selective packaging of ferricyanide within thermoresponsive microgels

Selective Packaging of Ferricyanide within ThermoresponsiveMicrogelsOlga Mergel,† Arjan P. H. Gelissen,† Patrick Wunnemann,‡ Alexander Boker,‡ Ulrich Simon,§

and Felix A. Plamper*,†

†Institute of Physical Chemistry II, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany‡Lehrstuhl fur Makromolekulare Materialien und Oberflachen, RWTH Aachen University, DWI - Leibniz Institut fur InteraktiveMaterialien, 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 thatthermoresponsive, cationic poly(N-isopropylacrylamide-co-methacrylamidopropyltrimethylammonium chloride) P-(NIPAM-co-MAPTAC) microgels act as selective, closablecarriers for trivalent hexacyanoferrate(III) (ferricyanide). Atthe same time, the microgel disregards even higher chargedhexacyanoferrate(II) (ferrocyanide). This is seen by inves-tigating the electrochemistry of hexacyanoferrates in thepresence of porous microgel particles with help of cyclicvoltammetry (CV), hydrodynamic voltammetry (rotating diskelectrode, RDE), and electrochemical impedance spectroscopy(EIS). For analysis, temperature-corrected parameters for eachtechnique are introduced. Assuming incorporation/complexation between hexacyanoferrates and microgels, different limitingscenarios 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 fulladdressability of all counterions by injection of electrons into the microgels), and full entrapment (scenario 3: only remainingfree counterions are addressable). Also, negligible interaction between hexacyanoferrates and microgels can be postulated, asfound 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 moredetail, the attraction between ferricyanide and microgel is enhanced at elevated temperatures due to the collapse and increasingcharge density induced by the thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) component, which in turn acts moreas an insulator in the heat. Hence, only the free hexacyanoferrates are electrochemically accessible in the heat. In addition, EISand CV indicate only a minor contribution of permanent entrapment (scenario 2) during charge transport.

1. INTRODUCTION

Stimulus responsive microgels (μGs) are nanometer-to-micro-meter-sized soft and porous polymeric particles. Theystructurally respond to changes in the environment, such aschanges in temperature, pH, ionic strength, solvent composi-tion, or to external fields, such as light and electric fields.1−6

These stimuli-responsive “smart” polymers have rapidly gainedimportance in materials science owing to their potentialapplications in biomedical technologies, such as drug releasesystems,7−12 separation and purification technologies,13 or insensor technology.2 The variety of μG-applications arises fromtheir stimulus-responsive nature due to their ability to undergoreversible volume phase transitions (VPT) in response toenvironmental changes.2

One of the most investigated stimuli-sensitive polymers ispoly(N-isopropylacrylamide) (PNIPAM),14 which exhibits anendothermic entropy-driven phase transition in aqueous

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), thehydrogen bonding between amide groups of non-cross-linkedlinear PNIPAM and water molecules is broken and the polymerbecomes more hydrophobic. Also, PNIPAM-based μGs arethermoresponsive and exhibit a characteristic volume phasetransition temperature (VPTT) in conjunction with the LCST:the μGs show a reversible decrease in size by expelling waterfrom the μG interior over a narrow temperature range.Additionally to competing van der Waals forces (polymer−polymer vs polymer−solvent interactions), the osmoticpressure and the cross-linking density, in other words, the

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

Article

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

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network elasticity, play an essential role in the volumetransition behavior of μGs. The VPTT thus depends on theμG composition and can be adjusted by introducing a suitablecomonomer, whereby ionic comonomers often increase theVPTT compared to pure PNIPAM (32 °C), due to increasedhydrophilicity of the polymer.A polyelectrolyte μG consists of a charged network with

partially confined (monovalent) counterions inside the gel. Inthis work, we introduced permanent charges by copolymerizingNIPAM with N-[3-(dimethylamino)propyl]methacrylamide(DMAPMA) and by consecutive quaternizing the DMAPMAunits (leading to methacrylamidopropyltrimethylammoniumchloride units − MAPTAC, also known as (3-methacrylamido-N,N,N-trimetylpropan-1-ammonium) chloride MATPAC).Then, a dry polyelectrolyte gel can swell 100- to 1000-fold byabsorbing water.16 The large swelling of polyelectrolyte gels isascribed mostly to the osmotic pressure arising from theelectrostatically confined small ions located in the interior ofthe gel and to the effective repulsive electrostatic interactionsbetween the charged groups.17−20 This can lead to asuppression of the thermosensitive properties at a high degreeof ionization.21

These charged polyelectrolyte μGs can also interact withoppositely charged multivalent counterions leading to theformation of μG-counterion complexes.22−26 The incorporationof the multivalent counterions into the charged polyelectrolyteμG leads to a release of a concomitant number of monovalentcounterions into a surrounding reservoir with low saltconcentration accompanied by a significant gain of entropy.27

Polyelectrolytes may take up large amounts of multivalentcounterions when the ionic strength is low and the absorptionbecomes weaker with increasing ionic strength.27,28 Hence, thiscomplex formation is favored by entropic contributionsaccompanied by a decrease in osmotic pressure leading to acollapse of the μG.29

Recently, a variety of polymer films consisting of thetemperature-responsive PNIPAM grafted on electrode surfacesin combination with the redox-responsive ferri-/ferrocyanidecouple was extensively studied by electrochemical means,combining the temperature sensitivity originating fromPNIPAM on the one hand with the redox-sensitivity of themobile ferri-/ferrocyanide counterions on the other hand.30,31

Especially the temperature-induced influence of PNIPAM-brushes and PNIPAM-based ultrathin films (UTFs) on thediffusion of the redox probe has been investigated.31,30 Chargeddeposited polymer systems were investigated by electro-chemical means in combination with the redox couple.32

Also, the ion transport properties of ferricyanide through apolyelectrolyte multilayer were studied as a function oftemperature and salt concentration.32 It was found that atlow salt concentrations the diffusion coefficients through themultilayer are significantly higher for ferricyanide compared toferrocyanide.Furthermore, electrochemically induced complexation be-

tween linear strong polyelectrolytes and hexacyanoferrates (i.e.,the redox couple hexacyanoferrate(III)/hexacyanoferrate(II)consisting of trivalent ferricyanide [FeIII(CN)6]

3− and tetrava-lent ferrocyanide [FeII(CN)6]

4−, respectively, Scheme 1) wasalready investigated in solution and at interfaces by Ansonduring the 1990s. Cationic polysiloxane/ferricyanide complexesbecome insoluble by anodic oxidation of ferrocyanide and canbe electrodeposited from solutions of the ferrocyanide/polycation complex as a thin film on the electrode surface,

which can be redissolved again by applying the reductionpotential to ferricyanide.33,34 In continuation, electrochemicallyinduced micellization of star-shaped polyelectrolytes wasachieved by changing the equilibrium potential accompaniedby a conversion of ferricyanide to ferrocyanide due to differentcomplexation behavior with the multivalent counterions.35

Thereby and in many other cases the hexacyanoferrate (HCF)redox couple exhibits an unexpected complexation behaviorwith strong cationic polyelectrolytes leading to a favoredcomplexation of ferricyanide compared to ferrocyanide. Theentropic contribution is supposed to be more pronounced incase of ferrocyanide due to the release of a higher amount ofmonovalent counterions by electrostatic attraction of the highercharged ferrocyanide. However, the observed stronger bindingof ferricyanide reflects the dominance of enthalpic factors, suchas solvation energies, hydrophobic interactions, and higherpolarizability, which apparently favor the uptake of the lesscharged counterion. This leads to ion-specific effects, as seen byentropy measurements.36,37 As another example, the strongeradsorption ability of ferricyanide to imidazolium-basedpolycationic polymers leads to integrated electrochemicalbiosensors for in vivo neurochemical measurements, showingthe potential of these polyelectrolyte−counterion complexesand their possible application as a glucose sensor inbiomedicine.38

