Switchable selectivity for gating ion transport with mixed polyelectrolyte brushes: approaching ‘smart’ drug delivery systems

IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 20 (2009) 434006 (10pp) doi:10.1088/0957-4484/20/43/434006

Switchable selectivity for gating iontransport with mixed polyelectrolytebrushes: approaching ‘smart’ drugdelivery systemsMikhail Motornov, Tsz Kin Tam, Marcos Pita, Ihor Tokarev,Evgeny Katz and Sergiy Minko

Department of Chemistry and Biomolecular Science, and NanoBio Laboratory (NABLAB),8 Clarkson Avenue, Clarkson University, Potsdam, NY 13699-5810, USA

E-mail: [email protected] and [email protected]

Received 18 May 2009, in final form 13 July 2009Published 2 October 2009Online at stacks.iop.org/Nano/20/434006

AbstractA pH-responsive mixed polyelectrolyte brush from tethered polyacrylic acid (PAA) andpoly(2-vinylpyridine) (P2VP) (PAA:P2VP = 69:31 by weight) was prepared and used forselective gating transport of anions and cations across the thin film. An ITO glass electrode wasmodified with the polymer brush and used to study the switchable permeability of the mixedbrush triggered by changes in pH of the aqueous environment in the presence of two solubleredox probes: [Fe(CN)6]4− and [Ru(NH3)6]3+. The responsive behavior of the brush was alsoinvestigated using the in situ ellipsometric measurements of the brush swelling, examination ofthe brush morphology with atomic force microscopy (AFM), and contact angle measurementsof the brush samples extracted from aqueous solutions at different pH values. The mixed brushdemonstrated a bipolar permselective behavior. At pH < 3 the positively charged P2VP chainsenabled the electrochemical process for the negatively charged redox probe, [Fe(CN)6]4−,while the redox process for the positively charged redox probe was effectively inhibited. On thecontrary, at pH > 6 a reversible redox process for the positively charged redox probe,[Ru(NH3)6]3+, was observed, while the redox process for the negatively charged redox species,[Fe(CN)6]4−, was fully inhibited. Stepwise changing the pH value and recording cyclicvoltammograms for the intermediate states of the polymer brush allowed electrochemicalobservation of the brush transition from the positively charged state, permeable for thenegatively charged species, to the negatively charged state, permeable for the positively chargedspecies. The data of ellipsometric, AFM and contact angle measurements are in accord with theelectrochemical study. The discovered properties of the brush could be used for thedevelopment of ‘smart’ sensors and drug delivery systems, for example, a smart drug deliverycapsule which could release negatively charged molecules of drugs in acidic conditions, whilepositively charged molecules of drugs will be released in neutral conditions.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Stimuli-responsive polymer systems change their propertiesupon changes in their environment (temperature, pressure,chemical composition, magnetic field, light, etc) [1]. Thisbehavior has been used for gating mass transport in different

applications ranging from biosensors to drug delivery devices.Different designs were explored to construct such devices. Forexample, stimuli-responsive polymer capsules were used torelease drugs upon changes in the pH of the environment [2].pH-responsive Pickering emulsions have been introduced togate field-driven ion transport upon inversion from water-in-

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Nanotechnology 20 (2009) 434006 M Motornov et al

oil to oil-in-water structures. The emulsion switching wascoupled with biocatalytic processes which allowed gating theion transport across the tube with the emulsion by addingproper amounts of glucose and enzymes [3]. In this workwe explore thin films for an external-stimuli-triggered gatingdevice.

