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A two-phase system consisting of an aqueous buffer solution and a toluene phase that includes the suspended capped magnetic nanoparticles is used to c...
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Langmuir 2004, 20, 9714-9719

Magnetoswitchable Controlled Hydrophilicity/ Hydrophobicity of Electrode Surfaces Using Alkyl-Chain-Functionalized Magnetic Particles: Application for Switchable Electrochemistry Eugenii Katz,† Laila Sheeney-Haj-Ichia,† Bernhard Basnar,† Israel Felner,‡ and Itamar Willner*,† Institute of Chemistry and Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91940, Israel Received June 21, 2004. In Final Form: August 5, 2004 Magnetic nanoparticles consisting of undecanoate-capped magnetite (average diameter ∼4.5 nm; saturated magnetization, Ms, 38.5 emu g-1) are used to control and switch the hydrophobic or hydrophilic properties of the electrode surface. A two-phase system consisting of an aqueous buffer solution and a toluene phase that includes the suspended capped magnetic nanoparticles is used to control the interfacial properties of the electrode surface. The magnetic attraction of the functionalized particles to the electrode by means of an external magnet yields a hydrophobic interface that acts as an insulating layer, prohibiting interfacial electron transfer. The retraction of the magnetic particles from the electrode to the upper toluene phase by means of the external magnet generates a hydrophilic electrode that reveals effective interfacial electron transfer. The electron-transfer resistance and double-layer capacitance of the electrode surface upon the attraction and retraction of the functionalized magnetic particles to and from the electrode, respectively, by means of the external magnet were probed by Faradaic impedance spectroscopy (Ret ) 170 Ω and Cdl ) 40 µF sm-2 in the hydrophilic state of the electrode and Ret ) 22 kΩ and Cdl ) 0.5 µF sm-2 in the hydrophobic state of the interface). The magnetoswitchable control of the interface enables magnetic switching of the bioelectrocatalytic oxidation of glucose in the presence of glucose oxidase and ferrocene dicarboxylic acid to “ON” and “OFF” states.

Introduction The modification of electrode surfaces with monolayer assemblies and thin films controls the interfacial properties such as the electrical properties of the electrodes or the hydrophilicity/hydrophobicity of the surfaces1 and consequently affects the electron transfer at the conductive interface.2 The assembly of densely packed long chain alkane thiols on Au surfaces generates hydrophobic interfaces,3,4 whereas the use of alkyl chains terminated with carboxylic acid (carboxylate) or hydroxyl residues yields hydrophilic interfaces.4 Assembly of a densely packed hydrophobic monolayer on electrodes blocks the interfacial electron transfer to solubilized redox labels.5 On the other hand, charged monolayers were reported to control the electron transfer at the electrode surface6 by the electrostatic attraction or repulsion of oppositely or similarly charged redox labels, respectively (known as * To whom correspondence should be addressed. Phone: 9722-6585272. Fax: 972-2-6527715. E-mail: [email protected]. † Institute of Chemistry. ‡ Racah Institute of Physics. (1) (a) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370-1378. (b) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156-161. (c) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370-2378. (d) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8, 2560-2566. (2) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: 1996; Vol. 19, pp 109-335. (3) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7267. (4) (a) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (b) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 71557164. (c) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175. (d) Laibinis, P. L.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167-3173. (e) Creager, E.; Clarke, J. Langmuir 1994, 10, 3675-3683.

