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Use of Paired, Bonded NdFeB Magnets in Redox Magnetohydrodynamics Prabhu U. Arumugam, Emily A. Clark, and Ingrid Fritsch*
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701
Bonded neodymium-iron-boron (NdFeB) permanent magnets in a paired configuration were successfully used to control mass transport in redox-based, magnetohydrodynamics (MHD). Control of fluid flow based on magnetic fields has potential for use in portable lab-on-a-chip (LOAC) and analytical devices. Bonded magnets, composed of magnetic powder and organic binder materials, are less expensive and easier to fabricate and pattern than electromagnets and sintered permanent magnets, which have been previously used in MHD studies on electrochemical systems. The ability to pattern bonded magnets near and around the electrodes is expected to allow for better control over the magnetic field distribution and solution flow. Current was generated at an 800-µm-radius platinum disk electrode in a solution of 0.06 M nitrobenzene and 0.5 M tetra-n-butylammonium hexafluorophosphate in acetonitrile. Increases in limiting current in the presence of the magnetic field, which indicate enhancement in mass transport, for sintered (210 ( 14%, N ) 4, where Br ) 1.23 T and magnetic field strength is 0.55 T) and bonded (94 ( 8%, N ) 4, where Br ) 0.41 T and magnetic field strength is 0.20 T) magnets, were similar to those obtained using an electromagnet with the same magnetic flux densities. The magnetic field strength and not the magnet type is important in controlling fluid flow, which is encouraging for integration of bonded permanent magnets into LOAC devices. The use of bonded neodymium-iron-boron (NdFeB) magnets in a paired configuration for control of convective mass transport with magnetohydrodynamics (MHD) is demonstrated here. Bonded NdFeB magnetic materials, which are common in motor and magnetic read head applications,1,2 have not previously been applied to chemistry applications. A bonded magnet is essentially a mixture of magnet powder and organic binder materials. Various magnetic powders such as ferrites, alnico, samarium cobalt, or NdFeB can be used, with NdFeB materials providing the highest magnetic fields of the group.3 Binder * To whom correspondence should be addressed. Tel: (479) 575-6499. Fax: (479) 575 4049. E-mail:
[email protected]. (1) Ormerod, J.; Constantinides, S. J. Appl. Phys. 1997, 84 (8), 4816-4820. (2) Roth, N.; Halsey, D.; Bauch, T. Applications of improved permanent magnet materials. In The 17th internal workshop on rare earth magnets and their applications, Newark, NJ; Hadjipanayis, G. C., Bonder, M. J., Eds.; Rinton Press, 2002; pp 1027-1033. (3) Skomski, R.; Coey, J. M. D. Permanent Magnetism, 1st ed.; IoP Ltd.: Philadelphia, 1999. 10.1021/ac048849b CCC: $30.25 Published on Web 01/15/2005
© 2005 American Chemical Society
materials can be a rubber or flexible thermoplastic resin, rigid thermoplastic, or rigid thermosetting resin.1 Although lower in magnetic field strength than sintered NdFeB, bonded NdFeB magnets are more easily fabricated into many shapes and sizes,4 patterned close to electrodes,5 and integrated to channels and cavities.6 Bonded magnets can be shaped using rolling, extrusion, compression or injection molding processes, where millimeter dimensions are easily achievable. A variety of designs can also be fabricated and patterned by spin coating (followed by masking and exposure, as in photolithographic processes)4 or screenprinting7 magnetic pastes, where features of several micrometers are possible. Generally, the smaller the dimensions of the magnet, the smaller the magnetic flux. However, the actual dependence of magnetic flux on the dimensions of the bonded magnets depends on many factors: particle size distribution, particle loading, surface treatment, alignment fields, compaction method, compaction pressure, and polymer matrix. Currently, the maximum residual induction that has been achieved in bonded magnets is 1.08 T.8 The design possibilities for bonded magnets are of interest in making portable devices that can benefit from an arrangement of magnetic fields, including those that use MHD-based convection. The time is right to focus on portable devices involving permanent magnets, especially with recent advances in improving the field strength of bonded magnets through the synthesis of NdFeB powder,8 nanocomposite magnets,9 and magnetic