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Reactive Layer-by-Layer Films from Solutions Containing Silicic Acid and a Ti(IV) Complex: Preferential Incorporation of Silica and Interactions of the Obtained Films with Hexacyanoferrate Anions Vincent Ball,*,†,‡,§ Laurence Daheron,† Claire Arnoult,† Valerie Toniazzo,† and David Ruch† † Department of Advanced Materials and Structures, Centre de Recherche Public Henri Tudor, 66 rue de Luxembourg, L-4002 Esch-sur-Alzette, Luxembourg, ‡Institut National de la Sant e et de la Recherche M edicale, Unit e Mixte de Recherche 977, 11 rue Humann, 67085 Strasbourg, France, and § Universit e de Strasbourg, Facult e de Chirurgie Dentaire, 1 Place de l0 H^ opital, 67000 Strasbourg, France
Received November 6, 2010. Revised Manuscript Received December 1, 2010 The concept of reactive layer-by-layer (LBL) deposition allows the build-up of films containing polycations and oxide particles, namely, silica and poorly crystalline anatase. Because polyelectrolyte multilayer films have been produced from blended polyanions or polycations solutions and since preferential incorporation of one of the partners of the blend has been found in most cases, one should wonder if a preferential polycondensation of either silica or titania should occur when the reactive deposition is performed from a solution containing a precursor of both inorganic species. X-ray photoelectron (XPS) and UV-visible spectroscopies show that the reactive LBL films made from the blend and poly(diallyldimethylammonium chloride) (PDADMAC) incorporate predominantly silica over TiO2 over the whole molar fraction range of the silicic acic/hydrosoluble Ti(IV) complex. The transparency of the films below 365 nm, corresponding to the band edge of TiO2, can easily be modulated. The silica/TiO2 films are all able to bind hexacyanoferrate owing to the presence of the polycation allowing the binding of the oxide particles to the substrate. However, the binding capacity of the film does not scale proportionally to its thickness. The films made from eight dipping cycles showed a sudden decrease in their binding capacity for hexacyanoferrate when the molar fraction of the titanium complex was higher than ∼0.6 in the blend. For the same films, electrochemical impedance spectra (EIS) showed marked differences with a change in film composition: the more TiO2 in the film, the higher the resistance to electron and to mass transfer. Therefore, EIS helps to explain the reduced surface concentration measured by means of cyclic voltammetry for films rich in TiO2.
Introduction The alternated deposition of mutually interacting polymers or nanoparticles, using electrostatic interactions,1,2 hydrogen bonds,3-5 or electron donor-electron acceptor interactions6 and host-guest interactions,7 allows for a versatile functionalization of solid liquid interfaces. The obtained films can be deposited not only on planar interfaces but also on colloids8,9 and in porous templates.10 When charged polymers are employed, the obtained films are called polyelectrolyte multilayers (PEMs). In the case of planar interfaces, the deposition method implies either alternated dipping of the *To whom all correspondence should be addressed. E-mail: vincent.ball@ medecine.u-strasbg.fr. (1) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569. (2) Decher, G. Science 1997, 277, 1232. (3) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (4) Laschewsky, A.; Wischerhoff, E.; Denzinger, E.; Ringsdorf, H.; Delcorte, A.; Bertrand, P. Chem.;Eur. J. 1997, 3, 34. (5) Sukhishvili, S.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (6) Shimazaki, Y.; Nakamura, R.; Ito, S.; Yamamoto, M. Langmuir 2001, 17, 953. (7) Van der Heyden, A.; Wilczewski, M.; Labbe, P.; Auzely, R. Chem. Commun. 2006, 3220. (8) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; M€ohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (9) De Geest, B. G.; Sanders, N. N.; Sukhorukov, G. B.; Demeester, J.; De Smedt, S. C. Chem. Soc. Rev. 2007, 36, 636. (10) Volodkin, D. V.; Larionova, N. I.; Sukhorukov, G. B. Biomacromolecules 2004, 5, 1962. (11) Chiarelli, P. A.; Johal, M. S.; Holmes, D. J.; Casson, J. L.; Robinson, J. M.; Wang, H. L. P. Langmuir 2002, 18, 168. (12) Schlenoff, J. B.; Dubas, S. T.; Farhat, T. Langmuir 2000, 16, 9968. (13) Izquierdo, A.; Ono, S. S.; Voegel, J.-C.; Schaaf, P.; Decher, G. Langmuir 2005, 21, 7558.