While the influence of polyelectrolytes in solutions and ofpolyelectrolyte-coated electrodes on the electrochemistry of theredox probe as well as the temperature-triggered manipulationof the electrochemical response of the redox probe werestudied before, to the best of our knowledge the present studyis the first on “smart” μGs, interacting with redox responsivecounterions. The purpose of this contribution is to combine theinfluence of electrostatic interactions of polyelectrolytes withthe temperature response and investigate the effect of thetemperature-responsive polyelectrolyte μGs on the electro-chemistry of the redox-responsive probe.In this work, temperature−responsive PNIPAM-based

cationic polyelectrolyte μGs were fabricated by precipitationpolymerization, which show a reversible collapse-swellingbehavior by modulating the temperature in the range of 20−60 °C. In addition, the accessibility and diffusivity ofelectrostatically attracted, redox-responsive counterions, as

Scheme 1. Schematic Illustration of the Cationic P(NIPAM-co-MAPTAC) Microgel with Reversible Temperature-Sensitive Collapse-Swelling Behavior and SelectiveCounterion Uptake of Reversibly Switchable FerricyanideLeading to an Encapsulation of the Confined Counterions atElevated Temperature

The Journal of Physical Chemistry C Article

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ferri- and ferrocyanide, can be modified, owing to thecontracted or expanded configuration of the polymer. Theremarkable selective incorporation of ferricyanide in the μGinterior as well as the tunable accessibility of these counterionsby modulation of temperature is presented here. An enrichmentof ferricyanide inside the μG was demonstrated at elevatedtemperatures leading to a full entrapment of the confinedmultivalent counterions at a final temperature of 60 °C. It isexpected that the strategy presented here can be employed tofabricate a variety of intelligent materials and possible usage ofthese smart μG systems for selective ion uptake in purificationtechnologies.39

2. EXPERIMENTAL SECTION2.1. Chemicals and Microgel Preparation. The used

reagents are summarized in the Supporting Information. Inaddition, a detailed experimental description of microgelsynthesis is given there.2.2. Experimental Techniques. Electrochemical measure-

ments were performed on the CH Instruments ElectrochemicalWorkstation Potentiostat CHI760D (Austin, Texas, U.S.A.).For rotating disk electrode measurements, the potentiostat wasconnected with a rotating ring disk electrode rotator (RRDE-3A from ALS Japan). The experiments were carried outtemperature-dependently from 20 to 60 °C in a conventionalthree-electrode setup in a water jacketed cell connected to athermostat (Thermo Scientific Haake A28). A platinum(rotating) disk electrode, 4 mm disk diameter, was used asworking electrode and an Ag/AgCl electrode stored in 1 M KClserved as reference electrode. All potentials in the text andfigures are referred to the Ag/AgCl couple. Two kinds ofcounter electrodes have been used. On the one hand, aplatinum gauze electrode for electrochemical impedancespectroscopy measurements and, on the other hand, a spirallyplatinum electrode, 23 cm, for cyclic voltammetry. Electro-chemical experiments of a 1 mM K3[Fe(CN)6], 1 mMK4[Fe(CN)6] (1:1) mixture as redox probe in a supportingelectrolyte solution of 0.1 M KCl were performed in presenceand absence of the μG P(NIPAM-co-MAPTAC). The samestock solution was used for both the reference experiment(without μG) and the preparation of the μG containingdispersion. Before performing each measurement, the workingelectrode was polished first with 1 μm diamond andsubsequently with 0.05 μm alumina polish, rinsed with water,and dried with a stream of argon. The solution was purged withAr for 10 min to remove dissolved oxygen.2.2.1. Cyclic Voltammetry (CV). Cyclic voltammetry

measurements were performed by scanning the potential inthe respective potential window (−0.2−0.6 V) at a scan rate of500 or 5 mV s−1. The potential scan was preceded by a 2 s“conditioning” at the start potential value. The temperature wasincreased from 20 to 60 °C at 5 K intervals.2.2.2. Rotating Disk Electrode (RDE). Hydrodynamic

voltammograms were recorded by sweeping the potential inthe range of −0.1 to 0.5 V versus Ag/AgCl at a scan rate of 5mV s−1. The rotation rate was increased from 100 to 1000 rpmat 100 intervals, whereas the temperature intervals stay thesame as in static voltammetry measurements.2.2.3. Electrochemical Impedance Spectroscopy (EIS). The

(dc) potential was held at the open circuit potential measuredat each temperature, while a small oscillating voltage of 5 mVamplitude was applied (leading to an alternating current−ac−readout). The measuring frequency f used for EIS measure-

ments ranged from 1 Hz to 100 kHz. Impedance data analysiswas performed according to proper transfer function derivationand identification procedures, which involved complex non-linear last-squares (CNLS) fitting based on the Marquardt−Levenberg algorithm using the CH Instruments Betasoftware.30

2.3. Dynamic Light Scattering (DLS). All experimentswere performed on an ALV setup equipped with a 633 nmHeNe laser (JDS Uniphase, 35 mV), a goniometer (ALV, CGS-8F), digital hardware correlator (ALV 5000), two avalanchephoto diodes (PerkinElmer, SPCM-CD2969), a light scatteringelectronics (ALV, LSE-5003), an external programmablethermostat (Julabo F32), and an index-match-bath filled withtoluene. Angle- and temperature-dependent measurementswere recorded in pseudocross correlation mode varying thescattering angle from 30° to 140° at 10° intervals and variationof temperature in the range of 20 to 60 °C at 2 K intervals andmeasurement time of 60 s. The samples were highly diluted toavoid multiple scattering. For data evaluation, the first cumulantfrom second order cumulant fit was plotted against the squaredlength of the scattering vector q2. The data were fitted with ahomogeneous linear regression, whereas the diffusion coef-ficient was extracted from the slope and the hydrodynamicradius Rh calculated by using the Stokes−Einstein equation.

2.4. Scanning Force Microscopy (SFM). The swellingbehavior of the μGs was observed with liquid-cell AFM (BrukerDimension Icon with MSCT tips; spring constant 0.1 N/m,resonant frequency 26−50 kHz) at room temperature, and 40and 60 °C via Peak Force QNM. The custom-made liquid-cellwas stabilized for ∼60 min on a temperature-controlled stage(ICONEC-V2-NOPOT, Bruker). For the liquid-cell experi-ments, 20 μL of an aqueous dispersion were spin-coated at1500 rpm for 30 s onto a silica wafer, which was activated viaplasma treatment for 10 s (Plasma Activate Flecto 10 USB, 100W, 0.2 mbar).All other experimental techniques are listed in the

Supporting Information.

