L.A. French

A lateral field excited liquid acoustic wave sensor

Lateral field excited (LFE) AT-cut quartz acoustic wave sensors in which the electrodes are located on the reference surface have been fabricated and tested in liquid environments. The sensing surface, which is opposite to the reference surface, is free allowing the electric field of the thickness shear mode (TSM) to penetrate into the liquid. This results in increased sensitivity to both mechanical and electrical property changes of the liquid. In the present paper, several 5-MHz LFE sensors with a range of electrode spacings were exposed to liquid environments in which the viscosity, relative permittivity, and conductivity were varied. The LFE sensors demonstrate sensitivity to viscosity that is more than twice that obtained for the standard quartz crystal microbalance (QCM), and sensitivity to relative permittivity and conductivity about 1.5 times that of the QCM sensors with modified electrodes. The present results clearly indicate that the LFE sensors may have a wide range of liquid phase applications in which sensitivity is crucial.

A lateral field excited acoustic wave biosensor

The rapid sensitive detection of biomolecules, microorganisms and cells is critical to human, animal, and plant health along with food and environmental safety. Available detection techniques such as enzyme-linked immunosorbent assays (ELISA) and polymerase chain reaction (PCR) have high sensitivity but require the acquisition of discrete samples and excessive lab personnel time. Recently a lateral field excited (LFE) sensor has been developed (1) which has a bare sensing surface allowing the measurement of both mechanical and electrical property changes in a target analyte selective film. In the present work the LFE sensor is evaluated as a biosensor using anti-rabbit IgG and Escherichia coli (E. coli) as target analytes. The LFE sensor properties are compared to those obtained using a QCM biosensor. I. INTRODUCTION

A lateral field excited acoustic wave pesticide sensor

Excessive use of pesticides such as organophosphates (OPs) on fruits and vegetables can have adverse effects on the environment and jeopardize the health of the consumer. As a result a need exists for an accurate, low cost, portable sensor to detect harmful pesticide levels. In the present work a lateral field excited (LFE) sensor (1), which has a bare sensing surface that allows the measurement of mechanical and electrical property changes in a target analyte selective film, has been used to detect phosmet, a commonly used OP. The acoustic energy distribution of this LFE sensor has been found to exhibit a circular pattern with maximum sensitivity at the center of the sensor. The LFE pesticide sensor is shown to be more sensitive than the standard QCM. Also, it is shown that the response time of the sensor can be drastically shortened by using the derivative of the frequency response. I. INTRODUCTION Organophosphates (OPs) are widely used in agriculture for pest control in fruits and vegetables, with about 25,000 brands of pesticides sold in the United States (2). Categorized as neurotoxins or cholinesterase inhibitors, they can affect neuromuscular transmission (2). This is especially critical for young children who consume large amounts of fruits and vegetables and have a lower tolerance than adults (3). In order to guard against the adverse effects of OPs, the Environmental Protection Agency (EPA) has determined the allowable concentration of pesticides. Depending on the crop and pesticide used, tolerances are normally restricted to the 0.1 - 100 ppm range (4). Since many countries do not have such regulations, a need exists to detect pesticides on imported fruits and vegetables. Currently, the two standard methods of testing for the presence of pesticides are gas chromatography/mass spectroscopy (GC/MS) and immunoassay. GC/MS is the testing procedure approved by the EPA. Although very precise and accurate, these tests are expensive to run, time consuming and have to be performed in a laboratory environment. Also significant training is required to operate the GC/MS machine. Recently, immunoassay tests have been introduced as a cheaper, quicker, and portable alternative to GC/MS. This test method is however not reusable and is qualitative in that it indicates only whether the measurand is above or below a particular level. Also, cross reactivity and reaction to broken down pesticides in these tests may lead to false positive tests (5). Therefore a need exists for a sensor that would combine the quantative and reusable properties of the GC/MS with the low cost, quick, and portable properties of the immunoassay tests.

Recent advances in lateral field excited and monolithic spiral coil acoustic transduction bulk acoustic wave sensor platforms

The quartz crystal microbalance (QCM) has been used extensively as a bulk acoustic wave (BAW) platform for applications such as chemical and biological sensors and rate monitors in thin film deposition systems. Although the QCM is capable of measuring mechanical property changes critical in many thin film deposition systems, it cannot measure electrical property changes that can occur in many sensor applications. In this paper we review the recent developments of two novel transducer configurations for BAW sensors. In the first sensor, called the lateral field excited (LFE) sensor, the transverse shear mode (TSM) in AT-cut quartz is excited by two electrodes on the reference surface, resulting in a bare sensing surface which allows both electrical and mechanical properties of target analytes to be measured. In the second sensor, called the monolithic spiral coil acoustic transduction (MSCAT) sensor, the TSM is excited by a photolithographically deposited spiral antenna on the reference surface which can excite high-order harmonics in the substrate, and potentially lead to increased sensitivity. The responses of both the LFE and MSCAT sensors to electrical and mechanical property changes of liquids have been examined and compared to the response of the standard QCM. In addition, results relating to the detection of chemical and biological target analytes using the LFE and MSCAT sensor platforms are presented.