pastes,7 resulting in high-energy particles. Yet, MHD studies have only been performed previously with the sintered form of NdFeB magnets.10,11 Those prior investigations have also focused mostly on a single magnet. The paired magnet configuration, which is used here, optimizes the magnetic field strength for the bonded magnet and provides a more uniform field.12,13 (4) Lagorce, L. K.; Allen, M. G. J. Microelectromech. Syst. 1997, 6 (4), 307312. (5) Arumugam, P. U.; Fritsch, I. IEEE Trans. Magn. 2004, 40 (4), 3063-65. (6) Arumugam, P. U.; Fritsch, I. Anal. Chem., in preparation. (7) Yuan, Z. C.; Williams, A. J.; Shields, T. C.; Blackburn, S.; Ponton, C. B.; Asbell, J. S.; Harris, I. R. J. Magn. Mater. 2002, 247, 257-269. (8) Honkura, Y.; Mishima, C.; Hamada, N.; Mitara, N. Anisotropic neo bonded magnets with high (BH)max. In Proceedings of the 17th international workshop on rare earth magnets and their applications, Newark, NJ, Hadjipanayis, G. C., Bonder, M. J., Eds.; Rinton Press Inc., 2002; pp 5261. (9) Jin, Z. Q.; Zhang, H.; Wang, A.; Hadjipanayis, G. C. Pr-Fe-B based nanocomposities with high energy product. In Proceedings of the 17th international workshop on rare earth magnets and their applications, Newark, NJ, Hadjipanayis, G. C., Bonder, M. J., Eds.; Rinton Press Inc., 2002; pp 760-70. (10) Leventis, N.; Gao, X. Anal. Chem. 2001, 73, 3981-3992. (11) Zhong, J.; Yi, M.; Bau, H. H. Sens. Actuators, A 2002, 96, 59-66.
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Control of convective mass transport on a small scale in the form of microfluidics (both linear fluid flow and stirring) is of growing interest in lab-on-a-chip (LOAC) applications in biological, chemical, medical, and environmental applications, which involve multistep analysis, mixing, sample preparation, or separations. Such systems can lead to tremendous improvements in product yields, high-throughput analysis, and decrease in waste and use of materials and power. However, propelling and stirring fluids on a small scale is challenging.14,15 The main methods of microfluidics presently used are electrokinetic,16 mechanical,17 and centrifugal.18 MHD is fairly new to microfluidics. It produces fluid flow due to a Lorentz force that is generated from the cross product of current and magnetic field vectors. MHD offers bidirectional (reversible) flow, low level of complexity (requires no mechanical valves or pumps), flow velocities that are comparable to the most popular method of electrokinetic pumping, and insensitivity to the physicochemical properties of the wall materials of the channel.19-24 We have recently introduced the concept of redox MHD microfluidics and its ability to stir ultrasmall volumes.5,25 The redox species added to the fluid carry current due to their oxidation and reduction. Redox MHD does not require high voltages, which can result in bubble generation, electrode degradation, or both. We have also demonstrated a practical application for redox MHD microfluidics in the detection of trace metals under circumstances in which reproducible mechanical stirring is impractical, such as in small volumes and in portable devices.26 There, redox MHD is used during the preconcentration step, instead of mechanical stirring, to enhance the sensitivity in anodic stripping voltammetry analysis. Magnets that are small and easily molded to form desired shapes around the working electrode are highly desirable in making portable, MHD-based trace metal analysis possible. In the last few decades, numerous papers have been published in the area of magnetic effects on mass transport in electrochemical systems27-34 and MHD mixers and pumps.19-24 But, applica(12) The website of Magnet Sales, Inc. http://www.magnetsales.com 2004. (13) Furlani, E. P. Permanent Magnet and Electrochemical Devices; Academic Press: San Diego, 2001. (14) Abraham, D. S.; Stephan, K. W. D.; Armand, A.; Igor, M.; Howard; Whitesides, G. M. Science 2002, 295 (25), 647-51. (15) Oddy, M. H.; J. G. Santiago; Mikkelsen, J. C. Anal. Chem. 2001, 73, 58225832. (16) Zeng, S.; Chen, C.