the Frumkin effect).7 The control of the hydrophilic/ hydrophobic properties of monolayer-functionalized supports or thin-film-coated surfaces has attracted recent research efforts.8-10 For example, the potential-induced bending or repulsion of carboxylate-terminated alkyl chains was reported to switch the interface to hydrophobic and hydrophilic states, respectively.8 Similarly, a bipyridinium-terminated alkyl chain monolayer revealed controlled interfacial hydrophilic/hydrophobic properties by potential-induced bending of the charged group followed by electrochemical reduction of the headgroup to the respective radical cation units.10 In all these systems, the hydrophobicity/hydrophilicity of the interfaces was con(5) (a) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 62336239. (b) Takehara, K.; Takemura, H. Bull. Chem. Soc. Jpn. 1995, 68, 1289-1296. (c) Nakashima, N.; Deguchi. Y. Bull. Chem. Soc. Jpn. 1997, 70, 767-770. (d) Krysinski, P.; Brzostowska-Smolska, M. J. Electroanal. Chem. 1997, 424, 1997. (e) Terrettaz, S.; Cheng, J.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 7857-7858. (f) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657-2668. (6) (a) Katz, E.; Schlereth, D. D.; Schmidt, H.-L. J. Electroanal. Chem. 1994, 367, 59-70. (b) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37-41. (c) Takehara, K.; Takehara, H. Bull. Chem. Soc. Jpn. 1995, 68, 1289-1296. (d) Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1994, 116, 7913-7914. (e) Katz, E.; Lion-Dagan, M.; Willner, I. J. Electroanal. Chem. 1995, 382, 25-31. (7) Delahay, P. In Double Layer and Electrode Kinetics. Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1965; Chapter 3. (8) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371-374. (9) (a) Wang, X.; Gershman, Z.; Kharitonov, A. B.; Katz, E.; Willner, I. Langmuir 2003, 19, 5413-5420. (b) Wang, X.; Katz, E.; Willner, I. Electrochem. Commun. 2003, 5, 814-818. (c) Wang, X.; Zeevi, S.; Kharitonov, A. B.; Katz, E.; Willner, I. Phys. Chem. Chem. Phys. 2003, 5, 4236-4241. (10) Wang, X.; Kharitonov, A. B.; Katz, E.; Willner, I. Chem. Commun. 2003, 1542-1543.

10.1021/la048476+ CCC: $27.50 © 2004 American Chemical Society Published on Web 09/29/2004

Alkyl-Chain-Functionalized Magnetic Particles

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Figure 1. (A) TEM image of the undecanoic acid functionalized magnetic particles. Inset: histogram showing the particle diameter distribution derived from the TEM image. (B) AFM image of the undecanoic acid functionalized magnetic particles.

trolled by a functional monolayer or thin film that was chemically bound to the surface. Functionalized magnetic particles were recently employed to control redox reactions or bioelectrocatalytic transformations at electrode surfaces.11 The electrical response of magnetic particles functionalized with electroactive groups could be switched “ON” and “OFF” by the cyclic attraction or retraction of the modified magnetic particles to and from the electrode surface, respectively.

Similarly, the mediated electrical activation of enzymes by redox relay units associated with magnetic particles could be switched ON and OFF by the electrochemical activation of the relay units through magnetic attraction or reduction of the magnetic particles to and from the electrode by means of an external magnet. Here, we wish to report on the application of magnetic particles functionalized with long alkyl chains as nanocomponents for controlling the hydrophobic properties of electrode surfaces

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Katz et al.

