Biomedical Engineering Reference
In-Depth Information
Fig. 12.4.
Taxonomy of optical effects potentially useful for biochemical sensing
no additional instrument was necessary. These optical sensing methods were
based on chemical reactions of the analyte with a receptor molecule that
resulted in a chemical product with optical absorption properties in the visible
spectral range. The resulting color change was then detected and evaluated by
the observer or an instrument. Since optical measurement techniques based on
spectral absorption are less sensitive than methods detecting phase changes
in light, optical biochips do generally not make use of spectral absorption.
12.3.2 Phase Shift
A change in the real part of the refractive index causes a phase change of
the electromagnetic wave that is used to probe the detection volume of the
sensor. This phase change can either be observed in an interferometer setup,
it can manifest itself as a change of the polarization properties of incident
linearly polarized light, it can change the optical field distribution close to an
interface, or it can change the propagation characteristics of a traveling wave.
This last effect is most commonly exploited in optical biochips, in particular,
through the modification of in- or out-coupling conditions, because it works
very well in the confining geometries at the surface of biochips, as illustrated
in Fig. 12.2, and it does not require active labeling of the analyte molecule.
12.3.3 Fluorescence
Because of the extremely high sensitivity of today's solid-state image sensors,
the detection of very low light levels down to individual photons is technically
not di
cult to achieve [15]. As a consequence, the use of fluorescent labels to
tag analyte molecules leads to a highly sensitive optical measurement method
for the concentration of the analyte. Incident light at a certain wavelength is
absorbed by the fluorescent labels, and the optical energy is re-emitted at a
longer wavelength where it is detected by a sensitive camera. For this reason,
fluorescence is one of the most widely used sensing methods in the life sciences
and, in particular, in optical biochips.
12.3.4 Luminescence
Instead of attaching a fluorescent molecule to the analyte, it is also possible to
bond a chemiluminescent molecule to it, such as the well-known luciferase [3].
