4506 DOI: 10.1021/la903527p Langmuir 2010, 26(6), 4506–4513Published on Web 12/11/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

Reversible “Closing” of an Electrode Interface Functionalized with a

Polymer Brush by an Electrochemical Signal

Tsz Kin Tam, Marcos Pita, Oleksandr Trotsenko, Mikhail Motornov, Ihor Tokarev,Jan Hal�amek, Sergiy Minko,* and Evgeny Katz*

Department of Chemistry andBiomolecular Science andNanoBio Laboratory (NABLAB), ClarksonUniversity,Potsdam, New York 13699-5810

Received September 17, 2009. Revised Manuscript Received November 14, 2009

The poly(4-vinyl pyridine) (P4VP)-brush-modified indium tin oxide (ITO) electrode was used to switch reversibly theinterfacial activity by the electrochemical signal. The application of an external potential (-0.85V vsAg|AgCl|KCl, 3M)that electrochemically reduced O2 resulted in the concomitant consumption of hydrogen ions at the electrode interface,thus yielding a higher pH value and triggering the restructuring of the P4VP brush on the electrode surface. The initialswollen state of the protonated P4VP brush (pH 4.4) was permeable to the anionic [Fe(CN)6]

4- redox species, but theelectrochemically produced local pH of 9.1 resulted in the deprotonation of the polymer brush. The producedhydrophobic shrunken state of the polymer brush was impermeable to the anionic redox species, thus fully inhibiting itsredox process at the electrode surface. The interface’s return to the electrochemically active state was achieved bydisconnecting the applied potential, followed by stirring the electrolyte solution or by slow diffusional exchange of theelectrode-adjacent thin layer with the bulk solution. The developed approach allowed the electrochemically triggeredinhibition (“closing”) of the electrode interface. The application of this approach to different interfacial systems willallow the use of various switchable electrodes that are useful for biosensors and biofuel cells with externally controlledactivity. Further use of this concept was suggested for electrochemically controlled chemical actuators (e.g. operating aselectroswitchable drug releasers).

Introduction

Reversible activation-inactivation of electrochemical inter-faces is a key element in many electrocatalytic,1 bioelectrocataly-tic,2 photoelectrocatalytic,3 biosensor,4 information processingbioelectronic,5 andmolecularmachine6 systems. Triggering of theelectrode interface transition between active and inactive stateswas achieved by the application of light signals resulting in thephotoisomerization of surface-confined organicmolecules,7 by anexternal magnetic field resulting in the translocation of magneticnanoparticles8 or the reorientation of magnetic nanowires9

associated with the interface, by chemical signals affecting theinterfacial properties,10 and by temperature alteration producingstructural changes in polymer thin films.11

Responsive polymer thin films immobilized on electrode sur-faces were used to regulate electrochemical processes. Harris andBruening10a demonstrated that the pH-induced swelling of multi-layers prepared from poly(allylamine hydrochloride) (PAH) andpoly(styrenesulfonate sodium salt) (PSS) bilayers affected thepermeability of the film for diffusive ionic redox species. Theswelling of themultilayer in basic solutions was accompanied by a10-fold increase in the film permeability. Stimuli-responsivehydrogel thin films with reversible tunable or switchable ionpermeability have been explored by several groups.10,11 Jaberand Schlenoff11b and later Akashi et al.11a reported on thereversible temperature-modulated ion permeability ofmultilayersassembled using ionically modified poly-N-isopropylacrylamide(PNIPAAM) copolymers. The changes in ion transport across thefilmswere attributed to the variations in film swelling arising fromthe phase transition of PNIPAAM.

Hydrogel thin films with reversible pH-switchable selectivityfor both cations (pH 10) and anions (pH 3) were reported byAdvincula and co-workers.10b The responsive multilayers were

*To whom all correspondence should be addressed. (S.M.) Fax: 1-315-2686610. Tel: 1-315-2683807. E-mail: sminko@clarkson.edu. (E.K.)Fax: 1-315-2686610. Tel: 1-315-2684421. E-mail: ekatz@clarkson.edu.(1) (a) Wang, J.; Musameh, M.; Laocharoensuk, R.; Gonzalez-Garcia, O.; Oni,

J.; Gervasio, D. Electrochem. Commun. 2006, 8, 1106–1110. (b) Wang, J.; Musameh,M.; Laocharoensuk, R.; Gonzalez-Garcia, O.; Oni, J.; Gervasio, D. Electrochem.Commun. 2006, 8, 1106–1110. (c) Wang, J.; Musameh, M.; Laocharoensuk, R.Electrochem. Commun. 2005, 7, 652–656.(2) (a) Katz, E.; Willner, I.Chem. Commun. 2005, 4089–4091. (b) Lee, J.; Lee, D.;

Oh, E.; Kim, J.; Kim, Y. P.; Jin, S.; Kim, H. S.; Hwang, Y.; Kwak, J. H.; Park, J. G.; Shin,C. H.; Kim, J.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 7427–7432.(3) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 4791–4794.(4) (a) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 4791–4794. (b) Cao,

Z.; Jiang, X.; Meng, W.; Xie, Q.; Yang, Q.; Ma, M.; Yao, S. Biosens. Bioelectron. 2007,23, 348–354.(5) (a) Baron, R.; Onopriyenko, A.; Katz, E.; Lioubashevski, O.; Willner, I.;

Wang, S.; Tian, H.Chem. Commun. 2006, 2147–2149. (b) Katz, E.; Willner, I.Chem.Commun. 2005, 5641–5643.(6) (a) Flood, A. H.; Ramirez, R. J. A.; Deng, W. Q.; Muller, R. P.; Goddard,

W. A.; Stoddart, J. F.Aust. J. Chem. 2004, 57, 301–322. (b) Katz, E.; Lioubashevsky,O.; Willner, I. J. Am. Chem. Soc. 2004, 126, 15520–15532.(7) (a) Browne,W. R.; Feringa, B. L.Annu. Rev. Phys. Chem. 2009, 60, 407–428.