-H.; Mikkelsen, J. C.; Santiago, J. G. Sens. Actuators, B 2001, 79, 107-114. (17) Lu, L.; Ryu, K. S.; Liu, C. J. Microelectromech. Syst. 2002, 11 (5), 462-469. (18) The website of Gyros AB. http://www.gyros.com, 2004. (19) Sadler, D. J.; Changrani, R. G.; Chou, C. F.; Daniel, Z.; Jeremy, W. B.; Frederic, Z. Proc. SPIE 2001, 4560, 162-69. (20) Bau, H. H.; Zhong, J.; Yi, M. Sens. Actuators, B 2001, 79, 207-215. (21) Jang, J.; Lee, S. S. Sens. Actuators, A 2002, 80, 84-89. (22) Lemoff, A. V.; Lee, A. P. Sens. Actuators, B 2000, 63, 178-185. (23) Lemoff, A. V.; Lee, A. P. Biomed, Microdevices 2003, 5:1, 55-60. (24) West, J.; Karamata, B.; Lillis, B.; Gleeson, J. P.; Alderman, J. Lab Chip 2002, 2 (4), 224-30. (25) Aguilar, Z. P. Development of bioassay platforms for small volumes. Ph.D. Dissertation, University of Arkansas, Fayetteville, 2002. (26) Clark, E. A.; Fritsch, I. Anal. Chem. 2004, 76 (8), 2415-2418. (27) Leventis, N.; Chen, M.; Gao, X.; Canalas, M.; Zhang, P. J. Phys. Chem. B 1998, 102, 3512-3522. (28) Leventis, N.; Gao, X. J. Phys. Chem. B 1999, 103, 5832-5840. (29) Ragsdale, S. R.; Lee, J.; Gao, X.; White, H. S. J. Phys. Chem. 1996, 100, 5913-5922. (30) Ragsdale, S. R.; Lee, J.; White, H. S. Anal. Chem. 1997, 69, 2070-2076. (31) Lee, J.; Ragsdale, S. R.; Gao, X.; White, H. S. J. Electroanal. Chem. 1997, 422, 169-177.
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tion to hand-held devices is still limited. Our interest is to miniaturize the entire setup including the magnets for microscale fluid control. Toward that goal, we performed studies that demonstrate the use of compact, externally placed sintered and bonded NdFeB permanent magnets, in contrast to bulky electromagnets that require a power source, for enhancing mass transport. In this report, two different kinds of NdFeB magnets were compared for use with redox MHD: commercial sintered (residual induction, Br ) 1.23 T)12 and in-house bonded (Br ) 0.41 T). A Pt disk electrode (800-µm radius) was used in a solution of 0.06 M nitrobenzene (NB) in an electrolyte of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile. The magnetic effects were studied in the orientation that optimizes MHD, where the magnetic field is perpendicular to the electrode axis. Nitrobenzene is a model redox species that was chosen to demonstrate the proof of concept, because it is known to exhibit strong MHD effects, which have been reported in several previous publications.31,35,36 The electrode type and size were selected based on convenience and the fact that MHD effects are easily observed at electrodes of these dimensions. Both the redox solution and electrode may be different for different applications (for example, MHD-enhanced ASV compared to microfluidic pumping or stirring). However, the results for the fundamental studies presented here may be used as a basis from which to predict MHD behavior in other systems and applications using paired bonded magnets. EXPERIMENTAL SECTION Chemicals and Materials. All chemicals were reagent grade and used as received. The TBAPF6 was obtained from Sigma, NB (99+%) from Aldrich, and CH3CN (99.99%) from Fisher Scientific. The working electrode was a Pt (99.998%) inlaid-disk electrode (800-µm radius, Bioanalytical Systems). The counter electrode was constructed from a platinum sheet (99.99%, 0.1 mm thick, Alfa AESAR). NdFeB particles (MQP-S, 40-µm median diameter) were obtained from Magnequench Inc. (Indianapolis, IN) and epoxy resin and hardener (Epo-Kwick-208136 and 208138) from Buehler Inc. Magnets and Measurements of Magnetic Fields. The magnetic flux density was measured using a GM1A gaussmeter (Applied Magnetics Laboratory, Inc., Baltimore, MD) with a PT 70 probe. A superconducting quantum interference device (SQUID) from Quantum Design Inc (San Diego, CA) was used for hysteresis measurements. The electromagnet (custom built by New England Techni-Coil, with a pair of 3-in. poles having an adjustable gap and tunable field) was configured to generate a field of 0.20 and 0.55 T from a source that was powered by an AL 7500 power supply (Alpha Scientific Laboratories). The sintered magnet (35NE646432) from Magnet Sales and Manufacturing Co (Austin, TX) was 2.5 cm × 2.5 cm × 1.2 cm thick. The dimensions of the bonded magnets were the same. Bonded magnets were fabricated from a mixture of isotropic spherical NdFeB particles (32) Aaboubi, O.; Chopart, J.-P.; Douglade. J.; Olivier, A. J. Electrochem. Soc. 1990, 137 (6), 1796-1804. (33) Aaboubi, O.; P, L.; Amblard; Chopart, J.-P.; Olivier, A. J. Electrochem. Soc. 2003, 150 (2), E125-130. (34) Pullins, M. D.; Grant, K. M.; White, H. S. J. Phys. Chem. B 2001, 105, 8989-8994. (35) Lee, J.; Gao, X.; Hardy, L. D. A.; White, H. S. J. Electrochem. Soc. 1995, 142 (6), L90-L92. (36) Ragsdale, S. R.; White, H. S. Anal. Chem. 1999, 71, 1923-1927.
