GCI is a surface-based, label-free biosensing technology. When target ligand molecules (e.g. proteins) are attached to the sensor surface, binding of injected analytes leads to an increase in mass and hence to a change in the refractive index near the sensor surface. The evanescent field generated across the whole wave-guide sensor ensures maximal detection sensitivity. In GCI, refractive index changes on a sensor surface are measured as time-dependent phase-shift signals (interferometry). The long light-to-sample interaction length of the waveguide provides intrinsically high signal-to-noise levels.