Biomedical Engineering Reference
In-Depth Information
Evanescent wave (EW)
Fiber-wave guide
Diode laser
source
Total internal reflection (TIR)
Laser
Fluorescent
molecules
Laser detector
EW
TIR boundary
FIGURE 18.3
Configuration and detection principle of fiber-optic biosensor.
are primarily bound to the fiber as opposed to those in the bulk solution. Fluorescent radi-
ation propagates back through the fiber in high-order modes.
Overall, EW-based fiber-optic biosensors exploit the measurement of fluorescent light
excited by an EW generated at the interface of an optical waveguide in which a laser light
undergoes TIR, to quantitatively detect biomolecules immobilized on the fiber surface (6). A
commercially available fiber-optic biosensor (Analyte 2000) has been developed using the
above principle by Research International (Monroe, WA). Now a portable integrated version
of this system called RAPTOR (Research International) is commercially available for the
detection of multiple foodborne and bioterrorism agents (41,42). The assay principle is based
on a sandwich immunoassay, where a capture antibody is immobilized onto the optical
fibers, and a Cyanine 5 fluorescent dye (Cy5) or Alexa-flour 647-labeled antibody is used for
detection of bimolecular attachment to the fiber. This assay has been used to detect staphy-
lococcal enterotoxin B (43-45),
C. botulinum
toxin (46) at nanogram concentrations, and
S. enterica
serovar Typhimurium at 10
4
cfu/ml (47). RAPTOR system was also used to detect
S
. Typhimurium from spent sprout-irrigation water after 67 h of initial inoculation of seeds
with 50 cfu/g (48). Possible use of this sensor for online monitoring of this pathogen in irri-
gation water used during sprouting was proposed. Fiber-optic sensor was also developed
for
E. coli
O157:H7 and was detected at 3-30 cfu/ml in spiked ground-beef samples (49). This
sensor was also developed to detect PCR products of
Listeria
species by allowing hybridiza-
tion of complementary sequence coated on the fiber surface (50). Later, this sensor was used
to detect
L. monocytogenes
cells at 1
10
8
cfu/ml (51). Most recently, an even more sensitive
fiber-optic assay for
L. monocytogenes
was developed using a combination of polyclonal and
monoclonal antibodies in a sandwich format (52). Rabbit polyclonal antibody was used as a
capture antibody on the fiber waveguide and subsequent detection was accomplished by
using a monoclonal antibody C11E9 (29). This combination improved the sensitivity of the
fiber-optic biosensor to a range of 4.3
10
4
cfu/ml of
L. monocytogenes
after 2.5 h
of sampling even in the presence of common food contaminants or stress conditions. This
sensor was able to detect
L. monocytogenes
from hotdogs or bologna that was naturally con-
taminated or artificially inoculated with 10
1
-10
3
cfu/g after enrichment in buffered Listeria
enrichment broth in less than 24 h. This biosensor was specific for
L. monocytogenes
and
showed significantly higher signal than other
Listeria
species or other microorganisms
and was an important advancement for detection of
L. monocytogenes
in ready-to-eat foods.
Varshney et al. (53) used immunomagnetic separation method to capture
S.
Typhimurium
10
3
to 4