Two major designs are broadly used for stimuli-responsivethin films: polymer gels—chemically or physically cross-linked thin polymer films [4] and polymer brushes—monolayers of end-tethered polymer chains [5]. In typicalexamples responsive thin polymer films are immobilized onthe surface of the pores of commercial filtration membranesor porous supports to form hybrid membranes in which thepermeability can be tuned or switched on and off by aspecific stimulus [6]. For example, a one-step method forthe fabrication of porous supports, surface functionalized withwell-defined responsive polymer brush coatings, was recentlydeveloped based on the use of copolymers with stimuli-responsive polymer blocks or graft chains in combinationwith the phase inversion method [7–9]. The regulationmechanism of mass transport in gating membranes originatesfrom swelling–shrinking transitions occurring in responsivepolymer brush layers which affect pore size and, hence,the permeability of the membranes. Gating membraneswith a ‘valve’ function that are sensitive to pH and ionicstrength [8, 10–14, 9], temperature [15–18], combined pH andtemperature [19], glucose [20], redox agents [21], and specificmolecules and ions [22, 23] have been developed by manyresearch groups.

Recently, we developed an alternative approach tothe fabrication of stimuli-responsive membranes based onpH-responsive macroporous poly(2-vinylpyridine) (P2VP)hydrogel thin films. The hydrogel films with continuoussubmicrometer pores were prepared on flat solid substratesusing phase separation in the polymer solutions and transferredon top of the mechanically stable porous substrate [24–26].Swelling of the hydrogel caused a reversible contraction ofthe pores (from fully open macropores down to completelyclosed). The gating membranes could be used for theregulation of mass transport of water and diffusive species in awide range of sizes from small molecules and ions to colloids.

Ruokolainen et al [27] used self-assembly of a polystyrene(PS)-block-poly(N-isopropylacrylamide) triblock copolymerto prepare a thermo-responsive hydrogel coating on top ofa stable mesoporous support. The short PS end-blocksformed spherical domains which acted as physical cross-linksin a continuous PNIPAAM hydrogel matrix. The resultingmembranes exhibited a thermo-switchable permeability forwater-soluble macromolecules. Photoswitchable ion gatingmembranes from polyelectrolyte multilayers with azobenzenechromophore groups were demonstrated by Kumar andHong [28]. Light-induced switching of the ion permeabilityof such membranes assembled on top of nanoporous aluminasupports was associated with changes in the size of themolecular pores in the multilayers that occurred during cis–trans photoisomerization of the azobenzene moieties.

In many reports, the responsive polymer thin filmswere immobilized on the surface of an electrode. Harris

and Bruening [29] demonstrated that the pH-inducedswelling of multilayered films prepared from poly(allylaminehydrochloride) (PAH) and poly(styrenesulfonate, sodium salt)(PSS) bilayers led to a drastic increase in the film permeability(up to ten-fold) for diffusive redox species.

The electrical charges on the surface or inside a responsivepolymer thin film render it ion permselective properties. Theearly demonstrations of ion permselective hydrogel films referto the studies of Willner et al [30] and Bruening et al[31]. In Bruening et al’s study, chemically cross-linkedmultilayered films composed of PAH and poly(acrylic acid)(PAA) favored the transport of ions of the opposite charge(cations) while inhibiting the permeability for ions of the samecharge (anions). This phenomenon is referred to the Donnaninclusion and exclusion [32].

Stimuli-responsive hydrogel thin films and polymerbrushes with reversible tunable or switchable ion permeabilityhave been explored by several groups [33–37]. Forexample, Jaber and Schlenoff [38] and later Akashiet al [39] demonstrated the reversible temperature-modulatedion permeability of multilayered films assembled usingionically modified PNIPAAM copolymers. Reversibleand fast switching of ion permeability in response totemperature, ionic strength, and pH were reported by Liet al [37] for electrodes modified with PNIPAAM-co-PAA brushes. Dual control over ion permeability (pHand temperature) was demonstrated for hydrogel thin filmsprepared from a photocross-linkable copolymer, PNIPAAM-co-poly(2-carboxyisopropylacrylamide), by Aoyagi et al [40].Huck et al [41] studied the effect of salts on the ionpermeability of a polyelectrolyte brush from quaternizedpoly[(dimethylamino)ethyl methacrylate] (Q-PDMAEMA).Some hydrophobic anions, such as ClO−

