Microgel Size Modulation by Electrochemical Switching - Chemistry of

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Microgel Size Modulation by Electrochemical Switching Olga Mergel,† Patrick Wünnemann,† Ulrich Simon,‡ Alexander Böker,§ and Felix A. Plamper*,† †

Institute of Physical Chemistry II, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany § Fraunhofer-Institut für Angewandte Polymerforschung (IAP), Lehrstuhl für Polymermaterialien und Polymertechnologie, Universität Potsdam, Geiselbergstraße 69, 14476 Potsdam, Germany ‡

S Supporting Information *

ABSTRACT: In this work we present the first e-microgel, whose size can be adjusted by application of an electrochemical potential, as seen by dynamic light scattering (3D-DLS in dependence of equilibrium potential) and scanning force microscopy (SFM). Hereby, polyelectrolyte microgels with attracted electroactive counterions provide an effective platform for the manipulation of the microgel size by electrochemical means. The reversible switching of guest molecules, namely, hexacyanoferrates, between oxidized ferricyanide [Fe(CN)6]3− and reduced ferrocyanide [Fe(CN)6]4−, influences the cationic host microgel, poly(N-isopropylacrylamide-co-methacrylamidopropyltrimethylammonium chloride) P(NIPAM-co-MAPTAC), and hence the swelling properties of the microgel. The combination of thermo- and redox-responsiveness in one particle leads to a novel type of multistimuli responsive material. In addition, the use of hydrodynamic voltammetry detects directly the preferred uptake of ferricyanide and enables the determination of the nominal charge ratio (ncr) between microgel and entrapped counterions at different states of switching. Further, electrochemical impedance spectroscopy allows a more detailed mechanistic insight into the microgel modulation.

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INTRODUCTION Microgels are soft polymeric particles typically in the nanometer to micrometer size range, consisting of a crosslinked and porous network.1 This network swells in water and adjusts its dimensions, density, and related properties according to the surrounding conditions.2 Their stimulus responsiveness arises often from their ability to undergo a volume phase transition (VPT) with respect to environmental changes like temperature, pH, ionic strength, solvent composition, and so on.1−4 These “smart” polymers have rapidly attracted significant research interest due to their wide field of potential applications in biomedical technologies, especially in drug delivery5−9 and controlled release systems, separation and purification materials,10 and sensor interfaces.2,4 In this work, we use strong cationic polyelectrolyte microgels (μGs), consisting of a permanently charged thermoresponsive network with the help of poly(N-isopropylacrylamide) (PNIPAM) as the major component. PNIPAM microgels undergo a reversible decrease in size by passing its volume phase transition temperature (VPTT). The VPTT can be adapted by the use of suitable comonomers, which typically increase11,12 or decrease13,14 the VPTT and the degree of swelling. The latter can be mainly attributed to the osmotic pressure arising from the confined counterions.15−20 Due to entropic reasons, these charged microgels21,22 attract multivalent counterions. This leads to a release of monovalent counterions into the surrounding reservoir and a concomitant © XXXX American Chemical Society

deswelling due to a decreased osmotic pressure within the gel, dependent on the ionic strength of the dispersion medium.23,24 In addition, bridging/cross-linking effects and enhanced hydrophobic interactions can also contribute to the collapse of the charged microgel in the presence of multivalent counterions.25 Though redox-active polymers have been known for decades, their switching in solution or in colloidal suspension by electrochemical means is still a rather unexplored field. At the current state of research, the full potential of electroactive polymers especially in dispersions/solutions is by far not exhausted.26 On the contrary, the above-mentioned classical stimuli were extensively studied in bulk solution. Further, electrochemical switching of polymer properties such as wetting27 or expansion/contraction behavior28 of deposited polymer films on electrode surfaces was covered as well.26,28−31 For example, we adapted the microgel used in the present study for increased biosensor sensitivity.32,33 In contrast, electrochemical bulk suspension manipulation is a rather new field. As an approach, multivalent redox-responsive counterions, such as the redox couple hexacyanoferrate(III)/(II) HCF, and their switchable complexation with polyelectrolytes are introduced.34 A number of studies show the stronger complexation abilities Received: July 17, 2015 Revised: October 6, 2015

