Anal. Chem. 2001, 73, 1577-1586
A Strategy for Chemical Sensing Based on Frequency Tunable Acoustic Devices Hayat S. Sindi, Adrian C. Stevenson, and Christopher R. Lowe*
Institute of Biotechnology, University of Cambridge, Tennis Court Road, Cambridge, CB2 1QT U.K.
A new acoustic sensor geometry, the magnetic acoustic resonant sensor (MARS), is described. The device comprises a circular 0.5-mm-thick resonant plate fabricated from a wide variety of nonpiezoelectric materials and coated on the underside with a 2.5-µm-thick aluminum film. Harmonic radial shear waves over at least a 2 orders of magnitude frequency range can be induced in the resonant plate by enhanced magnetic direct generation using a noncontacting rf coil and NdFeB magnet. Mass loading with adherent aluminum films produced frequency changes of 106 Hz/nm (40.8 Hz/ng‚mm-2), while contact with viscous fluids resulted in maximum changes of 15 446 Hz/cP. At an operating frequency of 50 MHz, the device detected viscosity changes as low as 0.0006 cP. The adsorption of proteins such as human IgG and the binding of a complementary antigen, goat anti-human IgG, on the upper nonmetallized surface of the device has been monitored with a detection limit of ∼75 ng/mL. The binding of substrates and allosteric effectors to glycogen phosphorylase b has provided evidence that the device is very sensitive to viscoelastic changes in adsorbed proteins. The MARS device generates radial shear acoustic waves over a broad bandwidth that are unaffected by the conductivity of the solution. These results suggest that simple metal, glass, crystalline, or polycrystalline plates can be used as a new type of tunable acoustic immunosensor. Acoustic wave biosensors fabricated from piezoelectric substrates have been extensively exploited to monitor electromechanical changes occasioned by binding complementary ligands to adsorbed films of biomolecules.1-3 Recognition of the measurand by the bioselective layer immobilized on the piezoelectric plate results in perturbations in mass loading, elasticity, viscosity, dielectric properties, or conductivity, which are detected electrically, following changes in the velocity of the propagating acoustic wave4-7 through the device. Acoustic wave transducers are * Corresponding author. Phone: +44.1223.334160. Fax: +44.1223.334162. E-mail:
[email protected]. (1) Toda, K. Sens. Actuators, A 1994, 44, 241-247. (2) Collings, A. F.; Caruso, F. Rep. Prog. Phys. 1997, 60, 1397-1445. (3) Lucklum, R.; Behling, C.; Hauptmann, P. Anal. Chem. 1999, 71, 24882496. (4) Cavic, B. A.; Chu, F. L.; Furtaldo, L. M.; Ghafouri, S.; Hayward, G. L.; Mack, D. P.; McGovern, M. E.; Su, H.; Thompson, M. Faraday Discuss. 1997, 107, 159-176. (5) Shons, A.; Dorman, F.; Najarian, J. Biomed. Mater. Res. 1972, 6, 565. 10.1021/ac000820u CCC: $20.00 Published on Web 03/01/2001
© 2001 American Chemical Society
conventionally divided into bulk acoustic wave (BAW)8 and surface acoustic wave (SAW)9,10 devices. The majority of the work performed on BAW sensors has exploited 0.2-0.5-mm-thick AT-cut quartz resonator disks coated with metal electrodes on both sides, which, on actuation, energize the crystal to vibrate at shear resonance frequencies in the lowmegahertz range related to the plate thickness.8-11 Operation of the resonant plate as a mass sensor has led to the common designation of the device as a quartz crystal microbalance (QCM) and its status as the most widely recognized acoustic biosensor.12 However, the principal limitation of the QCM as a biosensor is the detection limit of ∼1 ng‚cm-2, which is inadequate to allow the monitoring of low molecular weight drugs and hormones.13 SAW devices are fabricated from 0.5-mm-thick piezoelectric crystals coated with photolithographically patterned metal electrodes comprising interdigitated transducer (IDT) fingers and contact pads positioned at either end.14 Originally, it was believed that SAW devices operating at frequencies in the range 50-500 MHz would have sensitivities significantly in excess of the QCM. However, enhanced sensitivity has proven more elusive to realize than originally thought, owing to the increased background noise, which mitigates against improved detection limits.14 For example, the BAW device has a similar detection limit for analytes, despite a lower theoretical mass sensitivity.15,16 Nevertheless, the introduction of numerous orientations, material types, and configurations of piezoelectric