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 Halamek, Sergiy Minko,* and Evgeny Katz* Department of Chemistry and Biomolecular Science and NanoBio Laboratory (NABLAB), Clarkson University, 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 the interfacial activity by the electrochemical signal. The application of an external potential (-0.85 V vs Ag|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 initial swollen state of the protonated P4VP brush (pH 4.4) was permeable to the anionic [Fe(CN)6]4- redox species, but the electrochemically produced local pH of 9.1 resulted in the deprotonation of the polymer brush. The produced hydrophobic shrunken state of the polymer brush was impermeable to the anionic redox species, thus fully inhibiting its redox process at the electrode surface. The interface’s return to the electrochemically active state was achieved by disconnecting the applied potential, followed by stirring the electrolyte solution or by slow diffusional exchange of the electrode-adjacent thin layer with the bulk solution. The developed approach allowed the electrochemically triggered inhibition (“closing”) of the electrode interface. The application of this approach to different interfacial systems will allow the use of various switchable electrodes that are useful for biosensors and biofuel cells with externally controlled activity. Further use of this concept was suggested for electrochemically controlled chemical actuators (e.g. operating as electroswitchable drug releasers).
Introduction Reversible activation-inactivation of electrochemical interfaces is a key element in many electrocatalytic,1 bioelectrocatalytic,2 photoelectrocatalytic,3 biosensor,4 information processing bioelectronic,5 and molecular machine6 systems. Triggering of the electrode interface transition between active and inactive states was achieved by the application of light signals resulting in the photoisomerization of surface-confined organic molecules,7 by an external magnetic field resulting in the translocation of magnetic nanoparticles8 or the reorientation of magnetic nanowires9 *To whom all correspondence should be addressed. (S.M.) Fax: 1-3152686610. Tel: 1-315-2683807. E-mail:
[email protected]. (E.K.) Fax: 1-315-2686610. Tel: 1-315-2684421. E-mail:
[email protected]. (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.
4506 DOI: 10.1021/la903527p
associated with the interface, by chemical signals affecting the interfacial properties,10 and by temperature alteration producing structural changes in polymer thin films.11 Responsive polymer thin films immobilized on electrode surfaces were used to regulate electrochemical processes. Harris and Bruening10a demonstrated that the pH-induced swelling of multilayers prepared from poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate sodium salt) (PSS) bilayers affected the permeability of the film for diffusive ionic redox species. The swelling of the multilayer in basic solutions was accompanied by a 10-fold increase in the film permeability. Stimuli-responsive hydrogel thin films with reversible tunable or switchable ion permeability have been explored by several groups.10,11 Jaber and Schlenoff11b and later Akashi et al.11a reported on the reversible temperature-modulated ion permeability of multilayers assembled using ionically modified poly-N-isopropylacrylamide (PNIPAAM) copolymers. The changes in ion transport across the films were attributed to the variations in film swelling arising from the phase transition of PNIPAAM. Hydrogel thin films with reversible pH-switchable selectivity for both cations (pH 10) and anions (pH 3) were reported by Advincula and co-workers.10b The responsive multilayers were (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. B 2001, 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.