by means of an external magnet. We demonstrate the cyclic switchable control of the hydrophobic/hydrophilic properties of the electrode surface by the attraction or retraction of the functionalized magnetic particles to and from the electrodes, by means of the external magnet. In contrast to previous studies that used chemically modified electrodes, we control the hydrophobic/hydrophilic properties of the interface by physically associated or dissociated magnetic particles. We also describe the application of the magnetic switchable interfacial properties of the conductive surface to switch ON and OFF electrochemical and bioelectrocatalytic processes at the electrode surface. Experimental Section Chemicals and Materials. Undecanoic acid, potassium ferrocyanide, potassium ferricyanide, glucose oxidase (GOx) (EC 1.1.3.4; type X-S from Aspergillus niger), β-D-glucose, ferrocene dicarboxylic acid, and other chemicals were purchased from Sigma or Aldrich and used without further purification. Magnetic Fe3O4 particles coated with an undecanoic acid shell were synthesized according to the published procedure with the difference being that only a single capping layer was generated on the surface of the particles.12 Ultrapure water from Serapur PRO90CN source was used throughout all the experiments. Methods. Atomic force microscopy (AFM) images of the magnetic particles were acquired in tapping mode under air using a multimode atomic force microscope (Veeco-Digital Instruments). The magnetic particles for the AFM imaging were deposited on a mica support by evaporation of a toluene solution (1 mg mL-1). The room temperature magnetization measurements up to 5000 Gs were performed in a commercial (Quantum Design) superconducting quantum interference device (SQUID) magnetometer. Static contact angle measurements were performed with a droplet (10 ( 1 µL) of aqueous phosphate buffer solution (0.1 M, pH 7.0) using a CAM2000 optical contact angle analyzer (KSV Instruments). The contact angles were measured on a bare Aucoated glass plate and on a Au-coated glass plate covered with a layer of the undecanoic acid functionalized magnetic particles. A toluene solution of the modified magnetic particles (0.5 mL, 1 mg mL-1) was deposited onto the Au-coated glass plate and was dried at room temperature to yield a modified spot (∼1 cm diameter) on the surface, which was used as a support for the contact angle measurements. Electrochemical measurements were performed using an electrochemical analyzer (model 6310, EG&G) connected to a personal computer (EG&G 398 software for impedance spectroscopy or EG&G 270/250 software for cyclic voltammetry). A Au-coated (50 nm Au layer) glass plate (Analytical-µSystem, Germany) was used as a working electrode (0.3 cm2 area exposed to the solution). All the measurements were carried out under ambient temperature (25 ( 2 °C) in a conventional electrochemical cell consisting of a working electrode assembled at the bottom of the electrochemical cell, a glassy carbon auxiliary electrode, and a saturated calomel electrode (SCE) connected to the working volume with a Luggin capillary. All potentials are reported with respect to this reference electrode. Phosphate buffer (0.1 M, pH 7.0) was used as a background electrolyte. The undecanoic acid functionalized magnetic particles were added to the cell in a toluene solution (0.5 mL, 1 mg mL-1) yielding an upper organic solution layer immiscible with the aqueous electrolyte solution. The undecanoic acid functionalized magnetic particles were attracted to the Au-electrode surface from the upper organic layer or lifted back to the upper toluene layer by positioning a 12 mm diameter magnet (NdFeB/Zn-coated magnet with a remanent magnetization of 10.8 kG) below the bottom (11) (a) Hirsch, R.; Katz, E.; Willner, I. J. Am. Chem. Soc. 2000, 122, 12053-12054. (b) Sheeney-Haj-Ichia, L.; Katz, E.; Wasserman, J.; Willner, I. Chem. Commun. 2000, 158-159. (c) Katz, E.; Sheeney- HajIchia, L.; Bu¨ckmann, A. F.; Willner, I. Angew. Chem., Int. Ed. 2002, 41, 1343-1346. (d) Katz, E.; Willner, I. Electrochem. Commun. 2002, 4, 201-204. (e) Willner, I.; Katz, E. Angew. Chem., Int. Ed. 2003, 42, 4576-4588. (12) Shen, L.; Laibinis, P. E.; Hatton, T. A. Langmuir 1999, 15, 447453.

Figure 2. Magnetization curve of the magnetic particles in a toluene solution (1 mg mL-1). Inset: Calculation of the saturated magnetization of the magnetic fluid. electrode or above the electrochemical cell, respectively. Argon bubbling was used to remove oxygen from the solutions in the electrochemical cell. The cell was placed in a grounded Faraday cage. The Faradaic impedance measurements were performed in the presence of a 1 mM 1:1 K3[Fe(CN)]6/K4[Fe(CN)6] mixture as a redox probe. The Faradaic impedance spectra were recorded upon application of the bias potential that was equal to the redox probe formal potential, 0.17 V, while applying 5 mV alternative voltage in the frequency range 100 mHz-1.0 kHz. The Faradaic impedance spectra are plotted in the form of complex plane diagrams (Nyquist plots). The experimental impedance spectra were simulated using electronic equivalent circuits. For this purpose, commercial software (Zview, version 2.1b, Scribner Associates, Inc.) was employed. Cyclic voltammetry was measured in the presence of 1 mg mL-1 enzyme GOx, 1 × 10-3 M ferrocene dicarboxylic acid, and 60 mM glucose. The impedance and cyclic voltammetry measurements were performed at a bare Au-electrode surface, when the undecanoic acid functionalized magnetic particles were lifted up by positioning an external magnet above the electrochemical cell, and at the Au electrode coated with the modified magnetic particles upon their attraction to the electrode by positioning the magnet below the electrode.

