Polymer Brush-Modified Electrode with Switchable and Tunable

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Polymer Brush-Modified Electrode with Switchable and Tunable Redox Activity for Bioelectronic Applications Tsz Kin Tam, Maryna Ornatska, Marcos Pita, Sergiy Minko,* and Evgeny Katz* Department of Chemistry and Biomolecular Science, Clarkson UniVersity, Potsdam, New York 13699-5810 ReceiVed: February 5, 2008

A new signal-responsive interface with switchable/tunable redox properties based on a pH-responding polymer brush was studied. Poly(4-vinyl pyridine), P4VP, functionalized with Os-complex redox units was grafted to an indium tin oxide (ITO) conductive support in the form of a polymer brush. The modified electrode surface was responsive to the changes of the pH value of the electrolyte solution: at acidic pH ) 4.0 the redoxpolymer film demonstrated the reversible electrochemical process, E° ) 0.29 V (vs Ag/AgCl), while at neutral pH > 6, the polymer was not electrochemically active. The reversible transformation between the active and the inactive state originated from the structural changes of the polymer support. The protonation of the pyridine units of the polymer backbone at the acidic pH resulted in the swelling of the polymer brush allowing quasidiffusional translocation of the flexible polymer chains, thus providing direct contact of the polymer-bound redox units and the conducting electrode support. The uncharged polymer formed at the neutral pH values existed in the shrunk state, when the mobility of the polymer chains was restricted and the polymer-bound redox units were not electrically accessible from the conducting support, thus resulting in the nonactive state of the modified electrode. The reversible changes of the electrochemical activity of the modified electrode and the respective structural changes of the polymer-brush were characterized in details by electrochemistry, AFM, and ellipsometry. The stepwise changes of the pH value between 3.0 and 7.0 resulted in the reversible switching on and off of the electrode redox activity, respectively. The redox activity of the modified electrode was also tunable upon precise titration of the electrolyte solution between pH 3.0 and 7.0 demonstrating a titration-like curve for the amount of the redox-active group because of the smooth transition between the swollen and the shrunk states. The primary electrochemical activity of the modified electrode was coupled with a biocatalytic oxidation of glucose in the presence of soluble glucose oxidase (GOx), thus allowing reversible activation of the bioelectrocatalytic process. The modified electrode with the pH-controlled switchable/tunable redox activity was proposed as a “smart” interface for a new generation of electrochemical biosensors and biofuel cells with a signal-controlled activity. Introduction Chemically modified electrodes with variable on-demand properties have attracted recent attention because of their importance for many practical applications, particularly in various molecular electronic and optoelectronic systems.1 Switchable and tunable electrochemical interfaces were used in electroanalytical and bioelectroanalytical systems,2 electrooptical systems,3 magneto-electrochemical systems,4 fuel cell and energy storage systems,5 nano- or microactuators,6 information storage and processing systems,7 single-electron devices,8 molecular and biomolecular switches,9 and systems with controlled wettability.10 Special attention was given to the modified electrodes demonstrating switchable/tunable interfacial electron transporting properties controlled by external signals.11 These systems, usually called “command interfaces”, have two surface states: one of them allows easy access of soluble redox species to the conducting support, while another state has restricted access for the redox species. The different interfacial properties originate from various mechanisms controlling the access to the electrode surface, including switchable electrostatic interaction,12 variable wettability,13 and controlled thin-film or * To whom all correspondence should be addressed. Fax: 1-315-2686610 (Department). Phone: 1-315-2684421(E.K.), 1-315-2683807(S.M.). E-mail: [email protected] (E.K.), [email protected] (S.M.)

membrane porosity.14 Electrodes functionalized with command interfaces (particularly based on photoisomerizable monolayers) were extensively used in bioelectrochemistry and biosensors.15 It should be noted that the electrodes modified with the command interfaces control the electrochemistry of diffusional redox species providing their switchable/tunable access to the conductive support. Very few examples of redox species demonstrating switchable redox properties in the immobilized state are known.16 All of these redox molecules are subjected to isomerization (usually upon photoinduced processes) resulting in two molecular states with different redox properties. In this case, the switchable electrochemical processes are controlled by the intrinsic molecular properties rather than their restricted access to the electrode surface, allowing switchable redox properties in a monolayer or thin-film configuration at an electrode surface. The unique combination of the signal-induced isomerization process coupled with the change of the redox properties required for the switchable electrochemical processes results in the limited applicability of these rare molecules. Another approach to tailoring of the signal-responsive switchable redox system is based on supramolecular systems demonstrating mechanic translocation of molecular components resulting in the change of their redox properties.17 Actually, these systems represent isomerizable redox ensembles with the structural changes within the supramolecular system. Despite their origi-

