Tuning the Electrochemical Swelling of Polyelectrolyte Multilayers

Sep 17, 2014 - ... C. Internal composition versus the mechanical properties of polyelectrolyte multilayer films: the influence of chemical cross-linki...
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Tuning the Electrochemical Swelling of Polyelectrolyte Multilayers toward Nanoactuation Raphael Zahn,*,†,‡ János Vörös,† and Tomaso Zambelli† †

Laboratory of Biosensors and Bioelectronics, Institute for Biomedical Engineering, ETH Zurich, Zurich, Switzerland Biosurfaces Unit, CIC BiomaGUNE, Donostia-San Sebastian, Spain

‡

S Supporting Information *

ABSTRACT: We discuss physicochemical determinants of electrochemical polyelectrolyte multilayer swelling that are relevant to actuator usage. We used electrochemical quartz crystal microbalance with dissipation monitoring (EC-QCM-D) and cyclic voltammetry to compare the electrochemical swelling of two types of ferrocyanide-containing polyelectrolyte multilayers, poly(L-glutamic acid)/poly(allylamine hydrochloride) (PGA/PAH), and carboxymethyl cellulose/poly(diallyldimethylammonium chloride) (CMC/PDDA). We showed that ferrocyanide oxidation causes the swelling of PGA/PAH multilayers whereas it results in the contraction of CMC/PDDA multilayers. This behavior can be attributed to the presence of a positive and a negative Donnan potential in the case of PGA/PAH and CMC/PDDA multilayers, respectively. Using multilayers consisting of PGA and poly(allylamine) ferrocene (PGA/PAH-FC), we applied EC-QCM-D and demonstrated potentiostatic thickness control with nanometer precision and showed that the multilayer’s thickness depends linearly on the applied potential within a certain potential range.

1. INTRODUCTION Stimuli-responsive polymeric thin films are versatile systems for preparing functionalized, organized nanomaterials.1 Exposed to an external stimulus, these films undergo specific changes in their physical or chemical properties such as their thickness, elastic properties, permeability to small molecules, and wettability.2 Typical stimuli include heat, light, changes in pH or ionic strength, magnetic or electric fields, and electric currents/voltages.3 The latter is used for electroactive films that contain redox-active building blocks and thus can be oxidized and reduced in a reversible way. Electrochemical stimuli can be applied locally and in a controlled way, and as a result, electroactive thin films show a strong potential for applications. Among other responses, such as sol−gel transitions or colorimetric changes,4 most electroactive thin films show volumetric changes upon oxidation/reduction. This behavior is best known from so-called ionic electroactive polymers (ionic EAPs) that are used as conjugated polymer actuators.5,6 The volume changes are caused by an exchange of ions and solvent between the polymeric film and the electrolyte solution during the electrochemical reaction. This exchange is necessary to maintain electroneutrality in the polymer film.5 If ions are expelled in this process, then the film shrinks, and if ions are adsorbed, then the film expands. Electrochemically active polyelectrolyte multilayers operate on the same principle;7−9 however, they offer several advantages over the use of traditional ionic EAPs. Polyelectrolyte multilayers (PEMs) are obtained by the alternate deposition of positively and negatively charged building blocks © 2014 American Chemical Society

on a charged surface (so-called layer-by-layer assembly, LBL). While initially oppositely charged polyelectrolytes were used as building blocks,10 by now multilayers have been assembled using not only electrostatic attraction but all sorts of complementary interactions between the building blocks.11 This inherent structural flexibility combined with good environmental11 and electrochemical12 stability makes electroactive PEMs ideal for custom-designed electrochemical actuation systems. Recent applications of electrochemical multilayer actuation include controlled stiffness changes for the manipulation of cell adhesion,13 electrochromic windows,14 electrically induced multilayer disassembly,15 and redoxcontrolled permeability of microcapsules.16 Such applications are still in their early stages, yet the future use of PEM actuators can be envisioned for functions including artificial muscles,17 responsive coatings of nanoscale devices,12 microvalves, microswitches, data storage, and lab-on-a-chip devices.5 To implement electroactive PEMs in working devices successfully, it is necessary to have knowledge and control of the parameters that control the thickness changes in such multilayers. Here, we investigate how two selected parameters influence the swelling of electroactive multilayers. First, we show that by working with the same redox centers and applying the same voltages, the use of two different polyelectrolyte couples can induce opposite thickness changes (one time Received: July 30, 2014 Revised: September 14, 2014 Published: September 17, 2014 12057

