Effect of Thiocyanate Counterion Condensation on Poly(allylamine

Institut Charles Sadron, Centre National de la Recherche Scientifique, Unité Propre ... Alae E. El Haitami , David Martel , Vincent Ball , Hoan Cong ...
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Langmuir 2005, 21, 4129-4137

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Effect of Thiocyanate Counterion Condensation on Poly(allylamine hydrochloride) Chains on the Buildup and Permeability of Polystyrenesulfonate/Polyallylamine Polyelectrolyte Multilayers Vincent Ball,*,† Jean-Claude Voegel,‡ and Pierre Schaaf† Institut Charles Sadron, Centre National de la Recherche Scientifique, Unite´ Propre 22, 6 rue Boussingault, 67083 Strasbourg Cedex, France, and Institut National de la Sante´ et de la Recherche Me´ dicale, Unite´ 595, Faculte´ de Me´ decine, 11 rue Humann, 67085 Strasbourg Cedex, France Received September 27, 2004. In Final Form: February 21, 2005 In this study, we investigate the buildup of PEI-(PSS-PAH)n polyelectrolyte multilayers at pH 7.4 in the presence of either NaCl or NaSCN as a supporting electrolyte. It appears that in the presence of increasing thiocyanate concentrations (from 0.1 to 0.5 M), the thickness increment, obtained from optical waveguide lightmode spectroscopy experiments, increases whereas it stays practically constant for increasing sodium chloride concentrations (between 0.1 and 0.5 M). The hydration of the films differs also markedly between both electrolyte solutions. The differences in the construction of the polyelectrolyte multilayers in the presence of both supporting electrolytes are rationalized in terms of strong SCN- condensation on the PAH chains. The occurrence of this ion condensation is indirectly demonstrated by means of zeta potential measurements and directly demonstrated by means of attenuated total internal reflection infrared spectroscopy on the multilayer films. Moreover when the films are built up in the presence of SCN-, these ions are only slowly exchanged by the Cl- ions introduced in the bulk. Conversely the thick films obtained from 0.5 M NaSCN solutions do not deswell when the buffer solution is replaced by a 0.5 M NaCl containing buffer. The permeability of the films constructed in the presence of both sodium salts is also studied by means of cyclic voltametry and is found to be markedly different in the case of films made from five bilayers at 0.5 M salt concentration. This difference is due to the different morphology and porosity of the films constructed in the presence of 0.5 M NaCl and 0.5 M NaSCN.

Introduction Among the different functionalization methods of solid surfaces, the layer-by-layer deposition of polyelectrolyte multilayers, developed by Decher 13 years ago,1 has gained considerable importance due to its versatility.2,3 This deposition method can be applied to any interface (solid/ liquid and even liquid/air4) provided it carries a nonzero surface charge density. It can even be applied onto colloidal particles or onto biological cells or crystals.5 The buildup of the polyelectrolyte multilayers (PEMs) proceeds via the adsorption of a given polyelectrolyte until the surface charge density of the last adsorbed polyelectrolyte is overcompensated. This has been demonstrated by means of zeta potential measurements.6,7 These PEMs are now currently used to produce new materials with tailored properties in the field of electrooptical devices,8,9 semiconducting nanoparticles,10 separation membranes,11-13 and biomaterial coatings with sustained drug release14,15 or with procoagulant/anticoagulant properties16 and an* To whom all correspondence should be addressed. † Institut Charles Sadron. Centre National de la Recherche Scientifique, Unite´ Propre 22. ‡ Institut National de la Sante ´ et de la Recherche Me´dicale, Unite´ 595. (1) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (2) Decher, G. Science 1997, 277, 1232. (3) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (4) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871. (5) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; Mo¨hwald, H. Colloids Surf., A 1998, 137, 253. (6) Caruso, F.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 2011 (7) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. J. G. Langmuir 2000, 16, 1249.

tibacterial properties.17-19 These PEMs can also be used as reservoirs of active biomolecules20-22 and as a means to buildup enzymatic biosensors.23 The mechanical properties of such films can also be improved either by the incorporation of inorganic fillers, such as carbon nanotubes,24 or by cross-linking of the polyelctrolytes constituting the PEM.25 Besides their applications, a rather impressive amount of work has been done to understand the buildup mech(8) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (9) Eckle, M.; Decher, G. Nano Lett. 2001, 1, 45. (10) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (11) Stanton, B. W.; Harris, J. J.; Miller, M. D.; Bruening, M. L. Langmuir 2003, 19, 7038. (12) Krasemann, L.; Toutianoush, A.; Tieke, B. J. Membr. Sci. 2001, 181, 221. (13) Rmaile, H. H.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125, 6602. (14) Benkirane-Jessel, N.; Schwinte, P.; Falvey, P.; Darcy, R.; Haı¨kel, Y.; Schaaf, P.; Voegel, J. C.; Ogier, J. Adv. Funct. Mater. 2004, 14, 174. (15) Chluba, J.; Voegel, J. C.; Decher, G.; Erbacher, P.; Schaaf, P.; Ogier, J. Biomacromolecules 2001, 2, 800. (16) Serizawa, T.; Yamaguchi, M.; Matsuyama, T.; Akashi, M. Biomacromolecules 2000, 1, 306. (17) Mendelsohn, J. D.; Yang, S. Y.; Hiller, J.; Hochbaum, A. I.; Rubner, M. F. Biomacromolecules 2003, 4, 96. (18) Yang, S. Y.; Mendelsohn, J. D.; Rubner, M. F. Biomacromolecules 2003, 4, 987. (19) Boulmedais, F.; Frisch, B.; Etienne, O.; Lavalle, Ph.; Picart, C.; Ogier, J.; Voegel, J. C:, Schaaf, P.; Egles, C. Biomaterials 2004, 25, 2003. (20) Pei, R. J.; Cui, X. Q.; Yang, X. R.; Wang, E. K. Biomacromolecules 2001, 2, 463-468. (21) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427-3433. (22) Vazquez, E.; Dewitt, D. M.; Hammond, P. T.; Lynn, D. M. J. Am. Chem. Soc. 2002, 124, 13992. (23) Lahav, M.; Kharitonov, A.. B.; Katz, O.; Kunitake, T.; Willner, I. Anal. Chem. 2001, 73, 720-723,

10.1021/la047610n CCC: $30.25 © 2005 American Chemical Society Published on Web 03/22/2005

