Abstract
Resonant mechanical sensors are often considered as mass balance, which responds to an analyte adsorbed on or absorbed in a thin sensitive (and selective) layer deposited on the surface of the resonant device. In a more general sense, the sensor measures properties at the interface of the mechanical resonator to the medium under inspection. A phononic crystal (PnC) sensor employs mechanical resonance as well; however, the working principle is fundamentally different. The liquid medium under inspection becomes an integral part of the PnC sensor. The liquid-filled compartment acts as a mechanical resonator. Therefore, the sensor probes the entire liquid volume within this compartment. In both sensor concepts, the primary sensor value is a resonant frequency. To become an attractive new sensing concept, specifically as a bio and chemical sensor, the PnC sensor must reach an extraordinary sensitivity. We pay attention to the liquid viscosity, which is an important factor limiting sensitivity. The main part of our analysis has been performed on 1D PnC sensors, since they underlie the same material-related acoustic dissipation mechanisms as 2D and 3D PnC sensors. We show that an optimal relation of frequency shift to bandwidth and amplitude of resonance is the key to an enhanced sensitivity of the sensor-to-liquid analyte properties. We finally address additional challenges of 2D and 3D PnC sensor design concept. We conclude that the sensor should seek for a frequency resolution close to 10−6 the probing frequency, or a resolution with speed of sound approaching 1 mm s−1, taking water-based analytes as an example.
Highlights
Mechanical Resonant Sensors: A Short ReviewMechanical resonant sensors apply a mechanical element vibrating at one of its eigenfrequencies
A phononic crystal (PnC), an engineered material characterized by a periodic array of scattering inclusions in a homogeneous host matrix, exhibits bandgaps, where propagation of acoustic waves is forbidden or, in practice, at least significantly decreased; for example, this property has been employed in tethers of microelectromechanical systems (MEMS) resonators by incorporating a PnC
The PnC consists of Fe- and Al-plates
Summary
Mechanical resonant sensors apply a mechanical element vibrating at one of its eigenfrequencies. A phononic crystal (PnC), an engineered material characterized by a periodic array of scattering inclusions in a homogeneous host matrix, exhibits bandgaps, where propagation of acoustic waves is forbidden or, in practice, at least significantly decreased; for example, this property has been employed in tethers of MEMS resonators by incorporating a PnC It reduces anchor losses when designing the bandgap around the resonant frequency of the resonator. Very recently, Gao et al (2021) have exploited the surface phononic crystal (SPC) concept for SPC resonators operating in a liquid (Gao et al, 2021) They show the velocity of Rayleigh waves in the proposed sensor can be reduced to a value lower than the sound velocity in water, thereby suppressing the propagation of acoustic waves and eliminating acoustic radiation. The determination of sound velocity in this layer would provide the required accuracy
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