10.1021/jp801086w CCC: $40.75  2008 American Chemical Society Published on Web 05/03/2008

Polymer Brush-Modified Electrode nality and “beauty”, the synthetic complexity of these systems is an obvious drawback for their practical applications in electrochemical devices. A new powerful approach to the switchable redox materials would be possible if the redox function is separated from the isomerization (or conformation change) function and both of them are performed by different parts of the system. Polymer brushes18 are well-known systems used for functionalization of solid support to provide switchable properties for the modified interfaces. Long polymers tethered to solid supports have high flexibility of the chains and are capable to restructuring upon the change of the environment to minimize the system free energy. Polymer brushes made of polyelectrolyte molecules demonstrate restructuring at interfaces upon charging/ discharging their chains due to protonation/dissociation of the acid/base groups associated with the polyelectrolyte.19 Thus, upon changing pH value, the conformation of the polyelectrolyte polymer brush can change from the swollen state with diffusing segments around the grafting point to the shrunk state with the totally collapsed polymer chains.20 The polymer thin-film or polymer brush reorganization at an interface was used to control access of diffusional redox species to an electrode surface resulting in the gating of the electrochemical reactions by the pH-responsive polymer (actually operating as a “command interface”).14b,21 Derivatization of the polymer brushes with nanoparticles allowed their controlled movements upon cyclic shrinking/swelling of the polymer system.22 Very few studies were performed on the polymer brushes functionalized with redox units,23 including loading of a redox protein cytochrome c on the brush associated with the electrode surface.24 It was shown that the electrochemical response of the redox species loaded on the polymer brushes depends on the change of the background electrolyte, for example, on the nature of the cations and anions.25 This effect originated from the different conformation states of the polyelectrolyte support in the presence of various electrolytes. These results provided the background for the application of the polyelectrolyte brush as a mobile support for the immobilization of redox species, allowing their different interaction with the conductive support for the different states of the brush. The present paper is the first report on the modified electrode with the pH-controlled switchable/tunable electrochemical properties using a polymer brush as a signal-responsive scaffold for the immobilized redox species. Experimental Section Chemicals and Reagents. All chemicals were purchased from Sigma-Aldrich and used as supplied: glucose oxidase (GOx) from Aspergillus niger type X-S (E.C. 1.1.3.4), 4,4′dimethoxy-2,2′-bipyridine (dmo-bpy), (NH4)2OsCl6, poly(4vinyl pyridine) (P4VP, M.W. 160 kDa, density, F ) 1.101 g · cm-3), ethylene glycol, methylene chloride, ethanol, and bromomethyldimethylchlorosilane. Ultrapure water (18 MΩ· cm-1) from NANOpure Diamond (Barnstead) source was used in all of the experiments. Synthesis of Os(dmo-bpy)2Cl2 was performed according to the published procedure.26 P4VP was functionalized with Os(dmo-bpy)2 pendant groups in a solution according to the following procedure: Os(dmo-bpy)2Cl2, 50 mg, P4VP, 80 mg, were dissolved in a mixture of ethylene glycol and water (1:1 v/v), 8 mL, and refluxed for 3 days at 90 °C to yield Os-P4VP redox polymer (1). After that, the reaction mixture was centrifuged at 10 000 rpm for 40 min, and the supernatant was used for grafting onto ITO electrodes.

