Acoustic Biosensors: Principles, Types, and Applications
Week 6: Acoustic Biosensors
The principle behind acoustic biosensors is based on a mechanical wave propagating through piezoelectric or other materials. Any material changes at the surface will modify the wave’s propagation path, altering the electrical response of the sensor.
A key advantage of acoustic sensing is its ability to offer real-time measurement of surface interactions, which is particularly useful in studying protein binding events and immunochemical reactions.
Types of Acoustic Sensing Devices
- Thickness Shear Mode (TSM) resonators
- Acoustic Plate Mode (APM) device
- Surface Acoustic Wave (SAW) sensor
- Flexural Plate Wave (FPW) device
For acoustic sensing, a key aspect is the conversion between mechanical vibrations and electrical energy, made possible by the piezoelectric properties of the sensing element. The piezoelectric effect occurs when pressure applied to a dielectric material deforms its crystal lattice, causing a change in the distribution of charges in the atoms and bonds, generating a net macroscopic electrical polarization of the crystal.
Q Factor refers to a measure of the “quality” of a particular resonance, and represents the ratio of acoustic energy storage and dissipation.
Acoustic Devices
A typical acoustic device consists of a piezoelectric material with one or more metal transducers on its surface(s). These transducers launch acoustic waves (AWs) into the material at ultrasonic frequencies, ranging from one to hundreds of megahertz. The transducer metal is usually selected for either chemical inertness (e.g., gold) or for its acoustic match to the piezoelectric material (e.g., aluminum on quartz).
Common Acoustic Sensors
The TSM sensor is widely referred to as the Quartz Crystal Microbalance (QCM), and is undoubtedly the oldest and most recognized acoustic sensor. It consists of a thin disc of AT-cut quartz, with parallel circular electrodes on both sides (normally gold over chromium), acting as a one-port resonator.
Atomic Force Microscope (AFM)
The AFM head employs an optical detection system in which the tip is attached to the underside of a reflective cantilever. A diode laser is focused onto the back of the reflective cantilever.
As the tip scans the surface of the sample, the laser beam is deflected off the attached cantilever, and the photodetector measures the difference in light intensities between the upper and lower photodetectors, and then converts it to voltage.
Microcantilever Array Devices
Microcantilever array devices operate in static mode, dynamic mode, and heat mode.
Common to all modes is the fact that the cantilever’s surface is usually functionalized in such a way that one surface is rendered chemically active while the other surface is passivated by a chemically inactive substance.
Static Mode
In static mode, the chemical or physical reaction occurring on the cantilever surface is transduced into a nanomechanical bending of the cantilever.
Due to the asymmetric surface property of the cantilever, an asymmetric surface stress is produced upon chemical or physical reaction. This asymmetric loading causes the cantilever to bend. The detection of the reaction is finally achieved by measuring the nanomechanical bending by means of optical or electrical sensors.