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anism of the PEMs. The importance of the solution conditions such as the solution ionic strength3 and pH (in the case of weak polyelectrolytes)26 has been emphasized. It has also been discovered that there are two growing mechanisms of PEMs: (i) linearly growing multilayers in which the thickness increment is constant during each pair of layer deposition and the interpenetration of each of them by the neighboring layers extends only through one or two bilayers2,27 and (ii) layers in which the whole thickness of the film is an exponential function of the number of the pairs of layers that have been deposited.28-33 It was initially thought that this exponential buildup regime was related to an increase in the film roughness with an increasing number of deposited bilayers,28 but this assumption could be discarded by means of a systematic atomic force microscoopy (AFM) investigation on linearly and exponentially growing PEMs.31 It has been demonstrated later on by means of confocal laser scanning microscopy that the observed exponential buildup was due to the diffusion of at least one of the polyelectrolytes through the whole thickness of the PEM.30 A model has even been proposed to explain the difference between the linearly and exponentially growing PEMs: the difference may most probably lie in the existence of a surface potential barrier which is of the order of the thermal energy in the case of the exponentially growing films and which is much higher than this thermal energy in the case of the linearly growing films.34 It appears also that most, but not all,35,36 of the exponentially growing multilayers are observed in the case where at least one of the used polyelectrolytes is a homopolypeptide or a polysaccharide. It seems that the exponential behavior is related to the weak linear charge density of the polyelectrolytes as shown by the importance of the weak acid or base character of the monomers and the ionic strength dependence of the exponential growth. Moreover, the presence of hydrogen bond donors or acceptors, such as in the amide bonds, seems also to be important owing to the important numbers of exponentially growing PEMs in which at least one of the polyelectrolytes is an homopolypeptide.29-33 It has also been observed that a single change of the solvent properties, such as the use of water-ethanol mixtures with an increasing content in ethanol, is also able to induce the change of a linear to a supralinear-like behavior of the polystyrene sulfonate (PSS)/poly(allylamine) (PAH) combination of polyelectrolytes.37 (24) Mamedov, A.; Kotov, N. A.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. Nat. Mater. 2002, 1, 190. (25) Richert, L.; Boulmedais, F.; Lavalle, Ph.; Mutterer, J.; Ferreux. E.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Biomacromolecules 2004, 5, 284. (26) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (27) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (28) Elbert, D. L.; Herbert, C. B.; Hubbell, J. A. Langmuir 1999, 15, 5355. (29) Picart, C.; Lavalle, Ph.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C. Langmuir 2001, 17, 7414. (30) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J. C.; Lavalle, Ph. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531. (31) Lavalle, Ph.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458. (32) Boulmedais, F.; Ball, V.; Schwinte, P.; Frisch, B.; Schaaf, P.; Voegel, J. C. Langmuir 2003, 19, 440. (33) Lavalle, Ph.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J. C.; Mesini, P. J. Schaaf, P. Macromolecules 2004, 37, 1159. (34) Lavalle, Ph.; Picart, C.; Mutterer, J.; Gergely, C.; Reiss, H.; Voegel, J. C.; Senger, B.; Schaaf, P. J. Phys. Chem. B 2004, 108, 635. (35) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem. Mater. 2003, 15, 1575. (36) Schoeler, B.; Poptoshev, E.; Caruso, F. Macromolecules 2003, 36, 5258.

Ball et al.

The effect of the nature of the monovalent ions used to buildup the PEMs has also been illustrated for the polystyrene sulfonate (PSS)/poly (diallyldimethylammonium chloride) (PDADMAC) combination of polyelectrolytes,38 and it appeared that the thickness increment per bilayer increased with the “kaotropic” character of the negative ions of the used sodium salts. It is the major aim of this paper to compare the buildup of PEMs made from PSS and PAH in aqueous solutions containing either sodium chloride or sodium thiocyanate at a constant pH value of 7.4 as well as to provide an explanation for the observed differences. The effect of thiocyanate, a monovalent highly polarizable and kaotropic anion, was not investigated in the previous studies aimed to understand the small electrolyte effect on the PEM buildup process.38-40 In the case of the PSS/PDADMAC combination of polyelectrolytes, the polycation precipitated spontaneously in the presence of thiocyanate solutions, and this precluded the buildup of the PEM. This precipitation of PDAMAC might well have its origin in the occurrence of quaternary ammonium groups and, hence, to a charge density independent of pH. It will be shown here that such a precipitation does not occur in the case where the polycation is a weak one, like PAH. Nevertheless, PAH seems to form some aggregates in solutions of increasing SCN- concentration. Moreover, the permeability of potassium ferrocyanide through PEI/(PSS-PAH)n-PSS multilayers built up in the presence of sodium salts (either sodium chloride or sodium thiocyanate) at different concentrations will be investigated in this work. We will suggest that the huge permeability differences are related to the differences in porosity of the PEMs built up in the presence of NaCl or NaSCN. Materials and Methods Polyelectrolyte Solutions and Chemicals. All solutions were prepared from doubly distilled and deionized water with a resistivity of 18.2 MΩ‚cm. The solutions were buffered at pH 7.4 by the use of 10 mM Tris (tris(hydroxymethylaminomethane) from Gibco BRL). The salt concentration in NaX (with X- being Cl- or SCN-) was varied between 0.1 and 0.5 M., but most of the experiments were performed at 0.5 M were the differences between the two electrolytes were found to be maximal. Moreover the PAH solutions turned turbid at higher NaSCN concentrations. NaCl and NaSCN were purchased from Prolabo and from Fluka, respectively, and used without further purification. These buffer solutions will be called Tris-NAX (x M) in the following, for instance, Tris-NaSCN (0.5 M) refers to a 10 mM Tris buffer at pH 7.4 and containing 0.5 M of NaSCN. The polyelectrolytes used to buildup the multilayers were poly(ethyleneimine) (PEI, Aldrich, catalog no. 17,198-8, weight averaged molecular weight 750000 g‚mol-1), poly(allylamine hydrochloride) (PAH, Aldrich, catalog no. 28,322-3, weight averaged molecular weight 6500070000 g‚mol-1) and poly(styrene sulfonate) (PSS, Aldrich, catalog no. 24,305-1, weight averaged molecular weight: 70000 g‚mol-1). All these polyelectrolytes were dissolved at a concentration of 1 mg‚mL-1 in the desired buffer solution. The polyelectrolyte solutions were characterized by means of zeta potential measurements and by means of conductimetry to estimate the sign of their surface charge density and their uptake of counterions from the electrolyte solution, respectively. The multilayer films were built up as will be described in the next paragraphs and were characterized in situ and in real time by means of optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microbalance with dissipation (QCM-D). The (37) Poptoshev, E.; Schoeler, B.; Caruso, F. Langmuir 2004, 20, 829. (38) Saloma¨ki, M.; Tervasma¨ki, P.; Areva, S.; Kankare, J. Langmuir 2004, 20, 3679. (39) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. J. Phys. Chem. B 2003, 107, 7998, (40) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Langmuir 2002, 18, 5607.

