, and dried with warm air for 3 min. They were then immersed for 8 min in the PSS solution and rinsed and dried in the same fashion. The process was repeated using an automated dipper (Riegler and Kirstein, Berlin, Germany) to obtain multilayers. A deposition cycle is defined in the following as a (PVABC/PSS) layer pair. Samples will be denoted in the following as (A/B)x to indicate x cycles of deposition of polycation A and polyanion B. Characterization Techniques. Absorption Measurements. UV/visible spectra of dye-doped multilayers were taken using a Cary 50 conc UV/vis spectrophotometer (Varian). Fluorescence Measurements. Excitation and emission fluorescence spectra were obtained with a Fluorolog-2 fluorimeter (Jobin Yvon - Horiba) in a front-face configuration. X-ray Reflectometry (XRR) Measurements. The experimental setup is based on a Siemens D5000 2-circle goniometer. X-rays of 1.5418 Å wavelength (Cu KR) were obtained from a rotating anode operated at 12 kW. Monochromatization was achieved with the help of a secondary graphite monochromator, complemented with pulse height discrimination (scintillation counter). Proper collimation of the beam was obtained by using slits adjustable with micrometer precision. The corrected intensity is reported versus kz0, the component perpendicular to the interface of the wavevector in a vacuum of the incident photons (i.e., kz0 ) (2π/λ) sin θ, where λ is the X-ray wavelength and θ is half the scattering angle). Data analysis was performed either by Fourier transforming the data after proper normalization to obtain a (26) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 3689. (27) Baussard, J. F.; Habib-Jiwan, J. L.; Laschewsky, A. Langmuir, submitted.

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Patterson function or by fitting the data by standard routines as described previously.28 Diffusion Experiments. Inward Diffusion of Fluorescein into Preformed Films. Two (PVBAC/PSS)10 films were prepared under different pH conditions. One was grown and rinsed in deionized water at pH ) 6; the other was built in hydrochloric acid solutions at pH ) 1 and rinsed in acidic baths (pH ) 1). At the end of the assembly process, films were rinsed for 2 min in deionized water in order to remove small ions. The films were then dipped in a 10-3 M solution of fluorescein disodium (pH ) 6). After the desired dipping time, the films were rinsed 3 × 30 s in deionized water and dried with warm air. Outward Diffusion of Dyes from Dye-Loaded Films. These experiments were conducted by immersing the films into water and by measuring the decrease of UV absorbance A(t) with time. As demonstrated by others29,30 and detailed in the Supporting Information, the initial absorbance due to the dye in the film (A(0)) and the diffusion coefficient D can be estimated according to

([ x )]

A(t) ) A(0) 1 -

2 d

Dt π

(1)

which is valid for times at which A(t) > 0.48A(0). In this equation, d is the thickness of the film. Accordingly,

(bd2 )

D ) -π

2

(2)

where b is the slope of A(t)/A(0) when plotted against t1/2. Since diffusion is slow, the thickness of the film was taken as the one measured after 2 days of immersion in water, close to the original thickness.

Results Diffusion of Fluorescein Disodium into Preformed LBL Films. We first attempted to introduce a fluorescent dye in a PVBAC/PSS multilayer by dipping a preformed multilayer film in a solution of the dye in water. Previous works conducted with weak polyelectrolytes24 or mixed weak/strong polyelectrolytes25 have demonstrated the permeability of LBL films and estimated the diffusion coefficient of dyes in the films. We used UV/visible spectroscopy to investigate the diffusion of the dyes in (PVBAC/PSS)10 films grown at two different ionic strengths (corresponding to pHs of 1 and 6, respectively). Ionic strengths higher than 10-2, corresponding to low pHs, strongly influence the structure and the thickness of LBL films31-33 and could thus affect the diffusion of small molecules in the film. Figure 2a,b displays UV/visible spectra of films grown at pH ) 6 and pH ) 1, after different immersion times in a 10-3 M solution of fluorescein disodium in water. The amount of fluorescein loaded in the film can be followed owing to the absorbance of the peaks at 470 and 228 nm. For both wavelengths, the absorbances evolve proportionally within experimental precision (insets in Figure 2). The thickness and roughness of the films, measured by XRR before diffusion and after 46 h of diffusion, are reported in Table 1. Before fluorescein diffusion, the UV/ visible absorbances normalized to unit film thickness were found to be independent of the pH of the deposition solutions, indicating that there is no preferential orienta(28) Arys, X.; Laschewsky, A.; Jonas, A. M. Macromolecules 2001, 34, 3318. (29) Liu, J. Y.; Simpson, W. T. Drying Technol. 1997, 15 (10), 2459. (30) Crank, J. The Mathematics of Diffusion, 2nd ed.; Oxford University Press: Oxford, 1975. (31) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (32) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (33) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153.