4 , PF−6 , and Tf2N−,

caused the collapse of the brush that was accompanied byan increase in the resistance of the brush-coated electrodetoward a redox probe. Hydrogel thin films with reversiblepH-switchable selectivity for both cations (pH 10) and anions(pH 3) were reported by Advincula and co-workers [33].The responsive multilayered films were assembled frombenzophenone-modified PAA and PAH under pH conditionswhen the polymers were partially ionized, resulting in a largefraction of loops and tails that contain free carboxyl andamino groups. The pH-dependent ionization and protonationof these groups, which enabled switching the net ioniccharge of the multilayer between negative and positive values,were responsible for the observed bipolar ion permselectiveproperties of the photocross-linked PAH/PAA films. Thefunctionality of the PAA/PAH films was further extended bythe grafting of a temperature-responsive PNIPAAM brush atopthe multilayer [34].

Transport of divalent and multivalent ions in poly-electrolyte (PE) multilayered films is accompanied by theincorporation of ions in the multilayer with oppositely chargedfunctional groups [32, 31, 36]. Depending on the nature of theions, they can cause a decrease in the film permeability becauseof the formation of additional ionic cross-links in the polymerfilm.

Responsive electrocatalytic coatings have been fabricatedby immobilization of various electrocatalytically active ions,

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macroions, and proteins into responsive PE brushes [42, 43]and hydrogel thin films [44–46]. The efficiency of the electrontransfer process between the immobilized redox centers and anelectrode surface is dictated by the swelling state of the hostpolymer material; thereby, it can be controlled by chemicalsignals or physical stimuli. If the distances between the redoxcenters in a hydrogel thin film are short, the electron transportproceeds through the electron hopping mechanism. In thecase of polymer brushes, the electron transport may involvethe quasi-diffusional translocation of the polymer chains thatenables a direct contact of the polymer-bound redox centerswith the electrode surface [43].

Binding of specific chemicals can be used for the gatedtransport in stimuli-responsive thin films. For example,we designed porous responsive thin films made of aP2VP gel, which reversibly contracted the pores during theswelling transition of the hydrogel due to the binding ofcholesterol and therefore regulated the ion transport throughthe film [25]. Furthermore, we recently demonstrated [47, 48]that the ion transport across stimuli-responsive thin filmsmay include logic processing of multiple biochemical inputsignals, e.g., using a specifically designed sequence ofenzymatic reactions (enzyme logic network), and convertingthem into a single output signal readable by the responsivepolymer. The potential applications of such ‘smart’ electrodecoatings include electrochemical (bio)sensors and devices withtunable/switchable (bio)electrocatalytic activity [49].

In this article we propose an application of the switchablethin polymer film from mixed PE brushes for the bipolar ionpermselective regulation of ion transport. Polymer brushesprepared from end-tethered PE chains represent an importantclass of PE brushes. Thin films obtained by a combinationof two different end-tethered polymers in the PE brush arereferred to as mixed PE brushes [50, 51]. Weak PE brushesare of special interest because the structure and propertiesof the thin film can be tuned by changing the pH of thelocal PE brush environment [52–56]. The pH-responsiveproperties of the weak mixed PE brushes were used totune the surface electrical charge or switch between oppositecharges of the brushes, regulating their swelling, wetting andadhesion to various probes as well as mass transport across thebrushes [51, 57–63, 3].

In this work the mixed PE brush was prepared fromtwo weak PEs: P2VP and PAA. We demonstrate here thatthe mixed brush can switch between selective gating of thetransport of positively and negatively charged ions. Theswitching is triggered by the pH variation of an aqueoussolution. The switching pH was specially adjusted to cover therange of pH changes found in a human body. We foresee, thatthis stimuli-responsive thin film, capable of selective switchingof transport for differently charged species, could serve asa robust and versatile platform for ‘smart’ devices such asbiosensors and drug delivery systems.