3. METHODS AND THEORYWe employ different electrochemical techniques such as cyclicvoltammetry (CV), hydrodynamic voltammetry (rotating ringelectrode, RDE), and electrochemical impedance spectroscopy(EIS). Their evaluations are based on various equations, whichagain depend on the electrochemical reversibility of the system.We assume Nernstian behavior in almost all cases.40 Theapplicability of this assumption is shortly discussed in theResults and Discussion and in the Supporting Information.Then, we establish four possible scenarios, how the

electrochemical properties of HCF can be affected by thepresence of the μG. Hereby we address and discuss (a)electrostatic interaction as such, (b) exchange of free diffusingand confined species, and (c) the accessibility of the confinedspecies. As we are investigating a temperature-responsivesystem and all electrochemical experiments were performedat different temperatures, the fundamental equations of CV,RDE, and EIS were corrected by all temperature-relevantparameters, to be able to consider only the influence ofinteraction, exchange, and accessibility of the redox probeuncoupled of the superimposed temperature effect. Hence, weestimate the trends in the temperature-corrected Warburgparameter σcorr (as obtained by EIS), the slope of the Levichplot mcorr (as obtained by use of an RDE), and the CV peakcurrents ipeak,corr for each of the scenarios. While mcorr gives



information about the interaction between μG and counterionas such, only the combination of σcorr and mcorr provides aresolution between exchange and accessibility. A more detailedderivation of the following equations is given in the SupportingInformation. In summary, the Randles-Sevcik equationdescribing the peak currents ipeak,O(T) in CV yields correctedpeak currents ipeak,O,corr upon regarding obvious temperature-dependent terms (like viscosity; as an example, index “O”assigns the oxidizing agent, while “R” assigns the reducingagent):41−43

π η= ·

· ·

= · ·

i i TN T

v F

n CAR

( )6 ( )

0.4463peak,O,corr peak,OA solvent1/2 3/2

3/2O

O (1)

with F as Faraday constant, n as number of electrons transferredper electroactive unit, A as the electrode area (here, thenominal area of 0.13 cm2), NA as Avogadro number, T astemperature, ηsolvent as dynamic viscosity of solvent, v as scanrate (e.g., 0.005 V/s), RO is the hydrodynamic radii of theelectroactive species, and CO is the bulk concentration ofelectroactive species (superscript “0” assigns total bulkconcentration as in, for example, CO

0).The same procedure can be also applied for the Levich

equation during RDE experiments, which will be the maintechnique to analyze the system together with EIS.40 TheLevich equation describes the limiting currents ilimit(T) at theextremes of the potential scan at a rotating disk electrode (RDEas used in hydrodynamic voltammetry) with the angularvelocity ω.44,45 As ilimit(T) scales linearly with ω1/2, we considerin the following the slope m(T) (actually its modulus) of a plotof ilimit(T) against ω1/2 (Levich plot). Again, upon correctionwith obvious temperature dependent terms (like T and η), weobtain a corrected Levich slope mcorr, which depends only on n,C, and R (and A). Hence, all changes in complexation upontemperature rise are reflected in mcorr:

π η η

ρ= ·

·· · = · ·m m T

F kTn C

AR

( )(6 )0.62 ( )O,corr O

2/3solvent2/3

2/3solut1/6

1/6 OO2/3

(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 spectrosco-py,47,48 assuming the validity of a modified Randles circuit.49

This equivalent circuit was also applied for other soft mattersystems (Scheme 2).50−54

Within this modified description, the double layercapacitance was exchanged by a constant phase element(CPE).55 By fitting of the impedance data, we obtained thebulk 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 theWarburg coefficient by σ = 1/(Y0·2

1/2). Hence, we can write forσcorr upon correcting the obvious temperature-dependentterms:

σ σπ η

= ··

=·

+⎛⎝⎜⎜

⎞⎠⎟⎟

TF

k N T T

n ARC

RC

( )3 ( )

1

corr

2

A solvent

201/2

0

R1/2

R (3)

In the following, we regard the four possible scenarios anddiscuss how the electrochemical properties of HCF can beaffected by the presence of the μG. By extracting σcorr, mcorr, andipeak,corr, we are able to elucidate the electron pathway withinthis 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 electrochemicalproperties of HCF due to negligible interaction of the cationicpolymer and the anionic HCF. As alternative, HCF is attractedto the μG. Then we need to distinguish three limiting cases. Asscenario 1 (“Fast Exchange”), there is preferential uptake ofHCF into the μG, but still all HCF ions are electrochemicallyaccessible due to rapid exchange of HCF ions on the time scaleof the relevant electrode processes. In the case of prolongedlifetime of the ions inside the μG, there are two furtheralternatives. As scenario 2 (“Permanent Entrapment”), still allHCF are electrochemically accessible, though there is hardlyany exchange between μG and bulk electrolyte. That meansthat there is direct electron injection into the μG, accompaniedby μG-related exchange of monovalent and multivalentcounterions to establish the new equilibrium. As a lastalternative, the entrapped counterions are no longer address-able in scenario 3 (“Full Entrapment”), allowing the electrontransfer only to/from the free HCFs.After introducing these possible limiting scenarios, we need

to discuss their effect on the measured, averaged HCFhydrodynamic radius R and accessible concentration C ofdiffusing electroactive species, allowing the transfer of nelectrons each. Scenario 0 does not change R, C, and n at all.Hence, also ipeak,corr, mcorr, and σcorr do not change with theaddition of μG. Also, the properties resulting from scenarios 1and 3, respectively, are rather obvious. Due to partial residenceof HCF inside the μG in scenario 1, R will increase and ipeak,corrand mcorr will decrease, while σcorr increases. The same trendholds for scenario 3, as the accessible HCF concentrationdecreases. Hence, on first (qualitative) sight, scenario 1 and 3are not distinguishable. This is in contrast to scenario 2, whichis, however, more complicated (a more elaborate discussion isgiven in the Supporting Information). When assuming n beingproportional to the average number of HCF per diffusingelectroactive species (noninteracting redox sites),56 then C isproportional to 1/n, while R is approximately proportional to n.

Scheme 2. Schematic Illustration of the Modified RandlesCircuit with Bulk Solution Resistance RS, the ChargeTransfer Resistance RCT, the Constant Phase Element CPE(with parameters Q and q), and Warburg Impedance ZwExpressed with Admittance Term Y0

a

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



With this knowledge, we can derive that ipeak,corr hardly changeswhen only scenario 2 takes place. However, mcorr would scalewith 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 thedominant electron pathway (Table 1). This table will be thebasis for the evaluation shown below.

4. RESULTS AND DISCUSSION4.1. Microgel Synthesis and Characterization. Thermo-

and pH-responsive μGs P(NIPAM-co-DMAPMA) weresynthesized by precipitation copolymerization of the monomersNIPAM and DMAPMA using a cationic initiator and adjustingthe pH between 8 and 9. The μG synthesis was performedusing the same copolymerization conditions as previouslyreported for thermo- and pH-responsive μGs P(NIPAM-co-APMH) (poly(N-isopropylamide-co-N-(3-aminopropyl)-meth-acrylamide HCl), bearing a primary amine function,29 whereas

adjusting the pH leads to a noticeable increase in yield.57 Thesize of the μG depends on both pH and temperature. The pHresponse with the accompanied decrease in hydrodynamicradius Rh arises from deprotonation of the μG in alkalineenvironment. To generate permanent positive charges, thetertiary amine function of the DMAPMA was quaternized(Scheme 4) generating a strong cationic pH-independentpolyelectrolyte μG.The verification of successful quaternization and the molar

amount of 12 mol % MAPTAI (related to NIPAM, in goodagreement with monomer feed ratio of 10 mol % DMAPMA)in the μG was obtained from 1H NMR spectrum. Thesuccessful ion exchange of iodide with chloride could be provenwith elementary analysis.The thermosensitivity of the unquaternized P(NIPAM-co-

DMAPMA) and the quaternized μG P(NIPAM-co-MAPTAC)was investigated with DLS measurements (Figure 1). Heatingabove VPTT leads to decrease in size, as described above. TheVPTT of pure NIPAM μGs (32 °C)58 is shifted to highertemperatures (VPTT = 40 °C) as a result of the hydrophilicityarising from permanent positively charged groups of P-(NIPAM-co-MAPTAC) or protonated groups of the aminecomonomer in case of the unquaternized μG.59 Also, theosmotic pressure of the entrapped counterions counteracts thetemperature-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 analmost full protonation of the unquaternized μG and, hence, tosimilar repulsion interaction among the charged amine groups.At elevated temperatures, the protonation equilibrium isinfluenced by the thermo-induced collapse, leading to a μGwith a low number of charges in the collapsed state.60 Hence,

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

aPossible 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 multiple-charged object like the oppositely charged μG).