Figure 2. UV/visible spectra of a (PVBAC/PSS)10 film, (a) built in deionized water at pH ) 6, (b) built at pH ) 1, recorded after different immersion times in a 10-3 M fluorescein disodium solution. (a) From bottom to top, immersion times are 0 min, 30 min, 2 h, 4 h, 6 h, 10 h, 16 h, 24 h, 46 h, and 94 h. (b) From bottom to top, immersion times are 0 min, 15 min, 1 h, 2 h, 4 h, 6 h, 10h, 16 h, 24 h, 46 h, and 96 h. The insets present the variation of absorbance with time, for wavelengths corresponding to the maxima of absorption in the visible (open symbols, 470 nm) and UV (closed symbols, 228 nm) ranges. Table 1. Structural and Optical Parameters of (PVBAC/ PSS)10 Films before and after Fluorescein Diffusion sample

d (nm)a

σ (nm)b

A(227 nm)c

(PVBAC/PSS)10, pH ) 6 (PVBAC/PSS)10, pH ) 6 + fluorescein (46 h) (PVBAC/PSS)10, pH ) 1 (PVBAC/PSS)10, pH ) 1 + fluorescein (46 h)

4.1 11.5

1.5 2.3

0.057 0.293

16.9 23.7

1.4 2.8

0.273 0.407

a Film thickness from XRR (estimated error, 0.3 nm). b Rootmean-square roughness from XRR (estimated error, 0.5 nm). c UV absorbance at 227 nm (estimated error, 0.0005).

tion in any of the films despite their different initial thicknesses (which differ by a factor of 4, Table 1). Following the diffusion of fluorescein, both films were found to swell significantly. After 46 h of diffusion time, the film prepared in neutral conditions almost tripled its average thickness, while the thickness of the film prepared from acidic solutions increased by about 50%. Film roughnesses were found to increase in parallel. The amount of fluorescein in the films can be estimated from the increase of absorbance at 228 nm. We measured the molar extinction coefficient 0 of the characteristic peaks of fluorescein in water at different pHs and found the following for the peak around 235 nm: 0 ) 45 830 L‚mol-1‚cm-1 at pH 6, and 0 ) 34 900 L‚mol-1‚cm-1 at pH 9. These values measured in aqueous solution may change a little when the chromophores are in the form of H-dimers, as they must be as judged from the shape and position of the peak at 470 nm.34 From the increments of absorbance (34) Arbeloa, I. L. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1725.

Polyelectrolyte Multilayers as Nanocontainers

at 228 nm and the values of film thickness obtained by XRR, taking into account that both sides of the substrates are covered by a film, the average concentrations of fluorescein in the films after 46 h of diffusion time (saturation values) are estimated to be 0.002-0.003 mol per cubic cm of film and 0.0006-0.0008 mol‚cm-3 for films grown from solutions at pH 6 or 1, respectively. These values can be translated in approximate thickness increments resulting from fluorescein diffusion, assuming a specific gravity of about 1.2 g‚cm-3 for fluorescein. Values of 6.4-9.5 and 4-5.3 nm are found for films grown from solutions at pH 6 or 1, respectively. These compare reasonably well with the experimental increments of thickness determined by XRR (7.4 and 6.8 nm for the same conditions), given the imprecision of the extinction coefficient, the large roughness of the films, and a possible nanoporosity of the films. However, if similar computations are performed based on the absorbance of the films at 470 nm, using the literature value 0 ) 47 000 L‚mol-1‚cm-1 for fluorescein H-dimers (close to our own experimental determination), thickness increments in the range of 0.5-0.75 nm are predicted, in strong disagreement with the experimental thickness increments obtained by XRR. Thus, the absorbance of the fluorophores in the visible range is much lower than expected, either due to aggregation of the molecules or to preferential orientation, prohibiting use of this spectral range to obtain quantitative information. The maximum concentration of dye loaded in the multilayer grown at pH 6 is about 3 times larger than in the one grown at pH 1, but the absolute quantities introduced happen to be close to each other. The concentrations in the films at saturation are about 3 orders of magnitudes larger than in water, indicating that partitioning in the film is favored: the films act as sinks for the dye. It is difficult to determine diffusion coefficients with precision from our data, essentially because film thicknesses are not constant. Assuming a constant film thickness (the thickness measured after diffusion of the dye), we have estimated the diffusion coefficients D as 7 × 10-18 and 4 × 10-17 cm2‚s-1 for films grown at pH 6 and pH 1, respectively (see Supporting Information for details). Sequential Deposition of Polyelectrolyte and Dyes. Loading functional organic molecules in multilayers of strong polyelectrolytes by diffusion presents two main drawbacks: diffusion times may be prohibitively long, and the internal structure of the multilayer may suffer from significant reorganization during the diffusion process, as testified by the large increments of thickness. An alternative, possibly better solution would consist of introducing the target molecules directly during multilayer formation. This can be done by alternated deposition of an ionic dye with a polyion of opposite charge, provided the dye is charged enough.10 Although this proved feasible for the doubly charged fluorescein,35 the even less charged rhodamine B and coumarins failed to give rise to multilayer growth. Adsorption of Polyelectrolyte/Dye Soluble Complexes. Therefore, a (PVABC/PSS) multilayer was built with rhodamine B added in the polyanion and the polycation dipping baths. Concentrations of polyelectrolyte and dye were 10-2 and 10-3 M, respectively. The excess of polyanion with respect to the positively charged rhodamine B ensures that only soluble complexes are formed in solution. Figure 3 shows the absorbance of the film at 560 nm (which corresponds to the characteristic (35) Nicol, E.; Moussa, A.; Habib-Jiwan, J.-L.; Jonas, A. M. To be submitted.

