A Quartz Crystal Microbalance instrument or QCM, as it is usually called, is essentially a balance for very small masses. Instead of measuring pounds or kilograms, like the balances that we are used to in our everyday life, a QCM and its related “cousins” measure nanograms. The mass is measured in real-time, allowing the user to closely follow changes, such as mass increase or mass decrease. Hence, it is possible to follow how extremely small masses are being added or removed to the surface that you are weighing.
Why would you want to weigh masses that small?
Figure 1. The QCM and its cousins are balances for very small masses. Instead of measuring pounds or kilograms, like a kitchen scale, the QCMs measure nanograms.
The pertinent question is why would anyone want to weigh nanogram variations of a material? Many products that we use daily, such as smart phone touch screens, have coatings that are extremely thin, and it can be useful to know exactly how thin or thick they are to ensure optimal function. Additionally, measuring how masses change over time can provide useful information about how a material, such as proteins in tears, can interact with and build up on the surface of a contact lens over time. This detection is possible due to the fact that when a surface of interest is exposed to molecules that interact with and eventually adhere to it, the result will be an increase in mass detected by the QCM. A change in mass confirms that the interaction occurred. Alternatively, if no mass increase is detected, it confirms that there was no interaction and no molecules have adhered to the surface. Thus, a QCM is a method for characterizing and quantifying interactions between molecules and a surface where mass changes occur in real-time.
What can you use a QCM instrument for?
Figure 2. Schematic illustration of molecules adhering to the QCM sensor surface (top) and the resulting detected mass increase (bottom).
Figure 3. Schematic illustration of molecules leaving the QCM sensor surface (top) and the resulting detected mass loss (bottom)
Sticking with the example of a contact lens used above, say we want to design a new type of contact lens that can be worn for an entire year. We need to test whether proteins from our tears will attach to the lens over time. In order to do so, we coat the QCM sensor with the material used in the contact lens and introduce a flow of tears, or proteins that are found in tears, over the QCM surface. We can then evaluate whether a change in the mass occurs, which would imply that proteins are attaching to the surface, Figure 2, and modify the contact lens materials until these prevent the adhesion of any proteins.
As mentioned above, a QCM not only detects mass increases, it can also monitor mass losses, Figure 3. A specific example where measuring mass loss is important is in analyzing how well dish washer detergents can clean and remove dirt. Say we want to develop a new dishwasher detergent and evaluate how effective it is in removing grease from a glass surface. We start by coating grease onto a QCM sensor. Next, we introduce a flow of our new dishwashing detergent over the grease to ascertain whether there is any loss of mass that would correspond to the stain being removed. The QCM measurement enables us to monitor, in real-time, how fast the cleaning process is, and whether the surface is completely clean once the flow ceases or if there is still grease remaining at the surface.