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ion exchange properties of the multilayers built up in the presence of SCN- versus a solution containing Cl- was investigated by means of Fourier transform spectroscopy in the attenuated total reflection mode (FTIR-ATR spectroscopy). The effect of ion exchange induced from the electrolyte solution on the multilayer thickness was also investigated by means of OWLS. Moreover the PEMs were characterized by atomic force microscopy (AFM) in the dry state after buildup from solution mainly to have an idea of the film roughness and porosity. Finally, their permeation for an electoactive ion, Fe(CN)64-, was measured by means of cyclic voltammetry. All these experiments, which are detailed in the following sections, were done with the aim to compare the PEM properties with respect to the presence of Cl- or SCN- at a concentration of x M in the aqueous solutions at pH 7.4. Optical Waveguide Lightmode Spectroscopy (OWLS). The OWLS technique allows determination of the optical thickness and the refractive index of an adsorbed layer on a Si0.8Ti0.2O2 waveguiding substrate.41 Briefly, a laser beam (λ ) 632.8 nm) is directed on a diffraction grating imprinted in the waveguiding layer. For oxide films about 200 nm in thickness, only one transverse electric (TE) and one transverse magnetic (TM) mode is allowed to propagate along the waveguide. 41 This incoupling is realized at discrete values of the incident angle of the laser beam by means of a diffraction grating embossed in the waveguiding film. To each incoupling angle corresponds an effective refractive index NTE or NTM. These values allow calculation of the optical thickness dA and the refractive index nA of the film in the framework of a homogeneous and isotropic layer model provided the thickness dF and refractive index nF of the waveguiding film have been previously calculated from the NTE and NTM values obtained when the waveguide is put in contact with buffer.41,42 The technique is based on an evanescent wave sensing the deposited film over a penetration depth of about 200 nm in our case (refractive index of the buffer nC between 1.334 and 1.341, refractive index of the waveguiding film nF ≈ 1.78). Before multilayer deposition, the optical waveguides were cleaned in a hot (about 60 °C) Hellmanex solution at 2% (v/v) during 10 min. They were then rinsed extensively with distilled water, immersed during 10 min in hot 0.1 M hydrochloric acid solution and finally rinsed with distilled water and dried under a stream of ultrapure nitrogen. The waveguides were then put in the measuring chamber of the homemade OWLS apparatus.42 The internal volume of this chamber was of about 37 µL, and it was made tight with an O-ring (Kalrez). The waveguide was then put in contact with a circulating buffer solution, and the positions of the peak positions corresponding to the NTE and NTM peaks were followed in real time by rotating the goniometer. As soon as the NTE and NTM values changed by less than 10-5, these values were taken as a baseline to calculate the thickness dF and refractive index nF of the waveguiding layer. This calculation was done by using a Fresnel model with three layers, the glass substrate with refractive index nS, the waveguiding film, and the buffer solution with refractive index nC. The nC value was measured for all the Tris-NaX (x M) solutions using a Belligham refractometer operating at the sodium D line, hence close to the wavelength of the He-Ne laser used for the OWLS experiment. The adsorbed amounts per unit area, Γ, were computed according to the relationship43

Γ ) (nA - nC)dA/(dn/dc)

(1)

where the significance of nA, nC, and dA is given in the text and where {nA, dA} are obtained from the OWLS experiments. dn/dc is the refractive index increment of the polyelectrolyte. Quite arbitrarily, we use the average value of the dn/dc of the individual polyelectrolytes PSS and PAH in the given electrolyte solution. The polyelectrolyte multilayer was then built up beginning with the injection of the PEI solution. At all stages of the buildup, the syringe pusher injecting the buffer over the waveguide surface was stopped during the polyelectrolyte adsorption. One hundred (41) Tiefenthaler, K.; Lukosz, W. J. Opt. Soc. Am. B 1989, 6, 209. (42) Picart, C.; Ladam, G.; Senger, B.; Voegel, J. C.; Schaaf, P.; Cuisinier, F. J. G.; Gergely, C. J. Chem. Phys. 2001, 115, 1086. (43) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759-1772.

microliters of polyelectrolyte solution was manually injected through an injection port. This injection lasted over 30 s, and the adsorption was then allowed to take place during the 9.5 min before buffer injection lasting over 10 min at a flow rate of 10.1 mL‚h-1. The measurement of the incoupling angles allowing propagation along the waveguide was never interrupted during the PEI-(PSS-PAH)n multilayer buildup. In some experiments, after the deposition of the last PAH layer and rinsing with the buffer in which the buildup was performed, the solution was changed to a buffer at the same pH but containing other negative counterions (with respect to PAH) as those used during the buildup. Finally after monitoring the effect of this ion exchange on the multilayer thickness (taking into account the changes in nC for the thickness calculations), the buffer solution was switched back to the solution used to buildup the PEM. Quartz Crystal Microbalance. QCM-D is a technique in which a quartz crystal is excited at its fundamental frequency (about 5 MHz) and observation takes place at the first, the third, fifth, and seventh overtones (denoted ν and corresponding to 5, 15, 25, and 35 MHz, respectively).44,45 A decrease in ∆f/ν is usually associated, in a first approximation, with an increase of the mass coupled to the quartz. Here, we use QCM-D only for qualitative analysis. At first glance, due to the complexity of our films, we do not necessarily expect Sauerbrey’s relation to be valid.46 A more refined quantitative analysis would require the use of the viscoelatic model of thin films developed by Voinona et al.47,48 Indeed we will assume that the Sauerbrey relation can be used if the reduced frequency shifts ∆f/ν are overlapping for the different overtones. The Sauerbrey equation is written as

Γ ) -C∆fν/ν

(2)

with C ) 17.7 ng‚cm-2‚Hz-1 as given by the manufacturer (Qsense AB, Go¨teborg, Sweden). The PEI-(PSS/PAH)n or the PEI-(PSS/PAH)n-PSS multilayers were built up on silica-coated quartz crystals (Q-Sense) or on quartz crystals coated with 100 nm of gold. The silicacoated crystals were cleaned with ethanol, put in the flow chamber of the quartz crystal microbalance, and then put in contact with a 2% (v/v) Hellmanex solution during 30 min at room temperature. However, the gold coated quartz crystals were cleaned as the gold electrodes used for cyclic voltammetry experiments (see later on). The crystal surface was then rinsed extensively with distilled water, with a 0.1 M hydrochloric acid solution, with distilled water again, and finally with a 10 mM Tris buffer at pH ) 7.4 containing either NaCl or NaSCN at different salt concentrations. The resonance frequencies of the crystal were then followed until the frequency drifts became lower than 1 Hz‚min-1 at all investigated frequencies. When this condition was fulfilled, we began the buildup of the polyelectrolyte multilayer by injecting 1.5 mL of the PEI solution during 1 min. This solution was allowed to rest over the QCM crystal for an additional 9 min. No observable frequency shifts were noted after 10 min of adsorption, meaning that the adsorption kinetics have reached a steady state. The crystal was then rinsed with buffer (1.5 mL during 1 min) and allowed to rest for 9 min. The buildup was then pursued by adsorption of PSS (a 1.5 mL injection over 1 min, the crystal being allowed to rest for 4 min), a buffer rinse, adsorption of PAH (a 1.5 mL injection over 1 min, the crystal being allowed to rest for 9 min), and a new buffer rinse. All buffer rinse steps consisted in the injection of 1.5 mL of buffer solution, which was then allowed to rest over the crystal surface for 9 min. All these durations were sufficient to reach a steady state in the measured frequency changes. Fourier Transform Infrared Spectroscopy in the ATR Mode. The Fourier transform infrared spectroscopy (FTIR) (44) Rodahl, M.; Kasemo, B. Sens. Actuators, B 1996, B37, 111-116. (45) Ho¨o¨k, F.; Rodahl, M.; Brzezinski, P.; Kasemo, B. J. Colloid Interface Sci. 1998, 208, 63-67. (46) Sauerbrey, G. Z. Phys. 1959, 155, 206. (47) Rodahl, M.; Ho¨o¨k, F.; Fredriksson, C.; Keller, C. A.; Kroser, A.; Brzezinski, P.; Voinova, M.; Kasemo, B. Faraday Discuss. 1997, 107, 229-246. (48) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Phys. Scr. 1999, 59, 391-396.