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Figure 3. Variation with number of deposition cycles of the absorbance of a (PVBAC/PSS) multilayer grown in the presence of rhodamine B in the polycation and polyanion solutions, recorded at the wavelength of maximal absorption in the visible range (rhodamine B characteristic band). The film was rinsed in deionized water. Half-cycles correspond to the deposition of the polycation.

absorption band of rhodamine B in the visible domain), as a function of the number of deposition cycles. The figure indicates that the dye adsorbs for each dip in the solution of the polyanion/dye complex but desorbs from the multilayer when the sample is immersed in the polycation bath. This observation, which is in good agreement with the so-called ion exchange model,36 resembles previous observations by Caruso et al. on pyrenetetrasulfonate (PSA) bound to poly(allylamine hydrochloride) (PAH)/PSS multilayers.37 The positively charged PVBAC and rhodamine B compete to form a complex with PSS at the film/water interface. Since polycation/polyanion complexes are entropically favored with respect to polymer/“small molecule” complexes, the dye is displaced by PVBAC and washed out during the rinsing step. This illustrates the dynamic nature of the complexation of rhodamine B. Interestingly, the PSS/ rhodamine B complex is stable enough to withstand rinsing in pure water. Starting from the fifth cycle on, the absorbance after PVBAC deposition increases slowly: the film becomes thick enough, and the total residence time in the solutions long enough, to allow the dye to diffuse slightly in the multilayer. The amount of dye present in the film remains however extremely low. For our purposes, this procedure is thus not better than the simple immersion of a preformed film in a solution of the dye. Similar results were obtained for fluorescein disodium. In that case, however, the negatively charged fluorescein forms a complex with the polycation and is displaced by the polyanion. Control of Dye Displacement. A solution to this nagging problem was found by redesigning the rinsing step. The goal of this step is to remove the excess of polyelectrolyte deposited over the top layer, but it has also the unpleasant side effect to wash out dye molecules displaced by the polyions. To reduce washing out, fluorophores were introduced at the same concentration in all baths, even in rinsing baths. The following samples were all prepared under the same conditions: The films were rinsed successively in three baths containing the dye, before being dried with a warm air stream at each half-cycle. After the final cycle of deposition, the multilayers were rinsed for 1 min in pure water, to remove a (36) Fleer, G. J.; Cohen-Stuart, M. A.; Schentjens, J. M. H. M.; Cosgrove, T.; Vincent, B. In Polymer at Interfaces; Chapman & Hall: London, 1993. (37) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317.

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Figure 4. UV-visible absorption spectra of (PVBAC/PSS)10 films grown from solutions of increasing concentrations in fluorescein (from bottom to top: 0, 10-4, 2 × 10-4, 4 × 10-4, 7 × 10-4, 10-3, and 2 × 10-3 M) and rinsed in solutions of identical concentrations. The top inset presents the wavelength corresponding to the maximal absorption in the visible range versus dye concentration in the solutions. The bottom inset plots the maximal absorbance in the visible range divided by the film thickness versus the concentration of the solutions in dye. The line is drawn to guide the eye.

