The frequency shift (Δf) measured by QCM-D reflects mass change at the surface. A decrease in Δf indicates mass uptake, and vice versa. The mass sensed by QCM-D is the so-called “hydrated mass”, i.e., the mass of the molecules plus the coupled solvent. If molecules sitting at the surface change conformation, the amount of coupled solvent will typically change. I.e., even if the molecular mass remains the same before and after the conformational change, the coupled mass will increase (for example in cases of swelling) or decrease (for example in cases of collapse, crosslinking, or molecular re-arrangement), resulting in an increase of the sensed mass, i.e. decrease in Δf, or vice versa. The magnitude of the shift is related to the mass change.
The Dissipation shift (ΔD) provides information about the viscoelastic properties of the layer at the surface. As a rule of thumb, the higher the D, the softer and/or thicker the layer. A typical scenario where there is an increase in mass, i.e. decrease in f, shows an increase in ΔD. This is due to the fact that an increase of coupled solvent generally means a more hydrated layer, which is softer and/or thicker than before the mass change.
The detection range of QSense technology spans from nanometers to micrometers, depending on the viscoelasticity of the applied film or medium. Molecules and entities that are typically analyzed are for example lipids, proteins and other biomolecules, surfactants, polymers, nanoparticles and cells.