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DOI: 10.1021/acs.chemmater.5b02740 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials toward strong polyelectrolytes of the less charged trivalent ferricyanide counterion [Fe(CN)6]3− compared to tetravalent ferrocyanide [Fe(CN)6]4− due to ion-specific effects.22,27,33,35 Although the electrochemical manipulation of the solubility of polyelectrolyte/counterion complexes can be achieved by electrolysis and therefore by adjusting the ratio of ferri- and ferrocyanide,35 this principle was applied only recently for the reversible aggregation of unimeric polymers to micellar/ vesicular aggregates with the help of block-like copolymers (miktoarm star polymers).36 Here, a decoupling of the electrochemistry at the electrode and of the polymer aggregation in bulk solution prevents polymer film formation at the electrode. “Electrode fouling” happens easily in the case of polymers with strongly bound electroactive species.26 Therefore, only a few reports on electrochemical addressing of microgels and microgels in external electric fields are available.37−39 Poly(vinylpyridine)-based microgels show a pHdependent turbidity switching, being controlled by the use of a polyaniline electrode.40 Redox-active poly(ferrocenylsilane)based poly(ionic liquids) can self-cross-link to nanogels.41 An amplification of the electrogenerated chemoluminescence (ECL) in ruthenium-bearing thermoresponsive microgels was observed due to a decreasing distance between the adjacent redox sites during collapse.42 Microgels based on hemicellulose/aniline pentamer were reported recently.43 In all examples, only little is known about morphological changes upon electrochemical switching. To the best of our knowledge, there are no reports available demonstrating the manipulation of microgel size by electrochemical means. Therefore, we would like to extend the concept of stimuli responsiveness by a novel electrochemical switching of microgel properties in colloidal suspension. This offers opportunities and possible applications as, e.g., electrotriggered actuators, or in drug release devices.

Figure 1. Hydrodynamic radius Rh vs temperature obtained by 3dimensional cross correlation dynamic light scattering (3D-DLS) for a cationic P(NIPAM-co-MAPTAC) microgel (6,3 g/L, 0.63 wt %) in the presence of [Fe(CN)6]4− (blue; 1 mM, icr = 1) or [Fe(CN)6]3− (green; 1 mM, icr = 0.75), or in the absence of the multivalent counterions (black; all in 0.025 M KCl).

Scheme 1. Schematic Illustration of Reversible Microgel Size Manipulation by Electrochemical Switching

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RESULTS AND DISCUSSION In this study we investigate thermoresponsive microgels with incorporated redox-responsive oppositely charged counterions leading to a multiresponsive microgel system. The experimental details can be found in the Supporting Information. The synthesis and characterization of the NiPAM-based microgel containing 10−12 mol % of permanent positive charges used in this study were described elsewhere.22 The resulting microgel can be denoted as poly(N-isopropylacrylamide-co-methacrylamidopropyltrimethylammonium chloride), P(NiPAM-co-MAPTAC). NiPAM-based microgels usually possess a VPTT with a sharp decrease in size over a narrow temperature range.2 In contrast, a broadening of the VPT is observed for charged systems at low ionic strength, as seen in Figure 1. Additionally, the charged groups influence the onset of the transition leading to an increased VPTT of P(NiPAM-co-MAPTAC) (40 °C compared to 32 °C).44 These positive charges within the microgel network can now be “compensated” by oppositely charged hexacyanoferrates. In our previous investigations we found a preferential complexation of the microgel with the trivalent [Fe(CN)6]3− due to ion-specific effects, especially at elevated temperatures upon microgel collapse.22 Hence, varying attraction of the host microgel toward the different guest counterions is accompanied by a change in osmotic pressure within the microgel, affecting, e.g., its size according to Scheme 1. In agreement with our previous investigations, we found smaller hydrodynamic radii for microgel/counterion complexes in the presence of exclusively trivalent [Fe(CN)6]3− counterions as compared to tetravalent [Fe(CN)6]4−/microgel