crystal transducer has led to more sensitive devices based on acoustic plate mode (APM),17 Lamb wave,18 surface skimming bulk wave,19 Love wave,20,21 and Bluenstein(6) Thompson, M.; Arthur, C.; Dhaliwal, G. Anal. Chem. 1986, 58, 1206-1209. (7) Andle, J.; Vetelino, J. Sens. Actuators, A 1994, 44, 167-176. (8) Sauerbrey, G. Z. Phys. 1959, 155, 206. (9) Auld, B. Acoustic waves and fields in solids; Wiley-Interscience: New York, 1973; Vol. II. (10) Benes, E.; Groschl, M.; Seifert, F.; Pohl, A. IEEE Trans. UFFC 1998, 45, 1314-1330. (11) King, W. Anal. Chem. 1964, 36, 1735-1739. (12) Guilbault, G.; Ngeh-Ngwainbi, J. Biotech. Res. Appl. 1988, 212-217. (13) Guilbault, G.; Flock, B.; Schmid, R. Biosens. Bioelectron. 1992, 7, 411419. (14) Vig, J. IEEE Trans. UFFC 1991, 38, 311. (15) Suri, C.; Mishra, G. Biosens. Bioelectron. 1996, 11, 1199-1205. (16) Lu, X.; Chew, F.; Li, S. Anal. Sci. 2000, 16, 107-114. (17) Ricco, A.; Martin, S. Appl. Phys. Lett. 1987, 50, 1474-1476. Martin, S.; Ricco, A.; Niemczyk, T.; Frye, G. Sens. Actuators, A 1989, 20, 253-268. (18) White, R.; Wider. P.; Wenzel, S.; Zeller, E. IEEE Trans. UFFC 1987, 2, 162-171. (19) Gizeli, E.; Stevenson, A. C.; Goddard, N. J.; Lowe, C. R. Sens. Actuators, B 1992, 6, 131-137. (20) Stevenson, A.; Gizeli, E.; Goddard, N.; Lowe, C. R. Sens. Actuators, B 1993, 13-14, 635-637.
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Figure 1. Schematic of the magnetic acoustic resonator sensor showing (a) the disposition of the metallized resonant disk, rf coil, magnet, and liquid sample and (b) the four layers of the analytical model: Layers 2 and 3 constitute the device; layer 1 describes the electromagnetic and magnetic field conditions required to generate acoustic waves; layer 4 is the sample film or fluid which forms the upper boundary.
Gulyaev22,23 geometries. However, while enhanced mass sensitivity has been the driving force behind the development of biosensors based on SAW devices, the complexity, cost, and difficulties of fabricating, mounting, contacting, and measuring such devices has also severely impeded their exploitation as practical biosensors. Recent innovations in actuation, packaging, measurement electronics, and temperature control have improved the operating frequency and performance of the devices23-26 but have done little to obviate the complexity of the SAW geometry itself. In the present work, the characteristics of a new, simplified acoustic sensor geometry known as the magnetic acoustic resonator sensor (MARS) are described and the key questions relating to sensing mechanism, cost-effectiveness, and sensitivity answered in relation to existing acoustic devices.27 The MARS transducer establishes an acoustic resonance in a free-standing plate with remote excitation and detection performed via magnetic and electromagnetic fields. The device exploits magnetic direct generation of acoustic waves in a metal film and requires a metal plate or film and a remote magnet and source of electromagnetic waves such as a coil connected to a signal generator.28 The transduction efficiency of magnetic direct generation is substantially improved by coupling electrical resonance in the coil with the acoustic resonance in the solid resonator and thin conductive film.27 This approach was found to be essential for making a unique type of electromagnetic/acoustic sensor, which retains the practical advantages of piezoacoustic sensors but which generates acoustic waves without direct mechanical or electrical contact. This paper reports the theoretical basis and performance characteristics of the MARS device in response to deposited metal overlayers, viscous fluids, and adsorption of proteins and other biomolecules. (21) Gizeli, E.; Liley, M.; Lowe, C. R.; Vogel, H. Anal. Chem. 1997, 69, 48084813. (22) Kondoh, J.; Matsui, Y.; Shiokawa, S. Jpn. J. Appl. Phys. 1993, 32, 23762379. (23) Welsch, W.; Klein, C.; VonSchickfus, M.; Hunklinger, S. Sens. Actuators, A 1997, 62, 562-564. (24) Weiss, M.; Welsch, W.; VonSchickfus, M.; Hunklinger, S. Anal. Chem. 1998, 70, 2881-2887. (25) Wang, A.; Kiwan, R.; White, R.; Ceriani, R. Sens. Actuators, B 1998, 49, 13-21. (26) Freudenberg, J.; Schelle, S.; Beck, K.; VonSchickfus, M.; Hunklinger, S. Biosens. Bioelectron. 1999, 14, 423-425. (27) Stevenson, A. C.; Lowe, C. R. Appl. Phys. Lett. 1998, 73, 447-449. (28) Quinn, J. Phys. Lett. 1967, 25A, 522.; Houck, J.; Bohm, H.; Maxifield, B.; Wilkins, J. Phys. Rev. Lett. 1967, 19, 224-227.
