Langmuir 2005, 21, 8565-8568
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Notes Magnetically Induced Carbon Nanotube-Mediated Control of Electrochemical Reactivity Mustafa Musameh and Joseph Wang* Departments of Chemical & Materials Engineering and Chemistry and Biochemistry, The Biodesign Institute, Arizona State University, Tempe, Arizona 85287 Received March 8, 2005. In Final Form: June 21, 2005
Introduction Carbon nanotubes (CNT) have generated considerable recent interest in bioelectronics and electrochemistry owing to their unique mechanical, electrical, and chemical properties.1,2 The electrocatalytic properties of these materials have been exploited as a means of promoting the electron-transfer reactions of a wide range of important biomolecules.3,4 For example, the greatly enhanced electrochemical reactivity of hydrogen peroxide and NADH at CNT-modified electrodes makes these nanomaterials extremely attractive for numerous oxidase- and dehydrogenase-based amperometric biosensors.5,6 The enhanced electrochemical reactivity has been coupled to resistance to surface fouling and hence to high stability.6,7 The use of CNT molecular wires offers great promise for achieving efficient electron transfer from electrode surfaces to the redox sites of enzymes.8 Herein we report on the magnetic field-stimulated activation and deactivation of CNT-based electrocatalytic processes. Willner and colleagues reported on the magnetic control of enzymatic reactions using magnetic particles functionalized with redox relay units.9,10 Ferrocenefunctionalized magnetic particles were thus used for electrically contacting glucose oxidase and the electrode. The concept was extended to the dual biosensing of glucose and lactate.10 The same group recently reported on the magnetoswitchable control of surface polarity using alkyl chain-functionalized magnetic spheres.11 We reported on reversible and cyclic magnetic field-stimulated DNA oxidation12 and on the magnetically induced solid-state * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 1-480-727-0399. (1) Rao, C. N.; Satishkumar, B. C.; Govindaraj, A.; Nath, M. ChemPhysChem 2001, 2, 79. (2) Wang, J. Electroanalysis 2005, 17, 7. (3) Zhao, G. C.; Zhang, L.; Wei, X. W.; Yang, Z. S. Electrochem. Commun. 2003, 5, 825. (4) Wu, F. H.; Zhao, G. C.; Wei, X. W. Electrochem. Commun. 2002, 4, 690. (5) Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. (6) Musameh, M.; J. Wang, J.; Merkoci, A.; Lin, Y. Electrochem. Commun. 2002, 4, 743. (7) Wang, J.; Deo, R. P.; Musameh, M. Electroanalysis 2003, 15, 1830. (8) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113. (9) Hirsch, R.; Katz, E.; Wasserman, J.; Willner, I. J. Am. Chem. Soc. 2000, 122, 12053. (10) Katz, E.; Willner, I. Electrochem. Commun. 2001, 4, 201. (11) Katz, E.; Sheeney-Haj-Ichia, L.; Basnar, B.; Felner, I.; Willner, I. Langmuir 2004, 20, 9714.
detection of DNA hybridization13 in connection with oligonucleotide-functionalized magnetic beads. In the following sections, we will demonstrate that the magnetic and catalytic properties of CNT14,15 can be exploited for the magnetoswitchable control of electrontransfer reactions without the need for functionalized magnetic particles. The use of magnetic fields to align CNT14 and to self-assemble CNT devices15 has been reported. However, there are no reports on magnetic fieldstimulated CNT-based electrocatalytic reactions. We will illustrate below that magnetically induced attraction and removal of CNT to and from the surface of the working electrode allows cyclic on and off activation of electrode reactivity (Figure 1). Such spatially controlled CNTinduced electrocatalysis will be demonstrated using closely spaced dual carbon working electrodes. By switching the position of an external magnet below the surface to attract the CNT, the electrocatalytic oxidation of ascorbic acid or ferrocyanide could be reversibly switched. This represents the first example of magnetoswitchable controlled electrochemical processes where the magnetic and electrocatalytic properties are combined in the same material, hence obviating the need for functionalized magnetic beads. Experimental Section Apparatus. Cyclic voltammograms were recorded with an Autolab PGSTAT12 electrochemical analyzer (Eco Chemie BV, Utrecht, Netherlands). The experimental setup (shown in Figure 1) involved 2.5 cm × 7.5 cm × 0.1 cm glass microscope slides onto which two rectangular carbon strip