Polymer Multilayer Films Obtained by Electrochemically Catalyzed click chemistry

Polymer Multilayer Films Obtained by Electrochemically Catalyzed click chemistry

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DOI: 10.1021/la902874k ALangmuir X, X(X), X–X pubs.acs.org/Langmuir ©XXXX American Chemical Society

Polymer Multilayer Films Obtained by Electrochemically Catalyzed Click Chemistry

Gaulthier Rydzek,† Jean-S ebastien Thomann,‡ Nejla Ben Ameur,‡ Loıc Jierry,† Philippe M esini,†

Arnaud Ponche,§ Christophe Contal,† Alae E. El Haitami, ) , Jean-Claude Voegel, )

Bernard Senger, ) , Pierre Schaaf,*,† Benoıt Frisch,‡ and Fouzia Boulmedais†

†Centre National de la Recherche Scientifique, Unit e Propre de Recherche 2, Institut Charles Sadron, 23 rue du Loess, BP 84047, 67034 Strasbourg Cedex 2, France, ‡Laboratoire de Conception et Application de Mol ecules Bioactives, UMR 7199, CNRS/Universit e de Strasbourg, Facult e de Pharmacie, 74 route du Rhin, 67401 Illkirch Cedex, France, §Institut de Science des Mat eriaux de Mulhouse (IS2M), CNRS LRC7228,

15 rue Jean Starcky - BP 2488, 68057 MulhouseCedex, France, ) Institut National de la Sant e et de la Recherche

M edicale, Unit e 977, 1 rue Humann, 67085 Strasbourg Cedex, France, and Universit e de Strasbourg, Facult e de Chirurgie Dentaire, 1 place de l’Hopital, 67000 Strasbourg, France

Received August 4, 2009. Revised Manuscript Received November 16, 2009

We report the covalent layer-by-layer construction of polyelectrolyte multilayer (PEM) films by using an efficient electrochemically triggered Sharpless click reaction. The click reaction is catalyzed by Cu(I) which is generated in situ from Cu(I) (originating from the dissolution of CuSO4) at the electrode constituting the substrate of the film. The film buildup can be controlled by the application of a mild potential inducing the reduction of Cu(I) to Cu(I) in the absence of any reducing agent or any ligand. The experiments were carried out in an electrochemical quartz crystal microbalance cell which allows both to apply a controlled potential on a gold electrode and to follow the mass deposited on the electrode through the quartz crystal microbalance. Poly(acrylic acid) (PAA) modified with either alkyne (PAAAlk)o r azide (PAAAz) functions grafted onto the PAA backbone through ethylene glycol arms were used to build the PEM films. Construction takes place on gold electrodes whose potentials are more negative than a critical value, which lies between -70 and -150 mV vs Ag/AgCl (KCl sat.) reference electrode. The film thickness increment per bilayer appears independent of the applied voltage as long as it is more negative than the critical potential, but it depends upon Cu(I) and polyelectrolyte concentrations in solution and upon the reduction time of Cu(I) during each deposition step. An increase of any of these latter parameters leads to an increase of the mass deposited per layer. For given buildup conditions, the construction levels off after a given number of deposition steps which increases with the Cu(I) concentration and/or the Cu(I) reduction time. A model based on the diffusion of Cu(I) and Cu(I) ions through the film and the dynamics of the polyelectrolyte anchoring on the film, during the reduction period of Cu(I), is proposed to explain the major buildup features.


Thestep-by-stepdepositionofinteractingpolymersleadstothe formation of so-called polymer multilayer films. The interactions can be of electrostatic origin,1-6 due to hydrogen bonds,7-1 controlled by electrochemical deposition,12,13 or even be covalent bonds.10,14 Covalent bonds between the different multilayer constituents, even with single component polymer,15-18 are introduced in order to increase the chemical and mechanical stability of the films.19,20 This can be done once the multilayer is built by cross-linking the electrostatically interacting polyelectrolytes,19-21 by cross-linking hydrogen-bonded multilayers, in particular poly(vinylpyrolidone) and poly(methacrylic acid) multilayers using carbodiimide chemistry, a system extensively investigated by the group of Sukhishvili,2-24 or step-by-step during the buildup process involving the formation of covalent bonds between the interacting polymers. For this latter case, various reactions were already used,25-28 and they share the e-mail schaaf@ics.u-strasbg.fr. (1) Decher, G. Science 1997, 277, 1232. (2) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (3) Ladam, G.; Schaad, P.; Voegel, J.-C.; Schaaf, P.; Decher, G.; Cuisinier, F. J.