Published on Web 12/11/2009
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assembled from benzophenone-modified poly(acrylic acid) (PAA) and PAH under pH conditions in which the polymers were partially ionized, resulting in a large fraction of loops and tails that contain free carboxyl and amino groups (i.e., not involved in the formation of the polyelectrolyte complex). The pH-dependent ionization and protonation of these groups, which enabled switching of the net ionic charge of the multilayer between negative and positive values, were responsible for the observed bipolar ion permselective properties of the photo-crosslinked PAH/PAA films. In later work,11c the functionality of the PAA/PAH films was further extended by the grafting of a PNIPAAM brush atop the multilayer. The resulting thin films with binary architecture enabled a dual control mechanism (pH and temperature) for ion permeability across the films. Polyelectrolyte polymer brushes tethered to surfaces12 can effectively control interfacial properties, being switchable between shrunken and swollen states depending on their charge as controlled by an external pH value.13 Application of the pH-controlled polymer brushes for switching electrode interfacial properties14 allowed modified electrodes that were reversibly activated-inactivated by pH changes produced in situ by biocatalytic reactions.15 These electrodes have found applications in biofuel cells with the power output controlled by enzymatic16 or immune-recognition17 processes coupled with polymer brush restructuring induced by the biocatalytic pH changes. The pH changes resulting in the interfacial switching of the polymerbrush-modified surfaces can be produced in a bulk solution15 or directly at the functional interface.18 Further developments in chemical, biochemical, and electrochemical means for the reversible activation-inactivation of the electrode interfaces functionalized with the restructuring polymer systems would be beneficial for future switchable biosensors, actuators, and controlled-release devices. Polyelectrolyte brushes with electroactive counterions were recently used as an effective platform for surfaces with electrochemically switchable wetting properties.19 The electroactuation of microcantilevers coated on one side with cationic polyelectrolyte brushes represents an exciting example of a miniaturized electromechanical device based on stimuli-responsive polymer brushes where the response was triggered by an electrical potential applied to the functionalized electrode.20 This (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. ACS Nano 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. ACS Appl. 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.
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article demonstrates for the first time the electrochemically induced reversible “closing” of the electrode interface functionalized with a poly(4-vinylpyridine) (P4VP) brush tethered to the electrode surface. The pH-triggered switching behavior of responsive P2VP and P4VP thin films has been the subject of a number of publications,21 which is rationalized by a sharp coil-to-globule transition of the weak hydrophobic polyelectrolyte chains.22 Unlike previous reports, in this work, the switching process was based on the local interfacial pH changes induced by an electrochemical reaction.
Experimental Section Chemicals and Supplies. Poly(4-vinyl pyridine) (P4VP, MW
160 000 g 3 mole-1, F = 1.101 g 3 cm-3, Sigma-Aldrich), L-(þ)lactic acid (Sigma-Aldrich), 2,20 -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS, Sigma-Aldrich), horseradish peroxidase (HRP) type VI (E.C. 1.11.1.7, Sigma-Aldrich), thionin acetate (Alfa Asear), bromomethyldimethylchlorosilane (Gelest), and other chemicals and solvents were used as supplied without any further purification. Indium tin oxide (ITO) singleside-coated conducting glass (20 ( 5 Ω/sq surface resistivity, Sigma-Aldrich) served as the working electrode for electrochemical measurements. Highly polished silicon wafers (purchased from Semiconductor Processing, Union Miniere USA Inc.) were used for AFM experiments. Ultrapure water (18.2 MΩ 3 cm) from NANOpure Diamond (Barnstead) was used in all of the experiments. Electrode Modification. The ITO electrodes were chemically modified with P4VP brushes using the grafting-to method23 according to the following procedure. The ITO-coated glass slides were cut into 25 mm 8 mm strips. They were cleaned with ethanol in an ultrasonic bath for 15 min and dried in a stream of argon. The cleaning step was repeated using methylene chloride as a solvent. The initial cleaning steps were followed by immersing the strips into a cleaning solution (heated to 60 °C in a water 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 extreme precautions must be taken upon its use.) Subsequently, the glass strips were rinsed several times with water and then dried under argon. The freshly cleaned ITO strips were reacted with 0.1% v/v bromomethyldimethylchlorosilane in toluene for 20 min at 70 °C. The silanized ITO was rinsed with several aliquots of toluene and dried under argon. Then 60 μL of the P4VP solution in nitromethane (10 mg 3 mL-1) was applied to the surface of each ITO glass strip, dried to form a polymer coating, and left to react in a vacuum oven at 140 °C overnight. The final cleaning steps to remove the unbound polymer consisted of soaking for 10 min in ethanol, followed by an additional 10 min in a dilute solution of H2SO4 (pH 3). Modified electrodes were stored under water. The Si wafers were cleaned and modified using the same procedure as for the ITO-coated glass. We did not observe any differences in the properties of the brushes prepared on the different substrates. Electrochemical Measurements. The measurements were carried out with an ECO Chemie Autolab PASTAT 10 electrochemical analyzer using the GPES 4.9 (General Purpose Electrochemical System) software package for cyclic voltammetry and differential pulse voltammetry. All measurements were performed (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. Macromolecules 2007, 40, 2086–2091. (d) Lupitskyy, R.; Motornov, M.; Minko, S. Langmuir 2008, 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. Langmuir 2004, 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.