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experiments were performed on an Equinox 55 spectrometer (Bruker, Wissembourg, France) using a liquid nitrogen cooled detector. For the buildup of the multilayers, H2O was used as a solvent instead of D2O, which was used in our previous studies.49 In these experiments, where polyelectrolyte multilayers were built up from solutions containing homopolypeptides, the amide I band of the polypeptides (between 1600 and 1700 cm-1) is strongly affected by the strong absorption of water around 1643 cm-1 (O-H bending mode) whereas the corresponding vibration in D2O is around 1209 cm-1.50 Indeed, the components of the PEI-(PSS-PAH)n multilayers do not absorb infrared radiation around the water band at 1643 cm-1. All the spectra during the multilayer buildup were collected by accumulating 512 interferograms at 2 cm-1 resolution. For these infrared spectroscopy experiments, the PEI-(PSS-PAH)n multilayers were built up in situ on a trapezoidal ZnSe ATR crystal. This crystal constituted the lower part of a 500 µL flow cell (Graseby Specac). The infrared spectra were collected after each rinsing step, and the evolution of the PEM buildup was appreciated by following the increase in the intensity of the peaks attributed to PSS at 1007 and 1035 cm-1.51 In the case of the buildup in the presence of Tris buffer containing 0.5 M NaSCN, the buffer was switched to Tris 10 mM containing NaCl at 0.5 M at the end of the buildup. This experiment was done in order to follow the ion exchange kinetics of SCN- present in the film (and in solution before the ion exchange) by the Cl- anions from the 10 mM Tris buffer with 0.5 M NaCl. Indeed, due to its spherical symmetry Cl- is not infrared active whereas SCN- displays strong absorption around 2060 cm-1. The reverse experiment, in which the film is built up in the presence of 0.5 M NaCl and then put in contact with a flowing 0.5 M NaSCN solution, was also performed. Atomic Force Microscopy. The PEI-(PSS-PAH)4-PSS multilayers were made by dipping 12 mm diameter glass slides alternatively in the polycation solution, in either the Tris-0.5 M NaCl or the Tris-0.5 M NaSCN buffer, in the polyanion solution and again in the buffer solution. Each adsorption or rinsing step lasted over 10 min. The glass slides were cleaned just before the multilayer buildup like the optical waveguides. After the end of the dipping experiment, the glass slides were stored in the buffer used for the buildup. In some of the experiments we directly used the quartz crystals used for the QCM-D experiments for AFM imaging. Before imaging with AFM, all the used surfaces were quickly rinsed with distilled water, to remove the sodium chloride or sodium thiocyanate, and dried under a nitrogen stream. The slides where then imaged in the contact mode in air (Nanoscope III, Digital instruments) using Si3N4 cantilevers. It might well be that some morphological changes are induced during the drying process. Nevertheless, the surface topography we obtained for the PEI-(PSS-PAH)4PSS multilayers is very close to that obtained in wet conditions.31 However, It remains possible that the drying effect affects differently the multilayers buildup in the presence of SCN- and Cl-. For this reason the AFM images will only be used in a qualitative way. Zeta Potential and Light Scattering Measurements. To evaluate the effect of the counterion nature on the electrophoretic mobility (and hence on the zeta potential) of the PSS and PAH polyelectrolytes, we performed some electrophoretic light scattering experiments by using a Zetasizer 3000 HS from Malvern Instruments. For these experiments the electrophoretic mobility was converted into the zeta potential by means of the Schmolukowski relation. Indeed, the Debye screening length in a 0.5 M NaCl or NaSCN solution is equal to 4.3 Å, which is much less than the hydrodynamic radius of the used PAH (between 10 and 15 nm). Hence the Schmolukowski relationship can be applied. Before the experiments, the device was calibrated by means of a sulfated latex particle (Polysciences Inc., catalog no. 19403, nominal particle diameter of 477 nm) suspension in a 1 mM NaCl solution. These latex particle solutions have a zeta potential of -60 ( 10 mV if the measurement cell is clean. The PAH and PSS solutions were injected in the measurement cell only when (49) Debreczeny, M.; Ball, V.; Boulmedais, F.; Szalontai, B.; Voegel, J. C.; Schaaf, P. J. Phys. Chem. B 2003, 107, 12734. (50) Venyaminov, S.; Kalnin, N. N. Biopolymers 1990, 30, 1259. (51) Yang, J. C.; Jablonsky, M. J.; Mays, J. W. Polymer 2002, 43, 5125-5132.

Ball et al. the zeta potentiel value of the latex particle suspension fell in this specified range of values. All measurements were performed by applying a constant current of 5 mA between the two electrodes of the measurement chamber. The dynamic light scattering experiments were performed with the same device at a constant scattering angle of 90°. The obtained intensity autocorrelation functions were then transformed in a scattered light intensity distribution versus the hydrodynamic radius by using an inverse Laplace transform. The PAH solutions were characterized at a solution concentration of 1 mg‚mL-1 in the presence of 0.5 M NaX (X- ) Cl- or SCN-) and 10 mM Tris buffer. Conductimetry Experiments. To study the influence of the SCN- and Cl- anions on PAH, we performed some conductimetric titrations by using a CDM 210 conductimeter (Radiometer Analytical, Copenhagen) fitted with a 690-12 UN conductimetry electrode. To this aim we measured the increase in conductivity of the solution by injecting small volumes of NaCl (at 0.5 M) or NaSCN (at 0.5 M) in 50 mL of 10 mM Tris buffer containing or not PAH at 1 mg‚mL-1. At each point of the titration, the difference in conductivity between the solution containing PAH and the blank experiment without PAH gives information about the intrinsic conductivity of PAH in the presence of the Cl- and SCN- electrolytes. Cyclic Voltammetry Experiments. The cyclic voltametry experiments were carried out by means of a CHI604B potenstiosthat (CH Instruments) using a gold electrode, a saturated calomel electrode (SCE), and a platinum wire as working, reference, and auxiliary electrodes, respectively. The gold electrode was polished successively with 1 µm and 0.1 µm diamond pastes. After each polishing step, lasting over 2 min, the gold electrodes were sonicated twice in distilled water baths for 5 min, the water bath being changed after each sonication step. Just before each experiment, the electrodes were cleaned with ethanol and distilled water before being subjected to 30 cycles in a 0.5 M sulfuric acid solution. Each cycle was performed between 0 and 1.6 V versus SCE and again between 1.6 and 0 V versus SCE at a scan rate of 10 V‚s-1. These fast oxidation and reduction cycles allowed the buildup of a gold oxide layer whose roughness can be estimated by integrating the area under the reduction peak close to 0.8 V versus SCE.52,53 This oxide layer was then characterized with a oxidation-reduction cycle between 0 and 1.6 V versus SCE at a scan rate of 0.1 V‚s-1. The PEI was then directly adsorbed onto the gold electrode, without adsorption of a self-assembled monolayer (SAM), by dipping the gold electrode in a PEI solution at 1 mg‚mL-1 (in Tris 10 mM with x M NaX, x varying between 0.1 and 0.5 M) during 10 min. The possibility of adsorbing PEI directly on the gold electrode without the presence of a SAM was demonstrated by QCM-D on gold electrodes submitted to the same cleaning method as the electrodes used for the cyclic voltammetry experiments. The electrodes were then dipped in a 10 mM Tris solution containing x M NaX and 1 mM K4Fe(CN)6‚3H2O (Sigma, CAS registry no. 14459-95-1, batch 033K0128). A cyclic voltammogram was then taken between 0 and 0.45 V versus SCE to check the effective adsorption of PEI. Indeed naked electrodes, without adsorbed PEI displayed very small (lower than 10-8 A) oxidation and reduction currents over the explored potential range whereas electrodes covered with PEI showed well-defined and reversible oxidation and reduction peaks. After this, the electrodes were put in a 10 mM Tris buffer containing x M NaX. The decrease of the oxidation and reduction currents was then followed as a function of time, and it was found that no appreciable signal due to the oxidation/reduction of K4Fe(CN)6 could be found after 15 min of immersion in the 10 mM Tris with NaX buffer. After that, the polyelectrolyte multilayer buildup was undertaken by dipping the gold electrode alternatively in a PSS solution, in pure buffer (two successive rinsing steps), in a PAH solution and again in pure buffer. Each dipping step lasted over 10 min, and the buildup was undertaken to reach a PEI-(PSS-PAH)4-PSS or a PEI(PSS-PAH)8-PSS multilayer. Then, the PEM-covered gold electrode was dipped in the same buffer used during the PEM (52) Oesch, U.; Janata, J. Electrochim. Acta 1983, 28, 1237. (53) Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1990, 35, 117.