Table 1. Influence of Microgel Addition on the Measured,Corrected Variables for the Different Scenarios (see Scheme3)a

scenario 0 1 2 3

ip,corr → ↓ →b ↓mcorr → ↓ ↓ ↓σcorr → ↑ ↓b ↑

aip,corr, corrected peak current (CV); mcorr, corrected slope of Levichplot (RDE); σcorr, corrected Warburg parameter (EIS), with →,constant; ↑, increase; ↓, decrease. bAssuming instantaneous multi-electron transfer per microgel.



the unquaternized μG P(NIPAM-co-DMAPMA) (containingpH-sensitive amine groups) has the ability to collapse stronger.In addition, scanning force microscopy (SFM) measurementsof the quaternized μG have been performed, as only thequaternized P(NIPAM-co-MAPTAC) was used for thefollowing electrochemical experiments. The μG was adsorbedon a negatively charged silica wafer. Figure 2 shows the SFMimages of the same single absorbed μG and its temperatureresponse below, at and above the VPTT revealing that theparticles deswell laterally and vertically with respect to the solid

support with increasing temperature. The height profile plot(Figure 2) confirms the assumed shapes of adsorbed μGsnamely the truncated spheres at all temperatures. The volumeof absorbed μG changed by a factor of ∼2.4 upon exceeding theVPTT, while the deswelling degree of dispersed μG determinedby means of dynamic light scattering exhibit a factor of ∼2.8.The degree of dewelling in the adsorbed state is presumablyreduced due to the interaction with the substrate and is in goodagreement with comparable μG systems.61 A part of the μG isimmobilized due to firm adhesion resulting from electrostaticattraction of the oppositely charged μG and substrate andtherefore unable to undergo swelling/deswelling. While strong(poly-)ionic interactions lead to a decrease of swelling capacityby an order of magnitude,62 weak, short-range van der Waalsand hydrogen bonding interactions exhibit only slightly reducedswelling capacities in the adsorbed state.63 Furthermore, thecross-linking density as well as the charge density of a μG alsoinfluence the strength of interaction between μG and substrate,that is, soft μGs (2 mol % cross-linker) absorbed on oppositelycharged 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 partialdetachment from the substrate.64 However, in the present studythe cross-linker and the positively charged moieties amount to5 and 12 mol %, respectively, leading to a pancake-like structureof the absorbed μG.

4.2. Cyclic Voltammetry (CV). We prepared all sampleswith 1 mM K3[Fe(CN)6] and 1 mM K4[Fe(CN)6] in 0.1 MKCl. In presence of polymer, 25 g/L P(NIPAM-co-MAPTAC)

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

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

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).



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 chargescompared to the μG charges). We will first consider thevoltammetric response, which was performed in the unstirredstate. Figure 3 shows the cyclic voltammograms of the ferri/ferrocyanide redox couple in absence and presence of thetemperature-sensitive cationic polyelectrolyte μG P(NIPAM-co-MAPTAC) at a certain temperature below the VPTT as well asthe corrected peak currents over the entire temperature range(20−60 °C) and the influence of the thermoresponsive μG onthe electrochemistry of HCF. The presence of μG provokes adecrease in peak currents especially above the VPTT (Figure 3)due to an increased interaction of the redox active counterionswith the collapsing μG.Taking into account a more detailed CV discussion in the

Supporting Information, we consider here only the majorpoints. The peak currents ipeak are reduced upon μG addition.This indicates a preferred interaction between HCF and μG infavor of limiting scenario 1 or 3 (see Scheme 3). The averagemobility/addressability of the counterions, which are partlylocated inside the μG, is consequently diminished. Thedifference between HCF solution in absence and solution inthe presence of μG is even more pronounced at elevatedtemperatures (see Figure 3), indicating an even strongerattraction between HCF and μG.65 This data suggests a ratherminor contribution of scenario 2 to the overall electron

pathway. However, there are still some indications of residualcontributions of scenario 2 (especially at low temperature).Besides the results of EIS (see below), a careful look on thepeak separations ΔE point out that the peaks are less separatedin the presence of μG than in absence of μG. Hence, theelectrochemical reversibility seems to be slightly increased inthe presence of μG, indicating some μG-facilitated multipleelectron transfer from/to one electroactive species (here aHCF/μG complex). Thus, a subordinated electron transfermechanism might be present (scenario 2). Here, the μG wouldserve as mediator for a small contribution of multiple electrontransfer directly into the μG, which is probably accompanied byelectron hopping within this HCF/μG complex.66 In sum, thisfacile hopping/multiple transfer promotes an apparent ease ofelectron transfer, as seen by a slightly decreased ΔE.

4.3. Hydrodynamic Voltammetry (Rotating DiskElectrode RDE). In contrast to the power of CV to obtainqualitative information, hydrodynamic voltammetry is a verysuitable method to extract quantitative information, that is, todetermine the diffusion properties of ferri- and ferrocyanideseparately. Ferricyanide is, for example, reversibly reduced toferrocyanide at the RDE, which is also seen in linear Levichplots. Temperature- and angular velocity-dependent measure-ments for both the pure redox couple and the redox couple inthe presence of the cationic polyelectrolyte μG P(NIPAM-co-MAPTAC) have been performed to determine the apparent

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 cathodicpeak 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(NIPAM-co-MAPTAC), icr = 0.5 (filled symbols), and absence of μG (open symbols) as a function of temperature extracted from cyclic voltammograms; scanrate 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−) = 1.0mmol/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-co-MAPTAC) 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 rpmintervals at a Pt RDE.



diffusion coefficient as well as the Levich slope m in order toinvestigate the temperature-induced influence of the μG on thediffusivity/accessibility of the redox couple. An example ofhydrodynamic voltammograms in the presence and absence ofμG is given in Figure 4 for a measurement at 20 °C. Thecurrent−voltage plots are sigmoidal, as expected for transportby convection-diffusion under well-defined hydrodynamicconditions at a certain rotation rate of the rotating disk andincrease with increasing rotation rate, because of facilitatedmass transport to the electrode surface.40 The hydrodynamicvoltammograms in absence of the μG exhibit higher limitingcurrents especially at −0.1 V compared to the limiting currentsin the presence of the μG. The determined diffusioncoefficients D(ferricyanide) = 7.3 × 10−6 cm2·s−1 andD(ferrocyanide) = 6.2 × 10−6 cm2·s−1 at 25 °C in absence ofμG 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 thepresence of the μG about 57% for ferricyanide and only about17% for ferrocyanide, indicating a preferable interaction of thetrivalent ferricyanide counterion with the μG.The apparent diffusion coefficient Dapp is accessible from the

limiting currents with the aid of the Levich equation. Dapp canbe extracted (Figure 5) assuming no change in the electro-chemical accessibility of the counterions (the overall bulkconcentration CO

0 equals the concentration CO of addressablecounterions). The calculated diffusion coefficient represents anapparent diffusion coefficient composed of at least two kinds ofredox species: on the one hand the entrapped counterionswithin the cationic μG (when still addressable) and on theother hand the mobile free diffusing counterions in solution,which are in equilibrium. While the diffusion of ferrocyanideseems to be unaffected by the presence of the μG, the diffusionof ferricyanide in contrast is noticeably decreased due tostronger electrostatic interaction of the trivalent counterionwith the cationic moieties of the polyelectrolyte μG (as alreadyindicated by reduced limiting currents at a potential of −0.1 Vof the hydrodynamic voltammograms, Figure 4) and remainsalmost constant above the VPTT. To obtain a clearer picture,we used again the temperature correction for the Levich slopemcorr (see Methods and Theory). Here we do not assumeconstant accessibility, but we summarize all effects into acorrected Levich slope. Here it is important to perform atemperature correction of the (macroscopic) kinematic