J. Phys. Chem. C, Vol. 112, No. 22, 2008 8439 Modification of Electrodes. ITO conductive glass (20 ( 5 Ω/sq) was purchased from Aldrich, cut in pieces (25 mm × 8 mm), and cleaned prior to the modification by sonication in ethanol and then in methylene chloride. The ITO glass was treated with a cleaning solution (NH4OH, 30%, H2O2, 33%, and H2O mixture 1:1:1 v/v) for 1 h at 60 °C, rinsed with water for 20 min, and then dried under nitrogen. The freshly cleaned ITO glass was immersed in toluene with 0.1% (v/v) bromomethyldimethylchlorosilane for 20 min at 70 °C. The silanized ITO glass was rinsed with toluene and dried under nitrogen. Os-P4VP (1) solution in toluene (10 mg · mL-1, 60 µL) was deposited on the surface of ITO. Another 60 µL of the Os-P4VP solution was added on the ITO surface after 1 h. Then the ITO glass was left to react overnight at 140 °C in a vacuum oven. The Os-P4VP-modified ITO glass was soaked in a solution of ethylene glycol and ethanol (1:4 v/v) for 30 min, followed by rinsing with acidic water (pH 3.0) to remove chemically unbound polymer. Electrochemical Measurements. Electrochemical measurements were performed with an ECO Chemie Autolab PSTAT 10 electrochemical analyzer using the software package GPES 4.9 (General Purpose Electrochemical System). The measurements were performed with a three-electrode system in a standard cell (ECO Chemie), using the Os-P4VP-modified ITO working electrode (geometrical area 1.2 cm2), a Metrohm Ag|AgCl|KCl 3 M as a reference electrode, and a Metrohm Pt wire as the counter electrode. AFM Experiments. AFM images were recorded using a MultiMode scanning probe microscope (Veeco Instruments, NY) operated in tapping mode in fluid cell (MTFML, Veeco Instruments, NY). Samples were scanned using NPS silicon nitride probes (Veeco Instruments, NY) with a spring constant of 0.32 N · m-1 and resonance frequency in aqueous media of ∼9 kHz. Typical radius of the tip curvature was 35 ( 10 nm. Scanning was performed in light tapping mode with the tapping force of 95–98% from the set point (the typical setpoint was 0.9 to 2.4 V), integral gain 0.2–0.6, proportional gain 2–6, speed of scanning 0.4–2.0 Hz. After each change of the phosphate buffer, the sample was allowed to equilibrate for 10 min before beginning first image capture. Ellipsometry. Polymer layer thickness was measured with an null-ellipsometer (Multiscope, Optrel, Germany). The wavelength of the laser was 632.8 nm. The film thickness was measured at an incident angle of 70°. For the data interpretation, we used a multilayer model of the grafted films according to the protocol described in literature.27 Ellipsometric thickness of the P4VP layer was estimated using a 3-layer model with the SiO2 layer typically 1.4 ( 0.2 nm (refractive index n ) 1.4598), the bromomethyldimethylchlorosilane layer typically 0.7 ( 0.1 nm (n ) 1.4650), and the P4VP layer usually 9 ( 0.4 nm (n ) 1.56). Results and Discussion Polyelectrolyte, poly(4-vinyl pyridine), (P4VP), was chemically functionalized with pendant redox units by reacting with Os(dmo-bpy)2Cl2 resulting in the complex formation with pyridine ligands associated with the polymer. The conditions of the reaction were optimized to involve only a small fraction of pendant pyridine groups in the complex formation and reserve a larger fraction of the pyridine group for future use in the pH responsive mechanism of the brush. The amount of the Os complex bound to the polymer backbone was estimated as 1 complex per 350 pyridine units based on the results of UV–vis spectroscopy (on average, 4 Os complexes in one P4VP chain).

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SCHEME 1: Functionalization of Poly(4-vinyl pyridine) (P4VP) with the Pendant Redox Groups and Modification of the ITO Electrode with the Resulting Redox Polymer

Figure 1. Cyclic voltammograms of the Os-P4VP-modified electrode obtained upon application of different potential scan rates: (a) 50, (b) 100, (c) 200, (d) 300, (e) 400, and (f) 500 mV · s-1 The measurements were performed in 0.1 M phosphate buffer, pH 4.0, under Ar.

