Biophotonics_Concepts_to_Applications

7.8 Surface Plasmon Resonance Biosensors

Surface plasmonsare electromagnetic waves that propagate along the surface of a
metallic-dielectric or a metallic-air interface and are evanescent in all other direc-
tions [ 66 – 71 ]. Because these waves travel along the surface of a thin metallic layer
(on the order of several ten nanometers thickness for visible light), they are very
sensitive to any changes in the boundary characteristic at the surface. For example,
changes in the boundary characteristic can occur when molecules suspended in a
liquid sample are adsorbed on the metal surface. Asurface plasmon resonance
(SPR) is the collective oscillation of the surface electrons when they are stimulated
by incident light. The resonance condition occurs when the frequency of the
photons matches the natural frequency of surface electrons that are oscillating
against the restoring force of positive nuclei in the surface material. By making use
of the SPR effect, biosensors with a high sensitivity can be created for applications
such as imaging, medical diagnostics, drug discovery, food safety analysis, and
environmental monitoring.
The basic concept of a commonly used SPR configuration is illustrated in
Fig.7.27. In this setup,first a thin metallicfilm, such as gold, is deposited on a
dielectric substrate. Thisfilm is then functionalized with a layer of specific selective
biosensing receptors, for example antibodies. By using a prism, polarized light that

Broadband

light source

Optical

detector

Optical

circulator

Serial FBG array

FBG 1 FBG 2FBG 3

Catheter

Optical

fiber

Fig. 7.26 Data acquisition equipment to record pressure changes along a catheter

Polarized light

Polarizer

Photodetector

Substrate for metal layer

Gold layer with receptor coating

Target molecules (analyte)

Light

source

Prism Reflected light without analyte

Reflected light with analyte

Fig. 7.27 Concept of a biosensor using a surface plasmon resonance effect

7.8 Surface Plasmon Resonance Biosensors 225