(b) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783–1790.(8) (a)Willner, I.; Katz, E.Langmuir 2006, 22, 1409–1419. (b) Katz, E.; Baron, R.;

Willner, I. J. Am. Chem. Soc. 2005, 127, 4060–4070.(9) (a) Wang, J. Electroanalysis 2008, 20, 611–615. (b) Laocharoensuk, R.;

Bulbarello, A.; Mannino, S.; Wang, J. Chem. Commun. 2007, 3362–3364. (c) Loaiza,O. A.; Laocharoensuk, R.; Burdick, J.; Rodriguez, M. C.; Pingarron, J. M.; Pedrero, M.;Wang, J. Angew. Chem., Int. Ed. 2007, 46, 1508–1511.

(10) (a) Harris, J. J.; Bruening,M. L. Langmuir 2000, 16, 2006–2013. (b) Park, M.K.; Deng, S. X.; Advincula, R. C. J. Am. Chem. Soc. 2004, 126, 13723–13731. (c)Gabai, R.; Sallacan, N.; Chegel, V.; Bourenko, T.; Katz, E.; Willner, I. J. Phys. Chem. B2001, 105, 8196–8202. (d) Zhou, J. H.; Wang, G.; Hu, J. Q.; Lu, X. B.; Li, J. H. Chem.Commun. 2006, 4820–4822. (e) Kang, E. H.; Liu, X. K.; Sun, J. Q.; Shen, J. C.Langmuir 2006, 22, 7894–7901. (f) Tokarev, I.; Orlov, M.; Katz, E.; Minko, S. J. Phys.Chem. B 2007, 111, 12141–12145. (g) Wang, B. Z.; Anzai, J. Langmuir 2007, 23,7378–7384.

(11) (a) Serizawa, T.; Matsukuma, D.; Nanameki, K.; Uemura, M.; Kurusu, F.;Akashi, M. Macromolecules 2004, 37, 6531–6536. (b) Jaber, J. A.; Schlenoff, J. B.Macromolecules 2005, 38, 1300–1306. (c) Fulghum, T. M.; Estillore, N. C.; Vo, C. D.;Armes, S. P.; Advincula, R. C. Macromolecules 2008, 41, 429–435. (d) Karbarz, M.;Gniadek, M.; Stojek, Z. Electroanalysis 2009, 21, 1363–1368.

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assembled from benzophenone-modified poly(acrylic acid)(PAA) and PAH under pH conditions in which the polymerswere partially ionized, resulting in a large fraction of loops andtails that contain free carboxyl and amino groups (i.e., notinvolved in the formation of the polyelectrolyte complex). ThepH-dependent ionization and protonation of these groups, whichenabled switching of the net ionic charge of the multilayerbetween negative and positive values, were responsible for theobserved bipolar ion permselective properties of the photo-cross-linked PAH/PAA films. In later work,11c the functionality of thePAA/PAH films was further extended by the grafting of aPNIPAAM brush atop the multilayer. The resulting thin filmswith binary architecture enabled a dual control mechanism (pHand temperature) for ion permeability across the films.

Polyelectrolyte polymer brushes tethered to surfaces12 caneffectively control interfacial properties, being switchable betweenshrunken and swollen states depending on their charge as con-trolled by an external pH value.13 Application of the pH-con-trolled polymer brushes for switching electrode interfacialproperties14 allowed modified electrodes that were reversiblyactivated-inactivated by pH changes produced in situ by bioca-talytic reactions.15 These electrodes have found applications inbiofuel cells with the power output controlled by enzymatic16 orimmune-recognition17 processes coupled with polymer brushrestructuring induced by the biocatalytic pH changes. The pHchanges resulting in the interfacial switching of the polymer-brush-modified surfaces can be produced in a bulk solution15 ordirectly at the functional interface.18 Further developments inchemical, biochemical, and electrochemical means for the rever-sible activation-inactivation of the electrode interfaces functio-nalized with the restructuring polymer systems would bebeneficial for future switchable biosensors, actuators, and con-trolled-release devices.