complexes (Figure 1).33 The results were obtained by help of 3-dimensional cross correlation dynamic light scattering setup (3D-DLS), which allows the reliable determination of hydrodynamic radii even in turbid media (hence in the presence of multiple scattering). Hereby, the adjusted initial charge ratio (icr, which is defined as the molar ratio of the total hexacyanoferrate charges in relation to the microgel charges) amounts in the case of the tetravalent [Fe(CN)6]4− counterion to icr = 1 and in the case of the trivalent [Fe(CN)6]3− counterion to icr = 0.75 due to different valency of the counterions, whereas the molar amount of 1 mM was maintained. Although the initial charge ratio for the two counterions is different, the ionic strength I is set rather constant due to an excess of potassium chloride, which is used as supporting electrolyte for electrochemical experiments (in the case of the trivalent counterion complex, I([Fe(CN)6]3−) = 0.037 mol/L; in the case of the tetravalent counterion complex, I([Fe(CN)6]4−) = 0.045 mol/L). Additionally, a slight shift in the VPTT to lower temperatures is observed, when ferricyanide is added. The different sizes in the presence of the different hexacyanoferrates were rechecked by scanning force microscopy (SFM). This experiment is not a trivial one, as the conditions during 3D-DLS (Figure 1), bulk electrolysis experiments and SFM should be similar for a good comparison. This includes a comparable total concentration of hexacyanoferrate and microgel. However, SFM at such high concentration B


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Chemistry of Materials

Figure 2. Scanning force microscopy image of P(NIPAM-co-MAPTAC) (6.3 g/L) in the presence of exclusively 1 mM K4[Fe(CN)6] in 0.025 M KCl (left) and exclusively of 1 mM K3[Fe(CN)6] in 0.025 M KCl (middle) absorbed onto silicon wafer in liquid state at 25 °C and average height profiles across the apex of the absorbed microgel (right).

of colloidal particles is nearly impossible. We mimicked the bulk scenario by introducing a two-chamber SFM custom-made cell for measurements. Hereby, one-half of the cell is filled with the double concentration of microgels, whereas the other is filled only with electrolyte and with a solid surface modified with the attached microgels (spin-coated) for SFM analysis in liquid. Both chambers are separated by a semipermeable membrane, allowing the visualization of the microgel size at an overall high microgel concentration. The best results due to most stable measurement conditions (25 °C) are shown in Figure 2 (for data at 37 °C see Supporting Information). It can be clearly seen that the microgels in the presence of ferrocyanide are considerably larger than the microgels in the presence of ferricyanide. Again, the size changes in 3D-DLS are reflected by very similar size changes in SFM both at different temperatures and in the presence of different hexacyanoferrates, though the adsorption alters the overall size of the microgels (see Supporting Information). At the same time, in a colloidal suspension, the measured zeta potential ζ drops from 4.5 ± 0.57 mV (pure ferrocyanide) to 2.6 ± 0.26 mV in the presence of ferricyanide. The counterion switching was performed by electrolysis experiments by application of a potential of 0.6 V (vs Ag/AgCl) to an initial solution of either 1 mM K4[Fe(CN)6] in the presence of 6.3 g/L P(NIPAM-co-MAPTAC) microgel (0.025 M KCl; see Supporting Information) or to the double concentrated solution 2 mM K4[Fe(CN)6] in the presence of 12.5 g/L P(NIPAM-co-MAPTAC) microgel (0.05 M KCl) at ∼37 °C and icr = 1 in the half cell containing the working and reference electrode. The influence of the counterion switching on the microgel size is given in Figure 3 according to Scheme 1. One can observe a decrease in microgel size with a gradually increasing amount of the trivalent counterion [Fe(CN)6]3− due to varying complexation abilities as already indicated by Figure 1. During the stepwise oxidation process, approximately 170 mC were transferred per step, and the process was repeated 9 times. This leads to 20 nm reduction in hydrodynamic radius induced by electrochemical means, which is already about 40% of the total size change, which could be induced by temperature. In future work, we aim to optimize the system to achieve larger changes in size by varying the charge content and the structure of the microgels. After oxidation, a stepwise reduction was performed by application of a potential of 0 V (see Supporting Information) in order to investigate the reversibility of the swelling behavior. The reversibility of microgel size modulation by counterion switching is shown in Figure 4 illustrating the reversible microgel size decrease by enrichment of trivalent [Fe(CN)6]3−

Figure 3. Hydrodynamic radius vs open circuit potential (OCP, equilibrium potential; top)/transferred charge (bottom) during stepwise oxidation of an initial solution of 2 mM K4[Fe(CN)6] in 0.05 M KCl in the presence of 12.5 g/L P(NIPAM-co-MAPTAC) (icr = 1; 37 °C); applied potential, 0.6 V.