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substrate in the corresponding solutions, alternated spin coating,11 or alternated spraying.12,13 An additional source of versatility in the film deposition comes from the possibility of using a blend of two polyanions or two polycations. The aim is to change the film composition and properties.14-22 Up to now, for obvious reasons related to quantification of the different polyelectrolytes present in the PEM, the blends were made from a mixture of either two polycations or two polyanions, the polyanion and polycation solution being then composed of a single polyelectrolyte. In most cases, but not systematically, preferential incorporation of one of the blend’s component over the other one was found. Such findings were most often deduced from infrared spectra in the total internal reflection mode (ATR-FTIR). A typical example was the blend made from poly(sodium-4-styrene sulfonate) (PSS) and sodium hyaluronate (HA) whose composition was changed from one experiment to the other.19 In this investigation, the (14) Debreczeny, M.; Ball, V.; Boulmedais, F.; Szalontai, B.; Voegel, J.-C.; Schaaf, P. J. Phys. Chem. B. 2003, 107, 12734. (15) H€ubsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J.-C.; Schaaf, P. Langmuir 2004, 20, 1980. (16) Cho, J.; Quinn, J. F.; Caruso, F. J. Am. Chem. Soc. 2004, 126, 2270. (17) Quinn, J. F.; Yeo, J. C. C.; Caruso, F. Macromolecules 2004, 37, 6537. (18) Quinn, A.; Tjipto, E.; Yu, A.; Gengenbach, T. R.; Caruso, F. Langmuir 2007, 23, 4944. (19) Francius, G.; Hemmerle, J.; Voegel, J.-C.; Schaaf, P.; Senger, B.; Ball, V Langmuir 2007, 23, 2602. (20) Ball, V.; Bernsmann, F.; Betscha, C.; Maechling, C.; Kauffmann, S.; B. Senger, B.; Voegel, J. C.; Schaaf, P.; Benkirane-Jessel, N. Langmuir 2009, 25, 3593. (21) Sun, J.; Wang, L.; Gao, J.; Wang, Z. J. Colloid Interface Sci. 2005, 287, 207. (22) Leporatti, S.; Gao, C.; Voigt, A.; Donath, E.; M€ohwald, H. Eur. Phys. J. E 2001, 5, 13.
Published on Web 01/04/2011
DOI: 10.1021/la104423c
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components of the blend were alternately deposited with poly-Llysine hydrobromide. Analysis of the ATR-FTIR spectra showed that the films were considerably enriched in PSS with respect to HA. The HA content of the film became significant only for blends containing >90% HA.19 The group of Caruso23,24 as well as our own team showed the possibility to build-up films by a reactive layer-by-layer process from the deposition of a polyamine and an hydrosoluble precursor of an oxide.25,26 Sodium silicate or Ti(IV)bis(ammoniumlactato) dihidroxyde (TiBislac) undergoes polycondensation only in the presence of the polycation in a manner similar to that found in biosilicate production.27-29 Interestingly, the films made from poly(ethyleneimine) and TiBisLac were found to contain poorly crystalline and monodisperse anatase nanoparticles of (4.9 ( 1.2) nm in diameter without any thermal treatment. To our knowledge, no investigations were done with the aim to deposit LBL films made from a blend of silicic acid and TiBislac. It is the aim of this Article to investigate the film composition of PEMs made from the alternated deposition of PDADMAC and a blend containing a molar fraction x of TiBislac and 1-x of sodium silicate. In our experiments, the whole concentration of the inorganic precursor was held constant at 10 mM. These films will be denoted (PDADMAC/Tix-Si1-x)m where m denotes the number of deposition cycles. The films will be denoted according to the molar fraction of TiBislac used during their deposition from the blend, even if the Ti/(SiþTi) fraction in the film may be different. Indeed, the composition of these films will be investigated by both UV-vis spectroscopy and X-ray photoelectron spectroscopy (XPS). UV-vis spectroscopy was chosen because the silica counterpart of the film is transparent between 200 and 700 nm, in the absence of light scattering, whereas films containing TiO2, with a band gap of 3.2 eV, absorb strongly below 387 nm. In addition, UV-vis spectroscopy allows us to probe the whole film thickness, whereas XPS probes the film