Optical Biochips - Biophotonics - page 213

Fig. 12.1. Principle architecture of an optical measurement system for chemical
and biochemical sensing, as used for the different types of optical biochips

system, which can be a single microcontroller chip in the simplest case, dif-
ferent optoelectronic, optomechanic, and microfluidic components interact to
detect the effects of a chemical reaction, which is sensed by an optical effect in
the system's transducer section [2]. A light source is emitting either modulated
or unmodulated light, which is conditioned by input optics such as focusing,
in-coupling, or angle-adjustment elements. In the transducer part, a chemical
reaction takes place between the analyte (the target molecule) and a suitable
receptor in the microfluidics part of the system. This reaction is observed by
measuring a suitable change of any of the optical properties in the fluid vol-
ume where the reaction is taking place. The light exiting the interaction region
is carrying the information about the analyte concentration, and it needs to
be conditioned by appropriate output optics, such as imaging, out-coupling,
or angle-adjustment elements, so that the light can be e ciently detected by
a photosensor. The resulting electric signal is acquired and converted by an
electronic analog/digital circuit, and the digital signal is the processed to ex-
tract and communicate the desired concentration information, for example,
by displaying it for the user on a screen.
Although it is possible to employ many optical effects (see Sect. 12.3 below)
in an extended volume of the transducer part, much more control can be exer-
cised if this interaction volume is restricted to the immediate vicinity (typically
less than 1 m) of a sensitized surface. This principle is illustrated in Fig. 12.2.

The target molecules together with other possibly bio-active molecules are in
solution, and the microfluidics system has the task to transport them to and
from the transducer volume. There, receptor molecules are chemically bonded
to a surface where light is passing to sense the effect of the chemical bonding.
These receptors should react in a highly specific way only with the analyte, so
that the analyte molecules remain attached to the surface, effectively changing
the mass density close to the surface, which can be subsequently measured.
Since no additional measure is necessary to determine the concentration of
the analyte, this approach is called label-free sensing.
A higher detection sensitivity can be obtained if the analytes are tagged
with photoactive molecules. Most often, these labels are fluorescent molecules

µ