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B DOI: 10.1021/la902874k Langmuir X, X(X), X–X

Article Rydzek et al.

common property to be fast. Cu(I)-catalyzed azide-alkyne [3 þ 2] cycloaddition leading to 1,2,3-triazole bonds is one of them. This reaction, first introduced by Huisgen, also known as “click reaction”, has captured over the past years a large interest from the community for functionalization purposes.29,30 It presents the advantages to be very chemoselective, to proceed with high yields in mild conditions and in highly diluted aqueous solutions. The resulting triazole functions are extremely resistant to hydrolysis, oxidation, and reduction.30 Because of the thermodynamic instability of Cu(I), Cu(I) is used to be reduced in situ to Cu(I), implying the addition of reducing agents (e.g., sodium ascorbate) and/or of organic ligands.31

This reaction was, in particular, used for the buildup of covalent layer-by-layer multilayers. Caruso’s group was the first to introduce this buildup method to construct films entirely constituted of poly(acrylic acid) (PAA), functionalized with azide oralkynegroups,32andtoprepare antifoulingmicrocapsules that respond to pH by changing their size.3,34 The swelling and shrinking of similar films was investigated by Tang et al.,35 who studied the dynamic response of the multilayers to pH changes through a Belousov-Zhabotinsky reaction. Bergbreiter et al.36,37 fabricated temperature-responsive films made of PNIPAM chains with grafted azide or alkyne groups. Hawker’s group made also use of this strategy to build layer-by-layer assemblies ofazide-andalkyne-terminateddendrimers.38Nevertheless,from a practical point of view, the step-by-step film construction using click chemistry commonly used in the literature is rather tedious. Indeed, the instability of both Cu(I) and sodium ascorbate in solution implies to prepare fresh catalytic mixtures at each step of the buildup.32-36,39 This drawback considerably reduces the interestofthismethodologyforthestep-by-stepbuildup ofthin films.

Electrochemically triggered Sharpless reaction was used by

Collman et al.40 to attach ethynylferrocene on azide-terminated self-assembled monolayers (SAMs). SAMs were deposited on a gold-coatedelectrode which was held at -300 mV vs Ag/AgCl referenceelectrodetoformCu(I) from Cu(I).Theelectrochemical productionof Cu(I) was also used by Ku et al.41 to pattern azidofunctionalizedglass substrates by scanning electrochemical microscopyusingagoldultramicroelectrode.Inbothstudies,theauthors usedCu(I) inthepresence of a ligand inan aqueous solutionor in DMF solvent. In another study, Finn’s group showed that the electrochemical monitoring of the Cu(I)-catalyzed alkyne-azide cycloadditionisalsopossiblebetweentwofunctionalizedmolecules in the bulk containingCuSO4 with or without ligand.42

Here, we report the covalent layer-by-layer construction of a polyelectrolyte multilayer (PEM) film by using the click reaction triggered by electrochemistry in the sole presence of CuSO4 (Cu(I) ions). By using an electric potential applied on the substrate (gold electrode), we induce and control the formation of Cu(I) ions from Cu(I) ions. The Cu(I)-catalyzed click reaction is thus performed without any reducing agent and ligand by applying a mild electric potential (Scheme 1). The deposition and dissolution of Cu(0) on and from an electrode are complex processes whose details are still under debate.43-46 However, what seems to be firmly established is that the reduction reaction of Cu(I) into Cu(0) is a multistep process that involves Cu(I) which diffuses in the solution near the electrode. This Cu(I) diffusion could induce a fast and localized click reaction and thus a fast and localized film buildup. Moreover, this could allow selectively and independently functionalizing microelectrodes from an array with different polymer multilayer films by simply applying a reduction potential on the desired microelectrodes. Electrically induced “conventional” noncovalent polyelectrolyte multilayer films were reported where the electric field was used to obtain selective deposition.12,13,47-53Our approachconstitutesan alternative to these methods with the advantage to create a “covalent” multilayer film.