The generated Os-P4VP redox polymer (1) was used for the ITO-electrode modification, Scheme 1. The ITO-electrode surface was reacted with bromomethyldimethylchlorosilane to yield Br-functionalized interface activated for the alkylation of the free pyridine residuals of the redox polymer. Then Os-P4VP (1) was reacted with the silanized ITO surface to bind the redox polymer through quaternized pyridine groups upon their reaction with Br-alkyls of the silane thin-film on the surface, yielding tethered polymer chains in the form of a polymer brush. The thickness of the Os-P4VP brush in a dry state was estimated by ellipsometry resulting in ca. h ) 9 nm that corresponds to the grafting amount of 9.9 mg · m-2. Thus, the surface concentration of the Os complex associated with the polymer brush can be estimated as approximately 3 × 10-11 mole · cm-2. Since the polymer chains are grafted randomly via pyridine rings, we speculate that in the crowded polymer layer the chains are grafted by at least one point. Thus, on average, the length of the grafted strands is half of the chain length. This layer can be modeled as a polymer brush-like layer constituted of different length grafted strands with some fraction of polymer loops (for chains with more than one grafting point). The estimated grafting density of strands for this model is σ ) h · F · NA/2 · M.W. ) 0.075 chain · nm-2, and the reduced grafting density is Σ ) 11 (the number of chains that occupy an area that a free nonoverlapping polymer chain would normally fill at the same experimental conditions, Σ ) σ · π(〈s2〉1/2)2, where 〈s2〉1/2 is the gyration radius of polymer chain estimated here as 〈s2〉1/2 ) 0.25 nm · N0.5 ) 7.1 nm for θ conditions). The estimated reduced grafting density (Σ > 1) shows that the grafted strands are in the brush regime.28 The electrochemical properties of the redox-functionalized polymer brush-modified electrode were studied by cyclic voltammetry. Figure 1 shows cyclic voltammograms of the Os-P4VP-modified electrode obtained at different potential scan rates at pH 4.0. The cyclic voltammograms demonstrate the reversible electrochemical process, E° ) 0.29 V, with a small peak-to-peak separation, ∆Ep < 50 mV (at potential scan rates, V < 200 mV · s-1) and a shape typical for a surface-confined electrochemical reaction (the current drops to the background value after the peak). The width of the current peak at halfheight, Ewphh, was approximately 145 mV at V ) 50 mV · s-1, which is greater than the theoretical value29 of 90.6/n (n ) 1, the number of electrons in a single electron transfer act characteristic of the Os-complex electrochemistry30). This

Figure 2. Electrochemical responses of the Os-P4VP-modified electrode obtained at different potential scan rates: (A) The peak current dependence on the square-root of the potential scan rate. (B) The charge associated with the peaks on the cyclic voltammograms as a function of the potential scan rate. The measurements were performed in 0.1 M phosphate buffer, pH 4.0, under Ar.

deviation could originate from the repulsive interaction between the positively charged Os-complex units or/and polymer chains.31 The peaks on the cyclic voltammograms are stable and reproducible after many potential cycles and after the electrode washing, thus confirming that the electrochemical process originates from the surface-confined redox species. One would expect that the peak current value, Ip, should linearly increase with the elevated potential scan rate as it is theoretically predicted for a surface-confined electrochemical process;29,32 however, surprisingly, we found that Ip is proportional to the square-root of the potential scan rate (V) as predicted by the

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Figure 3. Peak-to-peak separation as a function of the potential scan rate. The measurements were performed in 0.1 M phosphate buffer, pH 4.0, under Ar.

Figure 5. Cyclic voltammograms of the Os-P4VP-modified electrode obtained upon stepwise measurements performed in neutral and acidic solutions: (a) pH 7.0, (b) 3.0, (c) again pH 7.0, (d) again pH 3.0, and (e) again pH 7.0. Inset: Reversible switching of the Os-P4VP-modified electrode activity.

Figure 4. Cyclic voltammograms of the Os-P4VP-modified electrode obtained at different pH values of the background solution: (a) 3.0, (b) 4.0, (c) 4.35, (d) 4.65, (e) 5.0, (f) 5.5, (g) 6.0, and (h) 7.0. Inset: Peak current dependence on the pH value of the background solution. The measurements were performed under Ar in 0.1 M phosphate buffer titrated to different pH values with the potential scan rate, 500 mV · s-1.

