Surface plasmon resonance (SPR) is an optical, surface-sensitive technique used to study the label-free interaction of biomolecules in a complex environment in real-time. In a typical SPR experiment, ligands are immobilised on a SPR sensor surface which is exposed to a flowing solution of analytes in a microfluidic channel. The sensor surface is generally a glass prism covered with a thin metal layer, like gold or silver. A plane-polarized, monochromatic incident light is directed onto the sensor to which the ligands are attached, creating charged oscillations, called surface plasmons, at the metal surface. When analytes become bound to surface-immobilised ligands, the surface plasmon resonance conditions change, resulting in a change in the reflected angle or wavelength of the light, depending on the interrogation used. This change is captured and plotted vs. time to generate a sensorgram.
Sensograms
Sensorgrams are used to extract affinity and kinetic data of the interactions between the ligand and analyte. They can also reveal any specificity and concentration information through the magnitude of the SPR signal. In general, a sensorgram has five phases:
Baseline: The initial phase is the baseline. A running or flow buffer is used to condition the sensor surface and check for any sensor system instability.
Association: The second phase is where analytes begin to bind to immobilized ligands. It is indicated by the initial sharp rise of the SPR signal in the sensorgram and it is ideally a single exponential curve.
Steady state: This phase occurs at the top flat portion of the sensorgram where the net rate of bound analytes is zero.
Dissociation: This phase begins when the analyte solution is replaced by a wash buffer, which causes the specific interactions between the analytes and ligands to break
Regeneration: Finally, a low pH buffer such as glycine is flowed to reset the SPR baseline signal as the beginning of the experiment