Results and Discussion The magnetic particles were prepared according to the literature procedure12 using undecanoic acid as a hydrophobic capping layer. The undecanoic acid functionalized particles are freely suspendable in organic phases such as toluene and form a stable homogeneous suspension (a magnetic fluid13). Figure 1A shows the transmission electron microscopy (TEM) image of the particles. We observe particles with dimensions in the range of 3-9 nm. The inset of Figure 1A shows the histogram of the particle diameters, implying that the average diameter is ∼5.1 nm. Figure 1B shows the AFM image of the hydrophobically capped magnetic particles. The heights of the particles are very similar to the diameters observed by the TEM analysis, and the average height is ∼4.9 nm. Figure 2 shows the room temperature magnetization curve of the magnetic particle solution in toluene that reveals a saturation-like magnetization of 36.4 emu g-1. (13) (a) Berkovsky, B. M.; Medvedev, V. F.; Karkov, M. S. Magnetic Fluids: Engineering Applications; Oxford University Press: New York, 1993. (b) Rosenzweig, R. E. Ferrohydrodynamics; Cambridge University Press: Cambridge, U.K., 1985.

Alkyl-Chain-Functionalized Magnetic Particles

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Chart 1. Magnetocontrolled Reversible Translocation of the Functionalized Magnetic Particles between the Organic Phase above the Aqueous Electrolyte and the Electrode Surfacea

a (A) The electrode surface is blocked with functionalized magnetic particles attracted to the electrode by the external magnet. (B) The magnetic particles are retracted from the electrode surface, and the electrode surface is electrochemically active.

The magnetization (M) does not reach full saturation at this field (B), and by plotting M2 versus 1/B and extrapolating to 1/B ) 0, we obtain a saturation value of Ms ) 38.5 emu g-1, (Figure 2, inset). This value is smaller than Ms ) 85 emu g-1, which is the value of bulk Fe3O4, because of the microstructure of the material. The magnetization of a specimen consisting of small particles decreases with decreasing particle size due to the increased dispersion in the exchange integral, finally reaching a superparamagnetic state wherein each particle acts as a big spin with suppressed interaction between the particles.14 The effect of deposition of the functionalized magnetic particles on the hydrophobicity of a Au surface was determined by contact angle measurements (Figure 3). An aqueous droplet deposited on a bare Au electrode yields a contact angle corresponding to 38° (Figure 3A). The deposition of the functionalized magnetic particles on the Au surface by evaporating a 0.5 mL toluene droplet that contained 0.5 mg of magnetic particles resulted in a substantial increase in the contact angle, 111° (Figure 3B), implying that the surface became hydrophobic and the aqueous droplet was repelled by the surface. Realizing that the deposition of the magnetic particles on the electrode surface turns the interface into a hydrophobic medium that could block the electron transfer at the electrode surface, the possibility for the cyclic magnetoswitching of electrochemical processes by means of the functionalized magnetic particles was studied. Toward this goal, an electrochemical cell that included a

Figure 3. Contact angle measurements using a droplet composed of 0.1 M phosphate buffer, pH 7.0, on (A) a bare Auelectrode surface and (B) the Au-electrode surface coated with hydrophobic magnetic particles.

Au-coated glass plate in a horizontal position was constructed. A biphase liquid system consisting of two immiscible solutions, aqueous and toluene, was placed in the electrochemical cell. An aqueous buffer solution was used as the electrolyte, and the toluene solution included the magnetic particles. As this organic phase is lighter than the aqueous phase, it is not in contact with the electrode surface. Positioning an external magnet below the electrode attracts the magnetic particles to the electrode, and the interface is expected to become hydrophobic and to yield an electrically blocked interface (Chart 1A). Positioning the magnet atop the cell retracts the magnetic particles from the electrode, a process that is anticipated to remove the hydrophobic features of the electrode and to reactivate the electron transfer at the electrode interface (Chart 1B). The blocking of the interfacial electron transfer by means of the hydrophobically capped magnetic particles that were physically attracted to the electrode support was examined by Faradaic impedance spectroscopy. Impedance spectroscopy is an effective method for probing the interface properties of surface-modified electrodes.15 The complex impedance can be presented as the sum of the real, Zre(ω), and imaginary, Zim(ω), components originating mainly from the resistance and capacitance of the cell, respectively, measured at a variable circular frequency, ω. The typical shape of a Faradaic impedance spectrum (presented in the form of a Nyquist plot) includes a semicircle region lying on the Zre-axis followed by a straight line. The semicircle portion, observed at higher frequencies, corresponds to the electron-transfer-limited process, whereas the linear part of the spectrum is characteristic of the lower frequency range and represents the diffusion-limited electron-transfer process. The semicircle diameter in the impedance spectrum is equal to the electron-transfer resistance, Ret. The experimental Fara(14) Elliott, S. R. Physcis of Amorphous Materials; Longman: London, 1984; p 350. (15) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980. (b) Stoynov, Z. B.; Grafov, B. M.; Savova-Stoynova, B. S.; Elkin, V. V. Electrochemical Impedance; Nauka: Moscow, 1991.