Randles-Sevcik equation for a diffusion-controlled electrochemical process,32 Figure 2A. This unusual behavior of the surface-modified electrode originates from the quasi-diffusional translocation of the redox units bound to the flexible polymer chains tethered to the electrode surface. This phenomenon was already reported for ferrocene groups linked to the end of long polymeric chains bound to an electrode,33 and it was rationalized in the model of elastic bounded diffusion.34 However, the present system represents much higher structural complexity due to the multiple redox centers randomly bound to the electrode-tethered polymer chains at different distances from the electrode surface. This does not allow the quantitative treatment of the electrochemical data to derive the diffusional parameters of the flexible chains. However, qualitatively, we can make the following important conclusions: (i) The electron transport between the conductive support and the redox centers bound to the polymer brush proceeds upon the quasi-diffusional translocation of the polymer chains requiring their flexibility. (ii) The distances between the redox centers are too long for the electron hopping between them. (iii) Only at slow potential scan rates can most of the redox centers can exchange electrons with the electrode support, while upon elevation of the scan rate, the part of the electrochemically accessible redox centers is decreasing,

Figure 2B. On the basis of these conclusions, we calculated the surface coverage of the electrode with the redox species, Γ ) 3 × 10-11 mole · cm-2, by integrating the peaks on the cyclic voltammogram recorded at a low potential scan rate, V ) 50 mV · s-1. This value is in a good agreement with the surfaceconcentration derived from the brush thickness and polymer composition. To have an additional support for our conclusions summarized above, we performed a control experiment with the Os-P4VPmodified electrode when the free pyridine units of the polymer brush were cross-linked upon reacting with 1,4-diiodobutane (4% v/v in methylene chloride) at 60 °C overnight. After crosslinking, the modified electrode had completely lost its electrochemical activity, thus confirming the conclusion that the quasidiffusional mobility of the flexible polymer chains is essential for the electron exchange between the redox groups associated with the polymer brush and the conducting support. In order to analyze the electrochemical kinetics and for a better understanding of the reaction mechanism, we analyzed the peak potentials shift upon increasing the potential scan rate. The peak-to-peak separation (∆Ep) keeps almost constant very small value of 10 mV, characteristic for a surface-confined reversible electrochemical process,32 when slow potential scan rates were applied (ν < 100 mV · s-1) Figure 3. The ∆Ep is obviously smaller than the theoretically predicted value (59/n mV) for the reversible diffusion-limited electrochemical process32 (n ) 1 for the Os-complex electrochemistry30), reflecting prevailing surface-confined electrochemical kinetics at slow potential scan rates. The elevated potential scan rate resulted in the deviation of the electrochemical process from the reversibility yielding the increasing ∆Ep values. However, the increased ∆Ep values originate from a complex mixed kinetics, which cannot be considered as pure surface-confined or diffusion-limited. We roughly estimated the electron transfer rate constant, ket ) 14 s-1, using the Laviron’s formalism29 developed for the surface-confined electrochemical reactions taking into account only ∆Ep values measured at relatively low potential scan rates (0.3 < ν < 1 V · s-1), when the diffusional limitation is still not critical. It should be noted that the anodic and cathodic peak potentials (Epa and Epc) were shifted in the

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SCHEME 2: Reversible pH-Controlled Transformation of the Redox-Polymer Brush on the Electrode Surface between Electrochemically Active and Inactive States

positive and negative directions with the similar slopes: δEpa/ δ(log ν) ) 0.045 V and δEpc/δ(log ν) ) 0.042 V (Supporting Information, Figure SI-1) resulting in the similar anodic and cathodic electron transfer coefficients, Ra ≈ βc ≈ 0.5, and reflecting similar activation energies for the anodic and cathodic processes. The most intriguing electrochemical results for the Os-P4VPmodified electrode were obtained upon varying pH of the background solution. The cyclic voltammograms obtained in the pH range of 3–5 show the negative potential shift of the peaks and their diminishing upon increasing the pH value, Figure 4. Finally, the peaks have almost disappeared on the cyclic voltammograms at pH > 6. Both effects are very unusual and deserve serious discussion. The Os(dmo-bpy)2-pendant redox groups are subjected to the reduction–oxidation process without participation of protons in the reaction;30 thus, their redox potential should not be affected by the pH value! However, the redox potential, E1/2, derived from the cyclic voltammograms of the Os-P4VP-modified electrode is negatively shifted upon increasing pH with the slope of approximately 67 mV/pH, which is slightly bigger than the Nernstian value of 59 mV/pH for 1e-/1H+ redox reaction (Supporting Information, Figure SI-2). A similar phenomenon of the pH-dependence for the potential of redox species reduced/oxidized without protonation/deprotonation was already reported for a viologen derivative immobilized in a polyelectrolyte thin film.35a This effect was rationalized in the terms of the Donnan potential generated on the interface.35 The decrease and finally disappearance of the electrochemical activity of the Os-P4VP-modified electrode upon increasing the pH value of the background solution is even more interesting and important. Peak currents derived from the cyclic voltammograms obtained for a freshly prepared Os-P4VPmodified electrode rapidly decrease upon elevating the pH of the background solution from 3.0 to 7.0, Figure 4, inset. This originates from the shrinking of the polymer brush at the neutral pH resulting from the deprotonation of the pyridine groups.20a,36 The shrunken polymer losses the quasi-diffusional flexibility of the polymer chains essential for the electrochemical activity of the redox groups, thus resulting in the decrease and finally disappearance of the peaks in the cyclic voltammograms. It should be noted that the observed pH-controlled change of the electrochemical activity is a novel effect observed for the first time specifically for the brush-configuration of the organic modified film. The similar Os-P4VP redox polymer (1) randomly adsorbed on an electrode surface does not show this