Electrode optimization for a lateral field excited acoustic wave sensor

Lateral field excited (LFE) acoustic wave sensors have a distinct advantage over the standard, AT-cut, quartz crystal microbalance (QCM) in which the thickness shear mode (TSM) is excited by electrodes on the sensing and reference surfaces. The TSM in LFE sensors is excited by placing electrodes only on the reference surface of the crystal, leaving the sensing surface free. Due to the lack of a shielding electrode on the sensing surface, the TSM electric field is able to penetrate into an analyte selective film or an adjacent liquid thus enabling the detection of mechanical and electrical property changes in the film or liquid. Several electrode designs on the reference surface are examined and evaluated by measuring each sensors admittance and resonant frequency when exposed to a variety of analytes and liquids. These results are compared to the results obtained using a standard QCM.

Acoustic plate mode properties of rotated Y-cut quartz

Recent demands for compact, low cost, accurate sensors for fluid phase operation have been largely unsatisfied. Among the most promising technologies are piezoelectric sensors. The piezoelectric sensors directly detect mechanical and electrical property changes caused by the analyte and are thus amenable to continuous monitoring of fluid streams. The current effort is directed towards the development of trace ion (e.g. mercury) and biochemical (e.g. DNA, antibodies, toxins) detection. Several candidate structures have been proposed and many have been shown to be feasible for fluid phase sensing with the best experimental piezoelectric sensor results published to date employing the shear horizontal acoustic plate mode (SHAPM) structure. This type of sensor has detected approximately 10 ng/ml of such analytes as mercury, human IgG and cholera toxin and employed Z-cut X-propagating (ZX) lithium niobate (LiNbO/sub 3/) SHAPM devices. The ZX LiNbO/sub 3/ wafers provide low propagation loss, high mass sensitivity, high electrical coupling and a single electrically-dominant acoustic mode. However, the principal drawback is the poor temperature stability of the material (/spl sim/-70 ppm//spl deg/C). In order to obtain better results the residual temperature instability of LiNbO/sub 3/ must be overcome. The current work analyzes potentially temperature stable plate modes in quartz crystals for dominant, temperature-stable electrically-efficient, mass-sensitive acoustic modes with low propagation loss under fluid loading.

Comparison of surface transverse wave (STW) and shear horizontal acoustic plate mode (SHAPM) devices for biochemical sensors

Surface transverse wave (STW) and shear horizontal acoustic plate mode (SHAPM) geometries have been proposed and demonstrated as potential biochemical sensors. The STW provides simplicity of instrumentation by virtue of its single acoustic mode of operation, while the SHAPM affords ease of packaging since the electronics may be placed on the opposite surface from the liquid solution. First generation device designs using both technologies have previously demonstrated nanogram sensitivity using model biochemical systems such as human IgG/goat anti-human IgG and cytomegalovirus DNA. More recently, second generation devices have been designed and are being tested using bacteria and their toxins. This paper will outline the critical tradeoffs between the two approaches. Initial electrical properties and sensor results are reported.

An acoustic plate mode sensor for aqueous mercury

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 /spl mu/g/L) which is approximately five times the limit imposed by the safe drinking water act (SDWA). 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.

A Lateral Field Excited Acoustic Wave Sensor

A lateral field excited (LFE) acoustic wave sensor with a bare sensing surface was developed. In addition to sensing mechanical and electrical changes in liquids, film-coated LFE devices were used to sense mechanical and electrical film property changes caused by chemical and biological analytes in solution. The sensor was tested as a chemical and biological sensor, detecting phosmet and Escherichia coli, respectively. A method to acoustically isolate multiple LFE sensors on a single substrate is also described. The results show that the LFE sensor can be used to sense analytes critical to homeland security, the military, agriculture, and health.

Electrochemical piezoelectric sensors for trace ionic contaminants

Industrial processes, such as fossil fuel combustion and nuclear materials processing, have resulted in heavy metal contamination of soils and potentially of the surrounding groundwater. In particular, mercury contamination of groundwater is a serious threat to the ecosystem, cumulating in serious health problems for humans as well as wildlife. Monitoring of mercury contamination in groundwater requires a method of long-term verification. Sensors with lifetimes of months to years of operation without operator intervention are required. One sensor geometry, which is capable of detecting relevant concentrations of aqueous ionic contaminants, such as mercury, while withstanding typical environmental conditions, is the shear horizontal acoustic plate mode (SHAPM) sensor. This 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 sensing element. Electrochemical techniques are employed to impart reversibility and to accelerate response kinetics. Results indicate a sensitivity of approximately 2.4 ng/mL, which approaches the 2.0 ng/mL limit imposed by the Safe Drinking Water Act (SDWA).