Introduction to Biosensor Technology - Sensor Technology in Neuroscience - page 31

d n 4 t 3 n g | 1

Figure 1.24

(a) Kretschmann configuration for observance of surface plasmon
resonance. (b) Incident angle versus reflected intensity of radiation as a
result of surface plasmon resonance.

d n 3 .

observation that valence electrons of metals such as gold and silver exhibit
oscillations in their density. If these oscillations, in the form of surface waves,
are present at the interface of the metal with a material of a different dielectric
constant, they can be excited by the introduction of an agent such as elec-
tromagnetic radiation or an electron beam. The classical approach to the study
of this phenomenon was initially introduced by Kretchmann 116 and is shown
schematically in Figure 1.24(a). The incident radiation is reflected at the
metal-dielectric boundary resulting in an evanescent wave in the metal, much
as described above for optical fibers. At the correct (resonant) angle this wave
can couple in a resonant fashion with the frequency of the incoming radiation,
resulting in the excitation of electron density oscillation mentioned above,
leading to the term surface plasmon resonance (SPR). (In this context, the
reader will be familiar with nuclear magnetic resonance and, indeed, the
acoustic wave devices described above function via an analogous coupling of
electrical with mechanical energy.) In its simplest form this process can be
described on a physical basis by the following equation:

2p = l n p sin y ¼ W sp ð 1 : 7 Þ

where l is the wavelength, n p is the material refractive index, y is the angle of
incidence of the radiation and W sp is a parameter called the propagation
constant.
On an experimental basis, the SPR process can be affected by modification
of the metal surface (such as attachment/loss of biomolecules), which is
associated with a change in the dielectric constant of the surrounding medium.
The surface alteration is generally explored through detection of shifts in the
SPR angle, observed wavelength of absorption, or change in the position of
reflectivity. With respect to the first of these cases, which represents a common
approach, alteration of the angle of incidence of the incoming light with
concomitant monitoring of the intensity of the reflected light intensity is