Polymer Multilayer Films Obtained by Electrochemically Catalyzed click chemistry

Polymer Multilayer Films Obtained by Electrochemically Catalyzed click chemistry

(Parte 2 de 3)

Scheme 1. Schematic Representation of the Polymer Multilayer Buildup Using Electrochemically Controlled Click Chemistry Scheme 2. Structure of PAAAlk, PAAAz, and PEIAz, Used in This Study, Synthesized at a Grafting Ratio of 6%

(54) Marx, K. A. Biomacromolecules 2003, 4, 1099. (5) H€o€ok, F.; V€or€os, J.; Rodahl, M.; Kurrat, R.; Boni, P.; Ramsden, J. J.;

Textor, M.; Spencer, N. D.; Tengvall, P.; Gold, J.; Kasemo, B. Colloids Surf., B 2002, 24, 155. (56) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Phys. Scr. 1999, 59, 391.

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

Article Rydzek et al.

produce comparable images to ascertain that there is no sample damageinducedbythetip.Deflectionandheightimages(10μm 10μm)werescannedatafixedscanrate(2Hz)witharesolutionof 512 512 pixels.

XPS Spectroscopy. XPS analysis was performed on a Gammadata Scienta (Uppsala, Sweden) SES 200-2 X-ray photoelectronspectrometerunderultrahighvacuum(P<10-9mbar).The monochromatized Al KR source (1486.6 eV) was operated at a powerof420W(30mAand14kV),andthespectrawereacquired atatakeoff angle(TOA) of90 (anglebetweenthesamplesurface and photoemission direction). The samples were outgassed in several ultrahigh-vacuum chambers with isolated pumping systems and pressure control until transfer to the analysis chamber. During acquisition, the pass energy was set to 500 eV for wide scans and to 100 eV for high-resolution spectra. The 100 eV pass energy gives an overall resolution of 0.48 eV determined on the Fermiedgeofsilversample.WiththehelpofCASAXPSsoftware (Casa Software Ltd., Teignmouth, UK, w.casaxps.com), atomic composition (in %) of the sample surface is calculated using raw area corrected with classical Scofield sensitivity factors and transmission function of the spectrometer. All components on high-resolution spectra were referenced according to the CHx component at 285.0 eV.

Results

Electrochemical Reduction of Cu(I) to Cu(0). We first investigatedwhathappensonthesurfaceoftheworkingelectrode in contact with a Cu(I) solution when an electric potential is applied. After the adsorption of a PEIAz precursor layer on the gold-coated QCM crystal, the evolution of the frequency shift in contact with 0.3 mM Cu(I) solution was monitored during the step-by-step application of electric potentials ranging from þ50 to -350 mV (all the potentials are given versus Ag/AgCl (KCl sat.)) reference electrode, as shown in Figure 1a.

Between each investigated electric potential, a potential of þ600 mV was applied to dissolve the formed Cu(0) deposit. At an applied reduction potential E more negative than a critical potential (Ecrit), which lies between -50 and -150 mV, the frequency shift decreases rapidly over the whole time during which the reduction potential is applied, indicating the deposition of Cu(0) on the working electrode through the reduction of Cu(I). When the electric potential is raised to þ600 mV, the frequency shift increases and returns to its initial value corresponding to the bare electrode. This takes place in less than 300 s. The whole Cu(0) is removed from the substrate. The oxidation and reduction cycle of the catalyst is thus completely reversible.

When the applied potential E is more positive than Ecrit,o nly a small frequency shift decrease is observed initially corresponding to a small mass increase, and the signal no longer evolves with time. This small mass increase could be due to a small ion adsorption as the reduction potential is imposed, but it does not correspondtoaCu(0)deposition.Thisindicatesthatnoreduction ofCu(I)toCu(0) takesplaceatthesepotentials.ThevalueofEcrit for which the reduction of Cu(I) starts is in agreement with reported values obtained from cyclic voltammetry experiments.4 In addition, we found that for reduction potentials more negative than Ecrit, the decrease of the frequency shift and thus the deposition rate of Cu(0) on the electrode are fairly independent of the applied potential. We also monitored the evolution of the frequency shift during the consecutive applications of -350 mV (5min)andþ600mV(5min)atdifferentconcentrationsofCu(I) in the solution. As expected, the Cu(0) deposition rate increases with the Cu(I) concentration in solution (Figure 1b).

