Selective Packaging of Ferricyanide within Thermoresponsive Microgels

Article pubs.acs.org/JPCC

Selective Packaging of Ferricyanide within Thermoresponsive Microgels Olga Mergel,† Arjan P. H. Gelissen,† Patrick Wünnemann,‡ Alexander Böker,‡ Ulrich Simon,§ and Felix A. Plamper*,† †

Institute of Physical Chemistry II, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany Lehrstuhl für Makromolekulare Materialien und Oberflächen, RWTH Aachen University, DWI - Leibniz Institut für Interaktive Materialien, Forckenbeckstraße 50, 52056 Aachen, Germany § Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany ‡

S Supporting Information *

ABSTRACT: This study effectively demonstrates that thermoresponsive, cationic poly(N-isopropylacrylamide-comethacrylamidopropyltrimethylammonium chloride) P(NIPAM-co-MAPTAC) microgels act as selective, closable carriers for trivalent hexacyanoferrate(III) (ferricyanide). At the same time, the microgel disregards even higher charged hexacyanoferrate(II) (ferrocyanide). This is seen by investigating the electrochemistry of hexacyanoferrates in the presence of porous microgel particles with help of cyclic voltammetry (CV), hydrodynamic voltammetry (rotating disk electrode, RDE), and electrochemical impedance spectroscopy (EIS). For analysis, temperature-corrected parameters for each technique are introduced. Assuming incorporation/complexation between hexacyanoferrates and microgels, different limiting scenarios for the electron pathway are proposed by discussing different life times of the hexacyanoferrates within the microgel: fast exchange (scenario 1: full electrochemical addressability of all counterions), permanent entrapment (scenario 2: still full addressability of all counterions by injection of electrons into the microgels), and full entrapment (scenario 3: only remaining free counterions are addressable). Also, negligible interaction between hexacyanoferrates and microgels can be postulated, as found experimentally for ferrocyanide [Fe(CN)6]4−. In contrast for ferricyanide [Fe(CN)6]3−, temperature even allows a switching between a dominant scenario 1 (fast exchange) in the cold and the scenario 3 (full entrapment) in the heat. In more detail, the attraction between ferricyanide and microgel is enhanced at elevated temperatures due to the collapse and increasing charge density induced by the thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) component, which in turn acts more as an insulator in the heat. Hence, only the free hexacyanoferrates are electrochemically accessible in the heat. In addition, EIS and CV indicate only a minor contribution of permanent entrapment (scenario 2) during charge transport. environment in a narrow temperature range around 32 °C, that is, close to body temperature.15 Above this temperature, called the lower critical solution temperature (LCST), the hydrogen bonding between amide groups of non-cross-linked linear PNIPAM and water molecules is broken and the polymer becomes more hydrophobic. Also, PNIPAM-based μGs are thermoresponsive and exhibit a characteristic volume phase transition temperature (VPTT) in conjunction with the LCST: the μGs show a reversible decrease in size by expelling water from the μG interior over a narrow temperature range. Additionally to competing van der Waals forces (polymer− polymer vs polymer−solvent interactions), the osmotic pressure and the cross-linking density, in other words, the

1. INTRODUCTION Stimulus responsive microgels (μGs) are nanometer-to-micrometer-sized soft and porous polymeric particles. They structurally respond to changes in the environment, such as changes in temperature, pH, ionic strength, solvent composition, or to external fields, such as light and electric fields.1−6 These stimuli-responsive “smart” polymers have rapidly gained importance in materials science owing to their potential applications in biomedical technologies, such as drug release systems,7−12 separation and purification technologies,13 or in sensor technology.2 The variety of μG-applications arises from their stimulus-responsive nature due to their ability to undergo reversible volume phase transitions (VPT) in response to environmental changes.2 One of the most investigated stimuli-sensitive polymers is poly(N-isopropylacrylamide) (PNIPAM),14 which exhibits an endothermic entropy-driven phase transition in aqueous © 2014 American Chemical Society

Received: August 28, 2014 Revised: October 8, 2014 Published: October 9, 2014 26199

dx.doi.org/10.1021/jp508711k | J. Phys. Chem. C 2014, 118, 26199−26211

The Journal of Physical Chemistry C

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network elasticity, play an essential role in the volume transition behavior of μGs. The VPTT thus depends on the μG composition and can be adjusted by introducing a suitable comonomer, whereby ionic comonomers often increase the VPTT compared to pure PNIPAM (32 °C), due to increased hydrophilicity of the polymer. A polyelectrolyte μG consists of a charged network with partially confined (monovalent) counterions inside the gel. In this work, we introduced permanent charges by copolymerizing NIPAM with N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA) and by consecutive quaternizing the DMAPMA units (leading to methacrylamidopropyltrimethylammonium chloride units − MAPTAC, also known as (3-methacrylamidoN,N,N-trimetylpropan-1-ammonium) chloride MATPAC). Then, a dry polyelectrolyte gel can swell 100- to 1000-fold by absorbing water.16 The large swelling of polyelectrolyte gels is ascribed mostly to the osmotic pressure arising from the electrostatically confined small ions located in the interior of the gel and to the effective repulsive electrostatic interactions between the charged groups.17−20 This can lead to a suppression of the thermosensitive properties at a high degree of ionization.21 These charged polyelectrolyte μGs can also interact with oppositely charged multivalent counterions leading to the formation of μG-counterion complexes.22−26 The incorporation of the multivalent counterions into the charged polyelectrolyte μG leads to a release of a concomitant number of monovalent counterions into a surrounding reservoir with low salt concentration accompanied by a significant gain of entropy.27 Polyelectrolytes may take up large amounts of multivalent counterions when the ionic strength is low and the absorption becomes weaker with increasing ionic strength.27,28 Hence, this complex formation is favored by entropic contributions accompanied by a decrease in osmotic pressure leading to a collapse of the μG.29 Recently, a variety of polymer films consisting of the temperature-responsive PNIPAM grafted on electrode surfaces in combination with the redox-responsive ferri-/ferrocyanide couple was extensively studied by electrochemical means, combining the temperature sensitivity originating from PNIPAM on the one hand with the redox-sensitivity of the mobile ferri-/ferrocyanide counterions on the other hand.30,31 Especially the temperature-induced influence of PNIPAMbrushes and PNIPAM-based ultrathin films (UTFs) on the diffusion of the redox probe has been investigated.31,30 Charged deposited polymer systems were investigated by electrochemical means in combination with the redox couple.32 Also, the ion transport properties of ferricyanide through a polyelectrolyte multilayer were studied as a function of temperature and salt concentration.32 It was found that at low salt concentrations the diffusion coefficients through the multilayer are significantly higher for ferricyanide compared to ferrocyanide. Furthermore, electrochemically induced complexation between linear strong polyelectrolytes and hexacyanoferrates (i.e., the redox couple hexacyanoferrate(III)/hexacyanoferrate(II) consisting of trivalent ferricyanide [FeIII(CN)6]3− and tetravalent ferrocyanide [FeII(CN)6]4−, respectively, Scheme 1) was already investigated in solution and at interfaces by Anson during the 1990s. Cationic polysiloxane/ferricyanide complexes become insoluble by anodic oxidation of ferrocyanide and can be electrodeposited from solutions of the ferrocyanide/ polycation complex as a thin film on the electrode surface,