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THEORY It has been known for some time that acoustic waves may be generated in metal plates by the electromagnetic induction of flow in the conduction electrons and thus movement of the ions of the metal lattice.28,29 However, while such studies established the basis of all subsequent electromagnetic/acoustic coupling effects, they did not address the problem of low transduction efficiency. Greater conversion efficiencies have been achieved more recently by use of a technique known as enhanced magnetic direct generation (EMDG)27 with a device geometry shown in Figure 1a. A glass resonator plate metallized on the lower face is located on-axis immediately above an rf coil and a NdFeB magnet, which provides the electromagnetic and magnetic fields, respectively. On exposure of the metallized plate (Figure 1b; region 2/3) to the electromagnetic field (Figure 1b; region 1), acoustic waves are generated in the thin metal film as a consequence of radial Lorentz forces generated in its interior. Equation 1 relates the
F(z) ) jB
(1)
applied Lorentz force (F(z)) to the magnetic field strength B and the current density j induced in the metal film by the coil. However, an unused second Lorentz force, IB, where I is the coil current, appears in the coil itself. This force has been used elsewhere to make contact magnetoacoustic devices, where a vibrating coil or electrode30 is bonded or evaporated onto a resonator plate. The noncontact forces that appear in the metal film are conveyed through atomic contact to excite acoustic resonance in the glass plate in region 3 (Figure 1b). This process is described by eq 2, where the Lorentz forcing term is coupled to differential 2 F(z) ∂2u 2 ∂ u V ) s 2 2 C ∂t ∂x
(2)
terms representing the elastic properties of the plate, C is the elastic modulus of the plate, u is the particle displacement, and Vs is the shear velocity. The shear polarization of the acoustic (29) Rodriguez, S. Phys. Rev. 1963, 130, 1778-1783. (30) Butler, M. A.; Hill, M. K.; Spates, J. J.; Martin, S. J. J. Appl. Phys. 1999, 85, 1998-2000.
wave is dependent on the orientation of the static magnetic field, which must be aligned perpendicular to the film surface. If an electromechanical coupling constant and electric field are substituted for the forcing term in eq 2, an equation of the same form as the constitutive equation used to describe the piezoelectric generation of acoustic waves appears. This congruence of theory suggests that both piezoelectric and magnetic direct generation protocols produce similar conditions for the formation of acoustic waves. The solution of eq 2, assuming the film is of negligible thickness relative to the plate, gives the Helmholtz wave equation described by eq 3. This result demonstrates that two partial waves
u(z,t) ) u+ej(kz-ωt) + u-ej(kz+ωt)
(3)
form in the plate and travel through regions 2 and 3 perpendicular to the plate face. In this instance, k is the acoustic wave vector parallel to the z-axis, while ω is the angular frequency of the forcing term. By imposing free and driven boundary conditions, it is possible to deduce a solution, eq 4, which describes a standing wave of
u ) 2u0e
jωt
cos(kz)
(4)
frequency ω that forms in the interior of the plate. As only one side of the plate is driven, both the symmetric and asymmetric standing waves can be supported by a plate of thickness d, where k ) pm/d, and m is an integer. Based on k, and the relation ω ) Vs/k, the resonance frequency (fR) can be equated to the shear velocity Vs and the thickness of the plate (d) in a way similar to a QCM device. Equation 5 shows that these
fR ) mVs/2d where m ) 1, 2, 3, 4...
