What is QCM-D

Principle of QCM-D

Principle of QCM-D

3T-analytik has developed quartz sensor instruments, the qCell-family, based on Quartz Crystal Microbalance (QCM) technology. The high sensitivity to mass and structural property variation enables QCM to be a powerful tool for gaining real time insights into molecular interactions, biofilms, liquid properties and even the analysis of blood. Quartz sensors are highly sensitive to the mass and the material properties of deposited molecular layers as well as of the wetting liquids at their surface. Due to its sensi­tivity to mass, the technique is often referred to as Quartz Crystal Microbalance (QCM). The measuring principle of quartz sensor technique is based on the precise oscillation of the quartz sensors at their resonant frequency when an alternating voltage is applied. Depositions at the sur­face or wetting of the surface result in a frequency shift and – depending on the material properties – additionally in a damping of the oscillation. Both, the frequency shift and the damping (dissipation) of the oscillation are cap­tured with the qCell / qCell T instrument with high resolu­tion and in real time.

Frequency, Damping and Dissipation

QCM is employed as transducer consisting of a piezoelec­tric AT-cut quartz crystal coated with two gold electrodes, one on each side. Upon application of an alternating voltage between the electrodes, the crystal will generate a mechanical shear wave, which is why the sensors are also referred to as Thickness Shear Mode Transducers.  If the wavelength of the shear wave matches twice the thickness of the crystal, a standing wave is formed, which will propagate as an evanescent wave into the deposited layer on the upper sensor surface. The penetration depth of the wave is 179nm at 10MHz in water. The sensor is sensitive to mechanical properties of the deposited layers, such as density, thickness, viscosity and elasticity. Layer properties affect the resonance frequency fr as well as the damping parameter Γ, which is proportional to the energy dissipated in the layer. The relation between dissipation D and damping Γ is D = 2Γ/fr .

Rigid Layer in Gas Phase

In QCM applications including layer thickness measure­ment of thermal metal deposition and self-assemble monolayer attaching, in which either rigid materials were used or layer thickness is at molecular level, the damping parameter  varies in a tiny amount and is often negligi­ble . Therefore the mass change per area  can be directly related to the frequency shift ∆fSauerbrey  which is described in the Sauerbrey equation:

∆fSauerbrey  = -Cr  · ∆m

Here Cr  is a constant relating to quartz thickness, density and the resonance frequency and surface area of the quartz sensor. The relation between frequency shift and mass change in Sauerbrey relation is simple and straight­forward. However it is applied upon certain assumptions: the deposited layer is considered laterally homogeneous and thin; The layer is rigid and all stresses are propor­tional to strain; The QCM is working in vacuum. In reality it is often hard to satisfy when operation environment changes from air to liquid or the deposited layer is not viscoelastic. The frequency shift alone cannot provide sufficient information to surface changes. It is then necessary to take damping into consideration.­

Operation in Liquid Phase

When the QCM sensor top surface is immersed in fluids, unlike in vacuum, the oscillation of the quartz is damped. This results in changes in frequency and more importantly the damping parameter Γ. Furthermore, the frequency and damping shift depend on the fluid’s properties. The figure shows an example of the influence of different fluids. When immersed in different pure Newtonian fluids (e.g. water and water-glycerol mixture in the figure), the fluid with higher viscosity and higher density will cause a decrease in frequency and an equal amplitude increase in damping. This is defined by Kanazawa:

ΔfKanazawa  = -ΔΓKanazawa  = -Cf  √(ηl ϱl )

Cf  is a constant relating to quartz shear modulus, quartz density and resonance frequency. ηl is the dynamic viscosity of the fluid and ϱl is the density of the fluid.

Rigid Layer covered with Liquid

In this situation, the layer is completely determined by its density and its thickness. In this case, the two contributions above just add, which means Δf=ΔfSauerbrey+ΔfKanazawa and ΔΓ=ΔΓKanazawa.

Viscoelastic Layer covered with liquid

The damping of oscillation is no longer negligible when the thickness of the deposited layer increases. The mass as well as elastic property of the material influence both frequency and damping. Unlike in Newtonian fluids, the frequency and damping shift vary in opposite direction but in different amplitude.