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over the substrate for 5 min. Two polyelectrolyte adsorption steps were separated by a buffer rinse for 10 min. Polyelectrolyte and buffer circulation was allowed through Tygon tubing by means of a peristaltic pump. The infrared spectra of the film were acquired during each buffer rinsing step in total attenuated reflection mode by accumulating 128 interferograms at a resolution of 2 cm−1. The signal transmitted through the ZnSe crystal was collected with a deuterated triglycine sulfate (DTGS) detector. During the deposition of the PEI-(PGA/ PAH)5 multilayer film, the absorption spectrum was calculated as −log(Tfilm/Tbuffer), where Tfilm and Tbuffer represent the transmission of the film adsorbed on ZnSe substrate in contact with buffer and the buffer in contact with the native ZnSe crystal, respectively. After its buildup and characterization, the PEI-(PGA/PAH)5 film was placed in contact with a ferrocyanide-containing solution (10 mM Fe(CN)64− in 10 mM HEPES, 100 mM KCl-containing buffer) for 10 min. Then the spectrum of the Fe(CN)64−-containing PEI-(PGA/PAH)5 film was acquired in the presence of the KCl-containing buffer. We compared the AT-FTIR spectra of ferrocyanide-containing multilayers to a reference spectrum recorded for a 10 mM ferrocyanide solution without polyelectrolytes. To determine the concentration of ferrocyanide ions in the multilayer, the penetration depth of the evanescent wave, dp, is required

swelling, one time contraction). This effect is caused by different charge balances in the two multilayer systems and, as a result, different ion fluxes during the oxidation/reduction cycles. Second, we demonstrate that the thickness of a selected electroactive PEM linearly depends on the applied potential. This allows setting the multilayer’s thickness with precision in the nanometer range, an enabling feature for a wide range of applications.

