Abstract
Surface acoustic wave (SAW) devices are well known for mass-sensitive sensor applications. In biosensing applications, chemical and biochemically evoked binding processes on surfaces are detected in liquid environments using delay line or resonator sensor configurations, preferably in combination with the appropriate microfluidic devices. All configurations share the common feature of analyzing the transmission characteristic of the propagating SAW. In this paper, a novel SAW-based impedance sensor type is introduced which uses only one interdigital transducer (IDT), simultaneously as the SAW generator and the sensor element. Here, the input port reflection coefficient S11 is measured at the IDT instead of the commonly used S21 transmission forward gain parameter. Thus, a sharp and distinct peak of the S11 spectrum is obtained, enabling a comfortable direct readout of the sensor signal. Proof of the concept was gained by analyzing the specific binding of the 4-mercaptophenylacetic acid gold nanoparticles (MPA–AuNP) directly to the IDT surface. The corresponding binding kinetic of the MPA–AuNP on the functionalized gold surface has been analyzed and a sensitivity of 7.4 mΩ nM−1 has been determined.
Highlights
Surface acoustic wave (SAW) devices with interdigitated microelectrodes serving as interdigital transducers (IDTs) are commonly used in the information technology industry as electronic filters in delay line or resonator configurations [1,2]
We proposed a simplified type of a SAW impedance sensor for gravimetric measurements in liquid environments
The sensor is based on a single IDT configuration to measure the input impedance mismatch during mass deposition
Summary
Surface acoustic wave (SAW) devices with interdigitated microelectrodes serving as interdigital transducers (IDTs) are commonly used in the information technology industry as electronic filters in delay line or resonator configurations [1,2]. In the “SAW transmission” configuration, which has become the conventional approach for mass sensitive applications, the sensor concept is based on the interaction of a propagating SAW with the adsorbed mass on a sensor surface, leading to characteristic changes of SAW velocity, resonance frequency, and amplitude [8,9,10]. Shiokawa et al demonstrated that Rayleigh SAWs with wave modes propagating perpendicular to the surface suffer from strong attenuation in liquid environments, which is caused by acoustic radiation losses into the fluid [14]. A waveguide layer preferably made of SiO2 , PMMA, or polyimide, Sensors 2017, 17, 2931; doi:10.3390/s17122931 www.mdpi.com/journal/sensors
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