DOI: 10.1021/la903527p
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Tam et al. Scheme 1. Chemical Modification of the ITO Electrode with the P4VP Polymer Brush
at ambient temperature (23 ( 2 °C) in a standard three-electrode cell (ECO Chemie). The working electrode was a P4VP-modified ITO-glass electrode with a geometrical area of 1.2 cm2. (Note that the typical surface roughness factor for ITO electrodes is ca. 1.6 ( 0.1.24) A Metrohm Ag|AgCl|KCl 3M electrode served as a reference electrode, and a Metrohm Pt wire was used as a counter electrode. The background aqueous electrolyte solution for the experiments with the electrochemically switchable interfaces was composed of 1 mM lactic buffer (pH 4.4) and 100 mM sodium sulfate. The electrochemical measurements over a broad pH range (the titration experiments) were performed in 0.1 M phosphate buffer with the pH adjusted by additions of H2SO4 or KOH. Cyclic voltammograms were recorded in the presence of 0.5 mM potassium 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 for each measurement were obtained from a second scan. The potential of -0.85 V was applied to the P4VP-modified electrode for 20 min to close the polymer brush electrochemically. Thionin acetate (0.5 mM) was used as a pH-dependent redox probe to detect the local pH produced electrochemically at the electrode surface. The E° measurements for thionin were performed using differential pulse voltammetry (DPV) with a potential scan rate of 7 mV 3 s-1. The obtained E° values were compared with the thionin potential measured at a bare ITO electrode at various pH values (0.1 M phosphate buffer titrated to different pH values) to derive the respective interfacial pH value. The bulk pH measurements were performed with a Mettler Toledo SevenEasy pH meter. Deoxygenation (in the control experiments) was achieved by bubbling argon through the working solution for 15 min. Ellipsometry. The layer thickness and the amount of grafted material were evaluated at a wavelength of 633 nm and at an angle of incidence of 60° for the P4VP-ITO glass using an Optrel Multiscop (Berlin, Germany) null ellipsometer equipped with an XY-positioning table for mapping the sample surface. Atomic Force Microscopy. The AFM studies were performed on a Multimode scanning microscope (Veeco Instruments, NY) operating in tapping mode. The samples of the P4VP brush bound to Si wafers were scanned using NPS silicon nitride probes (Veeco Instruments, NY) with a resonance frequency of ∼9 kHz and a spring constant of 0.58-0.32 N/m in the aqueous 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 analysis was used to follow changes in brush swelling in the AFM experiments. In this analysis, we used a metal needle to make a scratch on the surface of the brush so that in the scratched area the grafted polymer was mechanically removed from the substrate by the needle. The scratched area was scanned in situ in different environments, and the brush thickness was evaluated
Results and Discussion The electrode modification with the polymeric brush was performed in two steps. The ITO electrode surface was reacted with bromomethyldimethylchlorosilane to yield a Br-functionalized interface. Then, P4VP was grafted to the functionalized ITO surface through quaternized pyridine groups, yielding tethered 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, was estimated by ellipsometry, which corresponds to the grafted amount of ca. 9.2 mg 3 m-2. The same value for the grafting amount was obtained for a similarly modified Si-wafer substrate. The obtained grafted amount corresponds to a grafting density value of 0.075 chain 3 nm-2, as reported elsewhere.13a This is a typical grafting density value for the grafting-to method that indicates a sufficient number of surface-binding sites and originates from the limitation of the grafting by the diffusion of polymer chains through the grafted brush layer.12b The polymer-brush properties were shown to be dependent on the protonation state of the polymer chains.25 To characterize the pH-switchable properties of the surfaces prepared in the present study, we followed the pH-controlled shrinking-swelling of the polymer thin film by AFM scratch analysis (Figure 1). All AFM measurements were carried out in aqueous solutions with an ionic strength similar to that of the solutions used in the electrochemical experiments. The smooth surface of a Si wafer was modified with the P4VP brush using the same method that was used for the modification of the ITO electrodes. (Note that the ITO surface is not smooth enough for the AFM analysis,; thus the Si wafer with a smooth surface was used instead.) The polymer layer was scratched with a sharp needle to delaminate the layer down to the Si-wafer surface. The sample was then scanned over the area with the scratched line to determine the actual polymer film thickness. The AFM scratch experiments revealed the sharp swelling transition of the P4VP brush upon lowering the pH from 5.3 to 4.5. The brush thickness was increased from 9.2 nm at pH 5.3 (Figure 1A,C) to 29.2 nm at pH 4.5 (Figure 1B,D) because of swelling associated with the ionized pyridine groups and counterions. The initial shrunken state was obtained again when the pH returned to 5.3. The polymer layer swelling transition was reversible, and it was repeated many times. One can conclude that the pH change causes the reversible switching of the brush morphology in aqueous solutions from a stretched swollen homogeneous brushlike layer to a collapsed monolayer of pinned micelles. These changes are accompanied by the polymer thin-film
(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.
