How Quartz Crystal Microbalance Works to Measure Surface & Thin Film Interactions

Quartz Crystal Microbalance allows for the accurate measurement of thin films interactions by utilising the unique properties of quartz. However, it is only with the addition of dissipation measurement (QCM-D) that you can truly unlock the power of QCM instruments.

How did Quartz Crystal Microbalance come about?

As early as 1880 Jacques and Pierre Curie discovered that when mechanical stress is applied to quartz a voltage proportional to the stress is also created.

However, the term Quartz Crystal Microbalance wasn’t coined until the late 1950s by Sauerbrey, who also demonstrated that the oscillation frequency of the quartz was dependant on changes in surface mass. This led to the discovery of the Sauerbrey equation.

The Sauerbrey equation shows that when voltage is applied to a quartz crystal to produce a specific oscillation, any change in the mass of the quartz surface is directly related to the change in frequency of the oscillating crystal.

Where is the Sauerbrey equation applicable?

The Sauerbrey equation says that the change in QCM frequency determines the mass of the adsorbed analyte. It is valid for elastic or rigid samples like:

• Metal oxides
• Metallic coatings
• Thin adsorbed layers

As these materials do not dissipate energy during oscillation. The equation relies on a linear sensitivity factor that is fundamental to the properties of the quartz, so in theory it doesn’t require calibration.

However, this is only applicable to uniform thin film samples. That’s why the Sauerbrey equation is not valid with inelastic materials like:

• Polymers
• Biomolecular systems
• Cells

As viscous dampening during oscillation causes energy loss, soft and viscoelastic films fail to adequately couple with the oscillating quartz crystal. This can lead to an underestimation of the mass using the Sauerbrey relationship.

To fully characterise these types of materials QCM-D (Quartz Crystal Microbalance with Dissipation) was created. Today, QCM-D instruments are among the leading tools for measuring thin film interactions (mass, thickness, density) and viscosity. Additionally, associated solvent or water content of adsorbed films can be measured by comparing the mass measured using QCM-D with that of complementary techniques such as ellipsometry or surface plasmon resonance.

What is a Quartz Crystal Microbalance?

A QCM consists of a thin disk made from quartz and sandwiched between coated electrodes. AC voltage oscillates the sensors. As the sample material flows across the sensors, the mass changes and so does the frequency (Sauerbrey equation).

The QCM acoustic waves must propagate perpendicular to the surface of the crystal. Therefore, in the manufacturing process there is a specific method where the crystal axes are cut in either:

• AT
• BT

With AT the most common method of manufacturing the QCM as it provides for almost zero temperature dependency.

Resonance sensitivity

The thickness or thinness of the quartz crystal determines the sensitivity to resonance. The thinner the QCM, the higher the resonant frequency. Conversely, thinner quartz is also more fragile.

Common uses of Quartz Crystal Microbalance

Though the technology has existed for over half a century, it is mainly in the last few decades that QCMs have seen a rapid uptake in adoption. Uses for Quartz Crystal Microbalance include both research and industrial application:

• Surfactant adsorption on surfaces
• Contamination monitors
• Chemical analytes detection
• Environmental pollutant measurements
• Biomolecule.and disease biomarker measurements
• Pathogen detection
• Hydrogen adsorption on metal films
• Metal detection in vacuum
• Electrovalency measurements
• DNA and RNA hybridisation studies
• Gas chromatography detection
• Antigen-antibody reactions

Advantages over other instrumentation

In the area of chemical detection analysis is typically carried out by the following instruments/techniques:

• FTIR – fourier transform infrared spectronomy
• GC – gas chromatography
• MS – mass spectrometry

All of these techniques can be cumbersome and expensive, not to mention time consuming and limited to offline analysis.

QCM offers several advantages over these techniques:

• Low overheads
• Rapid results
• Safe, real time procedure
• Portability

Biolin Q-Sense Analyser

The Q-Sense Analyzer by Biolin represents the ideal qualities of a QCM. Features include:

• Real time analysis of thin films
• QCM-D technology (Quartz Crystal Microbalance with Dissipation)
• Measurement across two parameters:
  • Frequency – Mass / thickness
  • Dissipation – Rigidity
• 4 flow modules that can be used serially or in parallel
• The ability to measure samples of thin films, polymers, surfactants, proteins, lipids and cells

Integrated software

Q-Sense Dfind software provides a single suite of integrated analysis tools especially designed to interface with the Q-Sense Analyser.

Dfind is an intuitive software experience that first provides the user with the raw data and then guides them through modelling and reporting structures on elements like:

• Mass
• Thickness
• Viscoelastic properties
• Adsorption rate

QCM-D

Unlike many QCM systems the Biolin Q-Sense Analyser contains dissipation measurement to provide data on structure and viscoelasticity of the sample. Without the dissipation measurement QCM’s can only rely on the Sauerbrey equation which severely hinders the capability of the instrument.

Quartz Crystal Microbalance with Dissipation, QCM-D, provides real-time, label free measurements of molecular adsorption and interactions on various surfaces with nanogram sensitivity. In addition to adsorbed mass, measured as changes in frequency of the quartz crystal, the dissipation parameter (D) provides novel insights regarding structural (viscoelastic) properties of adsorbed layers.

The best in scientific instrumentation

ATA Scientific is committed to sourcing and supplying the best in scientific instruments. The Biolin Q-Sense Analyser is one of many top-of-the-line instruments for precision lab and industrial sampling. Contact ATA Scientific today for a competitive quote on your next instrument purchase, and find out why we’re the specialists in analytical instrumentation.