Polyelectrolyte brushes with electroactive counterionswere recently used as an effective platform for surfaces withelectrochemically switchable wetting properties.19 The electro-actuation of microcantilevers coated on one side with cationicpolyelectrolyte brushes represents an exciting example of aminiaturized electromechanical device based on stimuli-respon-sive polymer brushes where the response was triggered by anelectrical potential applied to the functionalized electrode.20 This

article demonstrates for the first time the electrochemicallyinduced reversible “closing” of the electrode interface functiona-lized with a poly(4-vinylpyridine) (P4VP) brush tethered to theelectrode surface. The pH-triggered switching behavior of respon-siveP2VPandP4VP thin films has been the subject of a number ofpublications,21 which is rationalized by a sharp coil-to-globuletransition of the weak hydrophobic polyelectrolyte chains.22

Unlike previous reports, in this work, the switching processwas based on the local interfacial pH changes induced by anelectrochemical reaction.

Experimental Section

Chemicals and Supplies. Poly(4-vinyl pyridine) (P4VP, MW160 000 g 3mole-1, F = 1.101 g 3 cm

-3, Sigma-Aldrich), L-(þ)-lactic acid (Sigma-Aldrich), 2,20-azino-bis(3-ethylbenzothiazo-line-6-sulfonic acid) (ABTS, Sigma-Aldrich), horseradish perox-idase (HRP) type VI (E.C. 1.11.1.7, Sigma-Aldrich), thioninacetate (Alfa Asear), bromomethyldimethylchlorosilane(Gelest), and other chemicals and solvents were used as suppliedwithout any further purification. Indium tin oxide (ITO) single-side-coated conducting glass (20 ( 5 Ω/sq surface resistivity,Sigma-Aldrich) served as the working electrode for electrochemi-cal measurements. Highly polished silicon wafers (purchasedfrom Semiconductor Processing, Union Miniere USA Inc.) wereused for AFMexperiments. Ultrapure water (18.2MΩ 3 cm) fromNANOpure Diamond (Barnstead) was used in all of theexperiments.

ElectrodeModification. The ITO electrodes were chemicallymodified with P4VP brushes using the grafting-to method23

according to the following procedure. The ITO-coated glass slideswere cut into 25 mm � 8 mm strips. They were cleaned withethanol in an ultrasonic bath for 15 min and dried in a stream ofargon.The cleaning stepwas repeatedusingmethylene chloride asa solvent. The initial cleaning steps were followed by immersingthe strips into a cleaning solution (heated to 60 �C in awater bath)composed of NH4OH, H2O2, and H2O in a ratio of 1:1:1 (v/v/v)for 1 h. (Warning: This solution is very reactive, and extremeprecautions must be taken upon its use.) Subsequently, the glassstrips were rinsed several times with water and then dried underargon. The freshly cleaned ITO strips were reacted with 0.1% v/vbromomethyldimethylchlorosilane in toluene for 20min at 70 �C.The silanized ITO was rinsed with several aliquots of toluene anddried under argon. Then 60 μL of the P4VP solution in nitro-methane (10 mg 3mL-1) was applied to the surface of each ITOglass strip, dried to form a polymer coating, and left to react in avacuum oven at 140 �C overnight. The final cleaning steps toremove the unbound polymer consisted of soaking for 10 min inethanol, followed by an additional 10 min in a dilute solution ofH2SO4 (pH 3). Modified electrodes were stored under water.The Si wafers were cleaned and modified using the same proce-dure as for the ITO-coated glass. We did not observe anydifferences in the properties of the brushes prepared on thedifferent substrates.

Electrochemical Measurements. The measurements werecarried out with an ECO Chemie Autolab PASTAT 10 electro-chemical analyzer using the GPES 4.9 (General Purpose Electro-chemical System) software package for cyclic voltammetry anddifferential pulse voltammetry.Allmeasurementswereperformed

(12) (a) R€uhe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Gr€ohn, F.;Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.;Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.;Usov,D.; Zhang,H.Adv. Polym. Sci. 2004, 165, 79–150. (b) Brittain,W. J.;Minko, S.J. Polym. Sci., Part A 2007, 45, 3505–3512. (c) Tokarev, I.; Motornov, M.; Minko, S.J. Mater. Chem. 2009, 19, 6932–6948.(13) (a) Minko, S. Polym. Rev. 2006, 46, 397–420. (b) Luzinov, I.; Minko, S.;

Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635–698. (c) Luzinov, I.; Minko, S.;Tsukruk, V. V. Soft Matter 2008, 4, 714–725.(14) (a) Combellas, C.; Kanoufi, F.; Sanjuan, S.; Slim, C.; Tran, Y. Langmuir

2009, 25, 5360–5370. (b) Motornov, M.; Sheparovych, R.; Katz, E.; Minko, S. ACSNano 2008, 2, 41–52. (c) Tam, T. K.; Ornatska, M.; Pita, M.; Minko, S.; Katz, E.J. Phys. Chem. C 2008, 112, 8438–8445. (d) Yu, B.; Zhou, F.; Hu, H.;Wang, C.W.; Liu,W. M. Electrochim. Acta 2007, 53, 487–494. (e) Choi, E. Y.; Azzaroni, O.; Cheng, N.;Zhou, F.; Kelby, T.; Huck, W. T. S. Langmuir 2007, 23, 10389–10394.(15) (a) Tam, T. K.; Zhou, J.; Pita,M.; Ornatska,M.;Minko, S.; Katz, E. J. Am.