Figure 4. Hydrodynamic radius vs number of switches; initial solution of 1 mM K4[Fe(CN)6] in 0.025 M KCl in the presence of 6.3 g/L P(NIPAM-co-MAPTAC) microgel (icr = 1; 38 °C); oxidation potential, 0.6 V; reduction potential, 0 V; lines are a guide to the eye.

during the oxidation process and the reversible microgel swelling after subsequent reduction. C


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Chemistry of Materials If we consider the time scale of the oxidation and reduction processes (Supporting Information), one can state that the oxidation process is approximately 2 times faster than the reduction process (within the setup used, transfer of full 1.8 C takes approximately 30 min for oxidation and 60 min for reduction). However, major changes in size can be achieved already within 2 min of electrolysis and can be even shortened with, e.g., a proper engineering of the electrochemical cell. This envisions even a faster switching compared to the temperature induced trigger. The overall switching process can be accompanied by a more detailed electrochemical analysis. Hydrodynamic voltammetry was conducted at different states of the electrolysis with help of a rotating disk electrode, RDE (Figure 5). In a comparison of

Figure 6. Open circuit potentials (OCPs) against the transferred charge during the stepwise oxidation process in the presence of 12.5 g/ L P(NIPAM-co-MAPTAC (red ■) and absence of microgel (black ●); the difference of both OPCs is plotted as ΔOPC in gray ◇ (started with 2 mM K4[Fe(CN)6]; at 37 °C in 0.05 M KCl); lines are a guide to the eye.

complexing partner of the microgel. While the OPC is hardly altered upon microgel addition to a pure ferrocyanide solution, considerable effects can be seen for the pure ferricyanide solution upon microgel addition. Finally, we performed electrochemical impedance spectroscopy (EIS) at different switching states. EIS gives deeper mechanistic insight into the electron transfer processes (Supporting Information). Then, the electron transfer appears facilitated in the presence of the microgel. This is seen by the somewhat lower charge transfer resistance RCT compared to the one without microgel, especially in the presence of excess

Figure 5. Hydrodynamic voltammogramms in the presence of 12.5 g/ L P(NIPAM-co-MAPTAC) (dark colors) and absence of microgel (light colors) at different electrolysis states: 1 mM K4[Fe(CN)6], 1 mM K3[Fe(CN)6] (black/gray); ∼2 mM [Fe(CN)6]3− (green); ∼2 mM [Fe(CN)6]4− (blue) at 37 °C in 0.05 M KCl; 100 rpm at the rotating disk electrode.

the electrochemical response in the presence and absence of the microgel, it can be clearly observed that the microgel affects mostly the cathodic currents of the ferricyanide reduction, in agreement with the stronger complexation with ferricyanide. Only in excess of ferrocyanide, the (modulus of the) anodic currents of the ferrocyanide oxidation are reduced by microgel addition, while the anodic currents are hardly altered at an intermediate oxidation state. This indicates again the preferential interaction of ferricyanide with the microgel, which is only replaced from the microgel at an excess of ferrocyanide. Furthermore, the incorporated ferricyanides do not impose a very different own standard potential, which would otherwise have caused an intermediate shoulder in the voltammograms (mixture of two redox couples: one bound and one free couple). The proximity between the standard potentials is probably the main reason for this behavior, but also a contribution of “full entrapment” of the ferricyanides (which are not electrochemically accessible anymore in the time scale of the experiment) and of a “fast exchange” (compared to the time scale of the electrochemical experiment) between bound and unbound states cannot be excluded. However, the preferred incorporation of ferricyanide can be seen in a moderate negative shift of the equilibrium potential (which equals the open circuit potential OPC) upon addition of microgel at different oxidation/electrolysis states (Figure 6). This indicates a decrease of the ferricyanide activity upon complexation with the microgel: ferricyanide is the favorite

