Incident light is reflected inside the fiber through a phenomenon called total internal reflection (TIR). TIR occurs when light travels from a medium with higher refractive index (RI) (e.g.glass) to that with a lower RI(e.g.Au). The reflection at the medium interface creates surface plasmons: propagating electron density waves caused by interaction between incident photons and free electrons in the metal. The resonance caused by this wave delivers a decrease in the intensity of the reflected light at SPR wavelengths. This is referred to as the SPR dip. Because the SPR conditions depend on the RI in the vicinity of the interface, changes to the outer surface will result in shift of wavelengths required to meet the SPR condition. Thus, the consequent changes of the outer surface can be followed in real-time by detecting the shift in SPR wavelengths (Figure 3B).