Chem. Soc. 2008, 130, 10888–10889. (b) Zhou, J.; Tam, T. K.; Pita, M.; Ornatska, M.;Minko, S.; Katz, E. ACS Appl. Mater. Interfaces 2009, 1, 144–149. (c) Privman, M.;Tam, T. K.; Pita, M.; Katz, E. J. Am. Chem. Soc. 2009, 131, 1314–1321.(16) (a) Amir, L.; Tam, T. K.; Pita, M.; Meijler, M. M.; Alfonta, L.; Katz, E.

J. Am. Chem. Soc. 2009, 131, 826–832. (b) Tam, T. K.; Pita, M.; Ornatska, M.; Katz, E.Bioelectrochemistry 2009, 76, 4–9.(17) Tam, T. K.; Strack, G.; Pita, M.; Katz, E. J. Am. Chem. Soc. 2009, 131,

11670–11671.(18) Pita,M.; Tam, T.K.;Minko, S.;Katz, E.ACSAppl.Mater. Interfaces 2009,

1, 1166–1168.(19) Spruijt, E.; Choi, E. Y.; Huck, W. T. S. Langmuir 2008, 24, 11253–11260.(20) Zhou, F.; Biesheuvel, P. M.; Chol, E. Y.; Shu, W.; Poetes, R.; Steiner, U.;

Huck, W. T. S. Nano Lett. 2008, 8, 725–730.

(21) (a) Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc.2004, 126, 15950–15951. (b) Tokarev, I.; Orlov, M.; Minko, S. Adv. Mater. 2006, 18,2458–2460. (c) Orlov, M.; Tokarev, I.; Scholl, A.; Doran, A.; Minko, S. Macromole-cules 2007, 40, 2086–2091. (d) Lupitskyy, R.; Motornov, M.; Minko, S. Langmuir2008, 24, 8976–8980. (e) Tokarev, I.; Tokareva, I.; Minko, S. Adv. Mater. 2008, 20,2730–2734.

(22) (a) Roiter, Y.; Minko, S. J. Am. Chem. Soc. 2005, 127, 15688–15689. (b)Minko, S.; Roiter, Y. Curr. Opin. Colloid Interface Sci. 2005, 10, 9–15.

(23) (a) Draper, J.; Luzinov, I.; Minko, S.; Tokarev, I.; Stamm, M. Langmuir2004, 20, 4064–4075. (b) Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K.-J.;Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Langmuir 2002, 18, 289–296.

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at ambient temperature (23( 2 �C) in a standard three-electrodecell (ECO Chemie). The working electrode was a P4VP-modifiedITO-glass electrode with a geometrical area of 1.2 cm2. (Note thatthe typical surface roughness factor for ITO electrodes is ca. 1.6(0.1.24) A Metrohm Ag|AgCl|KCl 3M electrode served as areference electrode, and aMetrohm Pt wire was used as a counterelectrode. The background aqueous electrolyte solution for theexperiments with the electrochemically switchable interfaces wascomposed of 1 mM lactic buffer (pH 4.4) and 100 mM sodiumsulfate.The electrochemicalmeasurements over a broadpHrange(the titration experiments) were performed in 0.1 M phosphatebuffer with the pH adjusted by additions of H2SO4 or KOH.Cyclic voltammograms were recorded in the presence of 0.5 mMpotassium ferrocyanide,K4[Fe(CN)6], in the potential range from-0.9 to 0.5 V after equilibration for 4 s at the starting potential.The potential scan rate was 50 mV 3 s

-1. Peak currents foreach measurement were obtained from a second scan. Thepotential of-0.85 V was applied to the P4VP-modified electrodefor 20 min to close the polymer brush electrochemically. Thioninacetate (0.5 mM) was used as a pH-dependent redox probe todetect the local pH produced electrochemically at the electrodesurface. The E� measurements for thionin were performed usingdifferential pulse voltammetry (DPV)with a potential scan rate of7 mV 3 s

-1. The obtained E� values were compared with thethionin potential measured at a bare ITO electrode at variouspHvalues (0.1Mphosphate buffer titrated todifferent pHvalues)to derive the respective interfacial pH value. The bulk pHmeasurements were performed with a Mettler Toledo SevenEasypH meter. Deoxygenation (in the control experiments) wasachieved by bubbling argon through the working solution for15 min.

Ellipsometry. The layer thickness and the amount of graftedmaterialwere evaluated at awavelength of 633 nmand at an angleof incidence of 60� for the P4VP-ITO glass using an OptrelMultiscop (Berlin, Germany) null ellipsometer equipped with anXY-positioning table for mapping the sample surface.

Atomic Force Microscopy. The AFM studies were per-formed on a Multimode scanning microscope (Veeco Instru-ments, NY) operating in tapping mode. The samples of theP4VP brush bound to Si wafers were scanned using NPS siliconnitride probes (Veeco Instruments, NY) with a resonance fre-quency of∼9 kHz and a spring constant of 0.58-0.32N/m in theaqueous media at different pH values. The root-mean-square(rms) roughness for all samples was calculated over the 2� 2 μm2

scanned area using commercial software. AFM scratch analysiswas used to follow changes in brush swelling in the AFMexperiments. In this analysis, we used a metal needle to make ascratch on the surface of the brush so that in the scratchedarea the grafted polymer was mechanically removed from thesubstrate by the needle. The scratched area was scanned in situ indifferent environments, and the brush thickness was evaluated

using the substrate area that was uncovered by the polymer asa reference.