Scheme 1. Schematic Illustration of the Cationic P(NIPAMco-MAPTAC) Microgel with Reversible TemperatureSensitive Collapse-Swelling Behavior and Selective Counterion Uptake of Reversibly Switchable Ferricyanide Leading to an Encapsulation of the Confined Counterions at Elevated Temperature

which can be redissolved again by applying the reduction potential to ferricyanide.33,34 In continuation, electrochemically induced micellization of star-shaped polyelectrolytes was achieved by changing the equilibrium potential accompanied by a conversion of ferricyanide to ferrocyanide due to different complexation behavior with the multivalent counterions.35 Thereby and in many other cases the hexacyanoferrate (HCF) redox couple exhibits an unexpected complexation behavior with strong cationic polyelectrolytes leading to a favored complexation of ferricyanide compared to ferrocyanide. The entropic contribution is supposed to be more pronounced in case of ferrocyanide due to the release of a higher amount of monovalent counterions by electrostatic attraction of the higher charged ferrocyanide. However, the observed stronger binding of ferricyanide reflects the dominance of enthalpic factors, such as solvation energies, hydrophobic interactions, and higher polarizability, which apparently favor the uptake of the less charged counterion. This leads to ion-specific effects, as seen by entropy measurements.36,37 As another example, the stronger adsorption ability of ferricyanide to imidazolium-based polycationic polymers leads to integrated electrochemical biosensors for in vivo neurochemical measurements, showing the potential of these polyelectrolyte−counterion complexes and their possible application as a glucose sensor in biomedicine.38 While the influence of polyelectrolytes in solutions and of polyelectrolyte-coated electrodes on the electrochemistry of the redox probe as well as the temperature-triggered manipulation of the electrochemical response of the redox probe were studied before, to the best of our knowledge the present study is the first on “smart” μGs, interacting with redox responsive counterions. The purpose of this contribution is to combine the influence of electrostatic interactions of polyelectrolytes with the temperature response and investigate the effect of the temperature-responsive polyelectrolyte μGs on the electrochemistry of the redox-responsive probe. In this work, temperature−responsive PNIPAM-based cationic polyelectrolyte μGs were fabricated by precipitation polymerization, which show a reversible collapse-swelling behavior by modulating the temperature in the range of 20− 60 °C. In addition, the accessibility and diffusivity of electrostatically attracted, redox-responsive counterions, as 26200



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ments ranged from 1 Hz to 100 kHz. Impedance data analysis was performed according to proper transfer function derivation and identification procedures, which involved complex nonlinear last-squares (CNLS) fitting based on the Marquardt− Levenberg algorithm using the CH Instruments Beta software.30 2.3. Dynamic Light Scattering (DLS). All experiments were performed on an ALV setup equipped with a 633 nm HeNe laser (JDS Uniphase, 35 mV), a goniometer (ALV, CGS8F), digital hardware correlator (ALV 5000), two avalanche photo diodes (PerkinElmer, SPCM-CD2969), a light scattering electronics (ALV, LSE-5003), an external programmable thermostat (Julabo F32), and an index-match-bath filled with toluene. Angle- and temperature-dependent measurements were recorded in pseudocross correlation mode varying the scattering angle from 30° to 140° at 10° intervals and variation of temperature in the range of 20 to 60 °C at 2 K intervals and measurement time of 60 s. The samples were highly diluted to avoid multiple scattering. For data evaluation, the first cumulant from second order cumulant fit was plotted against the squared length of the scattering vector q2. The data were fitted with a homogeneous linear regression, whereas the diffusion coefficient was extracted from the slope and the hydrodynamic radius Rh calculated by using the Stokes−Einstein equation. 2.4. Scanning Force Microscopy (SFM). The swelling behavior of the μGs was observed with liquid-cell AFM (Bruker Dimension Icon with MSCT tips; spring constant 0.1 N/m, resonant frequency 26−50 kHz) at room temperature, and 40 and 60 °C via Peak Force QNM. The custom-made liquid-cell was stabilized for ∼60 min on a temperature-controlled stage (ICONEC-V2-NOPOT, Bruker). For the liquid-cell experiments, 20 μL of an aqueous dispersion were spin-coated at 1500 rpm for 30 s onto a silica wafer, which was activated via plasma treatment for 10 s (Plasma Activate Flecto 10 USB, 100 W, 0.2 mbar). All other experimental techniques are listed in the Supporting Information.

ferri- and ferrocyanide, can be modified, owing to the contracted or expanded configuration of the polymer. The remarkable selective incorporation of ferricyanide in the μG interior as well as the tunable accessibility of these counterions by modulation of temperature is presented here. An enrichment of ferricyanide inside the μG was demonstrated at elevated temperatures leading to a full entrapment of the confined multivalent counterions at a final temperature of 60 °C. It is expected that the strategy presented here can be employed to fabricate a variety of intelligent materials and possible usage of these smart μG systems for selective ion uptake in purification technologies.39

