Nanostructure-enhanced surface acoustic love wave devices for biosensing applications
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A combined experimental and simulation approach is used to address two critical issues for the development of a high sensitivity SAW biosensor. One is the waveguide thickness optimization and the other is the integration of nanostructures on the surface of the Love wave sensor. In experiments, a SAW sensor is connected in a two-port network configuration and the critical waveguide thickness for Love wave operation is determined by analyzing the changes in signal amplitude and resonant frequency of the device in terms of the insertion loss (IL) spectrum. In simulations, the effects of adding nanostructures and changing waveguide thickness on wave propagation characteristics are investigated using a finite element method with COMSOL Multiphysics. Experimental results show that the insertion loss of a 433 MHz sensor coated with 100 nm of parylene (waveguide layer) is lower than the insertion loss for a sensor without parylene coating. When the thickness of the parylene coating is increased beyond 100 nm, the insertion loss of the sensor increased when compared with the insertion loss of a sensor without parylene coating. This behavior is typical of a Love wave SAW device where the acoustic wave travels predominantly into the waveguide layer beyond the critical thickness. Simulation results show a similar behavior observed in the experiments. Simulation results also show that a SAW sensor with nanostructures is more sensitive than its counterpart without nanostructures by exhibiting more changes in insertion loss. The sensitivity of the SAW sensor increases with the increase of the number of gold nanostructures but the sensitivity does not increase with the increase of the height of gold nanostructures beyond 150 nm. This behavior may be different when nanostructures with different material properties are added to the SAW sensor surface (See Appendix V). Even though nanostructure-enhanced SAW sensors exhibits higher sensitivity, such SAW sensors tend to have higher signal attenuation and lower resonant frequency of operation. This work will serve as a design guideline for fabricating high sensitivity SAW sensors operating in an aqueous environment. It will also provide new insight into the wave propagation in Love wave SAW sensors integrated with nanostructures.