Results and Discussion

The electrode modification with the polymeric brush wasperformed in two steps. The ITO electrode surface was reactedwith bromomethyldimethylchlorosilane to yield a Br-functiona-lized interface. Then, P4VP was grafted to the functionalizedITO surface through quaternized pyridine groups, yielding teth-ered polymer chains in the form of a polymer brush (Scheme 1).The thickness of the P4VP brush in a dry state, 8.4( 1.1 nm, wasestimated by ellipsometry, which corresponds to the graftedamount of ca. 9.2 mg 3m

-2. The same value for the graftingamount was obtained for a similarly modified Si-wafer substrate.The obtained grafted amount corresponds to a grafting densityvalue of 0.075 chain 3 nm

-2, as reported elsewhere.13a This is atypical grafting density value for the grafting-to methodthat indicates a sufficient number of surface-binding sites andoriginates from the limitation of the grafting by the diffusion ofpolymer chains through the grafted brush layer.12b

The polymer-brush properties were shown to be dependent onthe protonation state of the polymer chains.25 To characterize thepH-switchable properties of the surfaces prepared in the presentstudy, we followed the pH-controlled shrinking-swelling of thepolymer thin film by AFM scratch analysis (Figure 1). All AFMmeasurements were carried out in aqueous solutions with an ionicstrength similar to that of the solutions used in the electrochemicalexperiments. The smooth surface of a Si wafer was modified withthe P4VP brush using the same method that was used for themodification of the ITO electrodes. (Note that the ITO surfaceis not smooth enough for the AFM analysis,; thus the Siwafer with a smooth surface was used instead.) The polymerlayer was scratched with a sharp needle to delaminate the layerdown to the Si-wafer surface. The sample was then scanned overthe area with the scratched line to determine the actual polymerfilm thickness. The AFM scratch experiments revealed the sharpswelling transition of the P4VP brush upon lowering the pH from5.3 to 4.5. The brush thickness was increased from 9.2 nm at pH5.3 (Figure 1A,C) to 29.2 nm at pH 4.5 (Figure 1B,D) because ofswelling associated with the ionized pyridine groups and coun-terions. The initial shrunken state was obtained again when thepH returned to 5.3. The polymer layer swelling transition wasreversible, and it was repeatedmany times. One can conclude thatthe pH change causes the reversible switching of the brushmorphology in aqueous solutions from a stretched swollenhomogeneous brushlike layer to a collapsed monolayer of pinnedmicelles. These changes are accompanied by the polymer thin-film

Scheme 1. Chemical Modification of the ITO Electrode with the P4VP Polymer Brush

(24) Carolus, M. D.; Bernasek, S. L.; Schwartz, J. Langmuir 2005, 21, 4236–4239.

(25) Ionov, L.; Zdyrko, B.; Sidorenko, A.; Minko, S.; Klep, V.; Luzinov, I.;Stamm, M. Macromol. Rapid Commun. 2004, 25, 360–365.

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transition from the hydrophilic to hydrophobic state due to thechanges in the degree of ionization of pyridine functional groups.

Thus, as confirmed byAFManalysis, the interfacial propertiesof the P4VP-functionalized ITO electrode can be controlled by anexternal pH value.15 The positively charged swollen hydrophilicstate of the surface-confined polymer brush at pH < 4.5 ispermeable to anionic species (i.e., [Fe(CN)6]

4-) allowing theirelectrochemical process (Scheme 2, left). The neutral shrunkenhydrophobic state of the polymer thin filmgenerated upon the pHincrease (pH>5.3) does not allow the penetration of ionic speciesto the electrode-conducting interface, thus inhibiting the electro-chemical process (Scheme 2, right).15 The reversible transition ofthe polymer-brush-modified interface between the ON statepermeable to the anionic redox species ([Fe(CN)6]

4-, 0.5 mM)and the OFF state inhibiting their electrochemical reactions wasfollowed by cyclic voltammogramsmeasured at different solutionpHvalues (not shown), resulting in the titration curve showing thepeak currents derived from the cyclic voltammograms versus thepH scale (Figure 2). The sharp transition of the electrode interfacebetween the ON and OFF states controlled by the solution pHallows the reversible activation-inactivation of the electrochemi-cal reactions at the modified surface. It should be noted that theelectrochemical properties of the P4VP-modified electrode arechanged in a narrow pH range, 4.5 < pH < 5.3.

In previous work, the interfacial electrode properties werecontrolled by the titration of the electrolyte solution to a desirablepH14b or by generating pH changes in situ by biochemicalreactions,15 both resulting in the bulk solution pH changes.