effect.26 Stepwise electrochemical experiments performed in neutral (pH 7.0) and acidic (pH 3.0) solutions, Figure 5, show reversible and reproducible switching between the inactive and the active states of the Os-P4VP-modified electrode, Figure 5, inset. This switch originates from the different states of the polymer brush: in one of them (pH < 4), the polymer is swollen and allows the quasi-diffusional translocation of the polymer chains providing the electrochemical accessibility for the pendant redox groups, while in another state (pH > 6), the polymer is shrunken and the chains movements are “frozen” restricting the electrochemical process (only a few redox groups that are adjacent to the electrode are still electrochemically active), Scheme 2. In order to visualize two different states of the polymer brush, we performed AFM measurements for a similar brush grafted on silica to eliminate the roughness of the ITO electrode support. The measurements were performed in phosphate buffer at different pH values. The AFM image obtained at pH 7.0 for the shrunken brush state reveals a nanodomain-segregated morphology (pinned micelles) with the average domain sizes 45 ( 5 nm, which are known to form by polyelectrolyte brushes in poor solvent,38 Figure 6A. Poly(4-vinylpyridine) is a weak polyelectrolyte with a hydrophobic backbone. At pH below pH 4, the polymer is swollen because of electrostatic interactions. Ionization degree for P(4VP) is pH dependent and approaches approximately 50% at pH 4.38 At pH values above pH 4, the polymer undergoes phase transition from swollen coil to a compact globule because of the strong hydrophobic interactions in the polymer backbone. The AFM image obtained at pH 4.0 shows smooth morphology typical for the swollen state of the brush, Figure 6B. The different morphological features of the brush support the mechanism suggested for the switchable electrochemical behavior of the modified electrode. This result is intact with the coil-to-globule conformational transition of P2VP chains around pH 4 observed in a single molecule experiment36 and with a transition of the thickness and wetting behavior of P2VP brushes reported elsewhere.20a The pH-controlled electrode with the switchable redox function was further coupled with a bioelectrocatalytic reaction to illustrate some future potential applications. The Os-complex associated with the polymer is known to mediate the electron transfer process between redox enzymes (e.g., glucose oxidase, GOx) and the conducting support, thus activating the bioelectrocatalytic oxidation of glucose.39 We obtained cyclic voltammograms for the modified electrode at pH 4.0 (when it is in

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Figure 6. AFM images of the Os-P4VP brush on the Si-wafer: (A) Topography and cross section at pH 7.0 (z range, 100 nm; rms roughness, 4 nm). (B) Topography and cross section at pH 4.0 (z range, 150 nm; rms roughness, 9 nm).