Figure 7. Fitting parameter RCT against the transferred charge during stepwise oxidation process of 2 mM K4[Fe(CN)6] in 0.05 M KCl; in presence (c(μG) = 12.5 g/L, icr = 1; red ■) and absence (black ●) of P(NIPAM-co-MAPTAC). The extremes (0 and −1.8 C) are the pure 2 mM K4[Fe(CN)6] or 2 mM K3[Fe(CN)6] containing solution in presence and absence of μG, respectively (without electrolysis); lines are a guide to the eye.

ferricyanide (right side of Figure 7). RCT can be written in first approximation45 R CT =

RT RT = n′+ α n′+ α 0 nFi0 n2F 2Akapp CO*[1 − n ]C R*[ n ]

(1)

with n as the total number of electrons transferred per electroactive species (n′ correlates to the number of electrons transferred before the rate-determining step), F as Faraday constant, A as electrode area (here the nominal area of 0.13 D


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Chemistry of Materials cm2), T as temperature, R as the gas constant, k0app as an apparent rate constant, C*O and C*R as the bulk concentrations of the redox-active partners (index “O” assigns the oxidizing agent, while “R” assigns the reducing agent), and α as the transfer coefficient (typically α is close to 0.5). Hence, n correlates approximately with the number of hexacyanoferrates within a microgel, which are still all electrochemically addressable by direct injection of electrons into such microgel/hexacyanoferrate complex. Such a scenario (“permanent entrapment”) is a limiting scenario together with two other scenarios: “fast exchange” (all hexacyanoferrates are electrochemically accessible by exchange in the time scale of experiment) and “full entrapment” (entrapped hexacyanoferrates are no longer accessible), according to the classification introduced in our previous paper.22 We can now assess the effect of the scenarios on RCT, starting with “permanent entrapment”. Upon incorporation of the hexacyanoferrates, the total number of electroactive, independently diffusing species is reduced (again in approximation, e.g., CO* ∼ 1/n). When n increases and still a rather symmetric reaction occurs (i.e., α close to 0.5; ratedetermining step rather at intermediate state of oxidation/ reduction of one electroactive species), then RCT scales with 1/ n at almost equal amounts of CO* and CR*. At an excess of one of the hexacyanoferrates and upon violation of the assumptions made, the RCT(n) scaling might deviate from this simple law. But in all cases, a reduction of RCT is expected upon a predominant direct multiple injection of electrons into the microgel/hexacyanoferrate complex (probably accompanied by electron hopping throughout the complex). In contrast, a predominant decrease in the electrochemical accessibility by hexacyanoferrate entrapment within the microgel would reduce CO* and CR*, leading to an increased RCT (“full entrapment”). Further, a predominant rapid exchange would hardly alter RCT (“fast exchange”). With this information, we can state that a contribution of multiple injection of electrons into the complex is present at the conditions used. Even more, this “permanent entrapment” scenario is mostly pronounced for the ferricyanide rich side, as indicated by a significant decrease in charge transfer resistance RCT upon microgel addition (20-fold, right side of Figure 7). This direct injection is facilitated in the presence of excess of ferricyanide probably due to a stronger binding of ferricyanide and therefore reduced distance between the redoxactive centers within the gel particles. Hence, one gains limited insight in the predominant electron pathway by using RCT, but an extraction of each contribution of mixed scenarios is more demanding. It is necessary to compare to other impedance data, like the Warburg coefficient. In analogy to RCT, a change in diffusivity can be seen when comparing the Warburg coefficient σ (Figure 8). It describes the diffusional processes and depends in the following way on the parameters σ=

RT 2 2 nFA 2

⎛ 1 1 ⎞ ⎜⎜ 1/2 + 1/2 ⎟⎟ DR C R* ⎠ ⎝ DO CO*

Figure 8. Warburg coefficient σ against the transferred charge during stepwise (−300 mC per step) oxidation process of 2 mM K4[Fe(CN)6] in 0.05 M KCl, in presence (c(μG) = 12.5 g/L, icr = 1; red squares) and absence (black circles) of P(NIPAM-coMAPTAC). The extremes (0 and −1.8 C) are the pure 2 mM K4[Fe(CN)6], or 2 mM K3[Fe(CN)6] containing solution in presence and absence of μG, respectively (without electrolysis); lines are a guide to the eye.