2. EXPERIMENTAL SECTION 2.1. Chemicals and Microgel Preparation. The used reagents are summarized in the Supporting Information. In addition, a detailed experimental description of microgel synthesis is given there. 2.2. Experimental Techniques. Electrochemical measurements were performed on the CH Instruments Electrochemical Workstation Potentiostat CHI760D (Austin, Texas, U.S.A.). For rotating disk electrode measurements, the potentiostat was connected with a rotating ring disk electrode rotator (RRDE3A from ALS Japan). The experiments were carried out temperature-dependently from 20 to 60 °C in a conventional three-electrode setup in a water jacketed cell connected to a thermostat (Thermo Scientific Haake A28). A platinum (rotating) disk electrode, 4 mm disk diameter, was used as working electrode and an Ag/AgCl electrode stored in 1 M KCl served as reference electrode. All potentials in the text and figures are referred to the Ag/AgCl couple. Two kinds of counter electrodes have been used. On the one hand, a platinum gauze electrode for electrochemical impedance spectroscopy measurements and, on the other hand, a spirally platinum electrode, 23 cm, for cyclic voltammetry. Electrochemical experiments of a 1 mM K3[Fe(CN)6], 1 mM K4[Fe(CN)6] (1:1) mixture as redox probe in a supporting electrolyte solution of 0.1 M KCl were performed in presence and absence of the μG P(NIPAM-co-MAPTAC). The same stock solution was used for both the reference experiment (without μG) and the preparation of the μG containing dispersion. Before performing each measurement, the working electrode was polished first with 1 μm diamond and subsequently with 0.05 μm alumina polish, rinsed with water, and dried with a stream of argon. The solution was purged with Ar for 10 min to remove dissolved oxygen. 2.2.1. Cyclic Voltammetry (CV). Cyclic voltammetry measurements were performed by scanning the potential in the respective potential window (−0.2−0.6 V) at a scan rate of 500 or 5 mV s−1. The potential scan was preceded by a 2 s “conditioning” at the start potential value. The temperature was increased from 20 to 60 °C at 5 K intervals. 2.2.2. Rotating Disk Electrode (RDE). Hydrodynamic voltammograms were recorded by sweeping the potential in the range of −0.1 to 0.5 V versus Ag/AgCl at a scan rate of 5 mV s−1. The rotation rate was increased from 100 to 1000 rpm at 100 intervals, whereas the temperature intervals stay the same as in static voltammetry measurements. 2.2.3. Electrochemical Impedance Spectroscopy (EIS). The (dc) potential was held at the open circuit potential measured at each temperature, while a small oscillating voltage of 5 mV amplitude was applied (leading to an alternating current−ac− readout). The measuring frequency f used for EIS measure-

3. METHODS AND THEORY We employ different electrochemical techniques such as cyclic voltammetry (CV), hydrodynamic voltammetry (rotating ring electrode, RDE), and electrochemical impedance spectroscopy (EIS). Their evaluations are based on various equations, which again depend on the electrochemical reversibility of the system. We assume Nernstian behavior in almost all cases.40 The applicability of this assumption is shortly discussed in the Results and Discussion and in the Supporting Information. Then, we establish four possible scenarios, how the electrochemical properties of HCF can be affected by the presence of the μG. Hereby we address and discuss (a) electrostatic interaction as such, (b) exchange of free diffusing and confined species, and (c) the accessibility of the confined species. As we are investigating a temperature-responsive system and all electrochemical experiments were performed at different temperatures, the fundamental equations of CV, RDE, and EIS were corrected by all temperature-relevant parameters, to be able to consider only the influence of interaction, exchange, and accessibility of the redox probe uncoupled of the superimposed temperature effect. Hence, we estimate the trends in the temperature-corrected Warburg parameter σcorr (as obtained by EIS), the slope of the Levich plot mcorr (as obtained by use of an RDE), and the CV peak currents ipeak,corr for each of the scenarios. While mcorr gives 26201



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information about the interaction between μG and counterion as such, only the combination of σcorr and mcorr provides a resolution between exchange and accessibility. A more detailed derivation of the following equations is given in the Supporting Information. In summary, the Randles-Sevcik equation describing the peak currents ipeak,O(T) in CV yields corrected peak currents ipeak,O,corr upon regarding obvious temperaturedependent terms (like viscosity; as an example, index “O” assigns the oxidizing agent, while “R” assigns the reducing agent):41−43

Scheme 2. Schematic Illustration of the Modified Randles Circuit with Bulk Solution Resistance RS, the Charge Transfer Resistance RCT, the Constant Phase Element CPE (with parameters Q and q), and Warburg Impedance Zw Expressed with Admittance Term Y0a

ϖ assigns here the angular frequency regarding the oscillation of the electrode potential; j assigns the complex number j2 = −1.

a

i peak,O,corr = i peak,O(T ) · = n3/2 ·CO·

NA 6π ηsolvent(T ) v1/2 ·0.4463·F 3/2 A RO

σcorr = σ(T ) · (1)

with F as Faraday constant, n as number of electrons transferred per electroactive unit, A as the electrode area (here, the nominal area of 0.13 cm2), NA as Avogadro number, T as temperature, ηsolvent as dynamic viscosity of solvent, v as scan rate (e.g., 0.005 V/s), RO is the hydrodynamic radii of the electroactive species, and CO is the bulk concentration of electroactive species (superscript “0” assigns total bulk concentration as in, for example, CO0). The same procedure can be also applied for the Levich equation during RDE experiments, which will be the main technique to analyze the system together with EIS.40 The Levich equation describes the limiting currents ilimit(T) at the extremes of the potential scan at a rotating disk electrode (RDE as used in hydrodynamic voltammetry) with the angular velocity ω.44,45 As ilimit(T) scales linearly with ω1/2, we consider in the following the slope m(T) (actually its modulus) of a plot of ilimit(T) against ω1/2 (Levich plot). Again, upon correction with obvious temperature dependent terms (like T and η), we obtain a corrected Levich slope mcorr, which depends only on n, C, and R (and A). Hence, all changes in complexation upon temperature rise are reflected in mcorr: mO,corr = mO(T ) ·

=

F2 k ·NA T 3π ηsolvent(T )

1/2 RR1/2 ⎞ 1 ⎛ R0 ⎜ ⎟⎟ + ⎜ CR ⎠ n2 ·A ⎝ C0

(3)