(5)
resonance frequencies occur at harmonics of the fundamental frequency (f0) that are evenly spaced and occur twice as often as in the QCM device. The absence of electrical damping of the higher frequency harmonics of piezoelectric crystals facilitates frequency tuning of the MARS device. A key issue for constructing usable sensors is the amplitude of the signal voltage that appears across the coil terminal when the film vibrates. It has been shown that, under nonresonance conditions, the geometry of the coil G, the plate density F, the shear velocity Vs, and the static magnetic field B determine the amplitude of the vibration.31 Under resonance conditions, this amplitude is enhanced if the effective attenuation coefficient R is reduced or the electrical Q factor of the coil (Qe) is increased.32 In practice, optimum enhancement at higher frequencies is provided by hard glassy or crystalline materials, resonated by coils tuned with low loss capacitors. Equation 6 combines these enhancement factors to give a received signal voltage V that appears as a vector sum with the rf source voltage at the coil terminals under resonance conditions (31) Gaerttner, M. R.; Wallace, W. D.; Maxfield, B. W. Phys. Rev. 1969, 184, 702-704. (32) Gordon, R. A.; Seidal, G. Phys. Lett. 1971, 35A, 102-103.
V)
GB2IQe
2 Rd FVs(1 + β ) 2
(6)
The factor β is an adjustment factor that accounts for phase differences across the film thickness; for thin films, β can be assumed to be close to zero when the acoustic wavelength is much larger than the film thickness. The rf source current (I) driven into the coil is directly proportional to the detected voltage. The net result is that the electrical impedance of the rf coil appears to vary significantly at the acoustic resonance frequency of the plate, in a manner similar to a piezoelectric element at resonance. If a viscoelastic fluid or film is present in region 4 (Figure 1b), the boundary conditions are modified and the resonance frequency is predicted to shift. Shear and compressional forces generated by the upper surfaces of the plate act to strain the fluid or film. These forces are related by a differential equation to the viscosity of the contacting liquid (ηL), where surface stress, the ratio of the surface force Fx over an area A, is proportional to Vx, the shear velocity of the fluid at that point:
Fx(z,t) ∂Vx(z,t) ) ηL A ∂z
(7)
This modification is broadly analogous to the Kanazawa boundary condition33 but differs because a radially directed force produces a compressional boundary condition, which acts in addition to the shear component (eq 8), where CL and CS represent separate
Fx(z,t) ∂Ux(z,t) ) (CL + CS) A ∂z
(8)
contributions from the longitudinal and shear modulus. The net result is that both the shear and compressional properties of the liquid affect the resonance frequency of the acoustic plate. These considerations show that the MARS geometry differs from conventional QCM devices by its extended harmonic behavior, the incorporation of a compressional surface component due to the radial field, and the absence of surface electrical fields. The Experimental Section of this paper investigates the consequences of these differences and defines the sensing characteristics of the unique MARS system. EXPERIMENTAL SECTION MARS Plates. The MARS sensing devices were prepared from plates of silica (Spectrosil B; 12 mm diameter, 500 µm thick, density 2.2 g‚cm-3; Comar, Cambridge, U.K.), aluminum (99.8% pure; Goodfellow Metals, Cambridge, U.K.), white diamond (GECMarconi Materials, Towcester, U.K.), stainless steel, zerodur glass, polycrystalline sapphire (Comar, Cambridge, U.K.), fused quartz (Vitreosil, Comar GV47), and magnetic steel. The nonmetallic samples were coated with a 2-µm-thick layer of aluminum (Edwards high vacuum unit, model 306A), which was protected with a 10-nm overlayer of silica. The diameters of the plates ranged from 10 to 12 mm and the thicknesses from 0.2 to 0.98 mm. (33) Grate, J.; Abraham, M. Sens. Actuators, B 1991, 3, 85-111.
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Figure 2. Schematic showing the radio frequency instrumentation required to generate and detect acoustic resonances in the plate.
Spiral Coil Assembly. Enamelled copper wire of diameter 0.085-0.125 mm (RS Electronics, UK) was manually wound into a planar spiral form, 6 mm in diameter (35 turns, 1 Ω dc resistance, 1 µH inductance) encapsulated in cyanoacrylate glue and wired in series with a tunable capacitor. High frequencies (>10 MHz) used a 50-pF surface mount capacitor (RS Electronics, UK), whereas low frequencies (