Figure 1. Hysteresis curve for bonded magnets. The mixture consists of 95 wt % NdFeB particles and 5 wt % epoxy resin and hardener.
and epoxy resin (resin and hardener in 5:1 ratio by wt %) in a ratio of 85:15 vol %. Upon mixing, the epoxy resin fills the air gaps between particles. Another way to describe the final mixture is by percent weight, which is 95:5 wt % of NdFeB particles to epoxy resin. This mixture was compression molded using an aluminum mold at a pressure of 3.15 MPa. The compacted composite was allowed to cure overnight at room temperature. The composites were then magnetized by Magnequench Inc. in a 4-T electromagnet.37 (Higher ratios than 95:5 wt % NdFeB particles to epoxy resin did not allow good adhesion between particles under the compression conditions that we used.) Electrochemical Measurements. Cyclic voltammetry (CV) in a solution of 0.06 M NB in CH3CN containing 0.5 M TBAPF6 as the supporting electrolyte was performed using an EG&G Princeton potentiostat/galvanostat model 273A (Princeton, NJ) and recorded using PAR research electrochemistry software (Version 4.23). A Ag/AgCl (saturated KCl) electrode was used as a reference. Experiments involving electrochemistry in the presence of a magnetic field were performed with the working electrode centered between the poles of the electromagnet or the pair of permanent magnets in attracting orientation with a 1.6-cm pole separation. A perpendicular orientation of the magnetic field and electrode surface normal was used. Between experiments, the electrode tip was polished with 1-µm diamond polish (Part No. MF-2054, BioAnalytical Systems) and with 0.05-µm alumina B (Part No. 40-6353-006, Buehler Inc.), sonicated (L&R PC3 Compact High Performance Ultrasonic Cleaning System) in DI water, and dried with N2 gas. RESULTS AND DISCUSSION Properties of the Bonded Magnets. Bonded magnets were constructed using NdFeB particles (Br ) 0.72 T) because of their high remenance.12 The average density of the completed bonded magnets, based on two data sets, was found to be 4.9 g mL-1.37 Density was obtained by dividing the mass of the bonded magnet by its geometric volume. Figure 1 shows the hysteresis loop of a bonded magnet that was obtained using SQUID measurements. The Br value obtained from that figure is 0.41 T. Brown et al.38 showed that higher Br values (0.65-0.77 T) could be achieved by using different grades of NdFeB “isotropic” particles, achieving high-density samples (>6 g mL-1) with compression pressures (37) Measurements were performed in Magnequench Technology Center, NC.
Figure 2. Experimental setup for an electrochemical cell with magnets placed externally. An 800-µm radius Pt disk working electrode was used for these experiments. The magnet was either sintered permanent, bonded permanent, or an electromagnet.