2. Experimental details

2.1. Materials

Carboxyl terminated poly(2-vinylpyridine) (P2VP-COOH)(Mn = 53 000 g mol−1, Mw = 56 000 g mol−1), polymer

coil root mean square radius of gyration in melt 〈h2〉1/2 =6 nm, synthesized by anionic polymerization, was purchasedfrom Polymer Source, Inc (Dorval, Canada). Polyacrylicacid (PAA), Mw = 100 500 g mol−1), 〈h2〉1/2 = 8 nm waspurchased from Aldrich. 3-Glycidoxypropyl trimethoxysilane(GPS) was purchased from Gelest Inc. and used as received.

Highly polished silicon wafers (purchased from Semi-conductor Processing, Union Miniere USA Inc.) were firstcleaned in an ultrasonic bath for 30 min with dichloromethane,then placed in a cleaning solution (prepared from NH4OHand H2O2. Warning: This solution is highly reactive andextreme precautions must be taken upon its use) at 60 ◦Cfor 1 h, and finally rinsed several times with Milliporewater (18.2 M� cm). Single-side ITO-coated glass slides(20 ± 5 �/sq surface resistivity) were purchased from Sigma-Aldrich and cleaned using the same procedure as for the Siwafers. For all experiments, the Si wafers and ITO-coated glasssubstrates were cut into 25 mm × 8 mm strips before cleaningand modification.

2.2. Preparation of the mixed brushes

Polymer brushes were grafted to the surface of the ITO glasssubstrates and Si wafers. Samples prepared on the ITO glasswere used for electrochemical experiments. Samples preparedon the surface of the Si wafers were used for AFM experiments,since the roughness of ITO glass is relatively high for theseinvestigations. Both series of samples were synthesized intactin the same reactor and were characterized with ellipsometryand contact angle experiments. We did not find any noticeabledifference in the properties of the brushes prepared on thedifferent substrates.

For the synthesis of the mixed polymer brushes (figure 1),we explored the ‘grafting to’ method of grafting of an end-terminated polymer from the melt [64–66]. In the previouslyreported procedure [51] the mixed brush was prepared byconsecutive grafting of carboxyl functional group terminatedP2VP-COOH and PTBA(poly(tert-butyl acrylate))-COOH,followed by the hydrolysis of PTBA to PAA. In this work weused the layer-assisted method for grafting mixed brushes [57]in which PAA was grafted directly after the grafting of P2VP.

GPS was chemisorbed on the surface of the cleaned Siwafers or ITO glass substrates from 1% (v/v) solution in driedtoluene for 16 h at room temperature. Afterward, the reactionsamples were carefully rinsed with toluene and ethanol toremove ungrafted GPS. In the next step, the GPS-modified Siwafers or ITO glass substrates were spin coated with ∼50 nmthick film of P2VP-COOH. This polymer was grafted at 130 ◦Cin a vacuum oven for 1 h. Then, the ungrafted polymer wasremoved by rinsing the samples in methylethylketone multipletimes. The second polymer, PAA was then deposited by spincoating on the P2VP-modified Si wafer or ITO glass from a 1%(w/w) aqueous solution. The grafting was performed at 130 ◦Cfor 2 h. The ungrafted polymers were removed by multiplerinses with water, acidic water at pH 3, and basic aqueoussolution at pH 9.

Reference homopolymer PAA and P2VP brushes wereprepared by grafting the respective polymers at 130 ◦C

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Figure 1. Scheme of the synthesis of the mixed P2VP–PAA brush by the ‘grafting to’ method.

overnight. Afterward, the ungrafted polymers were removedby rinsing the samples in a similar way as that for the mixedbrush samples.

