Biomedical Engineering Reference
In-Depth Information
The condition required for resonance, that is to say the condition required for the bulk
wave to be transformed to the SP wave, is that the bulk propagation vector be equal to the
SP propagation vector (39):
kk
sp
b
(20.3)
Furthermore, resonance depends on the angle of incidence,
, in the following way (39):
knk
sp
b
sin
(20.4)
p
where
n
p
is the refractive index of the prism.
At a particular angle of incidence,
k
sp
will equal
k
b
and the conditions for resonance will
be fulfilled (see Equation 20.3). At this angle, there is maximal coupling of incident light
into SPs, which results in maximal absorption of the incident light by the gold film and a
resultant sharp decrease (minimum) in the reflection coefficient of the incident light is
observed (see Figure 20.9A). The angle at which the SPR minimum occurs is greatly
dependent on all the refractive indices of all the boundary media, including the gold film,
the bulk solution in contact with the gold-coated prism, and additional layers such as mol-
ecules deposited on the gold surface (39). Generally,
k
sp
increases proportionately with
increasing biolayer thickness, resulting in increases in the resonance angle (see Equation
20.4) (39). Therefore, the thicker the biolayer at the surface, the greater the SPR-minimum
shifts to higher angles will be. These shifts can be measured in real time during the adsorp-
tion of biomolecules at the gold film surface.
The SPR technique is extremely sensitive to optical thickness changes, and it has been
shown that a change in protein surface concentration of 1 ng/mm
2
will generate a change
of SPR coupling angle of 0.1ยบ (40).
20.2.2
Concept
Generally, an SPR device consists of an optical system, a transducing medium, and an elec-
tronic system supporting the optoelectronic components of the sensor and allowing data
processing (32). The optical component of the SPR device contains a source of optical radia-
tion and an optical structure in which the SP wave is excited and interrogated. The trans-
ducing medium interrelates the optical and bio(chemical) domains, relating changes in the
refractive index to changes in the quantity of interest. Its properties also determine the selec-
tivity and response time of the sensor. The sensitivity, stability, and resolution of the sensor,
in contrast, depend on the properties of both the optical system and the transducing
medium (32). Most modern SPR devices use light-emitting diodes as light sources and lin-
ear arrays of charge-coupled devices to detect reflected light from the surface. The use of
array detectors allows reflected light to be measured at a wide range of angles, thereby
avoiding the need to mechanically control the angular position of the detector (39).
20.2.3
Applications
SPR has been applied to a number of fields, including physics, chemistry, and molecular
biology. In the field of physics, SPR sensing devices have been developed to exploit cer-
tain physical phenomena that occur in various optical transducing materials, such as
humidity-induced refractive index changes in porous thin layers and polymers; or tem-
perature sensors based on the thermooptic effect in hydrogenated amorphous silicon (32).
