Fluorescence intensity in surface-plasmon field-enhanced fluorescence spectroscopy

Reflection loss in surface-plasmon resonance (SPR) curve of an absorbing dielectric is due to the absorptions of the metal film and the absorbing dielectrics. The SPR curve is significantly distorted from that of the nonabsorbing dielectric (i.e., broader curve shape with higher reflectance minimum), especially for strong absorbing dielectrics. When absorption of the absorbing dielectric is too small (i.e., very thin or low concentration), insignificant change in SPR curve is observed as the absorbing dielectric binds onto a nonabsorbing dielectric film. However, if the absorbing dielectrics are fluorophores or fluorophore-labeled molecules, the presence of small amount of the absorbing dielectric can be detected by the highly sensitive surface-plasmon field-enhanced fluorescence spectroscopy (SPFS). The fluorescence signal can be expressed in terms of the dielectric constant of the absorbing dielectric and the SPR-generated evanescent field. Although the fluorescence quenching near the metal surface complicates the SPFS-fluorescence signal, the quantitative analysis can be performed on a well-defined dielectric layer.