QCM-D FAQ

QCM-D Technology

Use and performance of Q-Sense systems

What is the mass sensitivity in air/liquid with Q-Sense instruments?

What is the time resolution when measuring with Q-Sense instruments?

What is the frequency range of Q-Sense instruments?

What coatings/surfaces can be used?

What is the maximum thickness of an applied/adsorbed film?

What is the detection range from the surface?

What is the temperature range of Q-Sense instruments?

How large is the sensor area?

How many times can the sensor crystal be reused?

What sample volumes are needed?

What fluids can I use?

• What is the difference between QCM-D and traditional QCM?
The Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) technology measures the frequency and dissipation of the freely oscillating quartz crystal after excitation. As opposed to ordinary QCM, the QCM-D determines the dissipation factor, providing information about conformational changes and softness/rigidity (viscoelasticity) of the molecules studied. The major benefits with this procedure compared to other QCMS are:

- Extremely quick measurements that gives a kinetic resolution of up to 200 data points per second with the latest instrument. Higher measurement rate means higher sensitivity since several data points can be used to average data.

- Record the response of the crystal when it is not a part of a feedback loop (active oscillation circuit). This means that the instrument only record changes occurring at the surface. In conventional QCMs part of the recorded changes in resonant frequency is due to changes in the feedback loop (such changes occur when the mechanical load on the crystal changes) and not due to changes taking place at the surface.

• What is dissipation?
The dissipation (damping) is the sum of all energy losses in the system per oscillation cycle. It is defined as 1/Q—the energy dissipated per oscillation, divided by the total energy stored in the system. With QCM-D, the dissipation factor is measured every time the drive generator output is stopped and the sensor oscillation starts to decay exponentially. A soft film attached to the quartz crystal is deformed during oscillation, which gives high dissipation. In contrast, a rigid material gives low dissipation.

• Why is it so important to measure the dissipation?
The frequency response of a quartz crystal represents the change of total mass in the measurement. This mass always includes a certain amount of water. However, the amount of water may vary between 10% and 90% depending on the type of molecule and the way it adsorbs to the surface (an elongated protein that adsorbs flat to the surface gives low dissipation while the very same molecule standing up on the surface gives high dissipation). By measuring the dissipation, it is possible to determine if a soft film (water rich) or a rigid film (less water) has formed on the surface. Only when the film is fairly rigid does the Sauerbrey relation give a good estimation of adsorbed mass. Measuring the dissipation means that it is possible to determine whether the Sauerbrey relation is valid. The dissipation factor gives additional ”structural” information, compared to an ordinary QCM measurement, in that one can measure the conformational change of the film, e.g., crosslinking (collapse) and swelling.

• When is it correct to use the Sauerbrey relation to calculate the mass adsorbed on the quartz crystal?
The Sauerbrey relation describes the linear relationship between changes in frequency and changes in mass for thin films adsorbing to the sensor surface. It gives a good estimate of mass/ thickness, as long as the dissipation is relatively low. When the dissipation value typically reaches above 1×10-6 per 10 Hz, the film is too soft to function as a fully coupled oscillator—the regions distant from the surface do not couple to the oscillation of the sensor. This means that the Sauerbrey relation, which is normally used to calculate the mass directly from the change in frequency, will underestimate the mass. However, by measuring both dissipation and frequency at several harmonics, it is possible to extract the correct thickness estimations even in these cases. This also makes it possible to calculate the viscoelastic and structural properties using a viscoelastic model incorporated in the Q-Sense software, QTools.

• What are the benefits of taking measurements at several frequencies simultaneously?
Simultaneous measurement at multiple overtones is required to model viscoelastic properties and to calculate the correct thickness of films that do not obey the Sauerbrey relation. With the Q-Sense E4 system, 14 incoming parameters (seven frequencies and seven dissipation values) per sensor provide a well-determined model of the particular film properties. Moreover, the different overtones give information about the homogeneity of applied layers: as the detection range out from the sensor surface decreases with increasing overtone number, abnormal frequency behavior suggests vertical variations in film properties. The fact that the detection range from the sensor surface decreases with increasing frequency is also used by the modelling software to calculate an accurate thickness of films that do not fully couple to the oscillation of the sensor. For rather soft films, with high water content (e.g., films made of large proteins), you will not obtain accurate thickness information without taking measurements at several frequencies. Another advantage of using higher overtones is the decreasing noise to signal ratio, which is good when extra-high sensitivity is desired.

• What is the difference between QCM-D and SPR measurements?
The ability to evaluate kinetics is quite similar in both systems from a technical point of view, but when it comes to sensitivity, they differ quite a bit. QCM-D systems are more sensitive for water rich and extended layers, while the SPR system is favored for compact and dense layers. QCM-D, being an acoustic technique, allows measurement of thicker films, while SPR, being an optical technique, greatly limits the film thickness.

