Quartz Crystal Microbalance
Quartz Crystal Microbalance (QCM) is an instrument that allows a user to monitor small mass changes on the surface of a coated quartz crystal. Gamry’s line of Quartz Crystal Microbalance instruments include high-sensitivity mass sensors which measure frequency and dissipation while giving you the option to simultaneously run electrochemical experiments to gain insight into what drives mass changes within films.
The QCM-D QCM-I Principle
What is Quartz Crystal Microbalance?
The Quartz Crystal Microbalance (QCM), a recognized, sensitive technique, resulting in a mass variation per unit area by measuring changes in resonant frequency of a quartz crystal.
The Quartz Crystal Microbalance can be used under vacuum for monitoring the rate of deposition in thin film deposition systems or in liquid environments where it is highly effective at determining the affinity of molecules (proteins, in particular) to surfaces functionalized with recognition sites.
This technique is used to measure larger entities such as viruses or polymers and biological assemblies with a sensor surface, in air or liquid, label-free and in real time based on the change in resonant frequency of a quartz crystal sensor when it is covered with a thin film or liquid.
Quartz Crystal Microbalance: QCM-D vs. QCM-I
The quartz crystal microbalance with dissipation monitoring (QCM-D) is a type of quartz crystal microbalance (QCM) used in interfacial acoustic sensing. The QCM-D determines the dissipation factor, providing information about conformational changes and softness/rigidity (viscoelasticity) of the molecules studied.
Using the quartz crystal microbalance with impedance (QCM-I), the impedance of the quartz crystal sensor is measured using a network analyzer to accurately determine the frequency and bandwidth of the crystal resonance. This is done over multiple frequency ranges to measure the fundamental and overtone resonances. Changes in frequency and bandwidth allow calculating changes in mass and dissipation of a film. These frequency changes can be used to determine the hydrated mass coupled to the sensor surface, with sensitivity in the ng/cm2 range and thicknesses of nm to μm. The resonant bandwidth is directly related to the energy dissipation at the interface (QCM-D) and gives information about the viscoelastic properties and structure of the adsorbed layer or viscosity of the liquid.
What are Quartz Crystal Sensors
Quartz crystal sensors are thin discs of quartz with electrodes deposited on each side. Because quartz is piezoelectric, when a voltage is applied between the two electrodes a shear displacement is caused in the crystal. If the voltage is alternated, a resonant oscillation can be stimulated in the crystal when the wavelength of the shear displacement in the quartz corresponds to twice the thickness of the sensor. This means that a standing wave can be setup in the quartz with the top electrode moving in the opposite direction to the bottom electrode. The frequency at which this happens is the resonant frequency of the crystal. As material is added on top of the sensor, the wave propagates into this layer and the resonant frequency changes. This is the basis of the measurement.
Why Makes QCM-I Experiments Unique?
An experiment using a quartz-crystal microbalance (QCM) with impedance (QCM-I) is a unique technique popular in characterizing biological films that are structurally complex. While basic Quartz Crystal Microbalance measurements track only the shift in the resonant frequency upon film deposition or growth, QCM-I provides quantitative information on a film’s viscoelastic properties.
Another unique technique using the Quartz Crystal Microbalance with impedance is measuring the adsorption or binding of polymers, proteins, nanoparticles and other molecules to a surface. Multi-parametric data is obtained in real time, detailing changes of the hydrated mass and rigidity of layers coupled to the sensor surface. Here, the mass and viscoelastic changes can be understood for a range of adsorptive events, including the polymer modification of a surface, followed by the physisorption of biotin-BSA, binding of streptavidin functionalized quantum dots and further in-filling with streptavidin.