4508 DOI: 10.1021/la903527p
using the substrate area that was uncovered by the polymer as a reference.
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Figure 1. AFM topography images (A, B) with the corresponding cross-sectional profiles (C, D) of the P4VP-brush-modified Si wafer obtained 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 the location 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
transition from the hydrophilic to hydrophobic state due to the changes in the degree of ionization of pyridine functional groups. Thus, as confirmed by AFM analysis, the interfacial properties of the P4VP-functionalized ITO electrode can be controlled by an external pH value.15 The positively charged swollen hydrophilic state of the surface-confined polymer brush at pH < 4.5 is permeable to anionic species (i.e., [Fe(CN)6]4-) allowing their electrochemical process (Scheme 2, left). The neutral shrunken hydrophobic state of the polymer thin film generated upon the pH increase (pH >5.3) does not allow the penetration of ionic species to the electrode-conducting interface, thus inhibiting the electrochemical process (Scheme 2, right).15 The reversible transition of the polymer-brush-modified interface between the ON state permeable to the anionic redox species ([Fe(CN)6]4-, 0.5 mM) and the OFF state inhibiting their electrochemical reactions was followed by cyclic voltammograms measured at different solution pH values (not shown), resulting in the titration curve showing the peak currents derived from the cyclic voltammograms versus the pH scale (Figure 2). The sharp transition of the electrode interface between the ON and OFF states controlled by the solution pH allows the reversible activation-inactivation of the electrochemical reactions at the modified surface. It should be noted that the electrochemical properties of the P4VP-modified electrode are changed in a narrow pH range, 4.5 < pH < 5.3. In previous work, the interfacial electrode properties were controlled by the titration of the electrolyte solution to a desirable pH14b or by generating pH changes in situ by biochemical reactions,15 both resulting in the bulk solution pH changes. Langmuir 2010, 26(6), 4506–4513
The present work aims at the local interfacial pH changes produced on the electrode surface by electrochemical means to control the polymer thin-film permeability for ionic redox species. The experiment was started at pH 4.4 (lactic buffer, 1 mM) when the P4VP polymer brush was protonated, positively charged, swollen, and permeable to the soluble negatively charged redox species ([Fe(CN)6]4-, 0.5 mM). (It should be noted that 1 mM lactic buffer was selected as a model solution for physiological fluids, with an aim toward future biomedical applications.) The cyclic voltammogram obtained in this electrode state showed a reversible electrochemical process of ferrocyanide, E° = 0.2 V, and a cathodic current of oxygen reduction, E < -0.65 V (Figure 3, curve a). The well-defined electrochemical response of the soluble redox species confirms the electrode ON state due to the “open” thin film of the polymer brush on the interface. To change the electrode interfacial properties, we applied a potential of -0.85 V for 20 min, resulting in the electrochemical reduction of oxygen and a concomitant increase in the interfacial pH value due to the consumption of hydrogen ions in the electrochemical process (eqs 1 and 2): O2 þ 4Hþ þ 4e - f 2H2 O
ð1Þ
O2 þ 2Hþ þ 2e - f H2 O2
ð2Þ
It is well known that the electrochemical reduction of oxygen can result in the formation of water and hydrogen peroxide in a ratio DOI: 10.1021/la903527p
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Figure 2. Titration curve showing the peak current values derived from the cyclic voltammograms obtained on the P4VP-brushmodified ITO electrode in the presence of 0.5 mM K4[Fe(CN)6] as the function of the solution pH. The background electrolyte was composed of 0.1 M phosphate buffer titrated to the specific pH values. The potential scan rate was 100 mV 3 s-1.