In the following, we regard the four possible scenarios and discuss how the electrochemical properties of HCF can be affected by the presence of the μG. By extracting σcorr, mcorr, and ipeak,corr, we are able to elucidate the electron pathway within this complex μG/HCF mixture. We propose Scheme 3. These scenarios need to be discussed separately for ferricyanide and ferrocyanide in the Results and Discussion. As scenario 0, μG addition does not change the electrochemical properties of HCF due to negligible interaction of the cationic polymer and the anionic HCF. As alternative, HCF is attracted to the μG. Then we need to distinguish three limiting cases. As scenario 1 (“Fast Exchange”), there is preferential uptake of HCF into the μG, but still all HCF ions are electrochemically accessible due to rapid exchange of HCF ions on the time scale of the relevant electrode processes. In the case of prolonged lifetime of the ions inside the μG, there are two further alternatives. As scenario 2 (“Permanent Entrapment”), still all HCF are electrochemically accessible, though there is hardly any exchange between μG and bulk electrolyte. That means that there is direct electron injection into the μG, accompanied by μG-related exchange of monovalent and multivalent counterions to establish the new equilibrium. As a last alternative, the entrapped counterions are no longer addressable in scenario 3 (“Full Entrapment”), allowing the electron transfer only to/from the free HCFs. After introducing these possible limiting scenarios, we need to discuss their effect on the measured, averaged HCF hydrodynamic radius R and accessible concentration C of diffusing electroactive species, allowing the transfer of n electrons each. Scenario 0 does not change R, C, and n at all. Hence, also ipeak,corr, mcorr, and σcorr do not change with the addition of μG. Also, the properties resulting from scenarios 1 and 3, respectively, are rather obvious. Due to partial residence of HCF inside the μG in scenario 1, R will increase and ipeak,corr and mcorr will decrease, while σcorr increases. The same trend holds for scenario 3, as the accessible HCF concentration decreases. Hence, on first (qualitative) sight, scenario 1 and 3 are not distinguishable. This is in contrast to scenario 2, which is, however, more complicated (a more elaborate discussion is given in the Supporting Information). When assuming n being proportional to the average number of HCF per diffusing electroactive species (noninteracting redox sites),56 then C is proportional to 1/n, while R is approximately proportional to n.

2/3 1/6 (6π )2/3 ηsolvent ηsolut A · · = n·CO· 2/3 0.62·F (kT )2/3 ρ1/6 RO

(2)

with k as Boltzmann constant, ρ as density of solution (1.0 g/ mL), and ηsolut as dynamic viscosity of the dispersion. Finally, we adapted this procedure for the Warburg impedance Zw,46 using the Warburg coefficient σ(T),40 which is obtained by use of electrochemical impedance spectroscopy,47,48 assuming the validity of a modified Randles circuit.49 This equivalent circuit was also applied for other soft matter systems (Scheme 2).50−54 Within this modified description, the double layer capacitance was exchanged by a constant phase element (CPE).55 By fitting of the impedance data, we obtained the bulk solution resistance RS, the charge transfer resistance RCT, the parameter of the CPE Q (including its exponent q), and the “Warburg admittance” Y0, which is interconnected to the Warburg coefficient by σ = 1/(Y0·21/2). Hence, we can write for σcorr upon correcting the obvious temperature-dependent terms: 26202



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Scheme 3. Comparison of All Different Limiting Scenarios Possibly Occurring During the Electrochemistry of Redox-Active Ions in the Presence of Oppositely Charged Polyelectrolytesa

Possible changes (increase ↑, decrease ↓, remain constant →) in (a) n, number of electrons transferred per electroactive unit; (b) C, electrochemically addressable (bulk) concentration; and (c) R, hydrodynamic radius of the electroactive species (the macroion specifies a multiplecharged object like the oppositely charged μG). a

adjusting the pH leads to a noticeable increase in yield.57 The size of the μG depends on both pH and temperature. The pH response with the accompanied decrease in hydrodynamic radius Rh arises from deprotonation of the μG in alkaline environment. To generate permanent positive charges, the tertiary amine function of the DMAPMA was quaternized (Scheme 4) generating a strong cationic pH-independent polyelectrolyte μG. The verification of successful quaternization and the molar amount of 12 mol % MAPTAI (related to NIPAM, in good agreement with monomer feed ratio of 10 mol % DMAPMA) in the μG was obtained from 1H NMR spectrum. The successful ion exchange of iodide with chloride could be proven with elementary analysis. The thermosensitivity of the unquaternized P(NIPAM-coDMAPMA) and the quaternized μG P(NIPAM-co-MAPTAC) was investigated with DLS measurements (Figure 1). Heating above VPTT leads to decrease in size, as described above. The VPTT of pure NIPAM μGs (32 °C)58 is shifted to higher temperatures (VPTT = 40 °C) as a result of the hydrophilicity arising from permanent positively charged groups of P(NIPAM-co-MAPTAC) or protonated groups of the amine comonomer in case of the unquaternized μG.59 Also, the osmotic pressure of the entrapped counterions counteracts the temperature-induced collapse of the μG. However, the unquaternized and quaternized μG exhibit nearly the same hydrodynamic radii at low temperatures (Figure 1) due to an acidic environment (pH 6), leading to an almost full protonation of the unquaternized μG and, hence, to similar repulsion interaction among the charged amine groups. At elevated temperatures, the protonation equilibrium is influenced by the thermo-induced collapse, leading to a μG with a low number of charges in the collapsed state.60 Hence,

With this knowledge, we can derive that ipeak,corr hardly changes when only scenario 2 takes place. However, mcorr would scale with n−2/3. Under the prerequisite of multiple electron transfer, σcorr scales approximately like σcorr ∼ n−1/2. In summary, we can establish a kind of “character table” to distinguish between the different mechanisms and to extract the dominant electron pathway (Table 1). This table will be the basis for the evaluation shown below. Table 1. Influence of Microgel Addition on the Measured, Corrected Variables for the Different Scenarios (see Scheme 3)a scenario

0

1

2

3

ip,corr mcorr σcorr

→ → →

↓ ↓ ↑

→ ↓ ↓b

↓ ↓ ↑

b

a

ip,corr, corrected peak current (CV); mcorr, corrected slope of Levich plot (RDE); σcorr, corrected Warburg parameter (EIS), with →, constant; ↑, increase; ↓, decrease. bAssuming instantaneous multielectron transfer per microgel.

4. RESULTS AND DISCUSSION 4.1. Microgel Synthesis and Characterization. Thermoand pH-responsive μGs P(NIPAM-co-DMAPMA) were synthesized by precipitation copolymerization of the monomers NIPAM and DMAPMA using a cationic initiator and adjusting the pH between 8 and 9. The μG synthesis was performed using the same copolymerization conditions as previously reported for thermo- and pH-responsive μGs P(NIPAM-coAPMH) (poly(N-isopropylamide-co-N-(3-aminopropyl)-methacrylamide HCl), bearing a primary amine function,29 whereas 26203



