QCM-D based biosensing – exploring the possibilities

Licentiatavhandling, 2011

We are constantly increasing our understanding of all biological systems and how these highly complex systems interact with each other. The high degree of complexity of these systems can be investigated in greater and greater detail by increasingly sophisticated sensing techniques. One interesting technique in this context is quartz crystal microbalance with dissipation monitoring (QCM-D), biosensing capabilities of which have been explored in this thesis. The strength of the QCM-D technique, when compared to other biosensing techniques, is primarily its ability to sense viscoelastic, or nanomechanical, changes in the system under study. To allow the monitoring of biomolecular interactions with a surface sensitive technique, such as QCM-D, surface modifications are needed which allow the desired interactions to occur near the sensor surface while minimizing undesired effects. Here, a surface modification was developed based on mixed monolayers of oligo(ethylene glycol) (OEG) disulfides on gold, exposing a fraction of biotin groups from the inert OEG background. By introducing biotin groups on the surface the strong interaction between the biotin and streptavidin could be used for immobilizing biotinylated biomolecules since streptavidin has four binding sites for biotin. The biotin surface modification was further characterized and used for repeated antibody-antigen interactions between bovine serum albumin and anti-(bovine serum albumin) that had been immobilized to biotinylated protein A in QCM-D measurements. The biotin surface modification was also used in experiments where conformational changes in biotinylated plasminogen were detected as changes in viscoelastic properties upon exposure to low molecular weight lysine analogues. These results provided an attractive basis for the drug development industry to screen for interactions inducing multiple, reversible, conformational changes relying on molecular properties that cannot be easily detected by other techniques. Furthermore, preliminary results from carbohydrate-protein interactions are presented as an interesting application area for the QCM-D technique to study materials mimicking the extra cellular matrix environment. In the last study, morphological changes in fibroblast cells were monitored simultaneously by QCM-D and light microscopy as the cells were subjected to the actin perturbing agent cytochalasin D. The combination of the two methods provided a powerful basis for interpreting the QCM-D results, giving new insights into the mechanical behavior of cellular systems. Based on the procedures presented in this thesis, future work will be focused on finding biological systems where the QCM-D technique can provide new, unique biological information that has not been measured earlier with other methods.