I saw this method today in one paper. It’s used to detect the binding between a small ligand and Fc chains of human antibody. The principle is simple. But technology is cool! The following like is from YouTube, the video illustrated the pricinple.
The combination between ligands and antibody could affect the electron on the the surface, and then affect the reflect lights.
Just like many other methods, SPR magnifies the microscopic change of system situation to macroscopic phenomenon.