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Scheme 4. Schematic Reaction Equation for the Quaternization of the Tertiary Amine Function of DMAPMA to MAPTAI with Methyl Iodide MI in the Presence of the Base Potassium Bicarbonate

support with increasing temperature. The height profile plot (Figure 2) confirms the assumed shapes of adsorbed μGs namely the truncated spheres at all temperatures. The volume of absorbed μG changed by a factor of ∼2.4 upon exceeding the VPTT, while the deswelling degree of dispersed μG determined by means of dynamic light scattering exhibit a factor of ∼2.8. The degree of dewelling in the adsorbed state is presumably reduced due to the interaction with the substrate and is in good agreement with comparable μG systems.61 A part of the μG is immobilized due to firm adhesion resulting from electrostatic attraction of the oppositely charged μG and substrate and therefore unable to undergo swelling/deswelling. While strong (poly-)ionic interactions lead to a decrease of swelling capacity by an order of magnitude,62 weak, short-range van der Waals and hydrogen bonding interactions exhibit only slightly reduced swelling capacities in the adsorbed state.63 Furthermore, the cross-linking density as well as the charge density of a μG also influence the strength of interaction between μG and substrate, that is, soft μGs (2 mol % cross-linker) absorbed on oppositely charged surfaces reveal a nonspherical pancake-like structure,62 whereas almost neutral and stiff μGs (10 mol % cross-linker) exhibit full spheres above the VPTT as a result of partial detachment from the substrate.64 However, in the present study the cross-linker and the positively charged moieties amount to 5 and 12 mol %, respectively, leading to a pancake-like structure of the absorbed μG. 4.2. Cyclic Voltammetry (CV). We prepared all samples with 1 mM K3[Fe(CN)6] and 1 mM K4[Fe(CN)6] in 0.1 M KCl. In presence of polymer, 25 g/L P(NIPAM-co-MAPTAC)

Figure 1. Hydrodynamic radius Rh against temperature obtained from dynamic light scattering measurements for unquaternized P(NIPAMco-DMAPMA) μG at pH ≈ 6 (purple triangles), quaternized P(NIPAM-co-MAPTAC) μG (red hexagons) in 0.1 M KCl.

the unquaternized μG P(NIPAM-co-DMAPMA) (containing pH-sensitive amine groups) has the ability to collapse stronger. In addition, scanning force microscopy (SFM) measurements of the quaternized μG have been performed, as only the quaternized P(NIPAM-co-MAPTAC) was used for the following electrochemical experiments. The μG was adsorbed on a negatively charged silica wafer. Figure 2 shows the SFM images of the same single absorbed μG and its temperature response below, at and above the VPTT revealing that the particles deswell laterally and vertically with respect to the solid

Figure 2. Scanning force microscopy image of several quaternized P(NIPAM-co-MAPTAC) μGs absorbed onto silica wafer in liquid state at 25 °C (left) of a single absorbed μG at different temperatures (middle) and average height profiles across the apex of the absorbed μG (right). 26204



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Figure 3. Cyclic voltammograms of 1 mM K3[Fe(CN)6], 1 mM K4[Fe(CN)6] in 0.1 M KCl in the presence (red, solid line; c(μG) = 25 g/L, icr = 0.5) and absence (black, dashed line) of P(NIPAM-co-MAPTAC) μG, scan rate v = 500 mV/s, at 20 °C (left) and corrected anodic and cathodic peak currents ipeak,corr of 1 mM K3[Fe(CN)6] (green squares), 1 mM K4[Fe(CN)6] (blue circles) in 0.1 M KCl in the presence of 25 g/L P(NIPAMco-MAPTAC), icr = 0.5 (filled symbols), and absence of μG (open symbols) as a function of temperature extracted from cyclic voltammograms; scan rate v = 500 mV/s; dotted lines: theoretical ipeak,corr by taking R([Fe(CN)6]3−) = 0.31 nm, R([Fe(CN)6]4−) = 0.38 nm, n = 1, C([Fe(CN)6]3−) = 1.0 mmol/L, C([Fe(CN)6]4−) = 1.0 mmol/L and Anom = 12.6 × 10−6 m2, right).

Figure 4. Hydrodynamic voltammogram of 1 mM K3[Fe(CN)6], 1 mM K4[Fe(CN)6] in 0.1 M KCl in the presence of 25 g/L P(NIPAM-coMAPTAC) μG, icr = 0.5 (right), and absence of μG (left) at T = 20 °C, scan rate v = 5 mV/s, rotation rates, ω, 100−1000 rpm with 100 rpm intervals at a Pt RDE.

μG was added, leading to an initial charge-to-charge ratio (icr) of 0.5 (icr is defined as the molar ratio of the total HCF charges compared to the μG charges). We will first consider the voltammetric response, which was performed in the unstirred state. Figure 3 shows the cyclic voltammograms of the ferri/ ferrocyanide redox couple in absence and presence of the temperature-sensitive cationic polyelectrolyte μG P(NIPAM-coMAPTAC) at a certain temperature below the VPTT as well as the corrected peak currents over the entire temperature range (20−60 °C) and the influence of the thermoresponsive μG on the electrochemistry of HCF. The presence of μG provokes a decrease in peak currents especially above the VPTT (Figure 3) due to an increased interaction of the redox active counterions with the collapsing μG. Taking into account a more detailed CV discussion in the Supporting Information, we consider here only the major points. The peak currents ipeak are reduced upon μG addition. This indicates a preferred interaction between HCF and μG in favor of limiting scenario 1 or 3 (see Scheme 3). The average mobility/addressability of the counterions, which are partly located inside the μG, is consequently diminished. The difference between HCF solution in absence and solution in the presence of μG is even more pronounced at elevated temperatures (see Figure 3), indicating an even stronger attraction between HCF and μG.65 This data suggests a rather minor contribution of scenario 2 to the overall electron

pathway. However, there are still some indications of residual contributions of scenario 2 (especially at low temperature). Besides the results of EIS (see below), a careful look on the peak separations ΔE point out that the peaks are less separated in the presence of μG than in absence of μG. Hence, the electrochemical reversibility seems to be slightly increased in the presence of μG, indicating some μG-facilitated multiple electron transfer from/to one electroactive species (here a HCF/μG complex). Thus, a subordinated electron transfer mechanism might be present (scenario 2). Here, the μG would serve as mediator for a small contribution of multiple electron transfer directly into the μG, which is probably accompanied by electron hopping within this HCF/μG complex.66 In sum, this facile hopping/multiple transfer promotes an apparent ease of electron transfer, as seen by a slightly decreased ΔE. 4.3. Hydrodynamic Voltammetry (Rotating Disk Electrode RDE). In contrast to the power of CV to obtain qualitative information, hydrodynamic voltammetry is a very suitable method to extract quantitative information, that is, to determine the diffusion properties of ferri- and ferrocyanide separately. Ferricyanide is, for example, reversibly reduced to ferrocyanide at the RDE, which is also seen in linear Levich plots. Temperature- and angular velocity-dependent measurements for both the pure redox couple and the redox couple in the presence of the cationic polyelectrolyte μG P(NIPAM-coMAPTAC) have been performed to determine the apparent 26205