2. EXPERIMENTAL SECTION Materials. All chemicals were used as received unless otherwise specified. The following polymers were used: polyethylenimine (branched) (PEI, Sigma-Aldrich 408727, MW = 25 000), poly(Lglutamic acid) (PGA, Sigma-Aldrich 408727, MW = 15 000−50 000), poly(allylamine hydrochloride) (PAH, Sigma-Aldrich 283215, MW = 70 000; for PAH-FC synthesis: Sigma-Aldrich 283223, MW = 58 000), poly(diallyldimethylammonium chloride) (PDDA, Sigma-Aldrich 409014, MW = 100 000−200 000), and carboxymethyl cellulose (CMC, Sigma-Aldrich 419311, MW = 250 000). Poly(allylamine) ferrocene (PAH-FC) was synthesized according to the literature as described previously.18,19 Briefly, we dissolved 16 mg (0.075 mmol) of ferrocene carboxaldehyde (Fluka, Buchs, Switzerland) in 2 mL of anhydrous methanol. This solution was added dropwise over 1 h to 10 mL of a methanol solution (Fluka, Buchs, Switzerland) containing 80 mg (1.40 mmol) of PAH and 0.52 mL (3.7 mmol) of triethylamine (Fluka, Buchs, Switzerland). The resulting dark-red reaction mixture was slowly stirred for 2 h while being cooled in an ice bath. Sodium borohydrate (3.5 mg, 0.093 mmol, Fluka, Buchs, Switzerland) was slowly added and stirred for 2 h until the solution color became lighter. The solution was then dialyzed against ultrapure water using a dialysis tube (MW cutoff 14 000) for at least 48 h. The water was changed every 12 h. Polyelectrolyte powder was obtained by freeze drying the washed solution. Buffers were prepared with ultrapure water (Milli-Q gradient A 10 system, Millipore Corporation) and filtered (0.2 μm) prior to use. All buffers contained 10 mM 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES, Fluka, Buchs, Switzerland) and 0.1 M potassium chloride (KCl, Fluka, Buchs, Switzerland). The pH of the buffer was adjusted to 7.4 using 6 M KOH (Fluka, Buchs, Switzerland). The ferrocyanide-containing multilayers were exposed to buffer solutions that contained 10 mM HEPES, 0.1 M KCl, and 10 mM potassium hexacyanoferrate(II) trihydrate (ferrocyanide, [Fe(CN)6]4−, Fluka, Buchs, Switzerland). Surface Cleaning and Film Preparation. We used previously published procedures for surface cleaning and film preparation.19−21 The gold surfaces were cleaned by immersion in a 2% (w/w) SDS solution (>30 min), followed by rinsing with ultrapure water, drying under a stream of nitrogen, and cleaning with UV/ozone for 30 min. The polyelectrolyte multilayers were built by the alternating adsorption of positively and negatively charged polymers on the gold surface. All polyelectrolytes were used at a concentration of 0.5 mg/mL in a buffer at pH 7.4, which contained 10 mM HEPES and 100 mM KCl. PEI was used as a precursor layer followed by alternating depositions of polyanions and polycations, always separated by a rinsing step. The temperature during the assembly was 25 °C. Adsorption steps were 5 min long, and rinsing steps were 2 min. The ferrocyanide-containing multilayers were prepared as described previously.20 Briefly, the multilayers were exposed to a ferrocyanide solution (10 mM Fe(CN)64− in 10 mM HEPES, 100 mM KCl, pH 7.4) for 10 min followed by a rinsing step with KCl buffer (10 mM HEPES, 100 mM KCl, pH 7.4). Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded in attenuated total reflection (ATR) mode with a Vertex 70 (Bruker, Germany). For these experiments, D2O was used as the solvent instead of H2O. The penetration depth of the infrared radiation was calculated to be 1.26 μm at 1550 cm−1. The films were deposited on a trapezoidal ZnSe crystal (Graseby-Specac, Orpington, U.K.) located on the bottom of a flow cell (Graseby-Specac, Orpington, U.K.) by allowing each polyelectrolyte solution to circulate

dp =

λ 2πn1 sin 2 θ −

2

( ) n2 n1

where λ is the wavelength of the incident light, θ is the incident angle (45°), and n1 = 2.42 and n2 = 1.34 are the refractive indexes of the ZnSe crystal and the buffer solution, respectively. In the case of ferrocyanide (absorbance peak at λ−1 = 2033 cm−1), we obtain dp = 1.33 μm. The concentration of ferrocyanide in the film is then obtained by integrating the exponentially decaying field over the thickness of the multilayer and comparing the value to a solution with a known concentration (infinite medium, 10 mM of ferrocyanide). Our multilayers are very thin (70 nm) compared to the decay length of the infrared radiation (1330 nm), and the refractive index of polyelectrolyte multilayers (1.38)22 is similar to the refractive index of our buffer solution (1.34). Therefore, we neglect differences between the refractive index of the multilayer films and the buffer solution and use a uniform refractive index of 1.34 to determine dp. Electrochemical Quartz Crystal Microbalance with Dissipation Monitoring (EC-QCM-D). The EC-QCM-D studies of the polyelectrolyte multilayers were performed as described previously.19,20 The cleaned QCX 301 gold crystals (Q-Sense AB, Gothenburg, Sweden) were mounted in the QCM-D cells (QE 401 instrument with standard modules, Q-sense AB, Gothenburg, Sweden). A volume of 0.5 mL of temperature-equilibrated polyelectrolyte or buffer solution was injected into the QCM-D cells, and the PEM buildup was monitored by continuously recording the sets of resonance frequencies and dissipation factors (3rd, 5th, 7th, 9th, and 11th overtones). For the electrochemical measurements, gold crystals with the adsorbed PEM film were quickly transferred to the QEM 401 module to ensure that the PEM did not dry. The QEM 401 cell represents a conventional three-electrode setup with a platinum counter electrode and an Ag/AgCl (3 M KCl) reference electrode (Micro Dri-Ref reference electrode, World Precision Instruments, Sarasota, FL, USA). The working electrode of the module is the gold surface of the QCX 301 crystal that is also used for the QCM-D measurements. Cyclic voltammetry (CV) measurements were performed using an IPS Jaissle PGU10 V-1A-IMP-S potentiostat/galvanostat (Jaissle Elektronik GmbH, Germany). For typical cyclic voltammetry (CV) measurements, we scanned the electrical potential from 0 to 600 mV at a scan rate of 50 mV/s. For potentiostatic control of multilayer swelling, constant potentials (between −200 and +450 mV) were applied. For both types of measurements we simultaneously recoded the resulting changes in resonance frequency and dissipation of the QCM-D signal. During the EC-QCM-D measurement with constant applied potentials, the frequency and dissipation values showed continuous 12058