Electrochemically Triggered Click Chemistry Buildup of

PolymerMultilayerFilms:ProofoftheConcept.Tobuildthe film by electrochemically catalyzed click chemistry, we used PAA modifiedwitheitheralkyne (PAAAlk) orazide (PAAAz) functions with a grafting ratio close to 6%, a system close to that investigated by the group of Caruso.32 The multilayer was constructed on a PEIAz precursor layer used to anchor strongly the film on the gold electrode. Then, the substrate was alternately broughtintocontactwiththePAAAlkandPAAAzsolutions,both containing CuSO4 (Cu(I), 0.3 mM). The film buildup procedure is displayed in Scheme 3.

When the polyelectrolyte solutions were brought into contact with the film, the electrical circuit was first kept open for 60 s. After this time interval, the potential of the gold electrode was fixedtoagivenvalueEfor300s,duringwhichreductionofCu(I) resulting in the deposition of Cu(0) on the surface tookplace. The potential was then raised to 600 mV for 150 s, and the polyelec- trolyte solution was replaced by a CuSO4 solution for an additional 150 s still at 600 mV. Finally, the electrical circuit was opened again, the CuSO4 solution was replaced by the next polyelectrolyte solution, and a new deposition step is started.

Figure 2 represents the typical evolution of the normalized frequency shift monitored by EC-QCM during the film buildup of a PEIAz-(PAAAlk/PAAAz)2 multilayer as a function of time with E = -350 mV. After the adsorption of a PEIAz precursor

Figure 1. (a) Evolution of the normalized frequency shift in the presence of 0.3 mM CuSO4 solution, measured at 15 MHz (ν=3) by EC-

QCM,asafunctionoftime.AftertheadsorptionofamonolayerofPEIAz,weappliedelectricalpotentials,stepbystep,fromþ50to-350mV alternatedwiththeapplicationofþ600mV.(b)Evolutionofthenormalizedfrequencyshift,measuredat15MHzbyEC-QCM,asafunction of time where CuSO4 solutions of different concentrations were injected one after the other on a PEIAz monolayer. For each solution, we applied consecutively -350 and þ600 mV versus Ag/AgCl. For the sake of clarity, the mechanical perturbations of the signal due to rinsing steps, i.e., injection of 2 mL of a CuSO4 solution, were removed from the curve.

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

Rydzek et al. Article layer under “open circuit potential” (OCP) conditions, the first adsorption of PAAAlk is mainly controlled by electrostatic interactions of PAA with the precursor PEI layer leading to a high decrease of the frequency shift even before the application of E. A smaller decrease in frequency shifts is obtained during the following PAAAz and PAAAlk deposition steps. One can notice thattheformationofCu(0) duringtheapplicationofthereducing potential E, corresponding to a rapid decrease in the frequency shift, and its subsequent removal from the surface at þ600 mV.

The evolutions of the normalized frequency shift and of the energy dissipation during the step by step construction of the

PEIAz-(PAAAlk/PAAAz)7 film as a functionof deposited layer are shown in Figure 3. A regular decrease of the normalized frequency shift is observed as a function of the number of deposition steps. We already proved previously that during the application of þ600 mV, Cu(0) is totally removed from a

PEIAz-coated substrate (Figure 1). We checked that this remains valid in the presence of a PEIAz-(PAAAlk/PAAAz)7 film by imposingsuccessivecyclesofCu(0)depositionat-350mVfollowedby

Cu(0) removal at þ600 mV. No evolution of the frequency shift was observed between two consecutive cycles (see Figure S-1 in Supporting Information). We also performed XPS experiments on films constituted of 14 and 17 PAAAlk/PAAAz bilayers to determine the amount of copper remaining in the film. The film exhibits an atomic composition of 0.2% in Cu(I) and no trace of Cu(0). More details about the XPS experiments are given in

FigureS-2intheSupportingInformation.Theregulardecreaseof the frequency shift with the deposited layer can thus entirely be attributedtothePAAAlk/PAAAzfilmbuildupandnottoacontributionofasmallandregularCu(0) deposition.Thisprovesthebuild- up of multilayers by electrochemically triggered click-reaction. Influence of the Applied Reduction Potential on the

Buildup of PAAAlk/PAAAz Film. We investigated the film buildupasafunctionoftheappliedreductionpotentialE(Figure4).