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Figure 5. Calculated apparent diffusion coefficients from Levich equation (left, assuming full electrochemical accessibility: C0 stays constant) and corrected Levich slope (right: Anom = 12.6 × 10−6 m2; dashed lines, theoretical mcorr, in the absence of μG, by taking R([Fe(CN)6]3−) = 0.31 nm, R([Fe(CN)6]4−) = 0.38 nm, n = 1, C([Fe(CN)6]3−) = 1.0 mmol/L and C([Fe(CN)6]4−) = 1.0 mmol/L)) for ferricyanide K3[Fe(CN)6] (green squares) and ferrocyanide K4[Fe(CN)6] (blue circles) in the presence (c(μG) = 25 g/L, icr = 0.5; full symbols) and absence (open symbols) of P(NIPAM-co-MAPTAC) as a function of temperature in 0.1 M KCl.

diffusion coefficient as well as the Levich slope m in order to investigate the temperature-induced influence of the μG on the diffusivity/accessibility of the redox couple. An example of hydrodynamic voltammograms in the presence and absence of μG is given in Figure 4 for a measurement at 20 °C. The current−voltage plots are sigmoidal, as expected for transport by convection-diffusion under well-defined hydrodynamic conditions at a certain rotation rate of the rotating disk and increase with increasing rotation rate, because of facilitated mass transport to the electrode surface.40 The hydrodynamic voltammograms in absence of the μG exhibit higher limiting currents especially at −0.1 V compared to the limiting currents in the presence of the μG. The determined diffusion coefficients D(ferricyanide) = 7.3 × 10−6 cm2·s−1 and D(ferrocyanide) = 6.2 × 10−6 cm2·s−1 at 25 °C in absence of μG are in good agreement with the values reported in literature (D(ferricyanide) = 7.6 × 10−6 cm2·s−1 and D(ferrocyanide) = 6.5 × 10−6 cm2·s−1).40,67 The limiting current decreases in the presence of the μG about 57% for ferricyanide and only about 17% for ferrocyanide, indicating a preferable interaction of the trivalent ferricyanide counterion with the μG. The apparent diffusion coefficient Dapp is accessible from the limiting currents with the aid of the Levich equation. Dapp can be extracted (Figure 5) assuming no change in the electrochemical accessibility of the counterions (the overall bulk concentration CO0 equals the concentration CO of addressable counterions). The calculated diffusion coefficient represents an apparent diffusion coefficient composed of at least two kinds of redox species: on the one hand the entrapped counterions within the cationic μG (when still addressable) and on the other hand the mobile free diffusing counterions in solution, which are in equilibrium. While the diffusion of ferrocyanide seems to be unaffected by the presence of the μG, the diffusion of ferricyanide in contrast is noticeably decreased due to stronger electrostatic interaction of the trivalent counterion with the cationic moieties of the polyelectrolyte μG (as already indicated by reduced limiting currents at a potential of −0.1 V of the hydrodynamic voltammograms, Figure 4) and remains almost constant above the VPTT. To obtain a clearer picture, we used again the temperature correction for the Levich slope mcorr (see Methods and Theory). Here we do not assume constant accessibility, but we summarize all effects into a corrected Levich slope. Here it is important to perform a temperature correction of the (macroscopic) kinematic

viscosity, which enters the Levich equation due to a decreased convective mass transport in the presence of viscous media. Then it becomes also obvious that the viscosity term introduced by the Stokes−Einstein equation is just influenced by the microviscosity (temperature-dependent viscosity of pure solvent needs to be entered → HCF experience still normal Brownian diffusion). In addition, this procedure projects the ferrocyanide data in the presence of μG on the almost constant mcorr in the absence of μG (see Figure 5; deviation from constant mcorr might be caused by deviations from full electrochemical reversibility). Otherwise, by entering erroneously the macroscopic viscosity (which is increased upon polymer addition) into the Stokes−Einstein term, the mcorr plots of the polymer samples in Figure 5 would possess a much higher slope and also the ferrocyanide data would not superimpose. This indicates that diffusion of small entities is not obstructed by the presence of polymer unless complexation is considered. Even further, we can conclude that ferrocyanide is basically not interacting with the μG, neither at low nor at high temperature. Taking the pronounced reduction in mcorr for ferricyanide in the presence of μG, we can state that there is a strong discrimination between the two HCFs in respect of μG uptake. One might now be surprised that also the anodic currents during CV (Figure 3) are affected by the addition of μG, though the RDE results clearly indicate that there is hardly any interaction of ferrocyanide with the μG. But one needs to keep in mind that the decreased conversion of ferricyanide to ferrocyanide in the presence of μG also decreases the available amount of ferrocyanide used for the anodic peak at the otherwise depleted electrode (as one essential difference between CV and RDE, CV converts the same species for anodic and cathodic currents, though in RDE the species are always “fresh”, due to rotation induced convection). This is one reason why both peaks in CV are affected here. The result on preferential interaction of ferricyanide with the μG is corroborated by an independent determination of the amount of interacting counterions. As described in the Supporting Information, the supernatant of a microgel suspension was analyzed, finding only a reduced ferricyanide concentration in the bulk electrolyte. The ferrocyanide concentration in the supernatant was not affected after sedimentation of the microgel. Hence, the results obtained by RDE measurements are in line with these observations. 26206



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Figure 6. Nyquist diagram of 1 mM K3[Fe(CN)6], 1 mM K4[Fe(CN)6] in 0.1 M KCl in the absence (left) and presence of P(NIPAM-co-MAPTAC) microgel c(μG) = 25 g/L, icr = 0.5 (right); dc, open circuit potential; ac, 5 mV; and frequency range, 100 kHz to 1 Hz; full lines are fit data.

Figure 7. Fitting parameter Rct and corrected Warburg coefficient σcorr against temperature of 1 mM K3[Fe(CN)6], 1 mM K4[Fe(CN)6] in 0.1 M KCl, in the presence (c(μG) = 25 g/L; icr = 0.5; red hexagons) and absence (black asterisks) of P(NIPAM-co-MAPTAC) (Anom = 12.6 × 10−6 m2; right, black dashed line theoretical σcorr by taking R([Fe(CN)6]3−) = 0.31 nm, R([Fe(CN)6]4−) = 0.38 nm, n = 1, C([Fe(CN)6]3−) = 1.0 mmol/L, and C([Fe(CN)6]4−) = 1.0 mmol/L).