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Figure 1. Characterization of PEI-(CMC/PDDA)7 multilayers. (A) QCM-D data showing the multilayer buildup and ferrocyanide adsorption. (B) AFM micrograph showing the surface topology. (C) The multilayer was scratched with a razor blade to determine its thickness. (D) Line scan across the scratch in the film (indicated by the white line in the micrograph in C). drifts (up to +1.6 Hz min−1 and −0.16 × 10−6 min−1, respectively). These drifts had a constant value, and they were independent of the sign and the magnitude of the applied potential. Thus, they cannot be explained by a “break in” of ions.12,23 We attributed the drifts to the formation of small gas bubbles on the sensor surface and corrected our QCM-D data accordingly (by subtracting a straight line). The investigated multilayer systems form rigid thin films. This is apparent from the QCM-D signal for multilayer construction and electrochemical swelling. In both cases, the normalized overtones of the QCM-D resonance frequency (Δf n/n) showed only a little dispersion, and the ratio of the QCM-D dissipation values (ΔDn) to the corresponding normalized frequencies (ΔDn/−(Δf n/n)) was below 2.5 × 10−7 Hz−1 for all overtones.24 For rigid films, the Sauerbrey equation can be used to analyze the QCM-D data and to calculate the mass adsorbed to the sensor surface.25 Figures S2 and S3 in the Supporting Information demonstrate the applicability of the Sauerbrey equation to our data and show the frequency dispersion and dissipation values for PEI-(CMC/PDDA)7 and PEI-(PGA/PAHFC)8 multilayers. The typical thickness changes during the electrochemical multilayer swelling are below 10% and therefore do not change the multilayer’s mass density significantly (multilayer mass densities calculated from QCM-D and AFM data: ρCMC/PDDA = 0.82 ± 0.15 g cm−3, ρPGA/PAH‑FC = 1.4 ± 0.13 g cm−3). As a result, the resonance frequency shift of the QCM-D experiments corresponds to the multilayer thickness in arbitrary units. EC-QCM-D data is also used to determine the concentrations of electrochemically active ferrocene (cF0, reduced form) and ferrocenium (cF+, oxidized form) in

the multilayer. The frequency shift of the multilayer swelling is proportional to the number of adsorbed counterions and water molecules. Their number in turns is proportional to the number of oxidized redox sites (ferrocenium, cF+).20 Thus, the frequency shift of the swelling equals the number of oxidized redox sites in arbitrary units. (The proportionality constant is 3.5 × 10−11 (mole of cF+)/Hz for PEI-(PGA/PAH-FC)8 multilayers with a thickness of 530 nm deposited on an electrode with a diameter of 14 mm.) The total number of active redox sites in the film is calculated from the maximal frequency shift as measured by cyclic voltammetry using EC-QCM-D. For Nernst plots (Figure 6) only the ratio of oxidized to reduced active sites is important. Therefore, their concentrations can be given in arbitrary units as long as the same type of unit is used for both concentrations. Atomic Force Microscopy (AFM). We used AFM to characterize the thickness and surface morphology of the PEMs. Immediately after the QCM-D measurements, the gold crystals were taken from the QCM-D flow chambers and mounted in a custom-made Teflon liquid cell. We ensured that the films never dried before and during the AFM experiment. We used a Nanowizard I BioAFM (JPK Instruments, Berlin, Germany) and Mikromasch CSC38/noAl cantilevers (nominal k = 0.03 N/m) in both contact and in intermittent-contact mode. At least three different positions were examined per sample to obtain sufficient statistics. The thickness of the polyelectrolyte multilayers was determined by imaging a sample area previously scratched with a razor blade. The AFM images were processed using the JPK image 12059