viscosity, which enters the Levich equation due to a decreasedconvective mass transport in the presence of viscous media.Then it becomes also obvious that the viscosity termintroduced by the Stokes−Einstein equation is just influencedby the microviscosity (temperature-dependent viscosity of puresolvent needs to be entered → HCF experience still normalBrownian diffusion). In addition, this procedure projects theferrocyanide data in the presence of μG on the almost constantmcorr in the absence of μG (see Figure 5; deviation fromconstant mcorr might be caused by deviations from fullelectrochemical reversibility). Otherwise, by entering erro-neously the macroscopic viscosity (which is increased uponpolymer addition) into the Stokes−Einstein term, the mcorr

plots of the polymer samples in Figure 5 would possess a muchhigher slope and also the ferrocyanide data would notsuperimpose. This indicates that diffusion of small entities isnot obstructed by the presence of polymer unless complexationis considered. Even further, we can conclude that ferrocyanideis basically not interacting with the μG, neither at low nor athigh temperature. Taking the pronounced reduction in mcorr forferricyanide in the presence of μG, we can state that there is astrong discrimination between the two HCFs in respect of μGuptake. One might now be surprised that also the anodiccurrents during CV (Figure 3) are affected by the addition ofμG, though the RDE results clearly indicate that there is hardlyany interaction of ferrocyanide with the μG. But one needs tokeep in mind that the decreased conversion of ferricyanide toferrocyanide in the presence of μG also decreases the availableamount of ferrocyanide used for the anodic peak at theotherwise depleted electrode (as one essential differencebetween CV and RDE, CV converts the same species foranodic and cathodic currents, though in RDE the species arealways “fresh”, due to rotation induced convection). This is onereason why both peaks in CV are affected here. The result onpreferential interaction of ferricyanide with the μG iscorroborated by an independent determination of the amountof interacting counterions. As described in the SupportingInformation, the supernatant of a microgel suspension wasanalyzed, finding only a reduced ferricyanide concentration inthe bulk electrolyte. The ferrocyanide concentration in thesupernatant was not affected after sedimentation of themicrogel. Hence, the results obtained by RDE measurementsare in line with these observations.

Figure 5. Calculated apparent diffusion coefficients from Levich equation (left, assuming full electrochemical accessibility: C0 stays constant) andcorrected 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] (greensquares) 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) ofP(NIPAM-co-MAPTAC) as a function of temperature in 0.1 M KCl.



4.4. Electrochemical Impedance Spectroscopy (EIS).We performed impedance experiments to gain a deeper insightinto the charge transport within these μG suspensions atvarying temperatures and the possible blocking effect of thecollapsed NIPAM above the VPTT resulting in counterionentrapment inside the gel. Figure 6 shows impedance spectra ofthe ferri-/ferrocyanide redox probe in presence and absence ofthe P(NIPAM-co-MAPTAC) μG. Nyquist diagrams exhibit asemicircle at high frequencies describing, for example, thekinetically controlled processes of the electron transfer followedby a straight line in low frequency part of the impedance datawith a slope approaching −45°, which is also known asWarburg impedance describing the diffusional processes inbulk, which are mass-transfer controlled. At the frequencies

used (>1 Hz), the influence of any convection on theimpedance spectra is still negligible (as would have beenmanifested in a bending of the low frequency Warburg tailtoward the real axis). Thus, the kinetic electron transferprocesses and the diffusional characteristics can be readout anddiscriminated from the other parameters influencing theimpedance spectra. The intercept of the semicircle with thereal part of the impedance at high frequencies gives the solutionresistance, Rs, whereas the semicircle diameter corresponds tothe electron transfer resistance, Rct. Information about thecapacitance, Cdl, and accordingly about the double layer at theelectrode surface can be calculated from the location of themaximum of the semicircle.40 From the Nyquist plot in Figure6, one can easily declare the smaller semicircle diameter in the

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 MKCl, 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 mMK4[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).



presence of the μG due to faster/multiple electron transfer.Again, we suppose that the μG acts as a mediator favoring anelectron hopping mechanism into the μG (permanent entrap-ment scenario 2). This mechanism apparently facilitates theelectron transfer, which is in line with the reduced ΔE from CVas already discussed above. Here, we cannot exclude a minorcontribution of this consecutive electron transfer/hopping (intothe μG) on the apparent diffusion properties of thecounterions. Its contribution might even increase for colloidswith higher HCF loading due to higher charge density.However, the peak current from CV and the discussionbelow on the diffusion properties obtained by EIS indicate thatthis electron pathway is mainly resembled in the sensitiveparameters Rct and ΔE. Otherwise, the changes in mcorr, ipeak,corr,and σcorr (see below) can be well described by changes inaccessibility and hindered diffusion caused by preferentialuptake into the μG.To obtain detailed information about the kinetic and

diffusional processes at the electrode surface, the data werefitted with a modified Randles circuit shown in Scheme 2. Thefitting parameters derived from a modified Randles circuit asdescribed in the Methods and Theory are given in Figures 7and 8.One interesting fact that could be extracted from the fitting

parameters is that the solution resistance Rs as well as thecharge transfer resistance Rct is lower in the presence of the μGprobably due to a small contribution of multiple electrontransfer/hopping into the μG/HCF complex. Furthermore, itcould be observed that the rise in temperature induces adecrease in the barrier for interfacial electron transfer, leadingto an enhancement of the kinetics of the charge transferprocess65 evident in a decrease of both solution and chargetransfer resistance in the absence of the μG (see Figures 7 and8). The same trend of faster kinetics of the redox-sensitiveredox couple was already reported for grafted PNIPAM brusheson Au electrodes30 and double hydroxide/PNIPAM ultrathinfilms.31

In contrast, the charge transfer resistance Rct in the presenceof the P(NIPAM-co-MAPTAC) μG increases above the VPTT(Figure 7). The counterions located inside the μG becomedifficult to be accessed as the μG shrinks. In other words, theprobability for multiple electron transfer/hopping is reducedand the collapsing PNIPAM acts more like an insulator. Inaddition, the incorporated counterions are not able to exchangewith the free diffusing counterions located outside the μGanymore. The entrapment and reduction of accessibility of the

incorporated counterions leads to an increase of the chargetransfer resistance.Taking into account the fit parameters of the CPE, we can

conclude that the interface acts almost like an ideal double layercapacitance (q ≈ 1; see Supporting Information). Simulta-neously, effects of polymer adsorption are very mild, as Q isalways in the range observed in absence of polymer (especiallyin the heat).Finally, the “Warburg-admittance” Y0 could be extracted,

which resembles the diffusional properties of the involvedelectroactive species. The admittance was transformed to theWarburg parameter σ and then corrected for temperatureeffects σcorr. As a result, we obtained a constant σcorr for theHCF pair in absence of polymer, which fits well to the literaturedata (dashed line in Figure 7). Hence, the fitting routine isdeemed reliable in our case. In the presence of μG, σcorr isincreased and increases further at elevated temperatures. Thisagain indicates that the HCFs are attracted by the μG and thatthe injection of electrons into the μG is not the dominantelectron pathway (as also seen by CV). However, adifferentiation between the interactions of ferro- andferricyanide cannot be achieved by EIS alone. Fortunately,the combination of RDE and EIS is very well suited to extractmore information, as described below.

4.5. Discussion. As a starting point, hydrodynamicvoltammetry gives the clear answer that there is an increasedinteraction of ferricyanide with the porous colloids as adecreasing mcorr is observed. In contrast, mcorr of ferrocyanidedoes not change with μG addition, favoring negligibleinteraction between the μG and the tetravalent counterions(scenario 0). This analysis is not only valid for the comparisonof the polymer-free reference solution before μG addition andthe polymer dispersion after μG addition. Also, for atemperature rise, a decrease in mcorr would without doubtindicate an enhanced interaction of HCF with the μG. Hence,the scenarios in Scheme 3 can be adapted directly for atemperature scan. Regarding ferrocyanide, there is hardly anychange with temperature. Hence, ferrocyanide is not prone tocomplex the cationic μG under the investigated conditions,irrespective of the temperature. However, mcorr of ferricyanideshows a sigmoidal step, which indicates preferred complexationof ferricyanide at elevated temperatures. This preferentialbinding of ferricyanide over ferrocyanide to the microgels isattributed to the weaker polarizability of ferrocyanide and ionspecific effects, such as possible charge transfer complexformation.33,68

Figure 9. Concentration C of addressable ferricyanide ions (left) and the average hydrodynamic radius R (right) of ferricyanide as obtained bycombined 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 linecorresponds to the literature value of R([Fe(CN)6]

3−).