>4.1 MPa and heating the mold to 135 °C. With proper tooling, we anticipate that we could increase the Br of our bonded magnets from 0.41 to 0.77 T, an 88% increase. Also, if we were to use “anisotropic” particles, instead of “isotropic” ones, we expect that the bonded magnets would have Br values of up to 1.08 T,8 which would be a 163% increase from that which we have achieved so far. However, for the proof-of-concept studies demonstrated here, a Br of 0.41 T is sufficient. Redox MHD Effects on Disk Electrodes. The magnetic effects from a pair of sintered and bonded NdFeB magnets were compared to those from the electromagnet that was tuned to match the magnetic field strengths of the two types of permanent magnets. The field strengths measured with a gaussmeter between the pairs of permanent magnets were 0.55 and 0.2 T, for the sintered and bonded magnets, respectively. The electrochemistry system with an 800-µm-radius Pt disk working electrode employed a perpendicular orientation like that shown in Figure 2. CV in the NB solution involves a one-electron reduction to the paramagnetic radical anion, NB•-.39 Local density gradients seem to enhance the limiting current for our electrochemical system in the absence of a magnetic field. There are two sets of evidence for this statement. First, a scan rate study of NB solutions of 0.25, 0.06, and 0.0024 M in 0.5 M TBAPF6 in acetonitrile all show linear dependence of peak current with square root of scan rate at high scan rates, consistent with a dominance of mass transport by planar diffusion. However, at slow scan rates, the CV responses become more sigmoidal and the peak current (or limiting current) is no longer proportional to square root of scan rate. This transition region occurs at around 100 to 25 mV/s, even though the calculated diffusion length is less than the radius of the electrode, when mass transport is still expected to be dominated by planar (not spherical) contributions by diffusion. Second, in a solution of 0.25 M NB, as the electrode disk/insulator plane is tilted more and more upward, so that the (38) Brown, D. N.; Ma, B.-M.; Campbell, P. The comparison of anisotropic (and isotropic) powders for polymer bonded rare earth permanet magnets. In Proceedings of the 17th international workshop on rare earth magnets and their applications, Newark, NJ, Hadjipanayis, G. C., Bonder, M. J., Eds.; Rinton Press Inc., 2002; pp 62-73. (39) Bard, J. A.; Faulkner, L. R. Electrochemical methods: fundamentals and applications, 2nd ed.; John Wiley & Sons Inc.: New York, 2001.
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perpendicular to the plane is no longer parallel to the gravitational force, the limiting current at a slow scan rate (5 mV/s) increases. At the same time, the concentration is high enough so that we can visually observe the colored reduction product produced at the electrode flow upward along the tilted surface to the edge of the insulator and then continue upward into bulk solution. Both sets of results support the concept that local gradients, in the vicinity of the electrode, contribute to natural convective flow. The density gradients may be due to compositional gradients or thermal gradients or both in the depletion layer at the electrode surface. These results are consistent with those of Ragsdale and White,36 who have suggested microscopic density gradients to be responsible for dramatic convection effects in NB solution at electrodes of even smaller dimensions, 25-µm radius, than those used here. The solution adjacent to the electrode is less dense than that in bulk during the reduction. As this region builds, it moves in the direction opposite to that of gravity, drawing more bulk solution, containing primarily NB, to the electrode surface, and therefore increasing the measured current, as long as the diffusion layer is large enough to be affected (slow scan rates). The convection is easily observed in the work by Ragsdale and White, because the perpendicular to the plane of the electrode surface is orientated at 90° to the gravitational force. In the studies reported here, the perpendicular to the disk/insulator plane is kept fairly parallel to the force of gravity; but as a region of lower density evolves at the electrode surface, it spreads outward to the edges of the insulator, displacing the higher density solution at the surface of the insulator. If there is even a slight tilt, the natural convective flow in that direction will be enhanced. In the presence of a magnetic field, the Lorentz force FL (N m-3) is generated at right angles to the magnetic field with the direction of force governed by the right-hand rule and is given by
FL ) J × B
(1)
where J is the flux of ions (C cm-2 s-1) and B is the magnetic flux density (T). In the perpendicular orientation employed here, the fields are orthogonal to each other, and eq 1 predicts that FL should be parallel to the electrode surface, resulting in a flow of solution across its face. The flow driven by FL results in a large increase of limiting current. This reasoning is consistent with the experimental results obtained with both the electromagnet and permanent magnets. The limiting current increase for a flux density of 0.55 T (compared to the current in the absence of an applied magnetic field) is 212 ( 1.5% (N ) 2) for the electromagnet and 210 ( 14% (N ) 4) for the sintered magnet (see Figure 3, dotted curves). Similarly, for 0.20 T, it is 94 ( 0% (N ) 2) for the electromagnet and 94 ( 8% (N ) 4) for the bonded magnets. (The error represents half of the range between data points for N ) 2 and one standard deviation for N g 3.) An interesting observation is that magnets having the same B (0.55 or 0.20 T) gave a similar enhancement of limiting current, irrespective of the magnetic flux distribution near the electrodes. The only convective mechanism expected from both types of magnets is from B, because a uniform field is expected halfway between the pole centers (where the electrode is placed). Therefore, the solution close to the electrode should experience a nearly homogeneous field. The similar 1170 Analytical Chemistry, Vol. 77, No. 4, February 15, 2005
Figure 3. Cyclic voltammetric responses at 5 mV/s at an 800-µmradius Pt disk electrode in a three-electrode setup in a solution of 0.06 M NB and 0.5 M TBAPF6 in CH3CN, comparing the behavior in two different external magnetic fields produced by an electromagnet and permanent magnets. No magnet (solid curve) and with magnet (dotted curve).
