Biomedical Engineering Reference
In-Depth Information
through the same lens system through which the fluorescence light is collected
(so-called epi-fluorescence excitation setup) [4].
The high demands on the collection and detection e
ciency of the emitted
light in fluorescence (and luminescence) sensing could also be met with an
alternative, as yet little explored sensing approach. Instead of using an inactive
substrate such as glass or plastic for the fixation of the receptor molecule, the
receptors can be bonded directly to the surface of a solid-state image sensor.
In this way, a large part of the light that is emitted close to the pixel surface,
usually less than 1
m from the photosensitive semiconductor material, can be
collected and detected. Although this approach has been studied by several
companies, no product has yet appeared on the market. This is probably
due to the current cost of high-sensitivity image sensors. Since it is expected
that the dropping prices and the increased sensitivity of cell phone cameras
will soon have an influence on the use of these components in other areas,
in particular in the life sciences, this alternate approach to fluorescence (and
luminescence) sensing might become important in the future.
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12.5 Readout Methods for Evanescent Wave Sensors
As mentioned earlier, the analyte concentration is measured in surface plas-
mon or in dielectric waveguide sensors by determining the maximum of a
resonance effect. The following realizations of optical readout principles have
been successfully employed in practice for this purpose:
12.5.1 Angular Scanning
The most common methods to satisfy the in-coupling condition (12.4) or
(12.5) is to work with a convergent beam of light or to mechanically adjust
the angle of a parallel light beam. In both cases, it is assured that a prop-
agating mode is created at the sensor surface. While it is much simpler to
employ a convergent beam of light, the e
ciency is much lower compared to
a mechanically adjusted system because the in-coupling condition is fulfilled
only for a small fraction of the incident light.
No mechanical adjustment must be foreseen on the detection side, however,
because one- or two-dimensional solid-state image sensors are employed, which
can detect the resonance maximum without any loss of light.
In Fig. 12.7, a complete such system based on prism coupling is schemat-
ically illustrated, together with typical sensor signals acquired with a solid-
state line-sensor. In this example, a convergent beam of light is employed, so
that the complete system can be realized without any mechanically adjustable
components. Such systems are commonly used with surface plasmon sensing.