After having seen the increased interaction of ferricyanidewith the μG (especially at elevated temperature), the difficultquestion remains, whether one of the scenarios 1 or 3 can bepreferred for ferricyanide (or whether we even have a mixedmode). Knowing the negligible interaction of ferrocyanide withthe μG, we can use the original concentration of ferrocyanide(1 mM) and its hydrodynamic radius as part of the second termin σcorr, suggesting its minor contribution to overall σcorr. Theincrease in σcorr is then directly explained by either an increaseof the apparent R0 or by a decrease in C0 of the ferricyanide.Having now two equations with two unknown variables(assuming n = 1),

= · ·m n CA

RO,corr OO2/3

(2)

σ =·

+⎛⎝⎜⎜

⎞⎠⎟⎟n A

RC

RC

1corr 2

O1/2

O

R1/2

R (3)

we can now extract the RO and CO for ferricyanide at eachtemperature. However, one needs to be aware that alreadysmall errors in σcorr or mO,corr might produce larger deviations inRO and CO due to the power-law dependencies. For this reasonwe smoothed the experimental data by sigmoidal fitting (formO,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 thesmoothing is performed, we see a maximum both especially forRO between 45 and 50 °C (Figure 9). Though the absolutevalues might be error prone, the trends are clear: RO increasesdue to increased interaction upon approaching the VPTT. At acertain point, the trivalent counterions are entrapped, whichleads to a pronounced reduction of their electrochemicalaccessibility. Even more, the final RO is then very much in therange of the literature value of ferricyanide, indicating thepacking 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 smalltemperature window CO drops to 0.4 mmol/L. This is in linewith the centrifugation results (Supporting Information). Uponincrease in temperature, there is actually also a small increase inelectrochemical accessibi l i ty unti l 45 °C, as theCO(ferricyanide) shows a maximum. This does not conflictwith the centrifugation results, as centrifugation only detectshow many counterions are entrapped inside the μG. Incontrast, Figure 9 accounts also for an increased exchangebetween inside and outside the μG up to a certain temperature,leading to a slight increase in the number of addressablecounterions up to 45 °C. Still, the interaction of theferricyanide with the μG increases with increasing temperature,which is reflected in an increasing averaged RO. At lowtemperature, the RO is slightly higher than the one of pureferricyanide. This indicates already that the counterions residepartly within the μGs. At the same time, a portion of thecounterions can be exchanged, continuing their journey aloneuntil they will be taken up again. These two populations offerricyanide (exchanged and fully entrapped ions) within themicrogels can also be explained by the inhomogeneous chargedensity distribution due to inhomogeneous cross-linkingdistribution.69 At the core of a microgel, the charge density isincreased even at low temperature, whereas the periphery of themicrogels has a lower charge density (at higher temperature,the charge density becomes more homogeneous). Upon

temperature rise, the uptake is more pronounced and theaverage lifetime inside the μG increases, which is reflected in anincrease of RO. Simultaneously, more entrapped counterionstake part in this exchange upon temperature increase(maximum of CO). However, above a certain temperature,both RO and CO drop drastically, indicating a reducedelectrochemical accessibility. At this point, parts of ferricyanideare firmly entrapped inside the μG. This insulation of thecounterions in the interior of the microgel from thesurrounding solution can be explained by two possiblemechanisms. First, the reduction in mesh size aggravates theexchange of ions, leading not only to a rather permanent uptakeof ferricyanide but also to a restricted exchange of other ionsthan ferricyanide. The latter is required for electroneutralityafter electron transfer into the μG. Second, the collapsingPNIPAM could act as a real insulator, similarly to theconventional plastic-sheathed power lines. In sum, exchangeis hindered and also electron injection into the μG is not thedominant electron pathway in this state (as already discussedabove). In the end, only the freely diffusing counterions takepart in the electrochemical reaction, while the RO approachesthe one of free ferricyanide. We can conclude that 60% of theferricyanide is fully entrapped inside the μG at 60 °C.

5. CONCLUSIONThe results indicate that a mixed scenario 1/3 (fast exchange ofa part of ferricyanide in combination with full entrapment ofanother part of ferricyanide within the microgel) provide themain electron pathway at low temperature below the VPTT.The exchanged counterions are electrochemically addressable,while the fully entrapped counterions are inaccessible. Thisconclusion is resembled both in a reduced addressability and inan increased hydrodynamic radius of ferricyanide. Thecontribution of scenario 1 increases up to 45 °C, as theferricyanide addressability and its averaged hydrodynamicradius exhibit a maximum. However, at even higher temper-atures (above the VPTT), scenario 3 becomes the dominantelectron pathway, while the entrapped ferricyanide ions are nolonger electrochemically accessible. This indicates that themicrogel collapse leads to a substantial increase in chargedensity, facilitating the condensation of multivalent ions. At thesame time, the collapsed PNIPAM acts like an insulating,nonpermeable layer in the time scale of our experiments. Theseresults were obtained by a combination of electrochemicalimpedance spectroscopy and hydrodynamic voltammetry,which operate at different overpotentials but at comparabletime scales. The success of this combination is also caused bythe negligible migration of both hexacyanoferrate and microgel.Hence, both methods give evidence for the presence ofselective colloidal containers with switchable permeabilitytoward both hexacyanoferrates and electrons. The latter, thatis, the direct injection of electrons into the microgel, providesnot a dominant electron pathway.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental procedures on synthesis and characterization (likeelectrophoretic mobility); derivation of equations; discussion ofscenario 2 and its effect on ipeak,corr, mcorr, and σcorr; effect of pH;NMR; elementary analysis; detailed discussion on the electro-chemical reversibility at low and high scan rates and effect ofconvection in CV; additional information regarding RDEmeasurements including the determination of the sample



viscosity; independent determination of amount of entrappedcounterions by centrifugation; additional EIS data includingBode plots and the “Warburg-admittance” Y0; comment onnegligible migration; comment on the time scales of measur-ments. This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: 0049-241-80-94750. Fax: 0049-241-80-92327. E-mail:[email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors gratefully acknowledge the funding of the GermanResearch Foundation (DFG) within the collaborative researchcenter SFB 985 (Sonderforschungsbereich SFB 985, ProjectA6; Functional Microgels and Microgel Systems). The authorsthank also Walter Richtering, Wolfgang Schuhmann, and AndrijPich for fruitful discussions and Ian Huang-Tsai for help in thepreparation of the microgel.