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processing software (JPK Data Processing, Version spm-4.0.23) and Gwyddion software (Gwyddion 2.26, GNU General Public License). The AFM-measured thicknesses were used to calibrate the frequency data (corresponding to arbitrary thickness) of the ECQCM-D experiments. We divided the frequency of the multilayer buildup by the corresponding (AFM) thickness of the multilayer. Hereby, we obtained the multiplication factor that relates the resonance frequency to the thickness of the multilayers. This method was used to calibrate the voltage-controlled multilayer swelling as we show in Figure 5. (Figure S5 shows the uncalibrated EC-QCM-D data.)

we found that CMC/PDDA multilayers grow exponentially. This discrepancy in growth behavior is likely caused by the use of polyelectrolytes of different molecular mass and other differences in the experimental parameters. For the adsorption of seven bilayers, we measured a normalized frequency shift of −1420 ± 120 Hz (Figure 1A). The thickness of a PEI-(CMC/ PDDA)7 multilayer was 310 ± 30 nm as determined from AFM line scans across a scratch in the film (Figure 1C,D). AFM images showed a smooth surface (Figure 1B) with a low roughness of 13 ± 4 nm. The addition of 10 mM ferrocyanide for 10 min resulted in a frequency increase of around 130 Hz in the QCM-D after rinsing with buffer (Figure 1A). This corresponds to a relative frequency change of 9% and is significantly less than the change observed for PGA/PAH multilayers (35%). We investigated the electrochemical properties of ferrocyanide-containing PEI-(CMC/PDDA)7 multilayers using an ECQCM-D cell and applying cyclic voltammetry. Figure 2 shows

3. RESULTS AND DISCUSSION Ferrocyanide-Containing Multilayers: Charge Balance and Swelling Behavior Depend on the Used Polyelectrolyte Couple. We and others investigated the properties of polyelectrolyte multilayers containing ferrocyanide/ferricyanide ions as redox centers.19,20,26−29 Ball et al. showed that ferrocyanide is adsorbed to PGA/PAH multilayers and stays firmly incorporated when rinsed.26 Upon ferrocyanide binding part of the polyanion, PGA, is expelled from the film and replaced by the negatively charged redox ions. Thus, the ferrocyanide ions are an integral part of the multilayer, which explains their tight binding to the PEM.20 Within the PEM, the ferrocyanide ions can transport, either by ion hopping30 or by the diffusion of polyelectrolyte chains.19 Oxidation/reduction of ferrocyanide-containing PEMs causes reversible swelling/ contraction of the films. This behavior is explained by the adsorption of counterions and water to the multilayer in order to maintain charge neutrality.20,27 Anzai et al. demonstrated that ferrocyanide is also adsorbed to multilayers composed of other weak polyelectrolyte couples.28 They showed that the choice of polycation has a significant effect on the redox properties of ferrocyanide in the PEM. Depending on the used polycation, the redox potential of ferrocyanide in the multilayers is shifted to more positive or more negative values. Here, we compare the electrochemical swelling of two ferrocyanide-containing polyelectrolyte multilayer systems, PEI(PGA/PAH)5 and PEI-(CMC/PDDA) 7. The multilayer buildup was characterized using QCM-D and AFM. For PEI(PGA/PAH)5 we have described the multilayer construction and ferrocyanide adsorption in a previous publication.20 Briefly, the QCM-D measurements showed a total frequency shift of approximately −350 Hz and a dissipation shift of approximately 6 × 10−6 for the deposition of five bilayers of PGA/PAH (third overtone, data not shown here). The AFM measurements showed a film consisting of densely packed droplets that cover the whole surface and have a height of between 60 and 80 nm (data not shown, see ref 20.). Upon exposure to a solution containing 10 mM ferrocyanide, 47% of the PGA in the multilayer was replaced by ferrocyanide ions (shown by ATRFTIR in ref 20), leading to a thickness decrease of 35% (shown by QCM-D). Only approximately 50% of the ferrocyanide in PGA/PAH multilayers is electrochemically active. We show this by comparing the number of ferrocyanide ions in the multilayer as measured by ATR-FTIR spectroscopy (1350 mM for PEIPGA/PAH; see Figure S1 in the Supporting Information) with the number of ferrocyanide ions that are oxidized in cyclic voltammograms at low scan rates (scan rate 0.001 V s−1; 623 mM of electrochemically active ferrocyanide in PEI-PGA/ PAH). Figure 1 shows QCM-D and AFM data for the buildup and characterization of PEI-(CMC/PDDA)7 multilayers. In contrast to other groups working with this polyelectrolyte couple,28

