Biomedical Engineering Reference
In-Depth Information
mirror, while the emission light is reflected by the mirror onto a detector [16].
In Fig. 11.4c, a fiber bundle is used between a high numerical aperture sens-
ing probe and the optics. The 635 nm excitation light is channeled down the
central silica fiber while the emitted higher wavelength fluorescence is coupled
back up the surrounding plastic fibers [17]. In Fig. 11.4d, the fiber is illumi-
nated using a light source normal to the fiber, and the fluorescence is detected
at the distal end [18, 19].
11.4.2 Optrode Biosensors
Optrode biosensors grew out of a wealth of applications for chemical sensing
on the basis of absorbance, fluorescence, and even time-resolved fluorescence.
The fibers were relatively easy to work with and facilitated the separation of
excitation and emission light; frequently, the fibers themselves were used to
transmit only excitation to the sensing region or only emission away from the
sensing area as shown in the diagram below. Furthermore, a wide variety of
light sources and detectors were used. These included lamps, LEDs, lasers,
and diode lasers for excitation and photomultiplier tubes, photodiodes, and
CCDs for detection. The optrode configuration is one of the most flexible
in terms of the types of assays that can be performed. Optrodes have been
used not only with immunoassays and DNA hybridization assays, but also
with enzyme assays and for analysis of whole cell function. These assays and
configurations have been reviewed by Walt [1, 2] (Fig. 11.5).
Probably the most exciting application of optrode technology today is the
fiber optic bundles developed by Walt [20] and currently marked by Illumina
(www.Illumina.com). In this approach, the ends of the optical fibers in the
bundle are etched out, leaving the surrounding cladding to form microwells.
Coded beads are placed in the microwells so that hundreds of thousands of as-
says can be conducted simultaneously. The assays are being used for genomics,
proteomics, and biodefense.
11.4.3 Evanescent Fiber Optic Biosensors
The first use of an optical fiber as a biosensor, and one of the first optical
biosensors was described by Hirschfeld and Block [21, 22] in the mid 1980s.
This type of sensor was also the first biosensor to be fully automated and used
remotely. In 1996-1998, Ligler, Anderson, and colleagues developed a biosen-
sor payload for a small, unmanned plane and tested it at Dugway Proving
Ground, Utah. The payload included a biosensor with four fiber probes, a
RAM-air driven cyclone for collecting aerosolized bacteria, an automated flu-
idics system, batteries, and a radiotransmitter. They demonstrated that the
airborne biosensor could collect a biothreat stimulant, identify it, and radio
the data to an operator on the ground in 6-10 min [23, 24].
The best known of the commercially available fiber optic biosensors is the
RAPTOR-Plus, made by Research International (http://www.resrchintl.com)