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Langmuir, Vol. 21, No. 9, 2005 4133 Table 1. Zeta Potential of PAH and of PSS in Different Electrolyte Solutions at pH ) 7.4 nature and concn of the electrolyte and of the investigated polyelectrolyte

(ζ ( σ)/mVa

PSS in 10 mM Tris, 0.5 M NaSCN PSS in 10 mM Tris, 0.5 M NaCl PAH in 10 mM Tris, 0.5 M NaSCN PAH in 10 mM Tris, 0.5 M NaCl PAH in 10 mM Tris, 0.3 M NaSCN

-22.8 ( 0.3 -22.6 ( 0.9 -20.5 ( 1.3 8.0 ( 0.8 -9.3 ( 9.9

a The standard deviation σ is obtained from five measurements on a given solution.

Figure 1. Difference in conductivity between a 10 mM Tris solution containing 1 mg‚mL-1 PAH and a 10 mM Tris solution as a function of the volume of added electrolyte, either 0.5 M NaCl (0) or 0.5 M NaSCN (b). The error in these data is of the order of 0.05 mS. buildup and containing 1 mM K4Fe(CN)6. This defined the time 0 of the permeation experiments. The cyclic voltammograms between 0 and 0.45 V versus SCE were then recorded at a scan rate of 0.1 V‚s-1 at different time intervals until the oxidation and reduction currents reached saturation values. After that, the electrode was dipped in the same buffer but without K4Fe(CN)6, and the release rate of the electroactive species contained in the PEM was followed as a function of time.

Results and Discussion First of all we checked that PAH remains soluble in Tris 10 mM buffer with x M NaSCN (0.1 < x < 0.5 M.). However, at higher NaSCN concentrations, the PAH solutions turned turbid, which shows that the SCN- anions and the PAH polycations are interacting. Hence, SCNions at high concentration are able to induce phase separation of the PAH chains. This is rather surprising for monovalent ions. Indeed, it has been shown that the effective charge of poly(acrylic acid) is independent of the nature of the investigated monovalent cations (Li+ or Na+) present in solution.54 To the best of our knowledge, the interaction between SCN- anions and PAH have not yet been described. However, it has been demonstrated that the strength of counterion interaction with PAH is in the order of higher interactions with ClO4- than with NO3-, Cl-, Br-, and I-.55 It is hence not surprising that PAH interacts with SCN-, which is known to be highly chaotropic, even more than ClO4-.56 The observation that PAH precipitates in the presence of high SCN- concentrations precluded performing multilayer buildups from solutions containing more than 0.5 M NaX. It has been reported that PDADMAC, a strong polycation, precipitates in the presence of SCN-.38 In the present study, dynamic light scattering experiments performed at a constant scattering angle of 90° showed that the hydrodynamic radius of PAH slightly increased in the presence of increasing concentrations of NaSCN. Moreover, part of the PAH molecules have an hydrodynamic radius that is higher in the presence of NaSCN than in the presence of the same concentrations of NaCl (See Supporting Information, Figure 1). Moreover, in the presence of NaSCN, the zeta potential of PAH is negative (Table 1) and its absolute value increases with an increase (54) Boisvert, J. P.; Malgat, A.; Pochart, I.; Daneault, C. Polymer 2002, 43, 141. (55) Itaya, T.; Ochiai, H. J. Polym. Sci., Part B: Polym. Phys. 1992, 30, 587. (56) Cacace, M. G.; Landau, E. M.; Ramsden, J. J. Q. Rev. Biophys. 1997, 30, 241.

in the NaSCN concentration. This points to a possible progressive modification of the PAH chains in the presence of SCN- with adsorption of the negative ions to small aggregates made from PAH. This clearly shows that PAH, even if it carries positively charged amino groups at pH 7.4, behaves like a polyanion in solution. This effect might probably originate from ion condensation of the negative counterions to the polycationic chains as is also suggested by the conductimetric titration of the PAH solution (Figure 1). Indeed, the data in Figure 1 correspond to the difference between the conductivity of a PAH solution at 1 mg‚mL-1 and a 10 mM Tris solution as a function of increasing volumes of added NaX solutions at 0.5 M, hence as a function of increasing concentration of the X- (Cl- or SCNions). For instance when the added volume of electrolyte solution amounts to 5 mL, the total concentration in NaX amounts to 4.5 × 10-2 M. Even at such a small salt concentration, it appears that this difference in conductivity decreases in the case of NaSCN whereas it increases in the presence of NaCl. This means that the intrinsic conductivity of PAH decreases in the presence of increasing concentrations of SCN- ions and hence that the polycation strongly interacts with these counterions, as already suggested from the zeta potential measurements. Further experiments are necessary to understand why the difference in conductivity between the PAH solution and the buffer increases during the addition of NaCl solution. Even if PAH appears negatively charged in PAH solutions, the polyelectrolyte multilayer buildup takes places when PSS and PAH are alternatively put in contact with a Si0.8Ti0.2O2 waveguide modified with a monolayer of PEI. This remains true even at the highest salt concentration investigated, namely, 0.5 M (Figure 2). This means that the PAH molecules behave again like a polycation when they reach a negatively charged interface. This suggests a strong contribution of the counterion release to the negative free energy changes accompanying the interaction between a soluble polyelectrolyte and a polyelectrolyte already adsorbed at the film/solution interface. This effect is also probably the same as the one occurring when two oppositely charged polyelectrolytes interact in solution.57,58 It is apparent from Figure 2 that the layer growth is linear in the presence of both small electrolytes with a slope that is about three times higher in the presence of NaSCN at 0.5 M than in the presence of NaCl at the same concentration. Nevertheless, the data always appear more scattered for the experiments performed in the presence of SCN- than those performed in the presence of Clprobably due to the fact that the polyelectrolyte is aggregated in these former conditions. Indeed, the experiments performed in the presence of NaCl appear much (57) Record, M. T., Jr.; Lohman, T. M.; Haseth, P. L. J. Mol. Biol. 1976, 107, 145. (58) Ball, V.; Winterhalter, M.; Schwinte, P.; Lavalle, Ph.; Voegel, J. C.; Schaaf, P. J. Phys. Chem. 2002, 106, 2357-2364.