layer of loosely adsorbed molecules on the film surface. With these conditions, introduction of the dye in the multilayers was found to be successful. Figure 4 displays the UV/visible absorbance spectra of films prepared from solutions of increasing concentrations in fluorescein. The top inset plots the wavelength of the maximum absorption of the dye in the film versus the concentration of the solutions in dye. Compared with the spectrum of the dye in solution, a “red” shift in the maximum absorption peak of the dye (about 20 nm) is observed, characteristic for the formation of aggregates. Similar aggregates of fluorescein derivatives were reported for Langmuir and Langmuir-Blodgett films.38,39 In the present case, the aggregation number increases with the concentration of the solutions, up to a concentration of 4 × 10-4 M where it stabilizes. The structure of the multilayers was studied by X-ray reflectometry. Figure 5 presents X-ray reflectograms of the films grown in the presence of fluorescein at different concentrations (the derived film thickness and roughness are plotted versus solution concentration in the insets). For dye concentrations up to 10-3 M, strong Kiessig fringes are observed in the reflectograms, indicating a reasonably low film roughness. When the concentration reaches 2 × 10-3 M, fluorescein aggregates become too large to get smooth films and the Kiessig fringes tend to vanish. Multilayer thickness and roughness increase monotonically with the concentration of dye in the baths (insets of Figure 5). The thickness tends to a plateau for high dye concentrations, while roughness continues to increase. Obviously, the increase of thickness cannot be attributed to the effect of the ionic strength created by the dye, which remains very low. The bottom inset in Figure 4 reports the absorbance of the films as a function of the concentration of the solutions in dye, normalized to unit thickness, for the wavelength corresponding to the maximum of absorbance of the dye in the visible range. The normalized (38) Dutta, A. K.; Salesse, C. Langmuir 1997, 13, 5401. (39) Orbulescu, J.; Mello, S. V.; Huo, Q.; Sui, G.; Kele, P.; Leblanc, R. M. Langmuir 2001, 17, 1525.

Nicol et al.

Figure 5. X-ray reflectivity of (PVBAC/PSS)10 films grown from solutions of increasing concentrations in fluorescein (from bottom to top: 0, 10-4, 2 × 10-4, 4 × 10-4, 7 × 10-4, 10-3, and 2 × 10-3 M) and rinsed in solutions of identical concentrations. Curves were displaced vertically for clarity. The top inset presents the thickness of the films versus dye concentration in the solutions. The bottom inset plots the roughness of the film/ air interface versus the concentration of the solutions in dye. Lines are drawn to guide the eye.

absorbance first increases quasi-linearly with the concentration of the solutions and then saturates for concentrations higher than 0.001 M. Similar results were obtained for the normalized absorbance at a wavelength corresponding to the maximum of absorbance in the UV range. Table 2 summarizes the main characteristics of fluorescein-doped samples. The question which arises is whether the increase of thickness corresponds solely to the supplementary volume resulting from the presence of the dye in the multilayers, or whether other physicochemical effects conspire to increase film thickness, such as modifications in the conformation of polyelectrolytes. A related question is the value of dye concentration in the multilayers. Such questions are difficult to answer from the UV/visible results alone, due to the dependence of extinction coefficients on the environment of the fluorophores and their state of aggregation. It is instructive to compare the multilayers made from solutions 0.001 and 0.002 M in fluorescein. These have similar absorbances per unit thickness (within 15%), whereas their thicknesses differ by more than 50%. This indicates that the amount of polyion included in the multilayer is not constant but depends instead on the concentration of the dye in the solution. Nevertheless, crude estimations for the concentration in dye of the multilayers are possible, based on their absorbance and thickness, neglecting the variations of the amount of PSS and PVBAC included in the multilayers depending on the concentration of the solutions in dye. For solution concentrations 0.001 M in dye, for example, values of 0.0015-0.0035 mol per cubic cm of film are found. These values are in the same range as those found previously for the diffusion experiments (saturation values). However, contrasting with diffusion experiments, the new method used to introduce the dye allows one to easily tune the dye content below this value, as shown in Figure 4. Fluorescence measurements confirm that increasing amounts of fluorescein are embedded in the films when the concentration of the baths in dye increases. Figure 6 shows the excitation and emission spectra of fluoresceindoped multilayers. The excitation maximum wavelength

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Table 2. Structural Data, Spectroscopic Data, and Diffusion Coefficients of LBL Multilayers (10 Cycles) Grown on Fused Silica Slides in the Presence of Fluorescein Disodium in All Dipping Baths sample