not dominated by the multiple electron injections into the microgels (“permanent entrapment” scenario). However, there is even a reduction of σ upon microgel addition at a substantial excess of ferricyanide (right side of Figure 8). This again corroborates the increased contribution of multiple transfer at these specific conditions. These investigations show that the interaction behavior of counterion and microgel changes and hence the electron pathway is influenced during the electrolysis from [Fe(CN)6]4− /microgel complex to [Fe(CN)6]3−/microgel complex. In contrast to the results of our previous investigation,22 the increased icr and the prevalence of ferricyanide (at full oxidation) promote the “permanent entrapment” scenario (multiple injection of electrons into the microgels).22 The increased amount of ferricyanide inside the microgels required for electron hopping is also corroborated by analysis of the supernatant after centrifugation of the microgel complex in order to determine the nominal charge ratios (ncr, which is defined as the molar ratio of the entrapped hexacyanoferrate charges in relation to the microgel charges; see Supporting Information). In a comparison to the previous case (more diluted case with hardly any direct electron injection into the microgel complex), we see now an increase of the ncr from approximately 0.1 to 0.3 at the conditions used in this investigation, accompanied by an increased concentration of ferricyanide within the microgels.

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CONCLUSION In conclusion, we have shown that microgels respond to an electrochemical stimulus with a fully reversible swelling and collapse behavior triggered by electrochemical switching of the redox-responsive guest molecules from reduced to oxidized state. Due to ion-specific effects, [Fe(CN)6]3−/microgel complexes exhibit smaller hydrodynamic radii Rh compared to the more extended [Fe(CN)6]4−/microgel complex. Full reversibility was proven by light scattering (3D-DLS). In addition, electrochemical techniques verify the hypothesis of a higher amount of incorporated trivalent [Fe(CN)6]3− into the microgel leading to a more collapsed microgel−counterion complex as compared to the tetravalent [Fe(CN)6]4− complex

(2) 45

with D as the respective diffusion coefficients. We have discussed the effects of the different scenarios on σ in our previous paper:22 only a dominant multiple injection of electrons into the microgel/counterion complex (“permanent entrapment”) would result in a reduced σ upon microgel addition. The other scenarios would lead to an increase in σ. In Figure 8, we see slightly increased σ in the presence of microgel at intermediate switching steps. The electron pathways are still E