composition only over the last few nanometers at the film-vacuum interface. Complementary, we also investigated the interactions of the (PDADMAC/ Tix-Si1-x)m films with the electroactive hexacyanoferrate anion, Fe(CN)64-. These experiments were performed by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with the aim of investigating if the film is able to bind hexacyanoferrate. This may well happen because of the presence of the positively charged PDADMAC. Both SiO2 and TiO2 are negatively charged at pH 7.5. (Their point of zero charge is of 2 to 3 and 5 to 6 for SiO2 and TiO2, respectively30.) The negative charge of the inorganic particles at pH 7.5 may thus hinder the binding of Fe(CN)64-. There is a practical interest in the production of composite films containing a controlled fraction of TiO2 and Fe(CN)64- because hexacyanoferrate can subsequently interact with Fe3þ cations to form Prussian Blue (PB). PB-TiO2 composites display a fascinating photomagnetic behavior.31 In addition, PEM films containing PB may find interesting bioanalytical applications when combined with enzymes such as glucose oxidase.32 (23) Caruso, F.; Shi, X.; Caruso, R. A.; Susha, A. Adv. Mater. 2001, 13, 740. (24) Shi, X.; Cassagneau, T.; Caruso, F. Langmuir 2002, 18, 904. (25) Laugel, N.; Hemmerle, J.; Porcel, C.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2007, 23, 3706. (26) Laugel, N.; Hemmerle, J.; Ladhari, N.; Arntz, Y.; Gonthier, E.; Haikel, Y.; Voegel, J.-C.; Schaaf, P.; Ball, V. J. Colloid Interface Sci. 2008, 324, 127. (27) Coradin, T.; Durupthy, O.; Livage, J. Langmuir 2002, 18, 2331. (28) Sumerel, J. L.; Yang, W.; Kisailus, D.; Weaver, J. C.; Choi, J. H.; Morse, D. E. Chem. Mater. 2003, 15, 4804. (29) Sumper, M.; Brunner, E. Adv. Funct. Mater 2006, 16, 17. (30) Parks, G. A. Chem. Rev. 1965, 65, 177. (31) Yamamoto, T.; Saso, N.; Umemura, Y.; Einaga, Y. J. Am. Chem. Soc. 2009, 131, 13196. (32) Zhao, W.; Xu, J.-J.; Shi, C.-G.; Chen, H.-Y. Langmuir 2005, 21, 9630.
1860 DOI: 10.1021/la104423c
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Experimental Section Chemicals. All solutions were made from double-distilled and deionized water (Milli Q Plus system, Millipore, Billerica, MA, F = 18.2 MΩ 3 cm). The deposition of the reactive PEM films, (r-PEM) was performed from Tris(hydroxymethyl aminomethane) buffer (Tris, Sigma-Aldrich, ref T1503) whose pH was adjusted to 7.5 with concentrated hydrochloric acid. NaCl (Prolabo, VWR International, West Chester, Pennsylvania, USA) was added at a concentration of 0.15 M. This solution will be called Tris-NaCl buffer in the following. The pH of all solutions was measured with an HI8417 pH meter (Hanna Instruments, Tanneries, France). Poly(diallyldimethylammonium chloride) (PDADMAC, Aldrich, ref 409014) was dissolved at 1 mg/mL in the Tris-NaCl buffer. The precursors of silica and TiO2 were sodium silicate (Riedel de H€aen, ref 13729) and TiBisLac (Aldrich, ref 388165), respectively. The blended solutions were prepared from 10 mM sodium silicate and 10 mM TiBisLac in the Tris-NaCl buffer. We prepared fresh solutions before the beginning of each experiment by mixing the sodium silicate and TiBisLac solutions to obtain a blend with a molar fraction x in TiO2 precursor. The total concentration in oxide precursor was 10 mM in all experiments. The blends were stable for at least 1 day, owing to the absence of an increase in scattered light.25 Potassium hexacyanoferrate (Sigma-Aldrich, ref 9387) was dissolved in the Tris-NaCl buffer at a concentration of 1 mM. It was used as a redox probe to measure its permeation trough the r-PEM films and its retention in the film upon rinse with TrisNaCl buffer. Adsorption Substrates. The substrates used for UV-visible spectroscopy, for the thickness and the permeability measurements were quartz plates (4 1 0.1 cm, Thuet, Blodelsheim, France), silicon plates (Siltronix, Archamps, France), and amorphous carbon electrodes (CH Instruments, Austin, TX, ref 104), respectively. The quartz plates and silicon wafers were cleaned by immersion in freshly prepared piranha solution (2 volume fractions of H2SO4 and 1 volume fraction