[fluo] (mol‚L-1)a

d (nm)b

σ(nm)c

λmax (nm)d

2A0e

(PVBAC/PSS)10 (PVBAC/PSS)10 (PVBAC/PSS)10 (PVBAC/PSS)10 (PVBAC/PSS)10 (PVBAC/PSS)10 (PVBAC/PSS)10 (PDADMAC/PSS)10g

0 10-4 2 × 10-4 4 × 10-4 7 × 10-4 10-3 2 × 10-3 2 × 10-4

4.1 7.8 13 19.9 29.9 32.4 45 7.2

1.5 1.0 1.4 1.7 2.1 2.6 6 1.0

490 504 521 522 521 520 501

0.002 0.014 0.086 0.271 0.452 0.739 0.002

D (cm2‚s-1)f

1.0 × 10-18 2.0 × 10-19 1.0 × 10-18 6.7 × 10-19 3.4 × 10-18

A0/d (nm-1) 1.5 × 10-4 5.5 × 10-4 2.2 × 10-3 4.5 × 10-3 7.0 × 10-3 8.2 × 10-3 1.4 × 10-4

a Dye concentration in all dipping baths. b XRR-determined film thickness (estimated error, 0.3 nm). c XRR-determined root-meansquare film roughness (estimated error, 0.5 nm). d Wavelength corresponding to the maximum of absorption in the visible range. e Maximum absorption at λmax, which includes the absorption of both sides of the substrate (estimated error, 0.0005). f Diffusion coefficient (estimated error, 0.4 × 10-18 cm2‚s-1). gPDADMAC was used as the polycation instead of PVBAC.

Figure 6. Excitation (continuous lines) and emission (dashed lines) fluorescence spectra of (PVBAC/PSS)10 multilayers grown in the presence of fluorescein in all dipping baths. The dye concentrations in the baths are specified in the figure. λex ) 500 nm and λem ) 560 nm.

increases from 506 to 523 nm as the molecules are more and more aggregated, while the emission maximum wavelength decreases from 562 to 558 nm. The fluorescence intensity increases up to a solution concentration of 10-3 M and then drops for the 2 × 10-3 M dipping baths due to self-quenching resulting from aggregation. Similar experiments were conducted with the rhodamine B dye. First, a (PVBAC/PSS)10 multilayer was built in the presence of rhodamine B in all the dipping baths, at a concentration of 2 × 10-4 M. Films of very low quality were obtained, having roughnesses exceeding those measurable by X-ray reflectometry (about 5-6 nm). This was attributed to the fact that at pH 6, rhodamine B is in the zwitterionic form RB(, known to aggregate more readily than its purely cationic form RBH+.40 The presence of such aggregates in the film may be the reason for the observed high roughness. To avoid this aggregation effect, multilayers were built similarly at pH 3. At this pH, rhodamine B was assumed to be in its cationic form RBH+, whereas the ionic strength created by the chloride and hydronium ions is still too low to influence the multilayer structure. However, for solution concentrations of 2 × 10-4 M in rhodamine, the roughness of the film was again found to be too large for X-ray reflectometry, suggesting that aggregates of large size still existed in the film. Arbeloa et al.40 reported a RBH+ aggregation constant (Kd ) 1400) about 300 times larger than the value measured for fluorescein disodium (Kd ) 5.0).34 It becomes thus easily understandable why multilayers containing rhodamine B are much rougher than those containing fluorescein, for similar concentrations in dye of the dipping solutions. (40) Arbeloa, I. L.; Ojeda, P. L. Chem. Phys. Lett. 1981, 79, 347.

Figure 7. Variation of the thickness (a), roughness of the film-air interface (b), and absorbance at the maximum of the characteristic band of rhodamine B (c) for (PVBAC/PSS)10 multilayers grown in the presence of rhodamine B in all dipping baths.

Therefore, the pH was decreased further to 1 by addition of hydrochloric acid, allowing relatively smooth films to be obtained. Their thickness and roughness as obtained from XRR are presented versus the concentration of the solutions in dye in parts a and b of Figure 7, respectively. The influence of rhodamine B on the thickness and roughness of the film becomes negligible only for very low concentrations in dye (5 × 10-5 M). By contrast, for solution concentrations in dye exceeding 7 × 10-4 M, the roughness of the films was too large to be determined by X-ray reflectometry, due to the formation of aggregates of large size. In addition, the absorbance of the films tends to saturate to a plateau value (Figure 7c). For intermediate concentrations between 10-4 and 4 × 10-4 M, thickness and roughness are constant, but the amount of dye loaded in the film per unit surface increases quasi-linearly with the dye concentration in the dipping baths (Figure 7c). Fluorescence measurements confirm the higher aggregation of rhodamine B as compared to the fluorescein. Figure 8 depicts the excitation and emission spectra of rhodamine B doped multilayers. The maximum emission wavelength increases from 601 to 610 nm when dye concentration increases. Also, for the rhodamine B case, self-quenching of the aggregates occurs for lower concentrations as for fluorescein-doped multilayers: the fluorescence intensity begins to drop when the solution concentration in dye exceeds 10-4 M, which is roughly 10 times lower than the critical concentration of fluorescein for aggregate formation.