Figure 8. Fitting parameter Rs (left) and Q, q (right; q values indicated by open symbols) against the temperature of 1 mM K3[Fe(CN)6], 1 mM K4[Fe(CN)6] in 0.1 M KCl; in the presence (c(μG) = 25 g/L; icr = 0.5; red hexagons) and absence (black asterisks) of P(NIPAM-co-MAPTAC).

4.4. Electrochemical Impedance Spectroscopy (EIS). We performed impedance experiments to gain a deeper insight into the charge transport within these μG suspensions at varying temperatures and the possible blocking effect of the collapsed NIPAM above the VPTT resulting in counterion entrapment inside the gel. Figure 6 shows impedance spectra of the ferri-/ferrocyanide redox probe in presence and absence of the P(NIPAM-co-MAPTAC) μG. Nyquist diagrams exhibit a semicircle at high frequencies describing, for example, the kinetically controlled processes of the electron transfer followed by a straight line in low frequency part of the impedance data with a slope approaching −45°, which is also known as Warburg impedance describing the diffusional processes in bulk, which are mass-transfer controlled. At the frequencies

used (>1 Hz), the influence of any convection on the impedance spectra is still negligible (as would have been manifested in a bending of the low frequency Warburg tail toward the real axis). Thus, the kinetic electron transfer processes and the diffusional characteristics can be readout and discriminated from the other parameters influencing the impedance spectra. The intercept of the semicircle with the real part of the impedance at high frequencies gives the solution resistance, Rs, whereas the semicircle diameter corresponds to the electron transfer resistance, Rct. Information about the capacitance, Cdl, and accordingly about the double layer at the electrode surface can be calculated from the location of the maximum of the semicircle.40 From the Nyquist plot in Figure 6, one can easily declare the smaller semicircle diameter in the 26207



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Figure 9. Concentration C of addressable ferricyanide ions (left) and the average hydrodynamic radius R (right) of ferricyanide as obtained by combined evaluation of the RDE and EIS data (1 mM K3[Fe(CN)6], 1 mM K4[Fe(CN)6] in 0.1 M KCl, c(μG) = 25 g/L, icr = 0.5; green dashed line corresponds to the literature value of R([Fe(CN)6]3−).

presence of the μG due to faster/multiple electron transfer. Again, we suppose that the μG acts as a mediator favoring an electron hopping mechanism into the μG (permanent entrapment scenario 2). This mechanism apparently facilitates the electron transfer, which is in line with the reduced ΔE from CV as already discussed above. Here, we cannot exclude a minor contribution of this consecutive electron transfer/hopping (into the μG) on the apparent diffusion properties of the counterions. Its contribution might even increase for colloids with higher HCF loading due to higher charge density. However, the peak current from CV and the discussion below on the diffusion properties obtained by EIS indicate that this electron pathway is mainly resembled in the sensitive parameters Rct and ΔE. Otherwise, the changes in mcorr, ipeak,corr, and σcorr (see below) can be well described by changes in accessibility and hindered diffusion caused by preferential uptake into the μG. To obtain detailed information about the kinetic and diffusional processes at the electrode surface, the data were fitted with a modified Randles circuit shown in Scheme 2. The fitting parameters derived from a modified Randles circuit as described in the Methods and Theory are given in Figures 7 and 8. One interesting fact that could be extracted from the fitting parameters is that the solution resistance Rs as well as the charge transfer resistance Rct is lower in the presence of the μG probably due to a small contribution of multiple electron transfer/hopping into the μG/HCF complex. Furthermore, it could be observed that the rise in temperature induces a decrease in the barrier for interfacial electron transfer, leading to an enhancement of the kinetics of the charge transfer process65 evident in a decrease of both solution and charge transfer resistance in the absence of the μG (see Figures 7 and 8). The same trend of faster kinetics of the redox-sensitive redox couple was already reported for grafted PNIPAM brushes on Au electrodes30 and double hydroxide/PNIPAM ultrathin films.31 In contrast, the charge transfer resistance Rct in the presence of the P(NIPAM-co-MAPTAC) μG increases above the VPTT (Figure 7). The counterions located inside the μG become difficult to be accessed as the μG shrinks. In other words, the probability for multiple electron transfer/hopping is reduced and the collapsing PNIPAM acts more like an insulator. In addition, the incorporated counterions are not able to exchange with the free diffusing counterions located outside the μG anymore. The entrapment and reduction of accessibility of the

incorporated counterions leads to an increase of the charge transfer resistance. Taking into account the fit parameters of the CPE, we can conclude that the interface acts almost like an ideal double layer capacitance (q ≈ 1; see Supporting Information). Simultaneously, effects of polymer adsorption are very mild, as Q is always in the range observed in absence of polymer (especially in the heat). Finally, the “Warburg-admittance” Y0 could be extracted, which resembles the diffusional properties of the involved electroactive species. The admittance was transformed to the Warburg parameter σ and then corrected for temperature effects σcorr. As a result, we obtained a constant σcorr for the HCF pair in absence of polymer, which fits well to the literature data (dashed line in Figure 7). Hence, the fitting routine is deemed reliable in our case. In the presence of μG, σcorr is increased and increases further at elevated temperatures. This again indicates that the HCFs are attracted by the μG and that the injection of electrons into the μG is not the dominant electron pathway (as also seen by CV). However, a differentiation between the interactions of ferro- and ferricyanide cannot be achieved by EIS alone. Fortunately, the combination of RDE and EIS is very well suited to extract more information, as described below. 4.5. Discussion. As a starting point, hydrodynamic voltammetry gives the clear answer that there is an increased interaction of ferricyanide with the porous colloids as a decreasing mcorr is observed. In contrast, mcorr of ferrocyanide does not change with μG addition, favoring negligible interaction between the μG and the tetravalent counterions (scenario 0). This analysis is not only valid for the comparison of the polymer-free reference solution before μG addition and the polymer dispersion after μG addition. Also, for a temperature rise, a decrease in mcorr would without doubt indicate an enhanced interaction of HCF with the μG. Hence, the scenarios in Scheme 3 can be adapted directly for a temperature scan. Regarding ferrocyanide, there is hardly any change with temperature. Hence, ferrocyanide is not prone to complex the cationic μG under the investigated conditions, irrespective of the temperature. However, mcorr of ferricyanide shows a sigmoidal step, which indicates preferred complexation of ferricyanide at elevated temperatures. This preferential binding of ferricyanide over ferrocyanide to the microgels is attributed to the weaker polarizability of ferrocyanide and ion specific effects, such as possible charge transfer complex formation.33,68 26208



