Acoustic waves excited in a piezoelectric medium provide an attractive technology for realizing a family of biosensors that are sensitive, portable, cheap and small. In this paper a wide range of bulk and surface-generated acoustic waves are described and prototype sensing-element geometries are presented. Results obtained using several candidate acoustic wave biosensors are also discussed.

Acoustic wave biosensors

A prototype biosensor, which utilizes an acoustic plate mode (APM) in a piezoelectric crystal plate as the sensing element, has been designed, fabricated and tested. The APM is selectively excited and received by aluminum interdigital transducers (IDTs), which are fabricated on a Z-cut X-propagating lithium niobate (ZX-LiNbO3) substrate using standard photolithographic techniques. The biological film is bound to the face of the ZX-LiNbO3 plate opposite to the IDTs. Appropriate r.f. electronics are used with the APM device to form the complete biosensor. Tests relating to the detection of deoxyribonucleic acid (DNA) hybridization demonstrate that the biosensor exhibits a high level of sensitivity, selectivity and reproducibility.

An acoustic plate mode biosensor

Acoustic plate mode (APM) devices on Z-cut, X-propagating LiNbO3, a high piezoelectric coupling material, are investigated as detectors in dilute electrolyte or metal-ion solutions. The sensing wave is an APM, a slow shear wave coupled to the interdigital transducer, which results in a relatively strong acoustic-ionic interaction. The resulting electrical loading leads to a measurable perturbation in the wave-propagation characteristics, which can then be related to the liquid electrical properties. Experiments conducted at different frequencies with various solutions that compare well with theoretical results show a detector that is at least two orders of magnitude more sensitive than similar APM sensors on quartz. These promising results have also led to the investigation of the detection of aqueous transition-metal ions (Fe3+, Cu2+, etc.) using solution conductivity changes rather than the commonly used mass sensitivity. The method involves the bonding of metal ions in soluiton to a ligand-coated silica or LiNbO3 support, placed adjacent to the APM device and in the path of the liquid flow. The resulting frequency increase is then interpreted in terms of the concentration of the metal ions. X-ray photoelectron spectroscopy analysis of the coated surface is performed to confirm the binding.

On the use of ZX-LiNbO3 acoustic plate mode devices as detectors for dilute electrolytes

Acoustic waves excited in a piezoelectric medium provide an attractive technology for realizing a family of biosensors that are sensitive, portable, cheap and small. In this paper a wide range of bulk and surface generated acoustic waves are described and prototype sensing-element geometries are presented. Results obtained using several candidate acoustic wave biosensors are also discussed.

It is well known that interdigital transducers usually launch a substantial amount of bulk wave radiation as well as the required surface wave. This dual effect is hardly surprising, since some of the first acoustic applications of such structures were specifically as sources of bulk wave radiation. The standard interdigital structure was used for this purpose by Mortley in 1965, and the interdigitally poled version described by Thomann in 1970 had already been described as a bulk wave transducer by van der Pauw in 1966. Furthermore, arrays of grooves were used as a means of converting bulk wave signals to surface waves, and vice versa , long before their use in reflective array compressors. Bulk wave radiation is therefore an expected rather than a surprising phenomenon in all these devices.

Suppression of bulk wave radiation from surface acoustic wave devices

: Experimental measurements of 345 MHz surface acoustic waves propagating along the X-axis of Z-cut lithium niobate have revealed the existence of a low-loss, high-coupling mode whose insertion loss is less than the Rayleigh-type surface acoustic wave. When the double electrode of quarter-wavelength transducers were excited at the third overtone at 1035 MHz, the insertion loss of this mode was about 10 dB less than that of the SAW. The experimentally observed velocity of this mode was 4375 m/sec, considerably higher than the metalized SAW velocity of 3800 m/sec, but in excellent agreement with the theoretical calculated metalized pseudo SAW velocity. The experimentally determined coupling (v/v) for this mode is almost a factor of three times that of the SAW. This volume mode is in low-loss, broadband signal processing devices. (Author)

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P.H. Carr

Papers

New Low Loss High Coupling Mode Up to 1 GHz on LiNbO 3