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Figure 8. Excitation (continuous lines) and emission (dashed lines) fluorescence spectra of (PVBAC/PSS)10 multilayers grown in the presence of rhodamine B in all dipping baths. The dye concentrations in the baths are specified in the figure. λex ) 565 nm and λem ) 600 nm.

Figure 9. Absorbance at the wavelength corresponding to the maximum of the fluorescein band in the visible range, plotted versus the square root of time of immersion in pure water solutions, for (PVBAC/PSS)10 multilayers grown with fluorescein added to all dipping baths. Concentrations of dye in the dipping baths are 2 × 10-4 M (squares), 4 × 10-4 M (circles), 7 × 10-4 M (triangles up), 10-3 M (triangles down), and 2 × 10-3 M (diamonds).

Similar results were also obtained for two coumarins (4-hydroxycoumarin at neutral pH and coumarin 343 at pH ) 10), even though the amount of 4-hydroxycoumarin introduced in the multilayers was found to be very low (Supporting Information). This demonstrated the general applicability of the method. In addition, fluorescein was also introduced in multilayers of PDADMAC and PSS, from solutions 2 × 10-4 M in dye (Table 2). It was found by UV/visible spectroscopy and X-ray reflectometry that the amount of fluorescein introduced into (PDADMAC/ PSS) multilayers was about 4 times lower than in similar (PVBAC/PSS) multilayers, although the increments of thickness per deposition cycle are similar for both types of multilayers (4.1 Å for a (PVBAC/PSS) bilayer and 4.0 Å for a (PDADMAC/PSS) bilayer,28 in the absence of dye and at zero ionic strength). Interactions between aromatic cycles may thus act as a supplementary driving force for the partitioning of aromatic dye molecules during the selfassembly process. Outward Diffusion of Embedded Dyes. We measured the diffusion coefficient of the fluorophores from dye-loaded films to water. This was performed by recording the UV absorbance of the films after selected immersion times in water. The decrease of the absorbance versus the square root of immersion time is presented in Figure 9 for films prepared from dipping baths of different concentra-

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Figure 10. Outward diffusion coefficient of rhodamine B from (PVBAC/PSS)10 multilayers grown in the presence of rhodamine B in all dipping baths, plotted versus the concentration of the dipping baths in dye. The multilayers were built at pH ) 1 (open triangles) and pH ) 3 (closed triangles).

tions in fluorescein. The first 60 s of the release experiments is dominated by the fast desorption of molecules adsorbed at the multilayer surface. These parts of the curves were not taken into account for the calculation of the diffusion coefficient. Then, a slower linear decrease of the absorbance is observed, corresponding to the outward diffusion of molecules originally embedded in the film. For fluorescein embedded in (PVBAC/PSS)10 multilayers, the diffusion rate is so slow that less than 25% of the initial amount of dye is lost after 6 days of immersion in deionized water. The diffusion coefficients obtained from these data did not vary significantly with the initial content in dye of the multilayers; their average value was found to be 0.7 × 10-18 cm2‚s-1 (standard deviation, 0.4 × 10-18 cm2‚s-1; standard error, 0.2 × 10-18 cm2‚s-1). This value of D is about 10 times lower than the one found from the inward diffusion of the same dye in initially pristine films. However, given the uncertainties in the determination of D for inward diffusion experiments, resulting from the large thickness perturbation of the films, the agreement may be seen as rather satisfactory. To evaluate the influence of the surface charge on the release, the diffusion coefficient was also measured for a (PVBAC/PSS)10.5 multilayer (i.e., with the polycation at the surface) grown from solutions 7 × 10-4 M in fluorescein. No significant difference was found for D as compared to the previous case where the polyanion is at the film surface, contrasting with previous claims by others for PAH/PSA multilayers.15 The diffusion of rhodamine B out of (PVBAC/PSS)10 multilayers was studied similarly. Figure 10 presents the diffusion coefficient of rhodamine B grown from solutions at pH 1 and 3 as a function of the concentration of dipping baths in dye. The diffusion coefficients are about 2 orders of magnitude larger than for fluorescein. In addition, there is a clear decrease of D as films get more saturated in dye, and films grown at a higher pH are more permeable to the dye. This is probably due to differences in the aggregation state of the dyes in the different films as revealed by fluorescence spectra. Discussion As for weak polyelectrolyte multilayers, organic dyes can be loaded in films made of strong polyelectrolytes by diffusion into preformed films, and the release of the molecules can be studied. However, this loading method