electrodes (0.7 cm × 2 cm) were screen printed. The electrodes were printed at the center of the glass slide, with a 2 mm gap separating them. The working electrodes were hand printed using an Acheson carbon ink, a patterned stencil screen, and a rubber squeezer. The printed electrodes were subsequently cured for 1 h (at 150 °C) and were allowed to cool to room temperature. Copper wires provided the electrical contact for the two electrodes. The reference and counter electrodes were Ag/AgCl (model CHI111, CH Instruments, Austin, TX) and platinum wire, respectively. A plastic cylindrical cell (1.5 cm diameter, 1.5 cm height) was glued (via an epoxy) to the center of the glass slide (to accommodate portions of the two working electrodes). Magnetic attraction of the CNT was achieved by placing an external NdFeB/Ni-coated magnet (3/8 in. × 3/8 in., 12.4 kG) under the glass slide. Chemicals. All solutions were prepared from double-distilled water. Potassium dihydrogen phosphate, dipotassium hydrogen phosphate, ascorbic acid, dopamine, and potassium ferrocyanide were purchased from Sigma. Multiwalled carbon nanotubes (MWCNT) of ∼97% purity [and less than 1% metals (Ni,Co, Fe)] were obtained from NanoLab (Brighton, MA). Single-walled carbon nanotubes (SWCNT) of 90% purity (along with 5% amporphous carbon and 0.6% Co) were obtained from Nanostructured and Amorphous Carbon (Los Alamos, NM). Such metal impurities may affect the magnetic properties of the CNT. Procedure. Measurements were carried out in a phosphate buffer (0.05 M, pH 7.4) supporting electrolyte medium at ambient temperature (of ca. 24 °C). Cyclic voltammetry measurements (12) Wang, J.; Kawde, A. Electrochem. Commun. 2002, 4, 349. (13) Wang, J.; Xu, D.; Polsky, R. J. Am. Chem. Soc. 2002, 124, 4208. (14) Fujiwara, M.; Oki, E.; Hamada, M.; Tanimoto, Y.; Mukouda, I.; Shimomura. Y. J. Phys. Chem. A 2001, 105, 4383. (15) Long, D.; Lazorcik, J.; Shahidhar, R. Adv. Mater. 2004, 16, 874.
10.1021/la050625g CCC: $30.25 © 2005 American Chemical Society Published on Web 07/30/2005
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Figure 1. Schematic representation of the experimental setup, involving a dual carbon electrode assembly, for the magnetic switching of CNT-induced electrocatalytic processes. Insets show cyclic voltammgrams for 5 mM ascorbic acid on each electrode with the magnet on and off. Measurements were made using 100 µg of MWCNTs along with a 10 min switching time. Electrolyte solution, 0.05 M phosphate buffer (pH 7.4); scan rate, 10 mV s-1. were performed under quiescent conditions, with the magnet in either the on or off position. Most experiments involved 100 µg of CNT, along with a 10 min switching time. Slight agitation of the solution for 5 s (through dispensing of air bubbles with a pipet) was necessary to aid the magnetic removal of the CNT from the electrode surface upon removal of the external magnet. The electrochemical oxidation was triggered by positioning the magnet below the corresponding working electrode (L) used to attract the CNT in solution and recording a cyclic voltammogram between -0.40 and 1.0 V at 10 mV s-1. The removal of the CNT from the surface of the left electrode (L) to the right electrode (R) was accomplished by moving the magnet below the right electrode (R) while agitating the solution for 5 s. A second cyclic voltammetry measurement was performed 10 min after the switching. The process was repeated for additional cycles. All measurements were performed at room temperature.
Results and Discussion The attraction and removal of the CNT to and from the working electrode allows reversible on and off activation of electrocatalytic process. Such use of local magnetic fields for controlling the electrocatalytic reactivity of CNTs has not been demonstrated previously. A dual working electrode assembly has been used to illustrate the spatially controlled CNT-induced electrocatalytic response (Figure 1). Changing the position of the magnet from one electrode to another “relocates” the CNT and leads to the cyclic activation and deactivation of electrocatalytic processes. This is indicated from cyclic voltammograms obtained at the individual electrodes for the oxidation of ascorbic acid. Although well-defined oxidation peaks (Ep ) 90 mV) are observed upon placing the magnet below the electrode, significantly broader and smaller signals are observed in the absence of an external magnetic field (Ep ) 790 mV).
