QCM-D – with focus on biosensing in biomolecular and cellular systems

Doctoral thesis, 2013

The high degree of complexity in biological systems is being investigated in greater and greater detail by increasingly sophisticated sensing approaches. The quartz crystal microbalance with dissipation monitoring (QCM-D) technique stands out in this context, by probing the viscoelastic properties at interfaces. The simultaneous detection of changes in resonance frequency and energy dissipation in QCM-D allows for the extension from the plain mass detection of conventional QCM instruments to more sophisticated approaches looking at changes in both mass and viscoelastic properties. In this thesis, the biosensing capabilities of QCM-D have been explored in novel applications ranging from biomolecular to cellular studies. To allow for specific immobilization of biomolecules, 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. Three biotinylated biomolecular systems were studied using this surface chemistry: i) antibody-antigen interactions between bovine serum albumin and anti-(bovine serum albumin), ii) conformational changes of the fibrinolytic protein plasminogen and iii) enzymatic degradation of the glycosaminoglycans hyaluronan and chondroitin sulphate. All of these biotinylated molecules were irreversibly attached to the sensor surface via the tetravalent protein streptavidin. This approach provided robust biosensor surface chemistries that could withstand regeneration and also control the molecular orientation to offer greater control of retained biofunctionality of the bound molecules. The results from those studies point to an attractive platform for sequential or parallel screening of biomolecular interactions and/or conformational changes, for academic purposes as well as for the life science industry. QCM-D was also used in studies of stimulated morphological changes in fibroblast cells monitored by simultaneous light microscopy. The combination of the two methods facilitated the interpretation of the QCM-D results, giving new insights into how structural changes within the cells are transduced to the cell-sensor interface. The final study combined the grafting of biotinylated hyaluronan and cell adhesion of chondrocytes to these layers which indicated a degrading activity of the cells even though they were seemingly not attached to the QCM-D sensor. QCM-D has proven to have great potential as a biosensing technique and this thesis emphasizes the broad applicability of QCM-D for biosensor development ranging from molecular to cellular systems.