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Figure 2. Evolution of the layer thickness of a PEI-(PSSPAH)n multilayer with the number of deposited layers. The multilayers were built up from a 10 mM Tris buffer at pH ) 7.4 containing either 0.5 M NaCl (0) or 0.5 M NaSCN (circles, three independent experiments). The even numbers correspond to the deposition of the polycation whereas the odd numbers correspond to the deposition of polyanions. Layer “0” corresponds to the thickness reached at the end of the buffer rinse succeeding the PEI adsorption.

Figure 3. Evolution of the refractive index of a PEI-(PSSPAH)n multilayer with the number of deposited layers. The multilayers were built up from a 10 mM Tris buffer at pH ) 7.4 containing either 0.5 M NaCl (0) or 0.5 M NaSCN (circles, two independent experiments). The even numbers correspond to the deposition of the polycation whereas the odd numbers correspond to the deposition of polyanions.

more reproducible than those performed in the presence of NaSCN (See Supporting Information, Figure 1). In the presence of NaSCN at 0.5 M, during the deposition of a bilayer, the optical thickness increases strongly during the deposition of PAH whereas it remains almost constant or only slightly increases during the deposition of PSS. This is obviously not the case when the buildup is performed in the presence of NaCl at 0.5 M (Figure 2). This suggests that some desorption of PAH occurs when the PAH ending multilayer is put in contact with a solution containing PSS in a 0.5 M NaSCN solution. It might then well be that PAH interacts less strongly with PSS when it is adsorbed from a solution containing NaSCN at 0.5 M than from a solution containing NaCl at 0.5 M. Moreover, the refractive index of the films, somewhat unrealistic from a physical point of view when only one PSS/PAH bilayer has been deposited, are higher in the case of deposition made from NaCl at 0.5 M than from NaSCN at the same concentration (Figure 3). This shows that the structures of both type of films are different due only to the change of the negative counterion. The fact

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Figure 4. Reduced frequency changes close to 15 MHz (ν ) 3) during the buildup of PEI-(PSS-PAH)4-PSS multilayers in the presence of 10 mM Tris and 0.5 M NaCl (0) and 10 mM Tris and 0.5 M NaSCN (b) buffer. The vertical lines on top of the curve obtained in the presence of thiocyanate correspond to the injection of PAH whereas the lines on the bottom of the same curve correspond to the injection of PSS. The injection times are the same as in the experiment performed with NaCl. The double headed arrow corresponds to the time at which the buffer containing 1 mM ferrocyanide was injected. The single arrows correspond to the beginning of the final buffer rinse.

that the calculated refractive indexes are smaller in the case of the buildup made from NaSCN suggests a higher water content of these films with respect to that of films made from NaCl. We followed also the multilayer buildup by means of QCM-D. As in the case of the OWLS experiments, we found that the frequency changes are a linear function of the number of deposited pairs of layers for both electrolyte solutions Tris NaCl (0.5 M) and Tris NaSCN (0.5 M) (Figure 4). As in the case of the OWLS experiments, the slope of the linear growth was about three times higher in the presence of Tris-NaSCN (0.5 M) than the slope obtained in the presence of Tris-NaCl (0.5 M). This observation holds both on gold (Figure 4) and on quartz substrates (data not shown). Moreover, as in the OWLS experiments, the adsorption of PAH seemed to induce greater frequency changes as those associated with the deposition of PSS when the buildup was performed from Tris-NaSCN (0.5 M). In addition, if one compares the evolution of the reduced frequency changes, ∆fν/ν, for the different harmonics (ν ) 1, 3, 5, and 7), one observes that these curves superimpose in the limit of the experimental accuracy (a few hertz). This means that the PEM behave as rigid and nonslipping films in both electrolyte solutions and that the Sauerbrey relationship can be applied46 to calculate the deposited mass per unit area (including the strongly bound water) from the measured frequency changes (Table 2). The results of the calculations given in Table 2 again suggest that the hydration of the PEI-(PSS-PAH)5 multilayer is higher when the deposition is made from a Tris NaSCN (0.5 M.) buffer than from a Tris NaCl (0.5 M.) buffer. Indeed, the ratio of the adsorbed amounts measured with both techniques and for a given small electrolyte solution is higher in the case of SCN- (about 3) than in the case of Cl- (about 2). We then asked the question if the observed ion effect is reversible. To that aim, we followed the buildup of a PEI-(PSS-PAH)4 multilayer in the presence of 0.5 M NaCl, the layer was rinsed with Tris NaCl (0.5 M) and then with Tris-NaSCN (0.5 M). We observed a fast increase in NTE and in NTM during this buffer change. But when the change in refractive index of the buffer was taken into

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Langmuir, Vol. 21, No. 9, 2005 4135 Table 2. Adsorbed Amounts in the OWLS and QCM-D Experiments

c

nature and concn of the electrolyte and of the investigated multilayer

Γ/µg‚cm-2

PEI-(PSS-PAH)5 in Tris 10 mM NaSCN (0.5 M), OWLS experiments PEI-(PSS-PAH)5 in Tris 10 mM NaCl (0.5 M), OWLS experiment PEI-(PSS-PAH)5 in Tris 10 mM NaSCN (0.5 M), QCM-D experiment PEI-(PSS-PAH)5 in Tris 10 mM NaCl (0.5 M), QCM-D experiment

7.0a 3.5a 10.3b 3.4c

a Calculation done with dn/dc ) 0.205 cm3‚g-1 as measured with a refractometer. b Calculation done with ∆f /ν ) -580 Hz (Figure 4). ν Calculation done with ∆fν/ν ) -190 Hz (Figure 4).