Materials and Methods

Polyelectrolyte Solutions. We used PAA modified with either alkyne (PAAAlk: PAA-(EG)2-Alk) or azide (PAAAz:

PAA-(EG)12-Az) functions grafted onto the PAA backbone through ethylene glycol EG arms and PEI modified by azide functions(PEIAz)atagraftingratioof6%(Scheme2).Thedetails of their synthesis can be found in the Supporting Information.

The PEIAz-(PAAAlk/PAAAz)n polyelectrolyte architectures, where n is the number of deposition cycles corresponding to the deposition of n PAAAlk/PAAAz pairs of layers, were built as follows. Polyelectrolyte solutions at 0.5 mg/mL were prepared by dissolution of the adequate masses of polyelectrolytes in filtered (Millex GV membranes, pore diameter of 0.2 μm) solutions of CuSO4 (0.3 mM) adjusted to pH 3.5 ( 0.05 using diluted HNO3 solution. In the case of lower CuSO4 concentrations, NaNO3 salt was added in aqueous solution to maintain the ionic strength constant (the ionic strength of 0.3 mM of CuSO4, i.e.,1.2mM).Thelayer-by-layer(LbL)assemblywasstartedonto a bare gold substrate by adsorbing positively charged poly-

(ethylenimine)-Az (PEIAz) to obtain a first electrostatic layer in order to promote further film buildup. The pH of the PEIAz solution was 7.0.

ElectrochemicalQuartz CrystalMicrobalance(EC-QCM) with Dissipation Monitoring.The electrochemical quartz crystal microbalance (EC-QCM) experiments were performed on a Q-Sense E1 apparatus from Q-Sense AB (Gothenburg, Sweden)

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DOI: 10.1021/la902874k CLangmuir X, X(X), X–X

Rydzek et al. Article by monitoring the changes in the resonance frequency f and the dissipation factor D of an oscillating quartz crystal upon adsorption of a viscoelastic layer.54,5 The quartz crystal is excited at its fundamental frequency (5 MHz), and the measurements are performed at the first, third, fifth, and seventh overtones, corresponding to 5, 15, 25, and 35 MHz. The QCM measurement is sensitive to the amount of water associated with the adsorbed molecules and senses the viscoelastic changes in the interfacial material.54 In the Results section, only the normalized frequency shifts of the third overtone (Δf3/3) are presented. The thickness of the adsorbed layers can be estimated using the viscoelastic Voigt model.56 For the evaluation, the fluid density (1009 kg/m3), fluid viscosity (0.91 mPa3s), and layer density (1000 kg/m3) are kept constant. The thickness is estimated using the third, fifth, and seventh overtones. The Q-Sense Electrochemistry Module, QEM 401, allows simultaneous QCM and electrochemistry measurements. The gold-coated QCM sensor acted as working electrode. Aplatinumcounterelectrodeonthetopwallofthechamberanda no-leak Ag/AgCl reference electrode fixed in the outlet flow channel were used respectively as counter and reference electrodes. Before the buildup of the polymer film, in order to test the quality of the EC-QCM cell, a capacitive current and a faradic currentof1mMofpotassiumhexacyanoferrate(I),K4[Fe(CN)6] (Sigma, CAS 14459-95-1), aqueous solution were recorded. A

Tris-NaNO3 buffer solution of tris(hydroxymethylaminomethane) (Tris, 5 mM, Gibco BRL, catalog no. 15504-020) and sodiumnitrate(NaNO3,0.15M)adjustedatpH7.4wasprepared to measure the capacitive current of the EC-QCM cell. 1 mM of potassium hexacyanoferrate(I) was prepared in Tris-NaNO3 buffersolutionand put intocontact with the crystal tomonitorits cyclic voltammogram. The buildup was operated as described in the text. The change in pH of the solution in contact with the film was obtained by injecting under flow rate of 1 mL/min 10 mM

NaNO3 solution at the desired pH. Atomic Force Microscopy. Multilayersamples,tobestudied by atomic force microscopy (AFM) in a liquid cell, were built on the QCM quartz crystal inside the EC-QCM apparatus. AFM images were obtained in contact mode in liquid conditions in the presenceof10mMNaNO3atpH3.5orpH9withtheNanoscope IVfromVeeco(SantaBarbara,CA).NoCu(I)waspresentinthe solution while performing the AFM measurements. Cantilevers with a spring constant of 0.03 N/m and silicon nitride tip (model MSCTAUHW, Veeco) were used. We always performed several scans over a given surface area. These scans had to

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