increase in limiting currents shows that we can use either paired sintered or bonded NdFeB permanent magnets, instead of bulky electromagnets. CONCLUSIONS In this paper, we show how paired, bonded NdFeB magnets could be an alternative to electromagnets and sintered magnets for MHD-powered convection. In particular, bonded magnets, which are compact (good for portability), strong, and can be easily fabricated, molded, and patterned close to electrode surfaces can be used for small-scale fluid mixing5,40 and pumping.6 The critical parameter in such a setup is the magnetic field strength, and not the magnet composition. The higher the magnetic field, however, the greater the MHD effect, as expected. The rapid progress in the area of magnetic materials, such as anisotropic NdFeB particles and nanocomposite magnets could lead to more powerful bonded magnets (Br g 1 T). Also, by optimizing magnet geometries, we can realize a still larger enhancement in mass transport. Ongoing work in our laboratory includes the integration of bonded magnets with hand-held trace metal analysis and electrode arrays, as well as chip-based mixing devices that may have advantages over electrohydrodynamic15 and chaotic mixers.14,41 Although the studies presented here using bonded magnets are focused on the MHD phenomenon, bonded magnets are likely to have a wider interest in chemical applications. For example, bonded magnets could be used in LOAC devices to construct microvalves42 and micropumps43 (in conjunction with microelec(40) Arumugam, P. U.; Fritsch, I. J. Phys. Chem., submitted. (41) Liu, R. H.; Stremler, M. A.; Olsen, M. G.; Santiago, J. G.; Adrian, R. J.; Aref, H.; Beebe, D. J. J. Microelectromech. Syst. 2000, 9 (2), 190-197. (42) Gongora-Rubio, M.; Sola-Laguna, L.; Smith, M.; Santiago-Aviles, J. A MesoScale Electro-magnetically Actuated Normally Closed Valve Realized on LTCC Tapes. SPIE Conference on Microfluidic Devices and Systems II, Santa Clara, CA, 1999; pp 230-238. (43) Kim, M.; Ananthasuresh, G. K.; Bau, H. H. Magnetically Actuated, Circular Diaphragm Pumps Fabricated with Low-Temperature, Co-fired Ceramic Tapes and Kapton Polyimide Films: Experiments and Theory. ASME International Mechanical Engineering Congress and Exposition, New York, November 11-16, 2001; pp 41-49.
tromagnets), which until now have involved sintered magnets. Bonded magnets could also be custom designed for analyses involving magnetic beads,44,45 which can be used to achieve small and fast immunoassays and DNA hybridization assays. ACKNOWLEDGMENT We acknowledge the National Science Foundation (CHE0096780) for financial support. We thank Dr. Mark Filipkowski (44) Wijayawardhana, C. A.; Wittstock, G.; Halsall, H. B.; Heineman, W. R. Anal. Chem. 2000, 72, 333-338. (45) Heineman, W. R.; Wijayawardhana, A.; Purushothama, S.; Schlueter, K.; Halsall, H. B.; Ridgway, T. H.; Kradtap, S.; Choi, J.-W.; Lannes, C.; Helmicki, A.; Ahn, C.; Nevin, J.; Henderson, T., Biomems immunoassay with electrochemical detection. Book of Abstracts, 219th ACS National Meeting, San Francisco, CA, March 26-30, 2000.
for assistance in making SQUID measurements. We acknowledge Kyle Puska and Ben Smith of Magnequench Inc. for providing the magnetic particles and magnetizing the bonded magnets. P.U.A. is a student in the Microelectronics and Photonics (MEPH) Graduate program and thanks MEPH for encouraging multidisciplinary research. E.A.C. acknowledges The Electrochemical Society, Inc., for support through the F.M. Beckett Summer Fellowship Award.
Received for review August 4, 2004. Accepted November 22, 2004. AC048849B
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