ABSTRACT
Surface-acoustic-wave resonators (SAWRs) have found widespread usage in various modern consumer radio frequency (RF) communications electronics, such as cellular phones, wireless devices, GPS devices, frequency control, and sensing applications. External mechanical vibrations modify an SAWR relative dimensions and the substrate's elastic properties, which alter the device's acoustic wave propagation velocity and ultimately cause the SAWR RF response to change. Detecting vibrations are desirable for dynamic strain or vibration sensing applications, whereas external mechanical excitations can result in spurious signals which compromise SAW-based filters and oscillators used in RF communication, frequency control, and sensors targeting measurands such as temperature and pressure. Therefore, understanding and characterizing the SAWR's response to external vibration is relevant for establishing device operation, and assisting in device design and packaging to either mitigate the impact of vibrations for RF communications and frequency control or enhance the SAWR response for sensor applications. This paper presents an in-phase and quadrature demodulation technique (I-Q technique) to detect, quantify, and analyze the effect of externally induced mechanical vibrations on an SAWR. The I-Q technique disclosed reveals that the mechanical vibrations cause both frequency and amplitude modulations of the SAWR RF response, which can be separated by this technique. Furthermore, the procedure also allows the direct measurement of vibration frequencies and vibration magnitude. The technique, measured results, and analysis established here provide a better understanding of the impact of external mechanical vibrations on an SAWR response, which is important in contemporary communications, frequency control, and sensing applications.
ABSTRACT
The toxigenic Escherichia coli O157:H7 bacterium has been connected with hemorrhagic colitis and hemolytic uremic syndrome, which may be characterized by diarrhea, kidney failure and death. On average, O157:H7 causes 73,000 illnesses, 2100 hospitalizations and 60 deaths annually in the United States alone. There is the need for sensors capable of rapidly detecting dangerous microbes in food and water supplies to limit the exposure of human and animal populations. Previous work by the authors used shear horizontal surface acoustic wave (SH SAW) devices fabricated on langasite (LGS) Euler angles (0 degrees, 22 degrees, 90 degrees) to successfully detect macromolecular protein assemblies. The devices also demonstrated favorable temperature stability, biocompatibility and low attenuation in liquid environments, suggesting their applicability to bacterial detection. In this paper, a biosensor test setup utilizing a small volume fluid injection system, stable temperature control and high frequency phase measurement was applied to validate LGS SH SAW biosensors for bacterial detection. The LGS SH SAW delay lines were fabricated and derivatized with a rabbit polyclonal IgG antibody, which selectively binds to E. coli O157:H7, in this case a non-toxigenic test strain. To quantify the effect of non-specific binding (negative control), an antibody directed against the trinitrophenyl hapten (TNP) was used as a binding layer. Test E. coli bacteria were cultured, fixed with formaldehyde, stained with cell-permeant nucleic acid stain, suspended in phosphate buffered saline and applied to the antibody-coated sensing surfaces. The biosensor transmission coefficient phase was monitored using a network analyzer. Phase responses of about 14 degrees were measured for the E. coli detection, as compared to 2 degrees due to non-specific anti-TNP binding. A 30:1 preference for E. coli binding to the anti-O157:H7 layer when compared to the anti-TNP layer was observed with fluorescence microscopy, thus confirming the selectivity of the antibody surface to E. coli.