■ REFERENCES(1) Pelton, R. Temperature-Sensitive Aqueous Microgels. Adv.Colloid Interface Sci. 2000, 85, 1−33.(2) Das, M.; Zhang, H.; Kumacheva, E. Microgels: Old Materials withNew Applications. Annu. Rev. Mater. Res. 2006, 36, 117−142.(3) Pich, A.; Richtering, W. Microgels by precipitation polymer-ization: synthesis, characterization, and functionalization. Adv. Polym.Sci. 2011, 234, 1−37.(4) Sawai, T.; Yamazaki, S.; Ishigami, Y.; Ikariyama, Y.; Aizawa, M.Electrical Control of Reversible Microgel Flocculation and ItsEstimated Performance as a Display Device. J. Electroanal. Chem.1992, 322, 1−7.(5) Fern, A. Motion of Microgel Particles under an External ElectricField. J. Phys.: Condens. Matter 2000, 12, 3605−3614.(6) Fernandez-Nieves, a; Marquez, M. Electrophoresis of IonicMicrogel Particles: From Charged Hard Spheres to Polyelectrolyte-Like Behavior. J. Chem. Phys. 2005, 122, 84702.(7) Nayak, S.; Lee, H.; Chmielewski, J.; Lyon, L. A. Folate-MediatedCell Targeting and Cytotoxicity Using Thermoresponsive Microgels. J.Am. Chem. Soc. 2004, 126, 10258−10259.(8) Nolan, C. M.; Reyes, C. D.; Debord, J. D.; García, A. J.; Lyon, L.A. Phase Transition Behavior, Protein Adsorption, and Cell AdhesionResistance of Poly(ethylene Glycol) Cross-Linked Microgel Particles.Biomacromolecules 2005, 6, 2032−2039.(9) Murthy, N.; Thng, Y. X.; Schuck, S.; Xu, M. C.; Frechet, J. M. J. ANovel Strategy for Encapsulation and Release of Proteins: Hydrogelsand Microgels with Acid-Labile Acetal Cross-Linkers. J. Am. Chem. Soc.2002, 124, 12398−12399.(10) Varma, M. V. S.; Kaushal, A. M.; Garg, S. Influence of Micro-Environmental pH on the Gel Layer Behavior and Release of a BasicDrug from Various Hydrophilic Matrices. J. Controlled Release 2005,103, 499−510.(11) Bromberg, L.; Alakhov, V. Effects of Polyether-ModifiedPoly(acrylic Acid) Microgels on Doxorubicin Transport in HumanIntestinal Epithelial Caco-2 Cell Layers. J. Controlled Release 2003, 88,11−22.(12) Morris, G. E.; Vincent, B.; Snowden, M. The Interaction ofThermosensitive Anionic Microgels with Metal Ion Solution Species.Prog. Colloid Polym. Sci. 1997, 105, 16−22.(13) Bromberg, L.; Temchenko, M.; Hatton, T. A. Smart MicrogelStudies. Polyelectrolyte and Drug-Absorbing Properties of Microgelsfrom Polyether-Modified Poly(Acrylic Acid). Langmuir 2003, 19,8675−8684.

(14) Liu, R.; Fraylich, M.; Saunders, B. R. ThermoresponsiveCopolymers: From Fundamental Studies to Applications. ColloidPolym. Sci. 2009, 287, 627−643.(15) Hofmann, C. H.; Plamper, F. A.; Scherzinger, C.; Hietala, S.;Richtering, W. Cononsolvency Revisited: Solvent Entrapment by N-Isopropylacrylamide and N,N-Diethylacrylamide Microgels in Differ-ent Water/Methanol Mixtures. Macromolecules 2013, 46, 523−532.(16) Schneider, S.; Linse, P. Swelling of Cross-Linked PolyelectrolyteGels. Eur. Phys. J. E 2002, 8, 457−460.(17) Katchalsky, A.; Michaeli, I. Polyelectrolyte Gels in SaltSolutions. J. Polym. Sci. 1955, XV, 69−86.(18) Wilder, J.; Vilgis, T. Elasticity in Strongly Interacting Soft Solids:A Polyelectrolyte Network. Phys. Rev. E 1998, 57, 6865−6874.(19) Frusawa, H.; Hayakawa, R. Swelling Mechanism Unique toCharged Gels: Primary Formulation of the Free Energy. Phys. Rev. E1998, 58, 6145−6154.(20) Vilgis, T. A.; Wilder, J. Polyelectrolyte Networks. Elasticity,Swelling and the Violation of the Flory-Rehner Hypothesis. Comput.Theor. Polym. Sci. 1998, 8, 61−73.(21) Pinheiro, J. P.; Moura, L.; Fokkink, R.; Farinha, J. P. S.Preparation and Characterization of Low Dispersity Anionic Multi-responsive Core-Shell Polymer Nanoparticles. Langmuir 2012, 28,5802−5809.(22) Gelissen, A. P. H.; Pergushov, D. V.; Plamper, F. A. Janus-LikeInterpolyelectrolyte Complexes Based on Miktoarm Stars. Polymer2013, 54, 6877−6881.(23) Plamper, F. A.; Gelissen, A. P.; Timper, J.; Wolf, A.; Zezin, A. B.;Richtering, W.; Tenhu, H.; Simon, U.; Mayer, J.; Borisov, O. V.; et al.Spontaneous Assembly of Miktoarm Stars into Vesicular Interpolye-lectrolyte Complexes. Macromol. Rapid Commun. 2013, 34, 855−860.(24) Plamper, F. A.; Walther, A.; Muller, A. H. E.; Ballauff, M.Nanoblossoms: Light-Induced Conformational Changes of CationicPolyelectrolyte Stars in the Presence of Multivalent Counterions. NanoLett. 2007, 7, 167−171.(25) Plamper, F. A.; McKee, J. R.; Laukkanen, A.; Nykanen, A.;Walther, A.; Ruokolainen, J.; Aseyev, V.; Tenhu, H. Miktoarm Stars ofPoly(ethylene Oxide) and Poly(dimethylaminoethyl Methacrylate):Manipulation of Micellization by Temperature and Light. Soft Matter2009, 5, 1812−1821.(26) Plamper, F. A.; Schmalz, A.; Muller, A. H. E. Tuning theThermoresponsiveness of Weak Polyelectrolytes by pH and Light:Lower and Upper Critical-Solution Temperature of Poly(N,N-Dimethylaminoethyl Methacrylate). J. Am. Chem. Soc. 2007, 129,14538−14539.(27) Wittemann, A.; Ballauff, M. Interaction of Proteins with LinearPolyelectrolytes and Spherical Polyelectrolyte Brushes in AqueousSolution. Phys. Chem. Chem. Phys. 2006, 8, 5269−5275.(28) Henzler, K.; Haupt, B.; Lauterbach, K.; Wittemann, A.; Borisov,O.; Ballauff, M. Adsorption of Beta-Lactoglobulin on SphericalPolyelectrolyte Brushes: Direct Proof of Counterion Release byIsothermal Titration Calorimetry. J. Am. Chem. Soc. 2010, 132, 3159−3163.(29) Gelissen, A. P. H.; Schmid, A. J.; Plamper, F. A.; Pergushov, D.V.; Richtering, W. Quaternized Microgels as Soft Templates forPolyelectrolyte Layer-by-Layer Assemblies. Polymer 2014, 55, 1991−1999.(30) García, T. A.; Gervasi, C. A.; Rodríguez Presa, M. J.; Otamendi,J. I.; Moya, S. E.; Azzaroni, O. Molecular Transport in ThinThermoresponsive Poly(N-iopropylacrylamide) Brushes with VaryingGrafting Density. J. Phys. Chem. C 2012, 116, 13944−13953.(31) Dou, Y.; Han, J.; Wang, T.; Wei, M.; Evans, D. G.; Duan, X.Temperature-Controlled Electrochemical Switch Based on LayeredDouble Hydroxide/Poly(N-isopropylacrylamide) Ultrathin FilmsFabricated via Layer-by-Layer Assembly. Langmuir 2012, 28, 9535−9542.(32) Ghostine, R. A.; Schlenoff, J. B. Ion Diffusion Coefficientsthrough Polyelectrolyte Multilayers: Temperature and ChargeDependence. Langmuir 2011, 27, 8241−8247.