Figure 2. Cyclic voltammograms of Fe(CN)64−/Fe(CN)63− recorded at a scan rate of 50 mVs−1 in 100 mM KCl on a gold electrode using EC-QCM-D. The current axis for the voltammogram of a 10 mM ferrocyanide solution (red solid line) is shown on the right (in red). On the left, the current axis (in black) for the ferrocyanide-containing multilayers is shown. Compared to the ferrocyanide in solution, the apparent redox potential is shifted to a higher positive value for the PEI-(PGA/PAH)5 multilayer (black dashes) and to a lower positive value for the PEI-(CMC/PDDA)7 multilayer (black dots). Potentials are measured vs an Ag/AgCl reference electrode. The voltammograms for the ferrocyanide-containing solution and the ferrocyanidecontaining PEI-(PGA/PAH)5 multilayer are adapted from ref 20 and reprinted with permission. Copyright 2010 American Chemical Society.

the cyclic voltammogram of ferrocyanide/ferricyanide in a PEI(CMC/PDDA)7 multilayer in comparison to the voltammogram for ferrocyanide/ferricyanide in a PEI-(PGA/PAH)5 multilayer and the voltammogram of ferrocyanide/ferricyanide in solution. For the free redox ions, the voltammogram shows a separation between oxidation and reduction peaks (ΔE = Eox − Ered) of more than 58 mV as expected for a diffusion-limited redox process. For the voltammograms of the ferrocyanide -containing multilayers, the small peak separation (ΔE ≪ 58 mV) indicates that the ferrocyanide ions are trapped in the multilayer.31 The apparent redox potential (E1/2 = 1/2Eox − 1 /2Ered) of ferrocyanide/ferricyanide in CMC/PDDA multilayers is E1/2 = 81 ± 5 mV compared to E1/2 = 395 ± 9 mV for ferrocyanide/ferricyanide in PGA/PAH multilayers. The 12060

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Figure 3. EC-QCM-D measurements showing frequency (Δf) and dissipation (ΔD) changes during cyclic voltammetry measurements (potential range 0−600 mV vs Ag/AgCl; scan rate 50 mVs−1). Potentials were first applied at t = 1 min. The potential cycling finished at t = 3 min (A) and at t = 2.6 min (B). See Figure 2 for the corresponding voltammograms. (A) Fe(CN)64−-containing PGA/PAH multilayers swell upon ferrocyanide oxidation (frequency decrease). (B) Fe(CN)64−-containing CMC/PDDA multilayers contract upon ferrocyanide oxidation (frequency increase).