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Chart 2. Equivalent Electronic Circuit Based on the Randles and Ershler Theoretical Model Used for the Computer Fitting of the Experimental Impedance Spectra

daic impedance spectra could be fitted by computer simulation using an equivalent electronic circuit based on the Randles and Ershler theoretical model (Chart 2).15,16 This equivalent circuit includes the ohmic resistance of the electrolyte solution, Rs, the Warburg impedance, ZW, resulting from the diffusion of the redox probe, the doublelayer capacitance, Cdl, and the electron-transfer resistance, Ret. Two components in the circuit, Cdl and Ret, depend on the dielectric and insulating features of the electrode/ electrolyte interface and are controlled by the surface modification of the electrode. In fact, the electron-transfer resistance, Ret, controls the interfacial electron-transfer

Katz et al.

rate between the redox probe in solution and the electrode support. The deposition of a hydrophobic, insulating layer of undecanoic acid functionalized magnetic particles on the electrode support is expected to introduce a barrier for the electron transfer at the electrode interface, resulting in an enhanced electron-transfer resistance at the electrode surface. On the other hand, the low dielectric constant value of the capping material should result in a significant decrease of the double-layer capacitance, Cdl. Indeed, in a series of reports, we have applied Faradaic impedance spectroscopy to probe the blocking of the electrode surface upon its modification with biomaterials or as a result of the formation of biorecognition complexes on the electrode surface.17 Figure 4A shows the Faradaic impedance spectrum of the system when the magnetic particles are retracted from the electrode surface. The impedance spectrum consists of a semicircle domain that corresponds to the electrontransfer-limited process, followed by a straight line observed at low frequencies that corresponds to diffusioncontrolled electron transfer. The diameter of the semicircle corresponds to an interfacial electron-transfer resistance of Ret ) 170 Ω, whereas the double-layer capacitance, Cdl, was found to be 42 µF cm-2, a value that is characteristic

Figure 4. Nyquist plot (Zim vs Zre) for the Faradaic impedance measurements performed upon (A) retraction of the functionalized magnetic particles from the electrode surface and (B) attraction of the functionalized magnetic particles to the electrode surface. The measurements were performed in the presence of a 1 mM 1:1 K3[Fe(CN)]6/K4[Fe(CN)6] mixture and upon biasing the working electrode at 0.17 V.

Figure 5. Reversible magnetoswitching of (A) the electron-transfer resistance, Ret, and (B) the double-layer capacitance, Cdl, of the Au electrode derived from the Faradaic impedance spectra. Steps 1, 3, and 5 correspond to the Au-electrode surface when the magnetic particles are retracted from the electrode. Steps 2 and 4 correspond to the Au-electrode surface coated by the attracted magnetic particles.

