Sensing of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 92

a b

j 0

j 0

prism

prism

air gap

metal

metal dielectric

Fig. 2.31 The matching of SPP and light wavevector components using half-cylindrical prisms:
( a ) Otto configuration, ( b ) Kretschmann configuration

Fig. 2.32 SPP launching
using a metallic grating

dielectric j 0

SPP

metal grating

with m an integer. As in the previous example with a prism, it is possible to excite
an SPP wave at the interface between the metallic grating and the dielectric at a
particular incidence angle.
Irrespective of the method of SPP excitation, the dependence of reflectivity on
the incidence angle of the optical excitation, i.e., R D f.'/, has a sharp minimum
at the resonance angle ' 0 , which indicates that SPP launching along the interface is
successful. This behavior, represented in Fig. 2.33 , is named SPP resonance.
The sharp reflectivity decrease at SPP launching is the basic concept for label-
free sensing in biomedical sciences ( Englebienne et al. 2003 ): The decrease in
reflectivity at the SPP resonance occurs when an additional molecular layer is
deposited on a metal surface, for example. Because the reflectivity change is
extremely sensitive to any variation of the thickness/conditions at the interface,
SPP resonance measuring is one of the most advanced techniques to measure DNA
hybridization, enzyme-substrate interactions, and other biomolecular processes,
such as the real-time kinetics of biomolecules.
A typical configuration of an SPP label-free biosensor is illustrated in Fig. 2.34
( Iwasaki et al. 2006 ). Here, the incident angle of the optical excitation through the
prism, ' 0 , is not varied, but the thickness and/or the refractive index of the sensing
layer varies as more and more biomolecules are captured on it. As a result, the