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may not be the best candidate for practical applications, because of the time required to load the assembly and because of the structural changes induced by the loading process. Polyelectrolyte multilayers swell up to 30% when immersed in water.41 This process is reversible, and the initial thickness and structure are recovered by drying the film. Much larger changes are induced by the diffusion of fluorophores, as the film volume can be multiplied by 3, although the consequences of the dye diffusion are less important when the film is grown from solutions of higher ionic strength. The reorganization process which inevitably must accompany the large swelling is a major drawback if one desires to introduce different functional molecules in well-defined multilayer compartments of predefined thickness. In this respect, the introduction of organic molecules simultaneously with the polyelectrolyte adsorption steps is a more promising method to obtain controllable functional multilayers. Furthermore, it does not require more time than for the growth of a simple multilayer. However, the displacement of small molecules by polyelectrolytes of identical charge strongly limits the potential of this method. This was solved by introducing in all baths, including the rinsing ones, the molecules to be loaded in the multilayer. Different kinds of water-soluble molecules could thus be easily introduced in the multilayers. The amount of loaded material is controlled by the nature of the target molecules and their concentration in the dipping baths (parameters which also influence the outward diffusion of these molecules). As the diffusion coefficients of these molecules are rather small, it becomes possible to construct complex multilayers made of compartments doped by different molecules. The loading of the dyes can be rationalized by considering the following simple arguments: Dye molecules interact with the polyelectrolyte chains in solution and when adsorbed on the substrate through, for example, electrostatic complexation with the polyelectrolyte bearing an opposite charge, π stacking, hydrophobic interactions, and so forth. Assuming thermodynamic equilibrium, that is, ignoring kinetic effects as a very rough approximation, one can write the following equations: K1

Polx + PolQ y\z Pol+-PolK2

Polx + Dye y\z Pol+-Dye K3

PolQ + Dye y\z Pol--Dye

(3) (4) (5)

In general, K1 is much larger than K2 or K3, because polymer/polymer interactions are favored with respect to polymer/dye interactions for entropic reasons. In addition, due to the expectedly slow kinetics of desorption of polyelectrolytes, K1 may effectively be considered as infinite over the normal processing times of the multilayers, except maybe for weak polyelectrolytes or at high ionic strengths.42 Therefore, small molecules are displaced by polyelectrolytes during multilayer growth. This is experimentally demonstrated by the fact that the dyes are not incorporated in significant amounts in the multilayers, even when the dye is present in all solutions except for the rinsing baths. However, in addition to eqs 3-5, the system tends to equilibrate the chemical poten(41) Ku¨gler, R.; Schmitt, J.; Knoll, W. Macromol. Chem. Phys. 2002, 203, 413. (42) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626.

Scheme 1. Schematic Description of the Growth of a (PVBAC/PSS) Multilayer with (a) Fluorescein Added to the Polycation and Polyanion Solutions and (b) Fluorescein Added to All Dipping Baths

tials of the dye in water and in the multilayer. This is why the dye diffuses in the film. This process is very slow, except over the few angstro¨ms at the outer surface of the growing film. When the film is rinsed with pure water, the osmotic pressure drives out of the film the dye which was included in the preceding step. By contrast, if the dye is added in the rinsing baths, the osmotic pressure drops to zero, avoiding the escape of the dye at each rinsing step. To embed more dye in the film, one must simply increase the dye concentration in all dipping baths. The concentration of the dye in the film is thus linearly related to the concentration of the dye in solution as found experimentally, except for very large contents, maybe due to aggregation of the dye molecules. Nevertheless, the concentrations in the multilayer and in the solutions are different, because the system tends to balance the chemical potentials, not the concentrations.43 Chemical potentials are related to concentration and specific interactions between the dye and its environment, and hence to solubility parameters. The size of the dye, its hydrophobicity, its net charge, and the structural and dynamical parameters of the film all influence the affinity of the dye for the multilayer and its activity coefficient. This explains why lower amounts of fluorescein are introduced in (PDADMAC/PSS) multilayers than in (PVBAC/PSS) multilayers, for the same concentration of the solutions in dye: aromatic interactions increase the affinity of dyes for the latter system. This also demonstrates that partitioning is far from being exclusively governed by ion exchange. Interestingly, the concentration in dye of the films is much larger than that of the solutions. This again illustrates the high affinity of the dye for the multilayers: multilayers behave as sponges capable of adsorbing extremely large amounts of small molecules. Scheme 1 compares the growth of the multilayers in the presence of fluorescein, without (a) or with (b) the fluorescein being introduced in the rinsing baths. (I) The substrate is immersed in the polycation/ fluorescein solution. The polycation/dye complex adsorbs, while the fluorescein diffuses slightly in the multilayer. (43) Guggenheim, E. A. Thermodynamics, 3rd ed.; North-Holland: Amsterdam, 1957.