depending on the applied potential,26 with both resulting in the consumption of hydrogen cations. The cyclic voltammogram measured after 20 min of the potential application showed a dramatic decrease in the electrochemical response of ferrocyanide, E° = 0.2 V, originating from the closing of the polymer thin film for the diffusional anionic redox species (Figure 3, curve b). At the same time, the cathodic wave of oxygen almost disappeared and a new cathodic wave of the electrochemically generated hydrogen peroxide was observed at Ep ≈ -0.75 V. It should be noted that the origin of this new cathodic wave was identified by three control experiments. In one experiment, oxygen was removed from the 0.5 mM [Fe(CN)6]4- solution by Ar bubbling (at pH 9.1 when the electrode interface was OFF for the ionic redox species), and then 15 μM H2O2 was added, resulting in a cyclic voltammogram very similar to the one observed after electrolysis. (Note that in this control experiment the bulk solution pH was changed to achieve the closed interface.) In another experiment, the solution produced by electrolysis (in the absence of ferrocyanide) was analyzed for the presence of H2O2 by a standard enzymatic assay27 in the presence of horseradish peroxidase (HRP, 0.5 units 3 mL-1) and ABTS (8.7 mM), resulting in an H2O2 concentration of ca. 18 μM. Another control experiment aimed at the origin of the closing effect of electrolysis was performed upon application of the -0.85 V potential in the absence of O2 in the solution (after Ar bubbling for 15 min). In this experiment, the electrochemical activity of the modified electrode was not changed, keeping the cyclic voltammogram unchanged after 20 min of electrolysis. This experiment confirmed that the closing effect for the redox reaction of ferrocyanide originates from the electrochemical reaction of oxygen. Importantly, the pH value of the bulk solution (1 mM lactic buffer) remained almost unchanged (reaching pH 4.7 after 20 min of electrolysis whereas the initial value was 4.4). The bulk pH values correspond to the swollen state of the P4VP brush and cannot result in the electrode interface closing. Therefore, the (26) Vetter, K. J. Electrochemical Kinetics: Theoretical and Experimental Aspects; 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.
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Figure 3. Cyclic voltammograms obtained on the P4VP-brushmodified 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. The background electrolyte was composed of 1 mM lactic buffer (pH 4.4) and 100 mM sodium sulfate. The potential scan rate was 50 mV 3 s-1. (Inset) Reversible switching of the peak current value upon closing the interface by the electrochemical signal and restoring the electrode activity by solution stirring.
local interfacial pH change generated by the electrolysis process was responsible for the closing of the modified interface. To analyze the interfacial pH changes upon electrolysis, we applied redox species with the E° potential being dependent on the pH. In this experiment, 0.5 mM thionin, known as the redox probe with a Nernstian dependence of E° on pH,18 was used instead of ferrocyanide and the differential pulse voltammograms (DPVs) were recorded before and after electrolysis (Figure 4, curves a and b, respectively). The negative shift of the peak potential, ΔE° = 217 mV, reflected the interfacial pH changes produced upon electrolysis. To translate the potential shift to the ΔpH value, we analyzed the pH dependence of the E° of thionin on a bare ITO electrode (Figure 4, inset). The dependence slope, δE°/δpH, of 46 mV per pH unit was found to be close to the theoretically expected value of 59 mV typical of the 2e-/2Hþ electrochemical process characteristic of thionin. The observed deviation of the experimental slope from the theoretical Nernstian value originates from the incomplete electrochemical reversibility of the thionin electrochemical process, which is typical for many quinonoid redox species.28 Then the local pH value generated by electrolysis after 20 min of the -0.85 V potential application was derived from the thionin DPV (Figure 4, curve b). This value was estimated to be pH 9.1, corresponding to the complete OFF state of the modified electrode on the basis of its titration curve (Figure 2). It should be noted that the thionin electrochemical process was only partially inhibited at this pH (compare the DPVs peak values before and after electrolysis in Figure 4, curves a and b), thus allowing the local pH analysis by the electrochemical measurements in the closed state of the interface. The smaller effect of the polymer-brush restructuring on the electrochemical process of thionin (comparing with the complete inhibition of the ferrocyanide redox process) could be explained by the uncharged aromatic structure of the thionin molecules allowing (28) Katz, E.; Shkuropatov, A. N.; Vagabova, O. I.; Shuvalov, V. A. J. Electroanal. Chem. 1989, 260, 53–62.