of H2O2 at 30% v/v, both from Sigma-Aldrich) for 30 min. This was followed by intensive rinse with Milli Q water and gentle drying under a stream of nitrogen. The amorphous carbon electrodes were rinsed with absolute ethanol (Aldrich) and polished with a 50 nm alumina paste (Buehler, Lake Bluff, IL, ref 40-6325-008). Each polishing step was performed during 2 min and was separated from the next one by intensive rinsing with water. Finally, the electrodes were sonicated at 35 kHz for 2 3 min in a Transonic TI-H-50 sonicator (Laval Lab, Laval, Canada). We checked the quality of the polishing by performing CV in the presence of Tris-NaCl buffer containing 1 mM hexacyanoferrate. The electrode was used for the deposition of a (PDADMAC/Tix-Si1-x)m film only if the separation between the oxidation and reduction peak potentials was 0.2. This change may be due to a decreased fraction of available binding sites for DOI: 10.1021/la104423c
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hexacyanoferrate and hence a decrease in “free” PDADMAC monomers or to a more compact structure of the films. The detailed structural changes of the film are out of the scope of this investigation, but we have clearly shown that the ability of the film to retain hexacyanoferrate can be modulated in an important manner by changing the molar fraction of TiBislac in the TiBisLac/ sodium silicate blend (Figure 6). These changes, namely, a decrease in the Donnan potential, a decreased retention in hexacyanoferrate, and an increase in the electron transfer resistance and Warburg impedance, are observed for x higher than ∼0.6. This molar fraction corresponds to films containing more than 20% of TiO2 (according to the y = f(x) relationship in Figure 2). We emphasize that PDADMAC was present in the films in an amount almost independent of x, as obtained from XPS spectra (Figure 3 of the Supporting Information). This suggests that it is probably the accessibility to the binding sites of PDADMAC that decreases when x is increased above 0.6.
Conclusions In this work, we investigated the reactive deposition of films during the alternated deposition of a polycation and a blend containing a precursor of SiO2 and a precursor of TiO2. The molar ratio of TiO2 in the film was found to be lower than the corresponding value in solution, meaning some preferential incorporation of SiO2 over TiO2. The transparency of the films as well as their ability to bind hexacyanoferrate anions can be modulated according to the molar fraction of TiO2 in the film. The more TiO2 in the film, the higher its absorption coefficient in the UV range of the spectrum and the less its ability to bind
1866 DOI: 10.1021/la104423c
the electroactive probes. These hybrid films of modulable properties may find interesting applications as photomagnetic coatings. Acknowledgment. V.B. acknowledges the FNR for the obtention of an Am2c mobility grant. Cosette Betscha (INSERM Unit 977, Strasbourg) is acknowledged for her technical assistance in the deposition of r-PEM films. We acknowledge the “CAPTOCHEM” proposal for financial support. Supporting Information Available: Deposition of a (PDADMAC/Tix=0-Si)m r-PEM film as followed by means of QCM-D, absorption spectra of (PDADMAC/Tix-Si1-x)m films after the deposition of m = 8 bilayers as a function of x, examples of XPS spectra obtained on (PDADMAC/TixSi1-x)m deposits after the deposition of 10 layer pairs for x = 0.25 and 0.75, examples of CVs measured on a (PDADMAC/Ti0.5-Si0.5)m=30 film after 30 min of contact with a 1 mM hexacyanoferrate containing buffer and after 1 and 5 min of rinse with Tris-NaCl buffer, evolution of the oxidation and reduction peak currents as a function of the scan rate for (PDADMAC/Ti0.5-Si 0.5)m=8 films, Nyquist plots corresponding to the electrochemical impedance spectra of (PDADMAC-Tix-Si1-x)m=8 films for different compositions of the blend used to buildup the film, and representation of a Randless equivalent circuit and its fit to an electrochemical impedance spectrum acquired on a film made from eight deposition cycles. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2011, 27(5), 1859–1866