2.3. Sample characterization

2.3.1. Ellipsometry. The layer thickness and the amountof the grafted material were evaluated at the wavelength of633 nm and at the angle of incidence of 70◦ for the Si wafersand 60◦ for the ITO glass using an Optrel Multiscop (Berlin,Germany) null-ellipsometer equipped with an XY -positioningtable for mapping the sample surface (lateral resolution isdefined by the beam spot of about 2 mm). The measurementswere performed for individual samples after each modificationstep; the measurements of the previous step were used as areference for the simulation of the ellipsometric data. In thecase of the Si wafers, the initial thickness of the native SiO2

layer (usually 1.4±0.2 nm) was calculated at refractive indicesn = 3.858 − i0.018 for the Si substrate and n = 1.4598 forthe SiO2 layer; the ITO coating was measured to be 142 nmthick, and the refractive indices were 1.52 and 1.856 for theglass and ITO coatings, respectively. The thickness of the GPSlayer (typically 1.0 ± 0.1 nm) was evaluated from the two-layer model: SiO2/GPS (or ITO/GPS) in which the refractiveindex of GPS of 1.429 was used. The thickness of P2VP asthe first grafted layer was evaluated from the three-layer modelcomprised of SiO2 (or ITO)/GPS/P2VP, with n = 1.59 forthe organic layer. Finally, the thickness of the whole polymerfilm after grafting of PAA was calculated using the two-layermodel comprised of SiO2(ITO)/polymer, considering the thinpolymer film as an effective optical medium with n = 1.49.We estimated (by simulations) that this calculation results inan error no larger than ±5% for the 5 nm thick films since thedifference in the refractive indexes of all organic ingredientsis small. From the obtained values, we calculated the graftingamount of each polymer, A = Hρ, and the grafting density,σ = ANA/Mw , where H is the ellipsometric thicknessof the dry brush sample, ρ is the density of the polymer(ρ = 1.17 g cm−3 for P2VP and 1.1 g cm−3 for PAA), NA

is Avogadro’s number. The thicknesses of films swollen inaqueous solutions at different pH values were measured ina sample liquid cell with a tube design (the extension tubesare fixed light guides on the laser- and detector-arms of theellipsometer which are dipped into the sample liquid cell) [67].The swollen layer thickness was calculated using the two-layer model comprised of SiO2(ITO)/swollen polymer. Thethickness and the refractive index were both obtained from

the fitting of the two-layer model. We estimated that thiscalculation results in an error no larger than ±10% for the two-fold swollen films in water.

The synthesized samples of the mixed brushes on the Siwafers and ITO substrates had the following characteristics:HPAA+P2VP = 23.8±2 nm; APAA = 18.4±1.8 g m−2; AP2VP =8.3 ± 0.8 g m−2; σPAA = 0.11 nm−2; σP2VP = 0.09 nm−2;σPAA+P2VP = 0.2 nm−2. The brush regime can be characterizedby the reduced grafting density (the number of chains thatoccupy an area that a free nonoverlapping polymer chainwould normally fill under the same experimental conditions)�PAA+P2VP = σπh2 = 30. Overlapping tethered chains arein the brush regime if � > 5 [68]. The reported experimentswere conducted with three series of samples of the polymerbrushes. For each samples we have performed at least threeconsecutive switching experiments to monitor changes in thebrush properties.

2.3.2. Contact angle (CA). The wettability of surfaces wascharacterized by contact angle measurements of sessile waterdroplets using a homemade system, which included a samplestage, long-focus microscope, light source, charge-coupleddevice camera, personal computer, and self-coded software forprocessing drop images. Advancing (adv) and receding (rec)contact angles from six individual droplets placed on six newsurface areas were measured by adding or withdrawing a smallvolume of water through a syringe. The needle was maintainedin contact with the drop during the experiments. All readingswere then averaged to give a mean advancing and recedingcontact angle for each sample. The accuracy of this techniqueis in the order of ±2◦.