CMC/PDDA, however, the Donnan potential is clearly negative. This difference in the Donnan potential for the two multilayers can be understood by looking at their construction. It has been shown that the ferrocyanide ions replace part of the polyanions in PGA/PAH multilayers.20 This exchange leaves the charge balance in the multilayer largely unaltered and preserves the positive net charge caused by the adsorption of PAH in the last deposition step. The uptake of ferrocyanide ions to PGA/PAH multilayers is accompanied by a significant frequency increase in the QCM-D signal (35%). We do not notice such a change for CMC/PDDA multilayers that are exposed to ferrocyanide (only 9%, Figure 1A). This indicates that in CMC/PDDA multilayers the ferrocyanide ions do not replace a substantial number of polyanions, but they are adsorbed to the multilayer in the same way as polyelectrolytes. This results in overcompensation by negative charges and thus a negative Donnan potential. The different adsorption behavior of ferrocyanide ions to PGA/PAH multilayers on the one hand and CMC/PDDA multilayers on the other hand is likely caused by different interaction strengths between the polyanions, the polycations, and the ferrocyanide ions in the two systems. For PGA/PAH, the binding of ferrocyanide to PAH is stronger than the interaction between PAH and PGA. Therefore, ferrocyanide can replace PGA. For CMC/PDDA, the binding of ferrocyanide to PDDA is weaker than the interaction between PDDA and CMC. Therefore, ferrocyanide cannot replace the polyanions in this system, and it solely binds to the “free” ammonium groups of PDDA. The different ferrocyanide binding strengths are probably caused by the presence of primary amine groups on PAH and quaternary ammonium groups on PDDA, a factor that is known to influence complex formation.37 The difference in the Donnan potential of the two ferrocyanide-containing multilayer systems has a strong influence on their swelling behavior. Figure 3 shows ECQCM-D measurements for the oxidation/reduction of ferrocyanide in a PEI-(PGA/PAH)5 multilayer (Figure 3A) and a PEI-(CMC/PDDA)7 multilayer (Figure 3B). The potential sweep rates (50 mVs−1) and scan ranges (0−600 mV vs Ag/AgCl) were identical for both measurements. For the PGA/PAH multilayer, the oxidation of ferrocyanide resulted in a decrease in Δf and an increase in ΔD, indicating a swelling of the multilayer. For the PDDA/CMC multilayer we observed the opposite reaction (decreasing Δf and increasing

apparent redox potential (E1/2) allows us to calculate the Donnan potential (ΔΦD) of the ferrocyanide-containing multilayers by using the redox potential of ferrocyanide ions in buffer solution (E° = 230 ± 3 mV) without a multilayer being present:20,32 E1/2 = E° + ΔΦD

Ferrocyanide ions in CMC/PDDA multilayers show a lower apparent redox potential compared to ferrocyanide ions in buffer solution, whereas the opposite (higher E1/2) is the case for ferrocyanide in PGA/PAH multilayers. This results in a negative Donnan potential (ΔΦD = −149 ± 5 mV) for CMC/ PDDA multilayers and a positive Donnan potential (ΔΦD = 140 ± 9 mV) for PGA/PAH multilayers. The Donnan potential is caused by an imbalance of fixed charges in the multilayer. An excess of fixed positive charges (polycation excess) results in a positive Donnan potential, and an excess of fixed negative charges (polyanion excess) results in a negative Donnan potential. The excess fixed charges are compensated for by the uptake of mobile ions from the electrolyte solution to maintain charge neutrality. For ferrocyanide-containing PGA/PAH multilayers this results in chloride ions being present in the multilayer, whereas potassium ions are largely excluded from it. In the case of ferrocyanide-containing CMC/PDDA multilayers, the opposite applies (potassium present, chloride excluded). The construction of most polyelectrolyte multilayers relies on charge overcompensation, meaning that the last adsorbed polyelectrolytes impose their charge on the whole film. (In some cases, a polyelectrolyte film forms even without net charge reversal,33 but for PGA/PAH34 and CMC/PDDA,35 multilayer charge reversal has been demonstrated.) We point out that both of our multilayers are exponentially growing systems. It is accepted that such films do not exhibit welldefined “stacked” layers but consist of a homogeneous bulk material allowing the free diffusion of the polyelectrolytes throughout the whole multilayer.36 The last adsorbed polyelectrolytes define the charge of the multilayer; however, this charge is not confined in the topmost layer but is smeared out throughout the multilayer as a consequence of polyelectrolyte diffusion. Both investigated multilayer systems, PEI(PGA/PAH)5 and PEI-(CMC/PDDA)7, have polycations adsorbed in the last deposition step; accordingly, they should have a positive Donnan potential. For ferrocyanide-containing 12061

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Figure 4. (A) Characterization of PEI-(PGA/PAH-FC)8 multilayers. (A) QCM-D data showing the multilayer buildup. (B) AFM micrograph showing the surface topology. (C) The multilayer was scratched with a razor blade to determine its thickness. (D) Line scan across the scratch in the film as indicated by the white line in the micrograph in C.