account, no change in film thickness or in its refractive index could be calculated. The same was true when the buffer was switched back to Tris-NaCl (0.5 M) (data not shown). In addition, we did the reverse experiment, namely, the PEI-(PSS-PAH)4 multilayer was built up in Tris-NaSCN (0.5 M) and the buffer was then switched to Tris-NaCl (0.5 M) to allow for ion exchange. Again no change in optical thickness could be calculated (Supporting Information, Figure 3). However, as expected in both experiments, when the buffer solutions were replaced by distilled water at the end of the experiment, the multilayer began to swell (Supporting Information, Figure 3). These observations clearly show that as soon as the PEM is built up in a given salt solution, 1, when it is put in contact with another salt solution, 2, it will not change its structure within the explored time scale (half an hour) to adopt the thickness and refractive index it would have if it had been constructed directly from the salt solution 2. However, one cannot expect that a film buildup from solution 1 and put in solution 2 will reach the same thickness as would be obtained if the same number of bilayers were deposited directly from solution 2. Nevertheless, a thickness change would be expected during this solution switch if the process is (partly) reversible. The reverse situation is also true when the film is built up in solution 2 and then put in solution 1. This means that the multilayer is in a kind of frozen state and it seems that its buildup is a partly irreversible phenomenon. To go further, we performed FTIR-ATR experiments in which the PEM is built up in the presence of 0.5 M SCNat pH 7.4. We first measured the absorbance of the buffer which displayed a strong absorption band around 2060 cm-1, which is attributed to the SCN- ions. This spectrum constitutes our reference spectrum to which all the other spectra were compared. When the polyelectrolyte multilayer is built up to reach a PEI-(PSS-PAH)7-PSS architecture, one observes a progressive increase in the intensity of the bands located at 1007 and 1035 cm-1, which are attributed to PSS (Figure 5A). This confirms again that the PEM buildup takes place on the ZnSe crystal covered with a PEI layer. Moreover one observes a strong increase in the band attributed to SCN- which means that the film is accumulating SCN- because the displayed spectra are the difference between the measured spectra and the reference spectra. Most interestingly, the SCNband decreases slightly when PSS is adsorbed on a PAH ending film whereas it increases strongly when PAH is adsorbing. This shows definitely the occurrence of a strong SCN- condensation on the PAH chains. This explains also why the thickness increases and the frequency decreases are higher when PAH is adsorbing in comparison with the changes observed for PSS in the presence of SCN-. PAH is adsorbing as an only slightly (negatively) charged chain, and when it reaches the surface terminated with PSS, only a part of the SCN- condensated ions are released. This explains that an important amount of PAH has to be adsorbed to ensure charge overcompensation and further multilayer growth. This ion condensation phenomenon seems to become very important when the

Figure 5. (A) Evolution of the ATR-FTIR spectra of a PEI(PSS-PAH)n multilayer built up from a 10 mM Tris, 0.5 M NaSCN buffer as a function of the number of layers: (‚‚‚) NaSCN containing buffer, (- - -) PEI-(PSS-PAH)2, and (s) PEI(PSS-PAH)7. (B) Time evolution of the ATR-FTIR spectra compared to the spectra of a PEI-(PSS-PAH)7-PSS multilayer built up from a 10 mM. Tris, 0.5 M NaSCN buffer, when this film is put in contact with a 10 mM Tris, 0.5 M NaCl buffer. The lower spectrum corresponds to the spectrum of a PEI(PSS-PAH)7-PSS multilayer built up in the presence of the 10 mM Tris, 0.5 M NaSCN buffer minus the spectrum of the 10 mM Tris, 0.5 M NaSCN buffer. The same differences were taken after (‚‚‚) 42.5 min and (- - -) 170 min of contact with the Cl--containing solution.

NaSCN concentration is between about 0.2 and 0.3 M since the slope of the thickness increase as a function of the number of layer pairs increases suddenly around this salt concentration (Supporting Information, Figure 4). We then investigated the effect of changing the SCNsolution in contact with the multilayer by a Cl--containing buffer (also at 0.5 M). As shown in Figure 5B, the reduction of the SCN- band is slow and seems to reach saturation after roughly 3 h of contact with the Cl--containing buffer. Moreover the reduction in the SCN- band is far from reaching the maximal value expected if all the initially present SCN- ions would have been exchanged by Cl(lower curve of Figure 5B). Integration of the area of these SCN- band as a function of time shows that only about 6-7% of the present SCN- can be exchanged (Supporting Information, Figure 5). This is in full agreement with the effect of the ion exchange experiments performed by OWLS, namely, that the multilayer thickness does not change significantly when the SCN--containing buffer is switched to a Cl--

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Figure 6. Oxidation and reduction waves of 1 mM Fe(CN)64in the 10 mM Tris-0.5 M NaSCN buffer onto a PEI-(PSSPAH)4-PSS multilayer as a function of the contact time between the Fe(CN)64- solution and the gold covered with the polyelectrolyte multilayer. Curves obtained after (s) 5 min, (9) 10 min, and (O) 20 min of contact between the film and the Fe(CN)64-containing buffer.

Figure 7. Cyclic voltametry of 1 mM Fe(CN)64- in the 10 mM Tris-0.5 M NaCl buffer onto a PEI-(PSS-PAH)4-PSS multilayer as a function of the contact time between the Fe(CN)64solution and the gold covered with the polyelectrolyte multilayer. Curves obtained after (s) 5 min and (O) 55 min of contact between the film and the Fe(CN)64--containing buffer.

containing buffer and vice versa. This confirms the irreversible character of the investigated films. We then investigated the penetration kinetics of ferrocyanide ions into PEI-(PSS-PAH)5 PEMs made from either 0.5 M SCN- or Cl- solutions. This was done in order to investigate the permeability of the multilayers as a function of the nature of the electrolyte solution used to buildup the films. To that aim we performed cyclic voltammetry experiments in the way explained in the Materials and Methods section. The multilayer was built up outside of the electrochemical cell and put in contact with the buffer solution at x M in NaX and 1 mM in ferrocyanide at time t ) 0. The evolutions of the cyclic voltamograms were then recorded as a function of time (Figures 6 and 7). As a main result, it appears that the films made from five bilayers prepared from SCNsolutions at 0.5 M are highly permeable to Fe(CN)64- ions. In addition, the electrochemical behavior seems to be reversible, namely, the intensities of the oxidation and reduction peaks are very close and the differences between the potential at which the maximal currents occurred were close to 60 mV as expected for a one-electron process at 298 K (data not shown). The multilayer seems to be

Ball et al.