2.3.3. Atomic force microscopy (AFM). AFM studies wereperformed on a Dimension 3100 microscope for dry samples.The tapping mode was used to map the film morphologyat ambient conditions. AFM tips of Veeco NP with aresonance frequency of 75 kHz and a spring constant of 0.58–0.32 N m−1 were used at ambient conditions. The root-mean-square roughness (RMS) for all samples was calculatedover the 2 × 2 μm2 scanned area using commercial software.The representative images for the thin film topography werecollected after examination of the sample surface in 3–5different spots for each series of samples.

2.3.4. Electrochemical measurements. The pH measure-ments were performed with a Mettler Toledo® SevenEasy pH-meter. Cyclic voltammetry measurements were carried out

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Figure 2. Proposed mechanism for switching of permselective properties of the mixed P2VP–PAA brush: the brush with swollen positivelycharged P2VP domains (dark, in blue) is permeable for anions (a), the brush with swollen negatively charged PAA domains (gray, in red) ispermeable for cations (c), and the brush in the state of P2VP–PAA uncharged PE complex is not permeable at 4 < pH < 6 (b).

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Figure 3. Swelling of the mixed brush in aqueous solutions (a) and water contact angles (advancing—squares and receding—circles) on thesurface of the dry samples of the brush extracted from aqueous solutions (b) at different pH.

with an ECO Chemie Autolab PASTAT 10 electrochemical an-alyzer, using the GPES 4.9 (General Purpose ElectrochemicalSystem) software. Cyclic voltammograms were recorded inthe potential range from −0.6 to 0.6 V. The potential scan ratewas 100 mV s−1. All the measurements were performed atroom temperature (23 ± 2 ◦C), in a standard three-electrodecell (ECO Chemie). The working electrode was a P2VP/PAA-modified ITO glass electrode with the geometrical area of1.2 cm2 (note that the typical surface roughness factor for ITOelectrodes is ca 1.6 ± 0.1 [69]). A Metrohm Ag|AgCl|KCl,3 M, electrode served as a reference electrode and a MetrohmPt wire was used as a counter-electrode. All solutions werebuffered (0.1 M phosphate buffer titrated to the pH valuesspecified in the text with the use of H2SO4 or KOH) andcontained 0.5 mM K4Fe(CN)6 and 0.1 mM Ru(NH3)6Cl3 asredox probes. Bubbling argon through the solution for 5 minwas performed prior to all measurements.

3. Results and discussion

In our previous reports, thickness, surface charge, morphologyand ionization of the 50:50 mixed P2VP–PAA brush has beenstudied as a function of pH and salt concentrations [51, 70, 71].It was demonstrated that the film swelled about 110% if thepH was changed from pH = 5 to either more acidic or basicconditions. The water CA decreased under both acidic and

basic circumstances. For the PAA–P2VP brush in the pH rangebetween pH = 4 and 7, the thickness of the brush correspondedto the dry film thickness, while the water CAs were muchhigher as compared to those at basic and acidic conditions. Inacidic solutions (pH < 3), P2VP was protonated and positivelycharged. A further decrease of pH resulted in an increase ofdensity of positive charges on P2VP chains. The PAA wasnegatively charged at pH > 5.5. In the range of 3.2 < pH < 7,the oppositely charged P2VP and PAA interacted in such amanner that, at pH = 4.9 (isoelectric point of the mixed brush),the charges were completely compensated, resulting in a zerosurface charge. These results were used to derive a conclusionthat the mixed PE brush demonstrated switching behaviorwhen the top of the brush was occupied by the negativelycharged PAA chains stretched away from the substrate, or bythe positively charged stretched P2VP chains at pH valuesabove and below the isoelectric point, respectively. It washypothesized that, around the isoelectric point, the mixed brushformed a compact PE complex with a zero charge.