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Figure 4. Differential pulse voltammograms obtained on the P4VP-brush-modified ITO electrode in the presence of 0.5 mM thionin (a) prior to the application of the potential to the electrode and (b) after application of -0.85 V to the electrode for 20 min. The background electrolyte was composed of 1 mM lactic buffer (pH 4.4) and 100 mM sodium sulfate. The potential scan rate was 7 mV 3 s-1. (Inset) Dependence of the thionin E° on the solution pH value derived from the DPVs on a bare ITO electrode.
their penetration through the shrunken state of the polymer thin film. After the electrode interface functionalized with the P4VP brush was closed for the soluble redox species, the ON state of the surface was returned and the electrochemical reaction was reactivated by disconnecting the applied potential and stirring the electrolyte solution in the electrochemical cell, resulting in the cyclic voltammogram being almost identical to that observed prior to electrolysis (Figure 3, curve a). Stepwise application of the -0.85 V potential on the modified electrode for 20 min followed by electrolyte stirring allowed the reversible inactivation-activation of the electrochemical reaction, respectively (Figure 3, inset). It should be noted that the inhibition-activation of the electrochemical reaction depends on two processes: the interfacial pH changes and the pH-induced restructuring of the polymer brush on the electrode surface. To analyze the kinetics of these processes, we followed the ferrocyanide redox process by cyclic voltammetry and the interfacial pH changes by thionin E° measured by DPV during the electrode transition between the ON and OFF states. Figure 5A shows a set of cyclic voltammograms obtained at the modified ITO electrode in the presence of 0.5 mM [Fe(CN)6]4- after different time intervals of the -0.85 V potential application. (Electrolysis was interrupted for the cyclic voltammetry measurements.) The inhibition kinetics of the ferrocyanide electrochemical process was characterized by the timedependent decrease in the peak currents derived from the cyclic voltammograms measured after different time intervals of the applied electrolysis (Figure 5A, inset). The half decay of the electrochemical activity, roughly characterizing the inhibition kinetics, was reached after ca. 1 min. Another experiment aimed at the analysis of the time-dependent local pH changes upon electrolysis. The DPVs of 0.5 mM thionin were observed at the different time intervals of the electrolysis (-0.85 V was applied; electrolysis was interrupted for the DPV measurements), resulting in the E° shift upon the progress in electrolysis. The E° values for thionin were translated into local pH values using the E° versus pH dependence (Figure 4, inset). The local pH values derived from the thionin DPVs were plotted as a function of electrolysis time (Figure 5B). The half change of the pH values, roughly Langmuir 2010, 26(6), 4506–4513
Figure 5. (A) Cyclic voltammograms obtained on the P4VPbrush-modified ITO electrode in the presence of 0.5 mM K4[Fe(CN)6] after different time intervals of the potential -0.85 V application: (a) 0, (b) 1, (c) 3, (d) 10, and (e) 20 min. The potential scan rate was 50 mV 3 s-1. (Inset) Time-dependent decrease in the peak current value derived from the cyclic voltammograms. (B) Local pH changes derived from the thionin DPVs measured after different time intervals of the potential -0.85 V application. The background electrolyte was composed of 1 mM lactic buffer (pH 4.4) and 100 mM sodium sulfate.