The reason for this difference is due to the different physical principles by which the coupled mass is measured. The mass-uptake estimate from SPR data is based on the difference in refractive index between the adsorbed biomolecules and water displaced by the biomolecules upon adsorption. This means that water associated with the protein film (e.g., the hydration shell) is essentially not included in the mass determination. In contrast, changes in frequency acquired with QCM-D measure water coupled as an inherent mass via direct hydration, viscous drag and/or entrapment in cavities in the adsorbed film. The SPR response is therefore proportional to the coupled "molar mass", while in QCM-D measurements the layer is essentially sensed as a "hydrogel" composed of the macromolecules and coupled water.

However, while SPR measures one parameter only, the additional information contained in energy dissipation data from QCM-D increases the capacity for a detailed interpretation. Changes in the dissipation are related to the shear viscous losses induced by the adsorbed layers, and thus provide information that has the potential to identify structural differences between different adsorbed systems, or structural changes in the same type of molecule during the adsorption process.

• What is the mass sensitivity of Q-Sense instruments?
The maximum mass sensitivity in liquid is about 0.5 ng/cm2 for Q-Sense E4 if measuring at a rate of one data point every five seconds. Q-Sense E4 operating with all four sensors and at three harmonics has a sensitivity of about 2 ng/cm2 if all data are collected in one second. For example, consider a monolayer (<100% surface coverage) of myoglobin (17.8 kDa): the monolayer corresponds to 177 ng/cm2 (change in frequency, 10 Hz).

• What is the time resolution?
The maximum rate is up to 200 data points per second with Q-Sense E4, giving you a high-resolution real-time measurement suitable for fast reactions.

• What is the frequency range of Q-Sense instruments?
The frequency range is 1 to 70 MHz with Q-Sense E4. A large range is important to be able to use the unique features of multiple frequency and dissipation sampling.

• What coatings/surfaces can be used?
The sensor can be coated with almost any material, as long as it can be applied as a thin (nm range), homogenous layer firmly attached to the underlying surface. The layer thickness can vary between nanometers and micrometers, depending on the viscoelastic properties of the applied material. Q-Sense offers pre-coated sensors with e.g., gold, Ti, SiO2, AlO3, stainless steel, hydroxyapatite, polystyrene and biotin. Several other materials are also available, e.g., most metals, metal oxides or spin-coated polymers.

• What is the maximum thickness of an applied/adsorbed film?
The maximum thickness of a coating depends on the viscoelasticity of the coating and may vary from a couple of hundred nanometers to a few micrometers. The more rigid the layer, the thicker the layer can be. It is always possible to contact Q-Sense to make a request for new surfaces.

• What is the detection range from the surface?
The detection range varies from nanometers to micrometers, depending on the viscoelasticity of the applied film. In pure water, the detection range is approximately 250 nm. Applying a very rigid film, such as a metal, still allows the same detection range in water. This means that the measurement principle is not affected when a thin film is coated on the surface prior to taking measurements. Compared to optical methods, the detection range of QCM-D is its main advantage. Consider, for example, polyelectrolyte multilayers several hundred nanometers thick. These are easily sensed by QCM-D.

• What is the temperature range of Q-Sense instruments?
Proper temperature stabilization and function of the chamber can be obtained at temperatures between 15°C (59°F) and 65°C (149°F) when the instrument is at normal room temperature around 20ºC. However, the Q-Sense High Temperature Chamber enables temperature control between 4°C (39.2°F) and 150°C (302°F).The stability of the actual temperature is +/- 0.02 K at 25°C.

• How large is the sensor area?
The standard sensor has a diameter of 14 mm. Because the sensor is most sensitive in the center, it is important to ensure that the same reaction is taking place over the entire surface in order to compare measurements, i.e., that your sample forms a homogeneous film.

• How many times can the sensor crystal be reused?
This depends on the application and the possibility of cleaning the sensor. If only mild cleaning conditions are needed and the application is insensitive to increased surface roughness, a sensor can be used for numerous experiments. However, for certain applications a standard surface can be used only once.

• What sample volumes are needed?
The volume above the sensor is 40 µl with the Q-Sense E4. With the E4, the total volume needed from inlet to outlet is about 150 µl.

• What fluids can I use?
Many different fluids including water, inorganic salt solutions, alcohols, and organic media (even e.g., hexane and toluene) can be used. Except for the titanium wall of the chamber, the fluid is exposed to tubing and O-rings that can easily be changed for different types of measurements (the so called high resistant kit). Check chemical compatibility charts for details.