We reported also that the isoelectric point and the pHrange of switching of the mixed brush depended on itscomposition [63]. In this work we prepared the mixedP2VP–PAA polymer brush of the composition P2VP:PAA =31:69% (w/w) to shift the isoelectric point and switchingthreshold toward an acidic pH. The prepared stimuli-responsive thin film is a promising material for in vivo ‘smart’

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Figure 4. AFM topography ((a), (c) and (e)) and cross-sections ((b), (d) and (f)) after extraction of the mixed brush samples from aqueoussolutions at pH 3 ((a), (b)), RMS = 1.2 nm, pH 6 ((c), (d)) RMS = 0.64 nm, and pH 4.5 ((e), (f)), RMS = 0.6 nm.

drug delivery devices. Different pH ranges in body can beexplored for triggered delivery systems. For example, alongthe gastrointestinal tract pH changes from pH 2 in the stomachto pH 7 in the small intestine. Thus, a ‘smart’ drug deliverycapsule applied by oral administration could explore thebipolar permselective properties to release negatively chargedmolecules of drugs in acidic conditions in the stomach, whilepositively charged molecules of drugs will be released inneutral conditions in the small intestine. Thus, we targeted

the brush composition when swollen positively charged P2VPdomains are permeable for anions at pH < 4 and swollennegatively charged PAA domains are permeable for cations atpH > 6 (figure 2).

The mixed brush synthesized in this work is in accordwith the mentioned above scenario. Evaluation of the mixedbrush swelling at different pH values provided evidence forthat (figure 3(a)). It was well documented, that the morphologyof the mixed brush samples obtained after the brush was

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Figure 5. Cyclic voltammograms for the ITO electrode decoratedwith the P2VP–PAA mixed brush in the electrochemical experimentswith 0.5 mM [Fe(CN)6]4− and 0.1 mM [Ru(NH3)6]3+ redox probesat pH 6.0 ((a), (c)), pH 3.0 ((b), (d)), and pH 4.35 (e). The insetillustrates a reversible character of the switching of the mixedpolymer brush for [Ru(NH3)6]3+ (a) and [Fe(CN)6]4− (b) (twocycles are shown).

extracted from the solvent demonstrates a memory effect—thedry sample morphology reflects the structure of the brush inthe solvent [72]. We used this behavior of the mixed brushto examine, with CA analysis and AFM, the dry samplesextracted from aqueous solutions at different pH values. CAdata (figure 3(b)) are in good agreement with the swellingexperiments. After being exposed to the acidic solution,the topmost layer was covered by the protonated P2VP anddemonstrated the wetting properties of P2VP (adv and rec CAs65◦ and 15◦, respectively), while after solutions with pH > 6,the brush was likely covered by PAA chains and the wettingbehavior was similar to the reference PAA brush (adv andrec CAs 50◦ and <10◦, respectively). In the range of pHvalues between 4 and 5 the surface was hydrophobic due tothe formation of the PE complex. The morphology of the drysamples examined with AFM provides evidence for the sameconclusions (figure 4). The surface with the lower roughnesswas obtained at pH 4.5 due to the formation of the compactlayer from the PE complex. At pH 3 and pH 6 the films havewell expressed laterally segregated domains that correspond toPAA domains in acidic pH and P2VP domains at basic pHvalues. No sharply pronounced laterally segregated domainswere observed in the pH range between pH 4 and pH 6.

Switchable permeability of the mixed brush was studiedin the electrochemical experiments. The switchable interfacialproperties of the modified electrode were observed bycyclic voltammetry measurements performed in the presenceof two soluble redox probes: 0.5 mM [Fe(CN)6]4− and0.1 mM [Ru(NH3)6]3+. The selected redox probes havenegative and positive charges providing different electrostaticinteraction with the polymer films at the electrode surface,while their redox potentials, E0([Fe(CN)6]4−) = +0.3 V,E0([Ru(NH3)6]3+) = −0.25 V, are different enough toallow separated waves in the cyclic voltammograms. Theconcentrations of the redox probes (ratio 5:1) were optimizedto observe similar electrochemical waves in the cyclicvoltammograms.