The PAAAlk/PAAAz film buildup by click chemistry is electrochemically triggered as long as the voltage conditions are favor- able for the reduction of Cu(I), the critical potential lying between -70 and -150 mV. For applied potentials lower than the critical value, the film buildup appears nearly independent of the applied voltage in accordance with the same independence of the Cu(0) deposition rate on a bare electrode (Figure 1). As a further control of the electrochemically triggered nature of the buildup mechanism, we also performed experiments at -350 mV whereonlyPAAAlkornonfunctionalizedPAAwasdepositedona

PEIAz precursor layer. As anticipated, in both cases the films did not buildup (see Figure S-3 in Supporting Information).

In the literature, it is not clear whether Cu(I) is formed during the reduction of Cu(I) to Cu(0) or during the oxidation of Cu(0) to Cu(I).43,4 In our standard procedure, the polyelectrolyte solutions are in contact with the film during a defined time at a reduction potential followed by 150 s at an oxidation potential of þ600mV;theclickreactioncouldtakeplaceunderreductiveand/ or oxidative conditions. To verify this point, we performed experiments where the polyelectrolyte solution was brought in contact with the film only once the electrode potential was set to þ600mV(see FigureS-3 inSupportingInformation). Besides the initial buildup due to electrostatic interactions between PEIAz and PAAAlk, no evolution of the frequency shift was observed, indicating that the film construction does not take place. This provesthatthe Cu(I) productiontakesplace onlyunder reductive conditions of Cu(I).

Effect of the CuSO4 Concentration on the Film Buildup. We varied the Cu(I) concentration of the buildup solutions by

Figure 2. Evolution of the normalized frequency shift, measured at 15 MHz (ν = 3) by EC-QCM, as a function of time during the buildup of a PEIAz-(PAAAlk/PAAAz)2 film. The film was built in the presence of 0.3 mM CuSO4 solution with the application of an electric potential of -350 mV during the adsorption of PAAAlk and PAAAz, followed by the application of an electric potential of þ600 mV. Each electric potential was applied for 5 min.

Scheme 3. Standard Procedure Used for the Buildup of Films

Composed of Poly(acrylic acid) Functionalized by Alkyne or Azide Functions, Using Electrochemically Catalyzed Click Chemistrya aThis procedure is applied after the adsorption of a precursor layer of PEI functionalized by azide functions. OCP means open-circuit potential.

Figure 3. Typical evolution ofthe normalized frequencyshift Δfν/ν and the energy dissipation Dν (in inset), measured by EC-QCM, as a function of the sequence of adsorbed polyelectrolyte layers ν=7at35MHz(1).Measurementsatthefundamentalfrequency (5 MHz) were not considered due its sensitivity to bulk solution changes. The functionalized polyelectrolytes (PAAAlk or PAAAz) were adsorbed at a reduction potential of -350 mV in the presence of 0.3 mM CuSO4 solution. A PEIAz precursor layer is used to anchor the film onto the substrate.

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

Article Rydzek et al.

keeping the applied reduction potential E fixed at -350 mV during the adsorption of PAAAlk and PAAAz (see Figure 5). The absolute value of the normalized frequency shift of PAAAlk/

PAAAz film increases upon increasing Cu(I) concentration in solution,indicatingthatthedepositedmass,foragivennumberof deposition steps, increases when the Cu(I) concentration is increased. This result correlates well with the evolution of the Cu(0) deposition rate with respect to the Cu(I) concentration in solution (Figure 1b) and thus with the production of Cu(I) as a function of the Cu(I) concentration in solution during the reduction step.