Apparently, the magnetic collection of the CNT greatly promotes the oxidation reaction. Note that such magnetoswitchable electrochemical transformations are accomplished without using magnetic particles. Apparently, the CNTs have two complementary functions, acting as catalytic as well as magnetic materials. The process can be reversed upon relocating the external magnetic field, through cyclic placement of the external magnet. The repeatability of such a activation/deactivation cycle is illustrated in Figure 2, which displays cyclic voltammograms for ferrocyanide obtained at the right (R) and left (L) electrodes in the presence (a) and absence (b) of the external magnetic field. Large, sharp redox peaks (∆Ep ) 210 mV) are observed in the presence of the magnet (a). In contrast, poorly defined voltammograms (∆Ep ) 370 mV) are indicated without the magnet (b). Switching the position of the magnet resulted in modulation of the electrochemical reactivity and hence modulation of the response. The process can be repeated multiple times upon changing the position of the magnet (from the right to left electrode) used to control the placement of the CNT, with defined and poorly defined voltammograms in the presence and absence of the magnetic field, respectively. Such repetitive switching of the electrochemical reactivity between on and off states upon attraction and retraction of the CNT to and from the electrode surface is illustrated in Figure 3 from the changes in the peak potentials and the separation of the peak potentials for ascorbic acid (A) and ferrocyanide (B), respectively. For example, the ascorbic acid peak potentials are modulated between 70, 746, 79, 809, 55, 766, 76, and 832 mV during eight such switching steps. Similarly, the ferrocyanide
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Figure 4. Influence of the level of CNTs using a 10 min switching time (A) and of the switching time using 50 µg of CNTs (B) upon the anodic peak potential for 5 mM ascorbic acid. Other conditions are the same as in Figure 1.
Figure 2. Cyclic voltammograms for 5 mM potassium ferrocyanide using the left and right electrodes with the magnet on (a) and off (b) for four cycles. Other conditions are the same as in Figure 1.
Figure 5. Cyclic voltammograms for a mixture of 5 mM ascorbic acid (a) and 0.1 mM dopamine (b) with the magnet on (A) and off (B). Other conditions are the same as in Figure 1.
Figure 3. Reversible on and off activation of CNT-promoted oxidation. Changes in the peak potential for 5 mM ascorbic acid (A) and in the peak-potential separation for 5 mM potassium ferrocyanide (B) during four activation/deactivation cycles. Other conditions are the same as in Figure 1.
peak separations are switched between 194, 407, 201, 443, 176, 392, 175, and 406 mV during similar activation/ deactivation cycles. These data suggest a reproducible placement and removal of the CNT with no apparent carryover.
Several experimental conditions affecting the CNTinduced magnetoswitchable electrocatalytic response were investigated and optimized. Both MWCNTs and SWCNTs displayed a similar magnetoswitchable electrocatalytic response; the former were preferred for subsequent work owing to their higher purity. The amount of CNT has a profound effect upon the response. The ascorbic acid oxidation peak decreases rapidly from 120 to 45 mV upon increasing the CNT content from 25 to 200 µg and remains stable for higher CNT levels (Figure 4A). The electrocatalytic response is also strongly influenced by the switching time (between placement of the magnet and recording the voltammogram). The peak potential decreases rapidly from 450 to 140 mV upon increasing the switching time between 0 and 4 min and very slowly to 100 mV thereafter up to 10 min (Figure 4B). Subsequent work employed a 10 min switching time in connection to 100 µg of CNTs. Higher CNT levels led to a small electrocatalytic response at the off electrode (indicating incomplete removal of the CNT in the absence of the magnet).
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The controlled surface reactivity induced by the CNTmediated magnetoswitchable operation enables ondemand control of the selectivity. Such capability was illustrated for the simultaneous measurement of dopamine and ascorbic acid, which commonly represents a major bioanalytical challenge. Earlier work illustrated that CNTmodified electrodes lead to voltammetric separation of the dopamine and ascorbic acid peaks.16 Figure 5 demonstrates that such selectivity can be reversibly controlled by magnetic stimulation. It shows cyclic voltammograms for a mixture containing 0.1 mM dopamine and 5 mM ascorbic acid in the absence (B) and presence (A) of an external magnet. A single, broad overlapping peak is observed in the absence of the magnetic field. In contrast, the magnetic attraction of the CNT leads to the separation of the anodic peaks and enables the measurement of (16) Wang, Z.; Liu, J.; Liang, Q.; Wang, Y.; Luo. G. Analyst 2002, 127, 653.
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dopamine (b) in the presence of a large excess of ascorbic acid (a). Conclusions The magnetic and electrocatalytic properties of CNTs have been exploited for magnetoswitchable control of electrochemical reactivity without the need for functionalized magnetic particles. Our data clearly demonstrate that the attraction of CNTs to the electrode by means of an external magnet generates an electrocatalytic surface and that the process can be reversed upon relocating the magnet. Such CNT-mediated magnetically induced triggering holds promise for a variety of future devices and applications. Acknowledgment. This work was supported by grants from the NSF (CHE 0209707) and the NIH (R01A 1056047-01 and R01A 1056047-01). M.M. acknowledges a fellowship from the Islamic Development Bank (IDB). LA050625G