Figure 6. The titration curves for the redox process on the ITOelectrode decorated with the P2VP–PAA mixed brush: for 0.5 mM[Fe(CN)6]4− (black squares) and 0.1 mM [Ru(NH3)6]3+ (red circles)derived from cyclic voltammograms recorded at different pH values.

The experiments were started at pH 6 when the polymericfilm is negatively charged because of dissociation of thePAA component of the mixed polymer brush. At this pH,a reversible redox process for the positively charged redoxprobe, [Ru(NH3)6]3+, was observed, E0 = −0.25 V, whilethe redox process of the negatively charged redox species,[Fe(CN)6]4−, was fully inhibited, figure 5, curve (a). UponpH change to 3, the polymer brush was re-charged and re-structured due to the protonation of the P2VP component ofthe mixed polymer brush. The positively charged polymer filmallowed the electrochemical process of the negatively chargedredox species, [Fe(CN)6]4−, E0 = +0.3 V, while the redoxprocess of the positively charged redox probe was effectivelyinhibited, figure 5, curve (b).

The interfacial changes allowing and inhibiting the redoxreactions of differently charged redox species were reversible,figure 5, inset, and similar cyclic voltammograms wereobtained repeatedly upon pH changes between 3 and 6,figure 5, curves (c) and (d). The reversible switching of theelectrochemical reactions for the differently charged redoxprobes originated from the re-charging of the polymer filmbetween the positive and negative charges. At pH 4.35 theminimum charge of the composite polymer film was produced,thus resulting in the inhibition of the both electrochemicalprocesses, figure 5, curve (e). Stepwise changing the pHvalue and recording cyclic voltammograms (not shown) forthe intermediate states of the polymer allowed electrochemicalmonitoring of the polymer brush transition from the positivelycharged state to the negatively charged state at the modifiedinterface. The experiment resulted in the titration curveswhere one of the redox probes is increasing the electrochemicalresponse, while another is getting deactivated upon re-chargingthe polymer film from its positive to the negative state, figure 6.

It should be noted that homopolymer brush films(composed of PAA or P2VP) electrochemically analyzedunder similar conditions in the presence of [Fe(CN)6]4− and[Ru(NH3)6]3+ have demonstrated the switching ON of theelectrochemical process only for one of the redox probes,while the second one was always inhibited. Specifically, the

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Figure 7. Reference cyclic voltammograms for the ITO electrode decorated with single-homopolymer brushes PAA (a) and P2VP (b)at pH 3 (i) and pH 6 (ii).

PAA-modified electrode was active for the positively charged[Ru(NH3)6]3+ redox probe at pH 6 when the polymeric filmwas dissociated and negatively charged, while the negativelycharged [Fe(CN)6]4− probe was inhibited. When the pH valuewas changed to 3, the both redox probes were inhibited by theneutral polymer film (figure 7(a)).

The P2VP-modified electrode was active for the nega-tively charged [Fe(CN)6]4− probe at pH 3 when the polymericfilm was protonated and positively charged, while the posi-tively charged [Ru(NH3)6]3+ redox probe was inhibited. Uponshifting pH value to 6, both redox probes were inhibited by thedeprotonated neutral polymer film (figure 7(b)).

4. Conclusions

The mixed PAA–P2VP brush (PAA:P2VP = 69:31 by weight)with bipolar permselective behavior has been synthesized inthis work. The mixed brush is permeable for positivelycharged species and inhibits transport of negatively chargedspecies in neutral and basic aqueous solutions. At pH <

4 the brush is permeable for the negatively charged speciesand blocks transport of positively charged species. Whenthe pH value is increased to pH > 5 the polymeric filmbecomes permeable for the positively charged species, whilethe transport of the negatively charged redox probe is inhibited.This ‘smart’ gating effect is of potential interest for biosensorsand delivery devices (for example, drug delivery devices)where the selective release of charged chemicals depends onthe local pH of the environment [73].

Acknowledgments

We acknowledge NSF awards DMR 0706209 and DMR0602528 for the financial support of this work.

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