Various different methods exist currently that enable the user to perform label free assays. The common denominator is that all of these rely on optical measurement. Specialised detection instruments are used to track in real time changes in the transmission of light waves at the sensor/analyte interface. Changes at the interface induced by analyte binding affect the transmission of light. The most popular methods currently are Surface Plasmon Resonance (SPR) and Biolayer interferometry (BLI). In the following the methods are briefly discussed.
In SPR sensors made of a thin gold film are used. Incoming light waves are absorbed and excite the electrons in the gold film into a state of collective oscillation, which is referred to as plasmon resonance. The properties of the plasmon resonance are extremely sensitive to the immediate surrounding. Binding events on the gold surface will affect the plasmon resonance, which then affect the way light waves travel. The sensors can be functionalized with biomolecules using various different approaches.
BLI is part of the general optical analysis technique called interferometry. A coherent light wave, i.e. light with only one well defined wavelength, is reflected from two surfaces separated by a small distance. The resulting diffraction pattern allows the determination of the distance between the two surfaces. In BLI the two surfaces are the sensor surface and the layer of immobilized biomolecules. Binding events result in a shift of the effective distance between the two surfaces.