Surface plasmon resonance (SPR) devices are the benchmark in optical sensing. They are used for detecting biomarkers of disease,discovering drugs, analyzing chemicals, and ensuring food quality and safety,and detecting pollutants in our environment. SPR devices can detect molecules within a few hundred nanometres of their metal surfaces. When a target molecule binds to sensing molecules placed on the device's surface, this alters the path of light travelling through the medium, or changing its "refractive index". This change of refractive index is used to indicate the molecule's presence.
Source: phys.org