Biomedical Engineering Reference
In-Depth Information
on beads, and chemiluminescence signals emitted from the bacteria were measured by using
a fiber-optic light guide.
18.3.4
Array Biosensor
Array biosensors can interrogate multiple samples at discrete regions on the sensing sur-
face and often it is facilitated by applying evanescence wave technology that can excite
multiple fluorophores. Taitt et al. (54) used a patterned array of antibodies against multi-
ple analytes immobilized on the surface of planar waveguide (microscope slides) to cap-
ture antigen. Fluorescent-labeled tracer antibodies were allowed to bind to the antigen
and subsequent excitation with a diode laser launched at the edge of the glass slide emit-
ted a fluorescent signal, which was captured by a charge-coupled device (CCD) camera.
Positive-signal-emitting antigens were identified by using an image-analysis software
(54). This system was developed primarily to address biothreat agents and simultane-
ously detected multiple pathogens including
B. anthracis
,
S. aureus
enterotoxin B, cholera
toxin, ricin,
Franciscella tularensis
and
Brucella abortus
(55). Bacterial cells were detected in
the range of 10
3
-10
6
cfu/ml, and toxins were in the ng/ml range. With appropriate mod-
ifications, this system could be applied for detection of foodborne pathogens and was
employed for rapid detection of
S.
Typhimurium with a detection limit of 8
10
4
cfu/ml
from different food matrices such as cantelope, chicken washings, sprouts, and liquid egg
(56). This system is now made as a portable device that contains a power supply, peri-
staltic pumps, interface circuit boards, diode laser, and CCD camera for onsite use (57).
An array biosensor was also configured to detect a single agent from different matrices.
Shriver-Lake et al. (58) detected SEB from six different types of food samples, including
beverages, homogenates of fruit and meat, and carcass washings, which could be com-
pleted in less than 20 min without sample preconcentration. However, the fluorescence
intensity varied between samples and the detection limit was 0.5 ng/ml. Later, the multi-
analyte array biosensor (MAAB) was developed. The MAAB can detect and identify mul-
tiple target agents in complex samples with minimal user manipulation. Upon binding of
a fluorescent analyte or fluorescent immunocomplex, the patterns of fluorescent spots
were detected using a CCD camera. The location of the spot and the mean fluorescence
intensity were used to determine the toxin or microorganism identity and concentration.
S.
Typhimurium and
L. monocytogenes
were detected at 10
3
10
6
cells. Staphylococcal
enterotoxin B, ricin, cholera toxin, botulinum toxoids, trinitrotoluene, and the fumonisin,
a mycotoxin, were detected at levels as low as 0.5 ng/ml (59,60).
Moreno-Bondi et al. (61) described another array sensor platform for multiple analytes
on a metal-oxide silicon biochip. Essentially, this system contains four-by-four microarrays
of antibodies. Binding of analytes to the capture antibody could be detected by using a sec-
ond antibody labeled with Cy-5 and by acquisition of signals using a diffractive optical
element and CCD camera.
18.3.5
Raman Spectroscopy
Raman spectroscopy is based on the phenomena of the shifted wavelength scattering of mol-
ecules excited with monochromatic light due to inelastic collisions of photons with mole-
cules. When the incident light is applied to bacterial cells, photons can be directly absorbed
or scattered. The scattered photons have the same energy (wavelength) as the incident pho-
tons. However, a small fraction of light (1/10
8
) is scattered at optical frequencies different
from the frequency of the incident photons. The process leading to this inelastic scattering is
called the Raman effect.
