J.C. Andle

Acoustic wave biosensors

Abstract 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.

An acoustic plate mode biosensor

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.

Improved acoustic-plate-mode biosensor

Abstract The design of an improved acoustic-plate-mode (APM) biosensor is presented. The biosensing element consists of a pair of APM delay lines fabricated on an ZX lithium niobate (LiNbO 3 ) substrate. Each delay line utilizes a single APM and exhibits adjacent-mode rejection superior to previous LiNbO 3 designs, without sacrificing the high sensitivity to electrical effects. The biosensing element is evaluated as an immunosensor using the vector voltmeter (VVM) measurement system. The results show that the improved biosensor is capable of selectively detecting antibody concentrations of 20 ng per ml.

Probing of strong and weak electrolytes with acoustic wave fields

Abstract The probing of strong and weak electrolytes using acoustic wave fields is described. Only a very small volume of the solution (≈ 10–100 μl) is required for the measurement. It is shown that ionic mobilities at infinite dilution can be determined for a given class of ions by accurately measuring the concentration at which the maximum attenuation occurs in the propagating acoustic wave under the probing action. This could serve as a means of identifying ion species of a given class. Experiments are performed using acoustic plate mode fields on ZX-LiNbO3, a relatively strong piezoelectric material, and various solutions of the alkali metal ions. For weak electrolytes, it is shown that the present device gives a linear response for low solution concentrations (up to 1 wt.%). Hence, the conductivity and the dielectric constant of the solution can be effectively determined. At higher concentrations, only a qualitative analysis of the variation of the solution parameters can be extracted.

The integration of a chemiresistive film overlay with a surface acoustic wave microsensor

Abstract The theory describing the operation of a surface acoustic wave (SAW) hydrogen sulfide (H 2 S) sensor was examined. This sensor employs a tungsten trioxide (WO 3 ) chemiresistive overlay deposited as the sensing element on a SAW delay line. Previous work has shown that a change in the films DC conductivity upon exposure to a target gas, such as H 2 S, has far more influence on the SAW than the films mechanical properties. However, early work also pointed out that higher sensitivities had been experienced experimentally than the current theory suggests. The objective of the current work was to reexamine the theory describing the effect of a films electrical properties on SAW velocity. Perturbation theory has been employed to characterize the effects of the films DC conductivity, dielectric constant and device operating frequency on the SAW gas sensor. Experimental work was conducted on a SAW delay line residing on a cut of quartz optimized for stability at high temperature.

An acoustic plate mode sensor for aqueous mercury

Abstract Many industrial processes have resulted in mercury contamination of soils and potentially of the surrounding groundwater. The remediation efforts for these sites requires a method of long-term verification. Sensors with lifetimes of months to years of operation without operator intervention are required to monitor these sites. One sensor geometry which is capable of detecting relevant concentrations of aqueous mercury while withstanding typical environmental conditions is the acoustic plate mode (APM) microsensor. This piezoelectric sensor protects the electronics from the potentially corrosive aqueous fluid environment while providing a significant interaction with the fluid. Gold films are employed to accumulate the mercury via surface amalgamation. The added mass is measured as a change in the resonant frequency of the piezoelectric element. A reference device helps compensate environmental factors, such as temperature drift, solution effects (viscosity, density and conductivity changes) and pressure fluctuations. Initial results indicate a sensitivity of approximately 10 ng/ml (10 μg l −1 ) which is approximately five times the limit imposed by the safe drinking water act (SDWA) in the US. Research is currently underway to lower this detection limit to allow the sensor to meet the requirements of environmental sensing, wastewater monitoring and drinking water testing.

Theory, design and operation of a surface acoustic wave hydrogen sulfide microsensor

Abstract The theory, design and operation of a surface acoustic wave (SAW) hydrogen sulfide (H 2 S) sensor that uses a gold-doped semiconducting metal oxide, tungsten trioxide (WO 3 :Au), film as the gas sensing element is described. It is shown experimentally that the dominant property change that occurs in the film when exposed to H 2 S is the electrical conductivity. Finally, it is pointed out that upon exposure to a target gas, a properly designed SAW gas sensor that monitors large changes in film electrical conductivity is much more sensitive than a similar sensor that monitors mechanical properties such as mass loading.

Selective acoustic plate mode DNA sensor

Abstract Acoustic wave sensors have been proposed for (bio)sensing applications for the past 20 years. One of the most attractive acoustic wave sensor geometries for these applications is the acoustic plate mode (APM) delay line. However, the use of APM delay lines has been hampered by the relative immaturity of the associated design techniques. The principle issue in the design of APM delay lines is to excite and detect electrically a single acoustic mode within the plate with distortion from intermode interference or multiple waveguide reflections. The use of single-phase unidirectional transducers (SPUDT) enables the excitation and detection of a single acoustic mode, reducing the distortions that occur in conventional transducer designs. The current work examines the sensing properties of the resulting APM device for the selective detection of chemically denatured, double-stranded deoxyribonucleic acid (ds-DNA) obtained through polymerase chain reaction (PCR) amplification of genomic viral DNA from cytomegalovirus (CMV).

An acoustic plate mode immunosensor

The application of an acoustic plate mode (APM) device as an immunosensor is presented. The APM sensor utilizes a Z-cut X-propagating lithium niobate (LiNbO/sub 3/) piezoelectric plate in which various types of acoustic waves are excited and received by means of a photolithographically deposited aluminium interdigital transducer (IDT). A judicious choice of the sensor operating frequency results in the selective excitation of an acoustic mode that is highly sensitive to fluid loading of the LiNbO/sub 3/ surface. This mode is then utilized as the sensing element in the sensor. Experiments have been performed using polyclonal antibodies for which the sensor has shown a strong response to the associated antigen. The biokinetics of the antigen-antibody reaction have also been studied. Experimental data compare favorably with theoretical results predicted by an affinity-purified human immunoglobulin antibody-antigen model. This immunosensor also has potential application in detecting various types of viruses such as AIDS (HIV), herpes simplex, and hepatitis.<<ETX>>

On the mass sensitivity of acoustic-plate-mode sensors

Abstract The mass sensitivity of acoustic plate modes (APMs) in general and in particular APMs on ZX-LiNbO3 is investigated for gas and liquid phase-sensing applications. It is shown that, unlike pure shear horizontal APMs, the modes on ZX-LiNbO3 consist of both propagating bulkwave components (with a linear frequency dependence of mass sensitivity) and evanescently trapped surface components from the pseudo surface acoustic wave (PSAW) (with f2 frequency dependence of mass sensitivity). Thus, for a given plate thickness, the overall frequency dependence of mass sensitivity depends on the frequency range of operation. Theoretical and experimental results are in good agreement. Results are also given for actual immunosensor experiments. The results show that, in sensor applications, an f2 dependence at relatively high frequencies, and hence higher mass sensitivity, is achievable. This will require using thinner plates and the dominant PSAW-derived APM.