saturated with Fe(CN)64- ions in only a few minutes, and a slight and unexplained decrease in the oxidation current was observed later on. When the electroactive ions are removed by replacing this solution with Tris 10 mM NaSCN (0.5 M) buffer, the decrease in the oxidation and reduction currents is fast again and after a few minutes no measurable peaks could be detected (data not shown). On the other hand, when a multilayer with the same architecture was built up from a Tris 10 mM NaCl (0.5 M) buffer, the oxidation and reduction currents were lower than those obtained in the case of the buildup made in the presence of SCN- by almost 2 orders of magnitude (Figure 7). It has to be noted that a PEI-(PSS-PAH)5 multilayer, hence being attractive for the negatively charged electroactive ions, displays the same electrochemical behavior in the presence of a 0.5 M Cl- solution as a PEI-(PSSPAH)4-PSS film. Hence the observed effect is not due to a repulsive electrostatic barrier impeding the entrance of the ferrocyanide ions in the film. This observation is in accordance with that made by Farhat and Schlenoff in the case of a PSS-PDADMAC multilayer: as soon as two bilayers were deposited, the PEM became essentially impermeable to electroactive ions.59 To further check the effect of ferrocyanide ions on the PEMs, we also performed QCM-D experiments. As is apparent from Figure 4, no apparent frequency changes are observed for both types of used support electrolyte. The dissipation changes were also very small (data not shown). For the case of the experiment performed in the presence of Cl- at 0.5 M, this result is in full agreement with the electrochemistry experiment: it seems as if the electroactive anions, carrying four negative charges, are not able to permeate through the multilayer. However in the presence of 0.5 M SCN-, there is an important electrochemical signal but no frequency change. This could mean that when ferrocyanide is penetrating in the multilayer, some SCN- ions and some water molecules are released in such a manner that not only the electroneutrality but also the whole mass of the film stays constant. Such a behavior has already been demonstrated by means of an electrochemical quartz crystal microbalance.60 Unfortunately, the occurrence of such a phenomenon cannot be evidenced by means of ATR-FTIR spectroscopy since the SCN- and Fe(CN)64- ions have quasiundistinguishable IR spectra. However another explanation can be found to explain the difference observed in electrochemistry and in QCM-D in the presence of 0.5 M SCN-. It might well be that the film buildup in the presence of SCN- displays some porosity which would allow the presence of channels allowing fast ion transport between the solution and the electrode surface. The transport of electroactive species should then be fast without the need to displace the SCN- ions already present in the film which could explain the constancy of the QCM-D signal upon exposure to a buffer solution containing 1 mM Fe(CN)64-. To check the validity of this second assumption, we performed AFM topography experiments in the dry state after buildup from either a Cl-- or SCN--containing buffer and intense water rinse. It appears clearly that the PEMs made from a 0.5 M SCN--containing buffer display considerable roughness and the presence of channels which penetrate deeply inside of the film and will probably reach the solid substrate (Figure 8A). The presence of deep pores appears even more evident from the AFM image provided in the Supporting Information, Figure 6, were (59) Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184-1192. (60) Calvo, E. J.; Wolosiuk, A. J. Am. Chem. Soc. 2002, 124, 8490.

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enough to allow permeation of the ferrocyanide anions to the gold electrode. The same kind of electrochemistry experiments were performed on PEI-(PSS-PAH)4-PSS multilayers built up from solutions containing either 0.1 or 0.2 M NaSCN to check the influence of the ionic strength on the permeability of the film. It appears that, in contrast to the experiment performed at 0.5 M NaSCN, the cyclic voltamograms look like purely capacitive curves (Supporting Information, Figure 7), hence like those obtained at 0.5 M NaCl. Such films built up at 0.1 or 0.2 M in NaSCN are considerably less rough and porous than those built up at 0.5 M in NaSCN (data not shown). All these, data taken together, show that the PSS-PAH film permeability can be tuned not only by the nature of the supporting electrolyte but also by the concentration of the counterions.

Figure 8. (A) AFM topographical image of a PEI-(PSS-PAH)4 multilayer built up from a 10 mM Tris buffer with 0.5 M NaSCN, pH ) 7.4, on a gold-coated quartz crystal microbalance crystal as described in the text. (B) AFM topographical image of a PEI-(PSS-PAH)4-PSS multilayer built up from a 10 mM Tris buffer with 0.5 M NaCl, pH ) 7.4, on a gold-coated quartz crystal microbalance crystal.

the PEI-(PSS-PAH)4 film has been scratched with a needle before imaging was performed. Such pores are not observed when the buildup of the PEM is made from a buffer containing 0.5 M Cl- ions (Figure 8B). It is remarkable that the PEMs built up from 10 mM Tris and 0.5 M NaSCN solutions are extremely rough but do nevertheless display a linear increase of their optical thickness with the number of deposited bilayers. It has been found in our group that exponentially growing PEMs such as (HA/PLL)n or (PGA/PLL)n display a quasi-constant roughness as well as a small roughness with respect to the layer thickness as soon as a minimal number of bilayers has been deposited.31 The example provided in this study is the opposite: the thickness increase is linear but the roughness is very high. This shows that at the present stage of knowledge no general relationship can be given between the roughness changes along the PEM buildup and the growth regime of the multilayers. Moreover, when performing cyclic voltametry experiments on a PEI-(PSS-PAH)8-PSS multilayer buildup in the presence of 0.5 M NaSCN, one obtains, in contrast to PEI-(PSS-PAH)4-PSS films, practically pure capacitive cyclic voltamograms (data not shown). These data show that as soon the porous PEI-(PSS-PAH)n multilayer buildup in the presence of 0.5 M NaSCN solutions become thick enough (Figure 2 shows that a PEI-(PSS-PAH)nPSS layer has an optical thickness of 130 and 250 nm when n ) 4 and 8 respectively), there is no channel wide

Conclusions In this study, we compared the buildup of PEI-(PSSPAH)n multilayers between Tris buffer solutions at pH 7.4 containing either NaCl or NaSCN. Huge differences in the thickness increments of the linear growth regimes were found: at 0.5 M in added salt the thickness increment was three times higher in the presence of SCN- as in the presence of Cl-. The same trend was found in terms of frequency changes in the QCM-D experiments. These differences were explained on the basis of strong SCNcondensation on the PAH chain. Evidence for this explanation came from zeta potential measurements of the PAH solutions as well as from conductimetry titration of PAH solutions with solutions containing either SCN- or Clions. Moreover, the ATR-FTIR experiments also showed that PEMs built up in the presence of SCN- are accumulating these small ions as long as the film is growing and that the exchange capacity of SCN- ions by Cl- ions is limited to a few percent. This small exchange capacity shows that these (PSS/PAH)n films are in a frozen state. It was also found in this study that the films built up in the presence of 0.5 M SCN- are more hydrated (on the basis of their refractive index and on the differences in the adsorbed amounts calculated from the QCM-D and OWLS experiments) and more porous as those built up in the presence of Cl- at the same concentration. This difference in porosity and hydration manifested in a drastically different electrochemical behavior: the PEI(PSS-PAH)5 multilayers built up from 0.5 M. SCNsolutions allowed fast and reversible permeation of ferrocyanide, most probably through pores, whereas the same architectures built up from 0.5 M Cl- solutions were totally impermeable to the electroactive ions. This high permeability can however be reduced either by the buildup of thicker films (at the same concentration in NaSCN) or by the buildup at smaller salt concentration in NaSCN. It hence seems that PSS-PAH multilayers built up in the presence of either NaCl or NaSCN are intrinsically impermeable to Fe(CN)6 4-. As soon as the film becomes thick enough, these pores are self-closing and there are no percolation channels for the electroactive species. A quantitative evaluation of the condensation of SCNanions to the PAH chains will be undertaken soon. Supporting Information Available: Figures showing distribution of the relative scattered intensity as a function of hydrodynamic diameter for PAH solutions, evolution of the optical parameters, optical thickness, and fraction of anions exchanged, an AFM image of a multilayer, and cyclic voltammograms of a multilayer on a gold electrode. This material is available free of charge via the Internet at http://pubs.acs.org. LA047610N