[riboswitch]

Hello fellow forum members! I'm having problems understanding the mechanism of piezoelectric biosensors. My biology background and few physics and chemistry knowledge are certainly not helping me.

According to my notes, piezoelectric crystals (like quartz) vibrate under the influence of an alternating electric field. Piezoelectric crystals vibrate (or oscillate) with a certain oscillation frequency (F). Each crystal has a characteristic resonant frequency (F₀). In a piezoelectric biosensor, molecular recognition elements (bioreceptors) are immobilized on the quartz crystal. A bioreceptor can be an antibody or an aptamer. The functionalized quartz crystal (the one with immobilized bioreceptors on its surface) has a characteristic resonant frequency. When the analytes bind to the bioreceptor, the resonant frequency changes. The correlation between the variation of the resonant frequency (ΔF) and the concentration of the molecules that bind to the bioreceptors is implied in the Sauerbrey equation:

[tex] \Delta F \ = - \frac{K \ F_{0}^{2} \  \Delta m}{A} [/tex]
   
In the Sauerbrey equation, ΔF is the variation of resonant frequency (in hertz), K is the proportional constant that depends on the properties of the quartz crystal (like its density and its "shear modulus", whatever that is), F is the fundamental resonant frequency of the quartz crystal, Δm is the mass of the analytes bound to the bioreceptors and A is the surface area of the quartz crystal.

The variation of the resonant frequency (ΔF) is directly proportional to the variation of the mass of the molecules present on the surface of the quartz crystal (Δm).

[tex]  \Delta F \  \propto  \  - \Delta m [/tex]
My question/s would be:

Did I explain the concepts properly? Did I write something wrong?

And

How can we measure ΔF?

By this so-called "quartz crystal microbalance", right? How does that work? The current materials under my possession briefly mentioned this so-called QCM but how can it detect this resonant frequency? What I know is that the frequency of vibration is dependent on the alternating electric field. But I don't know how to connect the concepts here...

P.S.: A typical material under my possession (at least the ones understandable enough for an undergraduate student like me) is something like this: Recent Progress in Nucleic Acid Aptamer-Based Biosensors and Bioassays. Thanks in advance for the helping me!

[Corribus]

Of course I have no idea how much detail you are supposed to provide but the description seems adequate.  You may consider a sentence or two describing what the piezoelectric effect is.

As to how QCMs work, there are plenty of sources on the web. For instance, what do you need that the Wikipedia article does not provide?

https://en.wikipedia.org/wiki/Quartz_crystal_microbalance

[Enthalpy]

The shear modulus, often noted G, relates the shear deformation (like: 1µm sideway per mm thickness) with the side force per area unit that provokes the deformation. It is hence given in GPa (or in ksi) and ranges from 20GPa for compliant metals to >200GPa for stiff ceramics.

Useful for many quartz resonators because most resonate in shear deformation rather then bending, in order to achieve 1-100MHz resonance or even more, as a fundamental mode or sometimes a partial 3 or 5, while bending served at 32768Hz for watches.

Frequency is the best property to measure in electronics, because a counter suffices often, and the excellent properties of quartz give references accurate to 10ppm, with temperature compensation 1ppm, in an oven <10ppb - and atomic clocks are better. A natural accuracy for voltages or currents would be 1% instead.

So if a sensor can exploit the resonant frequency of quartz, you've almost won. The quartz crystal itself is insensitive to temperature, ageing, and the extreme accuracy of the frequency lets it measure tiny added masses.

In your explanation, you can add that the shear resonance of an unloaded quartz crystal is just a wavelength condition between the thckness and the propagation speed of the shear wave, and that added mass load the resonator to lower its frequency. Both the bioreceptors and the sensed molecules do it.

Because the bioreceptors and the sensed molecules are supposedly very thin as compared to the quartz crystal, they act only through their mass, not their stiffness, so you can compute the effect as an extra quartz thickness that would add the same mass - and the frequency's reciprocal is proportional to the thickness.