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(II) The excess of polycation is removed during the rinsing step. (a) Most of the fluorescein molecules diffuse out the film, except for molecules complexed with the last PVBAC layer. (b) The multilayer remains saturated with fluorescein. (III) The substrate is immersed in the polyanion/ fluorescein solution. The polycation/polyanion complex being entropically favored, PSS chains displace dye molecules complexed with the PVBAC, while diffusion allows some fluorescein to penetrate in the multilayer. (IV) The PSS excess is removed during the rinsing step. (a) Fluorescein molecules diffuse out the film. (b) The multilayer remains saturated with fluorescein. We now turn to the values of diffusion coefficients. The diffusion coefficients reported in the present study are low and depend on the charge of the dye used to probe diffusion. Values of diffusion coefficients for monovalent dyes through polyelectrolyte multilayers were published before. For rhodamine B diffusing through (PAH/PSS) multilayers, D was reported to be 10-15-10-16 cm2‚s-1 with only a limited dependence on the ionic strength of the dipping solution.25 These values are about an order of magnitude larger than the values found in the present study, which may be due to a more compact structure of our multilayers grown from polystyrene-based strong polyelectrolytes. In addition, as was shown here, the concentration of the dye (and its aggregation state) are important factors affecting D. The diffusion of methylene blue through multilayers grown from weak polyelectrolytes (PAH/poly(acrylic acid) (PAA) at neutral pH) was also found to be in the 10-15-10-16 cm2‚s-1 range, with a strong dependence on the ionic strength of the solution containing the diffusing dye.24 When a divalent dye such as fluorescein is being used, the diffusion coefficients are found to decrease by more than 1 order of magnitude. Conversely, the diffusion of a small uncharged molecule was studied in (PAH/PSS) multilayers, and diffusion coefficients about 2 orders of magnitude larger than for the monovalent rhodamine B were estimated from the available data.25 This suggests that the model of reluctant exchange developed by Schlenoff and co-workers to describe the diffusion of ions into polyelectrolyte multilayers is applicable to charged dyes as well.21 Conclusions In this paper, we have investigated different ways to introduce a set of dyes in multilayers of strong polyelec-

Nicol et al.

trolytes. The inward diffusion of small organic molecules in preformed multilayers was found to be very slow and was accompanied by substantial thickness variations of the multilayer. This effectively rules out the practical interest of this procedure. We then attempted to coadsorb the fluorophores with the polyions. However, displacement of small molecules by polyions of identical charge and outward diffusion of the fluorophores during the rinsing step result in very limited inclusion of the dye by this procedure. This issue could be solved by introducing the fluorophore in all baths, including the rinsing ones. The amount of dye incorporated in the multilayers was found to be directly controlled by the concentration of the dipping solutions, with aggregation of the molecules being detected for the larger concentrations. The concentrations of the multilayer in dye were shown to be dependent on the nature of the dye and of the multilayer, for identical concentrations of the dipping solutions. The outward diffusion of the fluorophores was studied, and very low diffusion coefficients were determined. The ability to load rapidly polyelectrolyte multilayers with a variety of hydrophilic organic molecules of small molar mass is a major outcome of the present study. The possibility to control thickness down to a few nanometers, to attain low film roughness, and to incorporate a very large range of functional compounds in tunable concentration offers new opportunities to use these multilayers for the fabrication of functional nanodevices. Acknowledgment. The authors thank A. Laschewsky and B. Nysten for invaluable discussions concerning this work and H. Mo¨hwald and M. Van der Auweraer for interesting suggestions. Technical support provided by A. Moussa and J.-F. Baussard is also gratefully acknowledged. Financial support of the work was provided by the DG Recherche Scientifique de la Communaute´ Franc¸ aise de Belgique (Action de Recherches Concerte´e 00/05-261), by the Interuniversity Attraction Poles Program of the Belgian State (Federal Office for Scientific, Technical and Cultural Affairs), and by the Belgian National Fund for Scientific Research. Supporting Information Available: Derivation of the equations and approximations describing the diffusion; UV spectra of coumarin-loaded multilayers. This material is available free of charge via the Internet at http://pubs.acs.org. LA034855B