characterizing the kinetics of this process, was reached in less than 30 s, which is quite similar to the inhibition of the ferrocyanide process (compare Figure 5A, inset and Figure 5B). The similarity of both sets of kinetics means that the limiting step in the whole inhibition process is the pH change, which is rapidly followed by the restructuring of the polymer brush to reflect the electrochemically induced pH changes. In other words, the polymer-brush restructuring does not introduce a significant delay in the interface closing, which is mostly controlled by the rate of pH change. Because the closing of the electrode interface was controlled by the local pH value (note that the bulk pH of the buffered electrolyte solution was almost unchanged), solution stirring resulted in the rapid exchange of the thin layer of the electrolyte at the interface and the system returned to the initial state (“open”) for the ferrocyanide electrochemical reaction, thus allowing the reversible inactivation-activation cycle (Figure 3, DOI: 10.1021/la903527p
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Figure 6. (A) Cyclic voltammograms obtained on the P4VPbrush-modified ITO electrode in the presence of 0.5 mM K4[Fe(CN)6] after different time intervals following the -0.85 V potential disconnection, ranging from 0 to 50 min. (See the specific time intervals in B.) The potential scan rate was 50 mV 3 s-1. (B) Current values extracted from the cyclic voltammograms at 250 mV vs time.
inset). In the next step of the experiments, we studied the relaxation process of the electrode OFF state to the initial ON state without stirring, which is due to only the diffusional exchange in the solution. In this experiment, we produced the OFF state of the electrode interface by the application of -0.85 V for 20 min and then followed the ferrocyanide response by cyclic voltammetry and the local pH value by DPV (by measuring the thionin E°) after the potential was removed and the electrode stayed in the buffered electrolyte solution without stirring. Figure 6A shows a set of cyclic voltammograms (concentrated in the potential range of the ferrocyanide redox process) obtained after different time intervals following the potential disconnection. As we can see, the electrochemical response of ferrocyanide was slowly restored with the half-increase time of ca. 17 min, roughly reflecting the diffusional exchange of the thin layer of the electrolyte at the electrode interface (Figure 6B). Another experiment performed with a 0.5 mM thionin solution analyzed by DPV allowed us to follow the local pH changes upon the system’s return to its initial state. Figure 7A, curve a, shows the DPV obtained immediately after the electrochemically induced closing of the electrode interface, and the peak position in the DPV returned to the potential characteristic of the initial pH value after 50 min (Figure 7A, curve b). The kinetics of the local pH change upon relaxation of the 4512 DOI: 10.1021/la903527p
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Figure 7. (A) Differential pulse voltammograms obtained on the P4VP-brush-modified ITO electrode in the presence of 0.5 mM thionin (a) after the application of -0.85 V to the electrode for 20 min and (b) after 50 min of diffusional equilibration of the system. The potential scan rate was 7 mV 3 s-1. (B) Local pH changes derived from the thionin DPVs measured after different time intervals following the potential -0.85 V disconnection. The background electrolyte was composed of 1 mM lactic buffer (pH 4.4) and 100 mM sodium sulfate.
interface to the initial state was followed by measuring the DPVs with different time intervals after the end of the electrolysis (not shown). The local pH values derived from the DPVs reach the initial state in ca. 50 min (half-decay period of ca. 17 min) (Figure 7B). The similarity of both sets of kinetics suggests that the opening of the inhibited interface is kinetically controlled by the pH equilibration at the electrode surface with the bulk pH value, and the following restructuring of the polymer brush does not introduce any delay in the process.
Conclusions The studied electrochemical system based on the P4VP-brushfunctionalized ITO electrode allowed the reversible transition of the electrode interface between the active and inactive states for the electrochemical process of the anionic species (i.e., ferrocyanide anions). The switchable electrode activity was based on the reversible restructuring of the polymer brush induced by the local interfacial pH changes triggered by the electrochemical reduction of oxygen. The polymer brush being in the hydrophilic swollen Langmuir 2010, 26(6), 4506–4513
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state at pH