ΔD), meaning that the multilayer contracted upon ferrocyanide oxidation. This difference in behavior is caused by the opposite sign of the Donnan potential for the two multilayer systems. Upon oxidation of ferrocyanide, there is an excess of positive charge in both multilayer systems. In the PGA/PAH multilayer, the positive Donnan potential forbids the presence of small cations (potassium ions) in the multilayer. Therefore, the excess positive charge is compensated for by the adsorption of small anions (chloride ions) from the surrounding electrolyte solution. This leads to an increase in osmotic pressure in the multilayer and thus results in swelling. In PDDA/CMC multilayers the Donnan potential is negative; hence the excess positive charge upon ferrocyanide oxidation is removed by a release of small cations (potassium) from the multilayer. This in turns results in the observed contraction of the multilayer. We emphasize that our exponentially growing multilayers are homogeneous entities. Therefore, the ferrocyanide ions are not confined to the surface of the multilayer but are distributed throughout the film. As a consequence the whole multilayer, not only its surface layers, undergoes electrochemical swelling/ contraction. In addition, the ferrocyanide ions can move with the multilayer. This is possible by ion hopping30 or by diffusion together with the polyelectrolyte chains to which the ions are bound.19 Because the ferrocyanide ions in the films can move, they can be transported to the electrode where they are oxidized/reduced. In this way, not only the redox centers in

close proximity to the electrode but also a large part of the ferrocyanide ions within the whole multilayer (∼50% of the ferrocyanide ions for PGA/PAH, see above) are electrochemically active. The comparison between the two polyelectrolyte systems shows clearly that electroactive PEMs with the same type of redox centers can exhibit different swelling behavior. For potential applications of electroactive PEMs as nanoactuators it is important to keep in mind that the electrochemically induced swelling behavior and the Donnan potential of a multilayer are closely linked to each other. Thus, changes to the charge balance of a multilayer system can alter its swelling behavior. Such changes can be caused by the use of a different polyelectrolyte couple (as we show here) or a variation in the environmental parameters. Potentiostatic Thickness Control of Ferrocene Containing Multilayers: Swelling with Nanometer Precision. The covalent attachment of redox groups presents a common strategy for obtaining redox-active polymers. Polyelectrolytes are frequently modified with osmium38,39 or ferrocene18,40 derivatives, and the multilayers created with these polymers often contain additional redox molecules, e.g., enzymes or nanoparticles. Such electroactive multilayers have been used as glucose18,41 or ascorbic acid42 sensors, and there are detailed studies describing their charge transport19,39,43 and thermodynamic behavior.9,32 Also, their electrochemical swelling 12062

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Figure 5. Potentiostatic thickness control of PEI-(PGA/PAH-FC)8 multilayers. (A) Applying potentials of between 300 and 350 mV (vs Ag/AgCl) results in precise and reproducible changes in the multilayer’s thickness. The thickness changes are obtained by calibrating EC-QCM-D measurements with AFM data (text and Figure S5). Between the swelling steps the multilayer is reset to its unswollen state by applying a potential of −200 mV. The original EC-QCM-D data, displaying frequency and dissipation changes, is shown in Figure S5 in the Supporting Information. (B) For potentials of between 300 and 400 mV (vs Ag/AgCl) the multilayer’s thickness linearly depends on the applied potential (with a slope of 0.42 ± 0.01 nm V−1; see the red regression line).

the cyclic voltammetry experiments (0−600 mV, 50 mV s−1) we obtained a frequency change of 540 Hz corresponding to 70 nm or 13% of the multilayer’s initial thickness (data not shown). Better suited to characterizing the actuating performance of electrochemical PEMs are experiments that apply and hold a defined electrical potential and measure the resulting swelling/contraction of the multilayer. Figure 5 shows such a measurement where potentials of between 300 and 450 mV were applied for 2 min at a time and intercepted by the application of a reset potential of −200 mV (also 2 min long). The thickness changes shown in Figure 2A are calculated from EC-QCM-D data using the frequency-to-thickness conversion factor obtained from multilayer construction. (See above and also Figure S5 in the Supporting Information for the original EC-QCM-D data.) Figure 5 shows that the response time of the multilayer is fast (