The present work aims at the local interfacial pH changesproduced on the electrode surface by electrochemical means tocontrol the polymer thin-film permeability for ionic redox species.The experiment was started at pH 4.4 (lactic buffer, 1 mM) whenthe P4VP polymer brush was protonated, positively charged,swollen, and permeable to the soluble negatively charged redoxspecies ([Fe(CN)6]

4-, 0.5 mM). (It should be noted that 1 mMlactic buffer was selected as a model solution for physiologicalfluids, with an aim toward future biomedical applications.) Thecyclic voltammogram obtained in this electrode state showed areversible electrochemical process of ferrocyanide, E� = 0.2 V,and a cathodic current of oxygen reduction,E<-0.65 V (Figure3, curve a). The well-defined electrochemical response of thesoluble redox species confirms the electrode ON state due to the“open” thin film of the polymer brush on the interface. To changethe electrode interfacial properties, we applied a potential of-0.85 V for 20 min, resulting in the electrochemical reduction ofoxygen and a concomitant increase in the interfacial pHvalue dueto the consumption of hydrogen ions in the electrochemicalprocess (eqs 1 and 2):

O2 þ 4Hþ þ 4e- f 2H2O ð1Þ

O2 þ 2Hþ þ 2e- f H2O2 ð2ÞIt is well known that the electrochemical reduction of oxygen canresult in the formation of water and hydrogen peroxide in a ratio

Figure 1. AFM topography images (A, B) with the corresponding cross-sectional profiles (C, D) of the P4VP-brush-modified Si waferobtained under aqueous solutions (0.1 M phosphate buffer) and titrated to different pH values: (A,C) 5.3 and (B,D) 4.5.The lines show thelocation where the cross-sectional profiles were acquired.

Scheme 2. pH-Controlled Reversible Switching of the P4VP Brush between the ON (Left) and OFF (Right) States Allowing and Restricting the

Anionic Species’ Penetration to the Electrode Surface, Thus Activating and Inhibiting The Redox Process

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depending on the applied potential,26 with both resulting in theconsumption of hydrogen cations. The cyclic voltammogrammeasured after 20 min of the potential application showed adramatic decrease in the electrochemical response of ferrocya-nide,E�=0.2 V, originating from the closing of the polymer thinfilm for the diffusional anionic redox species (Figure 3, curve b).At the same time, the cathodic wave of oxygen almost disap-peared and a new cathodic wave of the electrochemically gener-ated hydrogen peroxide was observed at Ep ≈ -0.75 V.

It should be noted that the origin of this new cathodic wavewasidentified by three control experiments. In one experiment,oxygen was removed from the 0.5 mM [Fe(CN)6]

4- solution byAr bubbling (at pH 9.1 when the electrode interface was OFF forthe ionic redox species), and then 15 μM H2O2 was added,resulting in a cyclic voltammogram very similar to the oneobserved after electrolysis. (Note that in this control experimentthe bulk solution pHwas changed to achieve the closed interface.)In another experiment, the solution produced by electrolysis (inthe absence of ferrocyanide) was analyzed for the presence ofH2O2 by a standard enzymatic assay27 in the presence of horse-radish peroxidase (HRP, 0.5 units 3mL-1) and ABTS (8.7 mM),resulting in anH2O2 concentration of ca. 18 μM.Another controlexperiment aimed at the origin of the closing effect of electrolysiswas performed upon application of the -0.85 V potential in theabsence of O2 in the solution (after Ar bubbling for 15 min). Inthis experiment, the electrochemical activity of the modifiedelectrode was not changed, keeping the cyclic voltammogramunchanged after 20minof electrolysis. This experiment confirmedthat the closing effect for the redox reaction of ferrocyanideoriginates from the electrochemical reaction of oxygen.

Importantly, the pH value of the bulk solution (1 mM lacticbuffer) remained almost unchanged (reaching pH4.7 after 20minof electrolysis whereas the initial value was 4.4). The bulk pHvalues correspond to the swollen state of the P4VP brush andcannot result in the electrode interface closing. Therefore, the

local interfacial pH change generated by the electrolysis processwas responsible for the closing of the modified interface. Toanalyze the interfacial pH changes upon electrolysis, we appliedredox species with theE� potential being dependent on the pH. Inthis experiment, 0.5mM thionin, known as the redox probewith aNernstian dependence of E� on pH,18 was used instead offerrocyanide and the differential pulse voltammograms (DPVs)were recorded before and after electrolysis (Figure 4, curves a andb, respectively). The negative shift of the peak potential, ΔE� =217 mV, reflected the interfacial pH changes produced uponelectrolysis. To translate the potential shift to the ΔpH value, weanalyzed the pH dependence of the E� of thionin on a bare ITOelectrode (Figure 4, inset). The dependence slope, δE�/δpH, of 46mVper pHunit was found to be close to the theoretically expectedvalue of 59 mV typical of the 2e-/2Hþ electrochemical processcharacteristic of thionin. The observed deviation of the experi-mental slope from the theoretical Nernstian value originatesfrom the incomplete electrochemical reversibility of the thioninelectrochemical process, which is typical for many quinonoidredox species.28 Then the local pH value generated by electrolysisafter 20 min of the -0.85 V potential application was derivedfrom the thionin DPV (Figure 4, curve b). This value wasestimated to be pH 9.1, corresponding to the complete OFF stateof the modified electrode on the basis of its titration curve(Figure 2). It should be noted that the thionin electrochemicalprocess was only partially inhibited at this pH (compare theDPVspeak values before and after electrolysis in Figure 4, curves aand b), thus allowing the local pH analysis by the electrochemicalmeasurements in the closed state of the interface. Thesmaller effect of the polymer-brush restructuring on the electro-chemical process of thionin (comparing with the complete inhibi-tion of the ferrocyanide redox process) could be explained by theuncharged aromatic structure of the thionin molecules allowing

Figure 2. Titration curve showing the peak current values derivedfrom the cyclic voltammograms obtained on the P4VP-brush-modified ITO electrode in the presence of 0.5 mM K4[Fe(CN)6]as the function of the solution pH. The background electrolyte wascomposed of 0.1 M phosphate buffer titrated to the specific pHvalues. The potential scan rate was 100 mV 3 s

-1.

