224 Nanoplasmonics
observance that, although numerous commercial SPR systems are available, their
designs are very similar to that which was originally presented by Liedberg etal.
(1983). Figure 8.2 depicts a typical Kretschmann SPR sensor conguration with
angular modulation. In this setup, using a prism as an optical coupler, a focused
monochromatic incident light beam is totally reected at the metaldielectric inter-
face. An electromagnetic eld component, commonly termed an “evanescent wave,
excites surface plasmons within the metal layer. The strength of coupling between the
incident wave (K
i
) and the resulting surface plasmon wave (SPW or K
p
) is observed
at multiple angles of incidence of the incoming light wave. The angle of incidence
which yields the strongest coupling (i.e., maximum absorbance) can be identied by
monitoring the reected light using a simple light detector array which is used as the
sensor output. This output can then be calibrated to the corresponding RI value at
the sensor surface on the sample side. Figure 8.3 shows a typical angle-based SPR
sensor response.
Target
Nontarget
SPW (K
p
)
Specific binding
Gold layer
Prism
θ
Incident light (K
i
)
Reflected light
(K
r
to be detected)
Surface receptors
FIGURE 8.2 A typical Kretschmann SPR sensor setup.
SPR shift
Angle of incidence
Reflectance
FIGURE 8.3 A typical Kretschmann SPR response prole. A change in the sample RI causes
a corresponding shift in the SPR angle (i.e., the dip location of minimum reected intensity).

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