The film buildup seems to level off at large numbers of deposition steps. This is particularly striking at low Cu(I) concentrations. To check further this point, we performed two experiments: one at 0.075 mM and one at 0.3 mM of Cu(I) over up to 14 PAAAlk/PAAAz pairs of layers. When the film was constructed with solutions of 0.075 mM Cu(I), the buildup leveled off after n = 10 pairs of layers. With a concentration of 0.3 mM, we did not yet reach the leveling off up to n = 14, but the deposition rate clearly slowed down while n increased (see Figure S-4a in Supporting Information). Effect of the Reduction Time. We investigated the effect of the reduction time ΔtE over which the reduction potential E is applied. This potential was kept equal to -350 mV and we useda

Cu(I) concentration of 0.15 mM for this purpose. The evolution of the frequency shift relative to the film buildup for different times are given in Figure 6. While the film buildups are similar for 2 and 5 min, the film thickness increases with ΔtE at a given value of the number n PAAAlk/PAAAz pairs of layers. Whereas for 2 and 5 min the buildupstartstoleveloffaftersevendepositionsteps,notendency of leveling off is observed for longer application times up to n=

7. For ΔtE = 7 min,the levelingoff tookplace forn = 12(Figure S-4b in Supporting Information). One can thus assume that the value of the number of deposition steps at which the leveling off sets in increases with the reduction time ΔtE. A study of a precise relationship between n and ΔtE is however out of the scope of this paper.

Figure 4. Evolution of the normalized frequency shift, measured at 15 MHz (ν=3) by EC-QCM, as a function of the sequence of

0m V( 9) in the presence of 0.3 mM CuSO4 solution. A PEIAz precursor layer is used to anchor strongly the films onto the substrate. The crystal resonance frequency corresponding to the first PAAAlk deposition is taken as reference. For the three potential values where the films build up, the average and the standard deviation from at least three experiments are represented.

Figure 5. Evolution of the normalized frequency shift, measured at 15 MHz (ν = 3) by EC-QCM, as a function of the adsorbed polyelectrolyte layers for the PAAAlk/PAAAz system built in the presenceofCuSO4at0.3(b),0.15(O),0.075(2),and0.05mM(4). DuringtheadsorptionofPAAAlkandPAAAz,anelectricpotential E of -350 mV was applied. In the case of CuSO4 concentrations less than 0.3 mM of NaNO3 salt was added in aqueous solution to maintain the ionic strength constant at 1.2 mM (i.e., the ionic strength of 0.3 mM of CuSO4). For the concentrations equal to or largerthan0.075mMCuSO4,themeanandthestandarddeviation of at least three experiments are represented. A PEIAz precursor layer is used to anchor the films onto the substrate. The crystal frequency corresponding to the first PAAAlk deposition is taken as reference.

Figure 6. Evolution of the normalized frequency shift, measured at 15 MHz (ν = 3) by EC-QCM, as a function of the adsorbed polyelectrolyte layers for the PAAAlk/PAAAz system built in the presence 0.15 mM CuSO4 at a reduction potential E of -350 mV applied for 2 (b), 5 (O), 7 (1), and 20 min (4). For each reduction time, the average and the standard deviation from at least three experimentsarerepresented.NaNO3saltwasaddedintheaqueous solution of CuSO4 to have a constant ionic strength of 1.2 mM. AP EIAz precursor layer is used to anchor the films onto the sub- strate. The crystal frequency corresponding to the first PAAAlk deposition is taken as reference.

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

Rydzek et al. Article

Stability of the Films with Respect to Swelling. To test the stability and robustness of the multilayers, pH jumps were performed on a film constructed in the presence of 0.3 mM

CuSO4 at pH 3.5 with E = -350 mV by changing the pH of the contacting solution from 3.5 to 9 in the presence of 10 mM

NaNO3. The pH was then brought back to its initial value (i.e., pH3.5)inordertotestthereversibilityofthepHresponse.Partsa and b of Figure 7 represent respectively the evolution of the normalized frequency shift and dissipation of a PEIAz-(PAAAlk/

PAAAz)7 film determined by QCM. The film swells in a reversible and reproducible manner as the pH is increased. These observa- tionsare inagreementwith thoseofTangetal.35and are expected sincethecarboxylicgroupsbecomeionizedasthepHisincreased. The reversibility of the swelling indicates that no material is lost during swelling. The film is thus robust with respect to internal mechanicalstressasexpected from covalently bondarchitectures.

(Parte 2 de 3)

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