What is the QCM-D sensing depth?

Quartz crystal microbalance with dissipation monitoring, QCM-D, is a surface-sensitive technology based on tracking the oscillatory motion of a quartz crystal sensor. The sensing depth of the method, i.e., the detection range above the sensor surface, depends on the penetration depth of the oscillatory motion of the contacting medium above the sensor, and varies from nanometers to micrometers, depending on the properties of the contacting medium and the harmonic of oscillation.

The sensing range depends on the penetration depth of the shear wave

QCM measurements can be performed in both gas phase and in liquid phase. When the contacting medium above the sensor surface is viscous, the oscillatory motion of the sensor will couple to that medium and be transferred into the bulk. The transferred motion will be an induced, highly damped, sinusoidal shear wave that travels in the z-direction away from the sensor surface,¹ Fig. 1. The decay constant, δ, i.e. the penetration depth of the shear wave, is the sensing depth.

Figure 1. When there is a viscous medium above the sensor, the sensor oscillation will be transferred into that medium and induce a highly damped sinusoidal shear wave that travels away from the sensor surface. The illustration shows what the transverse fluid velocity profile for water would look like at three different points in time with different surface velocities, 'max', 'intermediate', and 'zero'.

The sensing depth is inversely proportional to the square root of the resonance frequency,¹ Eq. 1.,   

where ρ and η are the density and viscosity of the contacting medium, n is the harmonic, and f₀ is the fundamental frequency. This means that the higher the frequency, the shorter the sensing depth.

As an example, for a standard QSense sensor in water at 20°C, the penetration depth for the different harmonics would be according to the below table.

The sensing range of coated QCM sensors

So, what happens if the sensor is coated with an additional layer? If a rigid film, such as a thin layer of metal or similar, is added on top of the sensor, this will follow the oscillatory motion of the sensor. In this case, the sensing depth into the contacting medium will be the same as if the rigid layer had not been added, Fig. 2.

Figure 2. A schematic illustration of the sensing depth above (left) a bare QCM sensor in liquid and (right) a QCM sensor where a thin and rigid coating has been added on top. In the latter case, the solid layer will follow the oscillation of the sensor and the sensing depth will be the same as in the case of the uncoated sensor.

Concluding remarks

In summary, QCM-D is a surface-sensitive technology where the sensing depth of course is relevant for detecting changes above the sensor surface. Ranging from nanometers to micrometers, the sensing depth depends on the properties of the contacting medium and the harmonic of the oscillation used for the measurement. The versatility of the technology makes it valuable for diverse applications in both gas phase and liquid phase.