Adsorption and desorption processes occur everywhere. They play an important role in areas such as surface science, biomaterials, cell and molecular biology, and pharmaceutical development and production, where molecules and nanoparticles interact with various surfaces in different contexts.
Adsorption can be defined as the ‘adhesion’ of molecules from a liquid or gas phase onto a surface. Desorption is the reverse phenomenon, when adsorbed molecules are removed from a surface. QCM-D technology, which is essentially a balance for small masses, can monitor molecular adsorption and desorption processes in real-time by detecting the mass changes following the molecular uptake or release from the surface studied.
Characterizing adsorption and desorption processes on solid surfaces
Depending on the application and objective of the study, it may be relevant to either understand, characterize or optimize the adsorption or desorption events. Either way, it will be relevant to monitor the amount of material that is being added to or leaving the surface, and it may also be relevant to investigate the rate at which the process occurs. Each time material is added to or removed from a surface, there is a corresponding change in mass, which will be detected by QCM-D in real-time.
Example 1: Evaluating protein adsorption on glass and plastic
As an example, let’s have a look at protein adsorption on two different surfaces, one glass surface and one plastic. As outlined in Figure 1, we follow the steps below.
We run two measurements in parallel, one on glass and one on plastic. The measurements start with the respective surface in a background solution. At this point, the surfaces are bare, which means zero mass (Figure 1A).
Next, we introduce the protein solution and lets it flow over the surface. The QCM-D instrument captures two parameters, frequency and dissipation. The frequency shift represents the mass change in reverse, i.e. a negative shift means mass uptake, and positive shift means mass loss. When the protein reaches the surface, there is a mass uptake on both the glass and plastics surfaces. We note, however, that the adsorption is somewhat faster on the glass surface than on the plastic one (Figure 1B).
After 30 minutes, we rinse to remove any loosely bound protein. Here we see mass loss from both the glass and plastic surfaces (Figure 1C).
At the end of the experiment, we conclude that the final adsorbed amount was larger on the glass than on the plastic surface (Figure 1D).
Figure 1. (Top) Protein adsorption on plastic (PVDF) and glass (borosilicate) measured with QCM-D. (Bottom) Schematic illustration of the protein adsorption process.
Evaluating adsorption and desorption under different conditions
Monitoring the mass as a function of time, evaluating surface interaction processes is straightforward. It is also possible to compare behavior under different conditions by varying for example the concentration, temperature, pH and ionic strength.
Example 2: Comparing the protein adsorption with two different concentrations
Here we extend the first example and look at two different sample concentrations. Using the same experimental setup, we compare adsorption on glass and plastic with a low and high protein concentration to evaluate the influence of this parameter. Looking at the final adsorbed amounts, Figure 2, we see that the adsorbed amount more than doubled with the high protein concentration compared to the low one.
Figure 2. Protein adsorption measurements with two protein concentrations, high and low. The surface uptake was higher with the high protein concentration. For both concentrations, more protein was adsorbed on the glass than on the plastic.
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Surface interactions in other areas
In addition to these two examples, other types of adsorption and desorption events that could be characterized by measuring the mass uptake and mass loss include, for example, surface interaction of surfactants, polymers and nanoparticles.
The versatility of polyelectrolyte multilayers, PEMs, is high, which makes them interesting for e.g. biomedical applications. The functionality is largely determined by the layer properties, which needs to be understood to be tailored. Here, we show how PEMs can be characterized with QCM-D.
Biocompatibility, antibacterial qualities, and drug delivery can be achieved by for example polymer brushes, polyelectrolyte multilayers or hydrogels. When tailoring the interfacial properties of these thin films, the layer conformation, such as crosslinking and degree of hydration is important.