Figure 3. Cyclic voltammograms obtained on the P4VP-brush-modified ITO electrode in the presence of 0.5 mM K4[Fe(CN)6](a) prior to the application of the potential on the electrode and (b)after the application of -0.85 V to the electrode for 20 min. Thebackground electrolyte was composed of 1 mM lactic buffer(pH 4.4) and 100 mM sodium sulfate. The potential scan ratewas 50 mV 3 s

-1. (Inset) Reversible switching of the peak currentvalue upon closing the interface by the electrochemical signal andrestoring the electrode activity by solution stirring.

(26) Vetter, K. J. Electrochemical Kinetics: Theoretical and ExperimentalAspects; Academic Press: New York, 1967.(27) (a) Keesey, J. Biochemical Information; Boehringer Mannheim Biochemicals:

Indianapolis, IN, 1987; p 58. (b) P€utter, J. Becker, R.Methods of Enzymatic Analysis,3rd ed.; Bergmeyer, H. U., Ed.; Verlug Chemie: Deerfield Beach, FL, 1983; Vol 3,pp 286-293.

(28) Katz, E.; Shkuropatov, A. N.; Vagabova, O. I.; Shuvalov, V. A.J. Electroanal. Chem. 1989, 260, 53–62.

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their penetration through the shrunken state of the polymerthin film.

After the electrode interface functionalized with the P4VPbrush was closed for the soluble redox species, the ON state ofthe surface was returned and the electrochemical reaction wasreactivated by disconnecting the applied potential and stirring theelectrolyte solution in the electrochemical cell, resulting in thecyclic voltammogram being almost identical to that observedprior to electrolysis (Figure 3, curve a). Stepwise application of the-0.85 V potential on the modified electrode for 20 min followedby electrolyte stirring allowed the reversible inactivation-activationof the electrochemical reaction, respectively (Figure 3, inset).

It should be noted that the inhibition-activation of theelectrochemical reaction depends on two processes: the interfacialpH changes and the pH-induced restructuring of the polymerbrush on the electrode surface. To analyze the kinetics of theseprocesses, we followed the ferrocyanide redox process by cyclicvoltammetry and the interfacial pH changes by thionin E�measured by DPV during the electrode transition between theON and OFF states. Figure 5A shows a set of cyclic voltammo-grams obtained at the modified ITO electrode in the presence of0.5 mM [Fe(CN)6]

4- after different time intervals of the -0.85 Vpotential application. (Electrolysis was interrupted for the cyclicvoltammetry measurements.) The inhibition kinetics of the ferro-cyanide electrochemical process was characterized by the time-dependent decrease in the peak currents derived from the cyclicvoltammograms measured after different time intervals of theapplied electrolysis (Figure 5A, inset). The half decay of theelectrochemical activity, roughly characterizing the inhibitionkinetics, was reached after ca. 1 min. Another experiment aimedat the analysis of the time-dependent local pH changes uponelectrolysis. The DPVs of 0.5 mM thionin were observed at thedifferent time intervals of the electrolysis (-0.85 V was applied;electrolysiswas interrupted for theDPVmeasurements), resultingin the E� shift upon the progress in electrolysis. The E� values forthionin were translated into local pH values using the E� versuspH dependence (Figure 4, inset). The local pH values derivedfrom the thionin DPVs were plotted as a function of electrolysistime (Figure 5B). The half change of the pH values, roughly

characterizing the kinetics of this process, was reached in less than30 s, which is quite similar to the inhibition of the ferrocyanideprocess (compare Figure 5A, inset and Figure 5B). The similarityof both sets of kinetics means that the limiting step in the wholeinhibition process is the pH change, which is rapidly followed bythe restructuring of the polymer brush to reflect the electroche-mically induced pH changes. In other words, the polymer-brushrestructuring does not introduce a significant delay in the inter-face closing, which is mostly controlled by the rate of pH change.

Because the closing of the electrode interface was controlled bythe local pH value (note that the bulk pH of the bufferedelectrolyte solution was almost unchanged), solution stirringresulted in the rapid exchange of the thin layer of the electrolyteat the interface and the system returned to the initial state(“open”) for the ferrocyanide electrochemical reaction, thusallowing the reversible inactivation-activation cycle (Figure 3,

Figure 4. Differential pulse voltammograms obtained on theP4VP-brush-modified ITO electrode in the presence of 0.5 mMthionin (a) prior to the application of the potential to the electrodeand (b) after applicationof-0.85V to the electrode for 20min.Thebackground electrolyte was composed of 1 mM lactic buffer (pH4.4) and 100 mM sodium sulfate. The potential scan rate was7mV 3 s

-1. (Inset)Dependence of the thioninE� on the solutionpHvalue derived from the DPVs on a bare ITO electrode.