Figure 8. Reversible switching on and off of the bioelectrocatalytic oxidation of glucose in the presence of soluble GOx, 1 mg · mL-1, and the Os-P4VP-modified electrode upon changing the pH value of the background solution: (a) pH 4.0, (b) pH 7.0. The measurements were performed under Ar in 0.1 M phosphate buffer, the potential scan rate, 10 mV · s-1. Figure 7. Cyclic voltammograms corresponding to the oxidation of glucose biocatalyzed by soluble glucose oxidase (GOx), 1 mg · mL-1 and mediated by the redox-polymer brush: (a) in the absence of glucose, (b) in the presence of 5 mM glucose. Inset: The calibration plot showing the bioelectrocatalytic current derived from the cyclic voltammograms (at E ) 0.6 V) as a function of the glucose concentration. The measurements were performed under Ar in 0.1 M phosphate buffer, pH 4, the potential scan rate, 10 mV · s-1.

the swollen electrochemically active state) in the presence of GOx, 1 mg · mL-1, and variable concentrations of glucose, Figure 7. The cyclic voltammograms clearly show the anodic electrocatalytic current corresponding to the oxidation of glucose biocatalyzed by GOx and mediated by the Os-functionalized brush. The values of the anodic electrocatalytic current depend on the glucose concentration revealing saturation at the glucose concentration bigger than 1 mM. However, the most exciting result obtained in the study is the pH-controlled switchable bioelectrocatalysis. The pH increase from 4.0 to 7.0 results in the sharp inhibition of the bioelectrocatalytic process because of the inactivation of the Os-polymer brush performing the

mediator function in the process. Cyclic voltammograms recorded at pH 4.0 and pH 7.0 show the activated and deactivated bioelectrocatalytic process, Figure 8, curves a and b, respectively. The stepwise pH changes between pH 4.0 and 7.0 resulted in the reversible switching of the bioelectrocatalytic process on and off, respectively. It should be noted that the observed phenomenon originates from the restructuring of the polymer brush yielding the electrochemically active or inactive states, which activate or inhibit the electron transport from GOx to the electrode support, Scheme 3. The switchable behavior is based on the pH-controlled redox activity of the mediator units rather than on the physical accessibility toward GOx. Conclusions and Perspectives The present study demonstrates a novel approach to the development of switchable modified electrodes based on the restructuring of the polymer support for the redox units. The supporting polymer organized on the electrode surface in the form of the brush and loaded with redox units is able to

8444 J. Phys. Chem. C, Vol. 112, No. 22, 2008 SCHEME 3: Reversible pH-Controlled Activation and Inactivation of the Bioelectrocatalyzed Glucose Oxidation

change its structure allowing or restricting the electron transport between the redox centers and the conducting support. This behavior is possible because of the quasi-diffusional mechanism of the electron transport provided by the flexible polymer chains being in the swollen state. The reversible transition of the polymer to the collapsed shrunken state does not allow the mobility of the chains and restricts the electron transport between the redox species associated with the polymer and the conducting support. This phenomenon nicely demonstrates the novel application of polymer brushes and can be coupled with many electronic and optoelectronic functions. The functional groups with various electrocatalytic and optoelectronic properties composed of different redox species, dyes, nanoparticles, quantum dots, biorecognition elements could be associated with the polymer-brush electrodes allowing switchable properties controlled by the reconfigurable brush structure. The present study demonstrated the pH-switchable bioelectrocatalytic oxidation of glucose in order to illustrate the possible applications of the reconfigurable brush-modified electrodes. This process could be used in different switchable bioelectrocatalytic systems, for example, switchable glucose biosensors or biofuel cells. Much broader applications will be possible when electroactivated units with different electro/optical properties are introduced into the brush structure. Acknowledgment. This research was supported by the NSF Grant DMR-0706209. Supporting Information Available: Experimental plots showing the peak potential shift upon increasing the potential scan rate and the redox potential dependence on the pH value of the solution. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shipway, A. N.; Katz, E.; Willner, I. In Structure and Bonding. Molecular Machines and Motors, Sauvage, J.-P. Ed.; Springer-Verlag: Berlin, 2001; Vol. 99, pp. 237–281. (2) (a) Katz, E.; Weizmann, Y.; Willner, I. J. Am. Chem. Soc. 2005, 127, 9191–9200. (b) Katz, E.; Sheeney-Haj Ichia, L.; Bückmann, A. F.; Willner, I. Angew. Chem., Int. Ed. 2002, 41, 1343–1346. (c) Rodriguez, M. C.; Kawde, A. N.; Wang, J. Chem. Commun. 2005, 4267–4269. (d) Gooding, J. J.; Wasiowych, C.; Barnett, D.; Hibbert, D. B.; Barisci, J. N.; Wallace, G. G. Biosens. Bioelectron. 2004, 20, 260–268. (e) Liu, A.; Zhou,

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