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(33) Ohyanagit, M.; Anson, F. C. Electrochemical Behavior ofElectroactive Counterions in Solutions of Polyelectrolytes. J. Phys.Chem. 1989, 93, 8377−8382.(34) Plamper, F. A. Changing Polymer Solvatization by Electro-chemical Means. Adv. Polym. Sci. 2014, DOI: 10.1007/122014284.(35) Plamper, F. A.; Murtomaki, L.; Walther, A.; Kontturi, K.; Tenhu,H. E-Micellization: Electrochemical, Reversible Switching of PolymerAggregation. Macromolecules 2009, 42, 7254−7257.(36) Kobayasbit, J.; Anson, F. C. Association of ElectroactiveCounterions with Polyelectrolytes. 2. Comparison of Electrostatic andCoordinative Bonding to a Mixed Polycation-Polypyridine. J. Phys.Chem. 1991, 95, 2595−2601.(37) Plamper, F. A. Polymerizations under Electrochemical Control.Colloid Polym. Sci. 2014, 292, 777−783.(38) Zhuang, X.; Wang, D.; Lin, Y.; Yang, L.; Yu, P.; Jiang, W.; Mao,L. Strong Interaction between Imidazolium-Based PolycationicPolymer and Ferricyanide: Toward Redox Potential Regulation forSelective in Vivo Electrochemical Measurements. Anal. Chem. 2012,84, 1900−1906.(39) Kleijn, S. E. F.; Lai, S. C. S.; Koper, M. T. M.; Unwin, P. R.Electrochemistry of Nanoparticles. Angew. Chem., Int. Ed. 2014, 53,3558−3586.(40) Bard, A. J.; Faulkner, L. R. Electrochemical Methods:Fundamentals and Applications, 2nd ed.; John Wiley & Sons, Inc:New York, 2001.(41) Randles, J. E. B. A Cathode Ray Polarograph. Part II. TheCurrent−Voltage Curves. Trans. Faraday Soc. 1948, 44, 327−338.(42) Nicholson, R. S.; Shain, I. Theory of Stationary ElectrodePolarography. Single Scan and Cyclic Methods Applied to Reversible,Irreversible, and Kinetic Systems. Anal. Chem. 1964, 36, 706−723.(43) Sevcik, A. Oscillographic Polarography with PeriodicalTriangular Voltage. Collect. Czechoslov. Chem. Commun. 1948, 13,349−377.(44) Levich, B. The Theory of Concentration Polarization. ActaPhysicochim. URSS 1942, 17, 257−307.(45) Koutecky, J.; Levich, V. G. The Use of a Rotating Disk Electrodein the Study of Electrochemical Kinetics and Electrolytic Processes.Zh. Fiz. Khim. 1958, 32, 1565−1575.(46) Warburg, E. Ueber Das Verhalten Sogenannter UnpolarisirbarerElektroden Gegen Wechselstrom. Ann. Phys. 1899, 303, 493−499.(47) Hill, C. Impedance Spectroscopy. Ann. Biomed. Eng. 1992, 20,289−305.(48) Orazem, M. E.; Tribollet, B. . E. Electrochemical ImpedanceSpectroscopy; John Wiley & Sons: Hoboken, New Jersey, 2008.(49) Randles, J. E. B. Kinetics of Rapid Electrode Reactions. Discuss.Faraday Soc. 1947, 1, 11−19.(50) Gyepi-Garbrah, S. H.; Silerova, R. Probing Temperature-Dependent Behaviour in Self-Assembled Monolayers by Ac-Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2001, 3, 2117−2123.(51) De Lima, S. V.; de Oliveira, H. P. Melting Point of Ionic TernarySystems (Surfactant/Salt/Water) Probed by Electrical ImpedanceSpectroscopy. Colloids Surf., A 2010, 364, 132−137.(52) Tagliazucchi, M. E.; Calvo, E. J. Surface Charge Effects on theRedox Switching of LbL Self-Assembled Redox PolyelectrolyteMultilayers. J. Electroanal. Chem. 2007, 599, 249−259.(53) Tagliazucchi, M.; Grumelli, D.; Calvo, E. J. NanostructuredModified Electrodes: Role of Ions and Solvent Flux in Redox ActivePolyelectrolyte Multilayer Films. Phys. Chem. Chem. Phys. 2006, 8,5086−5095.(54) Araujo, E. S.; de Oliveira, H. P. Phase Inversions in EmulsionsProbed by Electrical Impedance Spectroscopy. J. Dispers. Sci. Technol.2011, 32, 1649−1654.(55) Brug, G. J.; van den Eeden, A. L. G.; Sluyters-Rehbach, M.;Sluyters, J. H. The Analysis of Electrode Impedances Complicated bythe Presence of a Constant Phase Element. J. Electroanal. Chem.Interfacial Electrochem. 1984, 176, 275−295.(56) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. ElectronTransfer to and from Molecules Containing Multiple, Noninteracting

Redox Centers. Electrochemical Oxidation of Poly(vinylferrocene). J.Am. Chem. Soc. 1978, 100, 4248−4253.(57) 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 Sponge-LikeAdsorption of Biomaterials for Biosensor Applications. Biomacromo-lecules 2014, 15, 3735−3745.(58) Schild, H. Poly(N-isopropylacrylamide): Experiment, Theoryand Application. Polym. Sci. 1992, 17, 163−249.(59) Heyda, J.; Soll, S.; Yuan, J.; Dzubiella, J. ThermodynamicDescription of the LCST of Charged Thermoresponsive Copolymers.Macromolecules 2014, 47, 2096−2102.(60) Polotsky, A. A.; Plamper, F. A.; Borisov, O. V. Collapse-to-Swelling Transitions in pH- and Thermoresponsive Microgels inAqueous Dispersions: The Thermodynamic Theory. Macromolecules2013, 46, 8702−8709.(61) Schmidt, S.; Zeiser, M.; Hellweg, T.; Duschl, C.; Fery, A.;Mohwald, H. Adhesion and Mechanical Properties of PNIPAMMicrogel Films and Their Potential Use as Switchable Cell CultureSubstrates. Adv. Funct. Mater. 2010, 20, 3235−3243.(62) Hofl, S.; Zitzler, L.; Hellweg, T.; Herminghaus, S.; Mugele, F.Volume Phase Transition of “smart” Microgels in Bulk Solution andAdsorbed at an Interface: A Combined AFM, Dynamic Light, andSmall Angle Neutron Scattering Study. Polymer 2007, 48, 245−254.(63) Hashmi, S. M.; Dufresne, E. R. Mechanical Properties ofIndividual Microgel Particles through the Deswelling Transition. SoftMatter 2009, 5, 3682−3688.(64) Tagit, O.; Tomczak, N.; Vancso, G. J. Probing the Morphologyand Nanoscale Mechanics of Single poly(N-Isopropylacrylamide)Microgels across the Lower-Critical-Solution Temperature by AtomicForce Microscopy. Small 2008, 4, 119−126.(65) Tsierkezos, N. G.; Ritter, U. Electrochemical and Thermody-namic Properties of hexacyanoferrate(II)/(III) Redox System onMulti-Walled Carbon Nanotubes. J. Chem. Thermodyn. 2012, 54, 35−40.(66) Tagliazucchi, M.; Calvo, E. J. Charge Transport in RedoxPolyelectrolyte Multilayer Films: The Dramatic Effects of OutmostLayer and Solution Ionic Strength. ChemPhysChem 2010, 11, 2957−2968.(67) von Stackelberg, M.; Pilgram, M.; Toome, W. Determination ofDiffusion Coefficients of Several Ions in Aqueous Solution in thePresence of Foreign Electrolytes. I. Z. Elektrochem. 1953, 57, 342.(68) Spruijt, E.; Choi, E.-Y.; Huck, W. T. S. Reversible Electro-chemical Switching of Polyelectrolyte Brush Surface Energy UsingElectroactive Counterions. Langmuir 2008, 24, 11253−11260.(69) Stieger, M.; Richtering, W.; Pedersen, J. S.; Lindner, P. Small-Angle Neutron Scattering Study of Structural Changes in TemperatureSensitive Microgel Colloids. J. Chem. Phys. 2004, 120, 6197−6206.



selective packaging of ferricyanide within thermoresponsive microgels

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