Biomedical Engineering Reference
In-Depth Information
11
Fluorescence-Based Optical Biosensors
F.S. Ligler
11.1 Introduction
Biosensors integrate biological molecules with a signal transduction device
to produce a signal when a molecular recognition event occurs. This tutorial
will focus entirely on biosensors that employ optical devices and incorporate
fluorescent mechanisms for signal transduction. Compared with the electro-
chemical glucose sensors found in pharmacies world wide, optical biosensors
are often more complex and more costly. However, optical biosensors are bet-
ter suited for repetitive analysis or continuous monitoring, for interrogation of
complex fluids, and for measuring binding events in real time. Optical imaging
methods have also been widely adapted to measuring microarrays of recog-
nition events; this experience provides a base for the development of highly
multiplexed optical biosensors.
Optical biosensors not included in the following discussion are, nonetheless,
worth mentioning. This group of biosensors is primarily directed at detection
of a target without the requirement for a label. Thus performing the assay
is simplified by requiring only the exposure of the sample (containing target)
to the biological recognition molecule. Noteworthy among these methods are
interferometry, surface plasmon resonance, resonant and antiresonance reflec-
tometry, and cantilever-based systems [1, 2]. All of these sensors measure a
change in optical properties, usually refractive index, at the sensing surface
where the recognition molecules are immobilized. Convenient to use, they
are excellent tools for measuring reactions in well-defined fluids. They tend
to be less sensitive to very small targets or to very large targets that have
most of their mass outside the sensing region; improvements in waveguide
technology are minimizing these problems. However, nonspecific adsorption
of components from complex samples can reduce the sensitivity by creating a
significant background signal that must be accurately subtracted.
The following tutorial on fluorescence-based optical biosensors is orga-
nized into five parts: (1) biological recognition molecules and assay formats,
(2) displacement immunosensors, (3) fiber optic biosensors, (4) bead-based