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Volume Transitions I; Dušek, K., Ed.; Springer: Berlin, 1993; Vol. 109, pp 123−171. (12) Schneider, S.; Linse, P. Swelling of cross-linked polyelectrolyte gels. Eur. Phys. J. E: Soft Matter Biol. Phys. 2002, 8, 457−60. (13) Hertle, Y.; Zeiser, M.; Hasenöhrl, C.; Busch, P.; Hellweg, T. Responsive P(NIPAM-co-NtBAM) microgels: Flory−Rehner description of the swelling behaviour. Colloid Polym. Sci. 2010, 288, 1047− 1059. (14) Wedel, B.; Zeiser, M.; Hellweg, T. Non NIPAM Based Smart Microgels: Systematic Variation of the Volume Phase Transition Temperature by Copolymerization. Z. Phys. Chem. 2012, 226, 737− 748. (15) Katchalsky, A.; Michaeli, I. Polyelectrolyte gels in salt solutions. J. Polym. Sci. 1955, 15, 69−86. (16) Wilder, J.; Vilgis, T. A. Elasticity in strongly interacting soft solids: A polyelectrolyte network. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1998, 57, 6865−6874. (17) Frusawa, H.; Hayakawa, R. Swelling mechanism unique to charged gels: Primary formulation of the free energy. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1998, 58, 6145−6154. (18) Kobayashi, H.; Winkler, R. Structure of Microgels with Debye− Hückel Interactions. Polymers 2014, 6, 1602−1617. (19) Mann, B. A.; Holm, C.; Kremer, K. Swelling of polyelectrolyte networks. J. Chem. Phys. 2005, 122, 154903. (20) Schneider, S.; Linse, P. Monte Carlo simulation of defect-free cross-linked polyelectrolyte gels. J. Phys. Chem. B 2003, 107, 8030− 8040. (21) Tiwari, R.; Walther, A. Strong anionic polyelectrolyte microgels. Polym. Chem. 2015, 6, 5550−5554. (22) Mergel, O.; Gelissen, A. P. H.; Wünnemann, P.; Böker, A.; Simon, U.; Plamper, F. A. Selective Packaging of Ferricyanide within Thermoresponsive Microgels. J. Phys. Chem. C 2014, 118, 26199− 26211. (23) Wittemann, A.; Ballauff, M. Interaction of proteins with linear polyelectrolytes and spherical polyelectrolyte brushes in aqueous solution. Phys. Chem. Chem. Phys. 2006, 8, 5269−75. (24) Henzler, K.; Haupt, B.; Lauterbach, K.; Wittemann, A.; Borisov, O.; Ballauff, M. Adsorption of beta-lactoglobulin on spherical polyelectrolyte brushes: direct proof of counterion release by isothermal titration calorimetry. J. Am. Chem. Soc. 2010, 132, 3159−63. (25) Quesada-Pérez, M.; Maroto-Centeno, J. A.; Martín-Molina, A. Effect of the Counterion Valence on the Behavior of Thermo-Sensitive Gels and Microgels: A Monte Carlo Simulation Study. Macromolecules 2012, 45, 8872−8879. (26) Plamper, F. Changing Polymer Solvation by Electrochemical Means: Basics and Applications. In Porous Carbons−Hyperbranched Polymers−Polymer Solvation; Long, T. E., Voit, B., Okay, O., Eds.; Springer International Publishing: New York, 2015; Vol. 266, pp 125− 212. (27) Spruijt, E.; Choi, E.-Y.; Huck, W. T. S. Reversible Electrochemical Switching of Polyelectrolyte Brush Surface Energy Using Electroactive Counterions. Langmuir 2008, 24, 11253−11260. (28) Grieshaber, D.; Vörös, J.; Zambelli, T.; Ball, V.; Schaaf, P.; Voegel, J.-C.; Boulmedais, F. Swelling and Contraction of Ferrocyanide-Containing Polyelectrolyte Multilayers upon Application of an Electric Potential. Langmuir 2008, 24, 13668−13676. (29) Tatsuma, T.; Takada, K.; Matsui, H.; Oyama, N. A Redox Gel. Electrochemically Controllable Phase Transition and Thermally Controllable Electrochemistry. Macromolecules 1994, 27, 6687−6689. (30) Yoshida, R.; Takahashi, T.; Yamaguchi, T.; Ichijo, H. SelfOscillating Gel. J. Am. Chem. Soc. 1996, 118, 5134−5135. (31) Hempenius, M. A.; Cirmi, C.; Song, J.; Vancso, G. J. Synthesis of Poly(ferrocenylsilane) Polyelectrolyte Hydrogels with Redox Controlled Swelling. Macromolecules 2009, 42, 2324−2326. (32) Sigolaeva, L. V.; Gladyr, S. Y.; Gelissen, A. P. H.; Mergel, O.; Pergushov, D. V.; Kurochkin, I. N.; Plamper, F. A.; Richtering, W. Dual-Stimuli-Sensitive Microgels as a Tool for Stimulated Spongelike Adsorption of Biomaterials for Biosensor Applications. Biomacromolecules 2014, 15, 3735−3745.

and an increased contribution of the direct injection of electrons into the microgel−counterion complex.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02740. Experimental details, further SFM, and electrochemical analytics (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0049-241-80-94750. Fax: 0049-241-80-92327. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the funding of the German Research Foundation (DFG) within the collaborative research center SFB 985 (Functional Microgels and Microgel Systems). The authors thank also Walter Richtering and Andrij Pich for fruitful discussions and Arjan Gelissen and Ian Huang-Tsai for help in the preparation of the microgel.

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REFERENCES

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Microgel Size Modulation by Electrochemical Switching - Chemistry of

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