Alkyl-Chain-Functionalized Magnetic Particles

for a bare polycrystalline Au electrode.18 Attraction of the magnetic particle to the electrode surface results in an impedance spectrum with a very large semicircle diameter, Ret ) 22 kΩ (derived from the computer fitting), and a very low double-layer capacitance, Cdl ) 0.5 µF cm-2 (Figure 4B). These results indicate that the attraction of the hydrophobically capped magnetic particles to the electrode by means of the external magnet blocks the interfacial electron transfer and generates on the electrode surface a film with a low dielectric constant. By the reversible retraction of the functionalized magnetic particles from the electrode surface and their attraction to the electrode support by means of the external magnet, the electrical properties of the electrode interface are controlled (Figure 5). The externally induced magnetic retraction and attraction of the functionalized magnetic particles from and to the electrode surface cycle the electrode from very low interfacial electron-transfer resistance and high double-layer capacitance to very high interfacial resistance and low double-layer capacitance, respectively (Figure 5). It should be noted that we cannot eliminate the possibility that a thin film of the organic solvent (toluene) is coadsorbed with the magnetic particles on the electrode surface and that this film co-contributes to the hydrophobic features of the interface. The fact, however, that the interfacial electron-transfer resistance does not change with time even though the organic phase is lighter than water suggests either that no organic solvent is associated with the magnetic particles or that the organic solvent is tightly bound to the particles and is nondissociable upon the translocation of the magnetic particles to and from the electrode support. It should be noted that the magnetic particles used to switch the interfacial properties of the electrode support exhibit a diameter of ∼5 nm, and the particles are capped with a hydrophobic undecanoate layer. The nanodimensions of the particles lead, upon their attraction to the electrode, to a dense “membranelike” film that insulates the electrode toward redox species in the electrolyte. For larger magnetic particles (diameter >200 nm), aggregation of the particles leads, upon their attraction to the electrode, to pinhole defects that enable electrical contact between the electrode and the electrolyte solution. The magnetoswitchable control of the electron transfer at the electrode surface by means of the functionalized magnetic particles may be applied for the switching of electrochemical transformations at the conductive support. This has been demonstrated by the magnetoswitchable bioelectrocatalytic oxidation of glucose in the presence of glucose oxidase (GOx) and ferrocene dicarboxylic acid, as diffusional components19 and using the hydrophobically capped magnetic particles (Figure 6). Retraction of the magnetic particles from the electrode surface allows for the oxidation of ferrocene dicarboxylic acid at the electrode surface, and the mediated bioelectrocatalyzed oxidation of glucose by GOx proceeds at the electrode support. This is reflected by the formation of a high electrocatalytic anodic current (Figure 6, curve a). The attraction of the magnetic particles to the electrode support blocks the (16) (a) Randles, J. E. B. Discuss Faraday Soc. 1947, 1, 11-19. (b) Ershler, B. V. Discuss Faraday Soc. 1947, 1, 269-277. (17) (a) Patolsky, F.; Filanovsky, B.; Katz, E.; Willner, I. J. Phys. Chem. B 1998, 102, 10359-10367. (b) Patolsky, F.; Zayats, M.; Katz, E.; Willner, I. Anal. Chem. 1999, 71, 3171-3180. (c) Alfonta, L.; Katz, E.; Willner, I. Anal. Chem. 2000, 72, 927-935. (d) Katz, E.; Willner, I. Electroanalysis 2003, 15, 913-947. (18) Champagne, G. Y.; Belanger, D.; Fortier, G. Bioelectrochem. Bioenerg. 1989, 22, 159-165. (19) Bartlett, P. N.; Tebbutt, P.; Whitaker, R. C. Prog. React. Kinet. 1991, 16, 55-155.

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Figure 6. Cyclic voltammograms obtained in the presence of 1 mg mL-1 GOx, 1 × 10-3 M ferrocene dicarboxylic acid×, and 60 mM glucose: (a) The functionalized magnetic particles are retracted from the electrode surface. (b) The functionalized magnetic particles are attracted to the electrode surface. The measurements were performed in 0.1 M phosphate buffer, pH 7.0, under argon with a potential scan rate of 10 mV s-1. Inset: The reversible switching of the bioelectrocatalytic current upon retraction (steps 1, 3, and 5) and attraction (steps 2 and 4) of the magnetic particles from and to the electrode surface, respectively.

oxidation of ferrocene dicarboxylic acid, and the bioelectrocatalyzed oxidation of glucose is prohibited (Figure 6, curve b). By the cyclic retraction and attraction of the magnetic particles from and to the electrode surface, the bioelectrocatalyzed oxidation of glucose is switched between the ON and OFF states, respectively (Figure 6, inset). In conclusion, the present study has described a new method for reversibly controlling the electrical properties of electrode interfaces using magnetic particles modified with a long chain hydrophobic capping layer. While previous studies have controlled the interfacial features of electrode surfaces by chemical modification of the conductive supports with monolayers or thin films and by triggering or switching the interface properties by pH,8 electrical stimuli,10 or photochemical signals,20 the present study introduces a physical means to control the electrode properties that involves the controlled translocation of functionalized magnetic particles with respect to the electrode. Acknowledgment. This research was supported by The Israel Science Foundation (Project No. 101/00). LA048476+ (20) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783-1790.