Figure 5. (A) Cyclic voltammograms obtained on the P4VP-brush-modified ITO electrode in the presence of 0.5 mM K4[Fe-(CN)6] after different time intervals of the potential -0.85 Vapplication: (a) 0, (b) 1, (c) 3, (d) 10, and (e) 20 min. The potentialscan rate was 50 mV 3 s

-1. (Inset) Time-dependent decrease in thepeak current value derived from the cyclic voltammograms. (B)Local pH changes derived from the thionin DPVs measured afterdifferent time intervals of the potential -0.85 V application. Thebackground electrolyte was composed of 1 mM lactic buffer(pH 4.4) and 100 mM sodium sulfate.

4512 DOI: 10.1021/la903527p Langmuir 2010, 26(6), 4506–4513

Article Tam et al.

inset). In the next step of the experiments, we studied therelaxation process of the electrode OFF state to the initial ONstatewithout stirring,which is due to only the diffusional exchangein the solution. In this experiment, we produced the OFF state ofthe electrode interface by the application of -0.85 V for 20 minand then followed the ferrocyanide response by cyclic voltamme-try and the local pH value by DPV (by measuring the thionin E�)after the potential was removed and the electrode stayed in thebuffered electrolyte solution without stirring. Figure 6A shows aset of cyclic voltammograms (concentrated in the potential rangeof the ferrocyanide redox process) obtained after different timeintervals following the potential disconnection. As we can see, theelectrochemical response of ferrocyanide was slowly restored withthe half-increase time of ca. 17 min, roughly reflecting thediffusional exchange of the thin layer of the electrolyte at theelectrode interface (Figure 6B). Another experiment performedwith a 0.5 mM thionin solution analyzed by DPV allowed us tofollow the local pH changes upon the system’s return to its initialstate. Figure 7A, curve a, shows the DPV obtained immediatelyafter the electrochemically induced closing of the electrode inter-face, and the peak position in the DPV returned to the potentialcharacteristic of the initial pHvalue after 50min (Figure 7A, curveb). The kinetics of the local pH change upon relaxation of the

interface to the initial state was followed by measuring the DPVswith different time intervals after the end of the electrolysis (notshown). The local pH values derived from the DPVs reach theinitial state in ca. 50 min (half-decay period of ca. 17 min)(Figure 7B). The similarity of both sets of kinetics suggests thatthe opening of the inhibited interface is kinetically controlled bythe pH equilibration at the electrode surface with the bulk pHvalue, and the following restructuring of the polymer brush doesnot introduce any delay in the process.

Conclusions

The studied electrochemical system based on the P4VP-brush-functionalized ITO electrode allowed the reversible transition ofthe electrode interface between the active and inactive states forthe electrochemical process of the anionic species (i.e., ferrocya-nide anions). The switchable electrode activity was based on thereversible restructuring of the polymer brush induced by the localinterfacial pH changes triggered by the electrochemical reductionof oxygen. The polymer brush being in the hydrophilic swollen

Figure 6. (A) Cyclic voltammograms obtained on the P4VP-brush-modified ITO electrode in the presence of 0.5 mMK4[Fe(CN)6] after different time intervals following the -0.85 Vpotential disconnection, ranging from 0 to 50min. (See the specifictime intervals in B.) The potential scan rate was 50 mV 3 s

-1. (B)Current values extracted from the cyclic voltammograms at250 mV vs time.

Figure 7. (A) Differential pulse voltammograms obtained on theP4VP-brush-modified ITO electrode in the presence of 0.5 mMthionin (a) after the application of -0.85 V to the electrode for20 min and (b) after 50 min of diffusional equilibration of thesystem. The potential scan rate was 7 mV 3 s

-1. (B) Local pHchanges derived from the thionin DPVs measured after differenttime intervals following the potential -0.85 V disconnection. Thebackground electrolyte was composed of 1 mM lactic buffer(pH 4.4) and 100 mM sodium sulfate.

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state at pH<4.5 demonstrated permeability for the anionic redoxspecies with respect to the conducting support, thus allowing theirelectrochemical process. Opposite to this, the hydrophobic shrun-ken state of the polymer brush that was generated electrochemi-cally introduced a barrier for the anionic species, thus restrictingtheir access to the electrode surface and inhibiting their redoxprocess. The electrochemically produced closed state of theinterface was returned to the active state by the exchange of thethin layer of the electrolyte with the locally changed pH valueupon stirring the solution orby the slowdiffusional exchangewiththe bulk solution. The bulk solution pH stayed almost unchangedupon switching the electrode activity by the electrochemicallyinduced local pH changes.

The first example of the electrochemically controlled electrodeinterface reported in this article allowed the interface closing

induced by the electrochemical signal. Other systems based on thisconcept, being currently studied in our laboratory, will allow theinterface opening controlled by electrochemical signals. Thedeveloped approach will find many applications in electrochemi-cally switchable biocatalytic electrodes (e.g., in switchable bio-sensors or biofuel cells). A similar approach could be adapted forelectrochemically controlled release systems (e.g., drug-deliveringimplantable devices).

Acknowledgment. This research was supported by theNational Science Foundation (grants DMR-0706209 andDMR-0602528), by the ARO under grant W911NF-05-1-0339,and by the Semiconductor Research Corporation (award 2008-RJ-1839G). T.K.T. acknowledges theWallaceH. Coulter scholar-ship from Clarkson University.