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temperature rise, the uptake is more pronounced and the average lifetime inside the μG increases, which is reflected in an increase of RO. Simultaneously, more entrapped counterions take part in this exchange upon temperature increase (maximum of CO). However, above a certain temperature, both RO and CO drop drastically, indicating a reduced electrochemical accessibility. At this point, parts of ferricyanide are firmly entrapped inside the μG. This insulation of the counterions in the interior of the microgel from the surrounding solution can be explained by two possible mechanisms. First, the reduction in mesh size aggravates the exchange of ions, leading not only to a rather permanent uptake of ferricyanide but also to a restricted exchange of other ions than ferricyanide. The latter is required for electroneutrality after electron transfer into the μG. Second, the collapsing PNIPAM could act as a real insulator, similarly to the conventional plastic-sheathed power lines. In sum, exchange is hindered and also electron injection into the μG is not the dominant electron pathway in this state (as already discussed above). In the end, only the freely diffusing counterions take part in the electrochemical reaction, while the RO approaches the one of free ferricyanide. We can conclude that 60% of the ferricyanide is fully entrapped inside the μG at 60 °C.

After having seen the increased interaction of ferricyanide with the μG (especially at elevated temperature), the difficult question remains, whether one of the scenarios 1 or 3 can be preferred for ferricyanide (or whether we even have a mixed mode). Knowing the negligible interaction of ferrocyanide with the μG, we can use the original concentration of ferrocyanide (1 mM) and its hydrodynamic radius as part of the second term in σcorr, suggesting its minor contribution to overall σcorr. The increase in σcorr is then directly explained by either an increase of the apparent R0 or by a decrease in C0 of the ferricyanide. Having now two equations with two unknown variables (assuming n = 1), A R O2/3

(2)

1/2 RR1/2 ⎞ 1 ⎛ RO ⎟⎟ ⎜ + ⎜ CR ⎠ n 2 · A ⎝ CO

(3)

mO,corr = n·CO·

σcorr =

we can now extract the RO and CO for ferricyanide at each temperature. However, one needs to be aware that already small errors in σcorr or mO,corr might produce larger deviations in RO and CO due to the power-law dependencies. For this reason we smoothed the experimental data by sigmoidal fitting (for mO,corr) or an arithmetic average of two linear regressions (intersecting as indicated in Figure 7) and polynomial fitting (third degree for σcorr). No matter whether or how the smoothing is performed, we see a maximum both especially for RO between 45 and 50 °C (Figure 9). Though the absolute values might be error prone, the trends are clear: RO increases due to increased interaction upon approaching the VPTT. At a certain point, the trivalent counterions are entrapped, which leads to a pronounced reduction of their electrochemical accessibility. Even more, the final RO is then very much in the range of the literature value of ferricyanide, indicating the packing and shielding of these ions inside the μG (ferricyanide “parcels” close at ∼60 °C). CO stays at approximately (0.6 ± 0.05) mmol/L up to 50 °C, before in a rather small temperature window CO drops to 0.4 mmol/L. This is in line with the centrifugation results (Supporting Information). Upon increase in temperature, there is actually also a small increase in electrochemical accessibility until 45 °C, as the CO(ferricyanide) shows a maximum. This does not conflict with the centrifugation results, as centrifugation only detects how many counterions are entrapped inside the μG. In contrast, Figure 9 accounts also for an increased exchange between inside and outside the μG up to a certain temperature, leading to a slight increase in the number of addressable counterions up to 45 °C. Still, the interaction of the ferricyanide with the μG increases with increasing temperature, which is reflected in an increasing averaged RO. At low temperature, the RO is slightly higher than the one of pure ferricyanide. This indicates already that the counterions reside partly within the μGs. At the same time, a portion of the counterions can be exchanged, continuing their journey alone until they will be taken up again. These two populations of ferricyanide (exchanged and fully entrapped ions) within the microgels can also be explained by the inhomogeneous charge density distribution due to inhomogeneous cross-linking distribution.69 At the core of a microgel, the charge density is increased even at low temperature, whereas the periphery of the microgels has a lower charge density (at higher temperature, the charge density becomes more homogeneous). Upon

5. CONCLUSION The results indicate that a mixed scenario 1/3 (fast exchange of a part of ferricyanide in combination with full entrapment of another part of ferricyanide within the microgel) provide the main electron pathway at low temperature below the VPTT. The exchanged counterions are electrochemically addressable, while the fully entrapped counterions are inaccessible. This conclusion is resembled both in a reduced addressability and in an increased hydrodynamic radius of ferricyanide. The contribution of scenario 1 increases up to 45 °C, as the ferricyanide addressability and its averaged hydrodynamic radius exhibit a maximum. However, at even higher temperatures (above the VPTT), scenario 3 becomes the dominant electron pathway, while the entrapped ferricyanide ions are no longer electrochemically accessible. This indicates that the microgel collapse leads to a substantial increase in charge density, facilitating the condensation of multivalent ions. At the same time, the collapsed PNIPAM acts like an insulating, nonpermeable layer in the time scale of our experiments. These results were obtained by a combination of electrochemical impedance spectroscopy and hydrodynamic voltammetry, which operate at different overpotentials but at comparable time scales. The success of this combination is also caused by the negligible migration of both hexacyanoferrate and microgel. Hence, both methods give evidence for the presence of selective colloidal containers with switchable permeability toward both hexacyanoferrates and electrons. The latter, that is, the direct injection of electrons into the microgel, provides not a dominant electron pathway.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures on synthesis and characterization (like electrophoretic mobility); derivation of equations; discussion of scenario 2 and its effect on ipeak,corr, mcorr, and σcorr; effect of pH; NMR; elementary analysis; detailed discussion on the electrochemical reversibility at low and high scan rates and effect of convection in CV; additional information regarding RDE measurements including the determination of the sample 26209



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viscosity; independent determination of amount of entrapped counterions by centrifugation; additional EIS data including Bode plots and the “Warburg-admittance” Y0; comment on negligible migration; comment on the time scales of measurments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 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 (Sonderforschungsbereich SFB 985, Project A6; Functional Microgels and Microgel Systems). The authors thank also Walter Richtering, Wolfgang Schuhmann, and Andrij Pich for fruitful discussions and Ian Huang-Tsai for help in the preparation of the microgel.

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Selective Packaging of Ferricyanide within Thermoresponsive Microgels

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