Biomedical Engineering Reference
In-Depth Information
and used for the sensitive and simultaneous detection of several different biological toxins,
including ricin, staphylococcal enterotoxin B (SEB), and yersinia pestis. 11 0 Attachment of
these antibodies was accomplished using a photoactivated optical adhesive that was placed
in small wells on the microscope slide. Once the captured antibodies were immobilized on
the slide, toxins were introduced, followed by a second fluorescently labeled antibody. Upon
binding of the fluorescently labeled secondary antibodies, the array was excited and the
resulting fluorescence image was captured on a CCD camera and correlated to the various
toxins. From this work it was found that the various toxins could be detected at concentra-
tions ranging from 5 to 25 ng/mL.
3.3.3
Aptamer-Based Biochips
A third class of bioreceptor that has recently been used in the development of biochip
arrays is a relatively new type of bioreceptor known as an aptamer. 111 Aptamers are syn-
thetic oligonucleotide sequences that fold into three-dimensional structures and are capa-
ble of binding proteins and other biological species with affinities approaching those of
antibodies. 112,113 By employing aptamers as biological recognition elements in a biochip
array, it has been possible to develop proteomic assays that require minimal washing steps
while providing a high degree of sensitivity and specificity. 11 3 In addition, to providing a
high degree of sensitivity and specificity, aptamers also offer the advantage of being able to
be synthetically fabricated and amplified via PCR, as well as the ability to be placed in
denaturing conditions and later be reconstituted. When employing aptamer arrays, analyte
proteins from a sample are placed in contact with aptamers immobilized in an array format
on a suitable substrate, and then washed with a fluorescent dye that selectively attaches to
proteins. Once the dye label has been attached to the immobilized proteins, fluorescence
images of the arrays are obtained using a CCD or other imaging device, and related to the
presence and concentration of analyte proteins, similar to other types of biochips.
3.3.4
Integrated Multifunctional-Based Biochips
When performing complex biological diagnostics, the need to monitor more than just one
type of analyte is often essential. To this end, biochip arrays have been developed that
employ a mixture of different types of bioreceptors (e.g., antibodies and oligonucleotides)
on the same substrate array. Biochip arrays formed using a mixture of bioreceptor mole-
cules are often termed “multifunctional biochips.” One early example of a multifunc-
tional biochip was demonstrated by Vo-Dinh and coworkers, 11 4 in which DNA and
antibodies were immobilized on a single-sensing array and used to detect various bio-
markers or disease end points. This work also demonstrated the development of one of
the earliest, self-contained bioreceptor-transducer array formats, in which a complemen-
tary metal oxide semiconductor (CMOS)-based photodiode sensor array with on-chip
amplifiers, discriminators, and logic circuitry were employed for the detection of optical
signals corresponding to analyte binding. 115,116 Unlike most previous biochip detection
systems, which employed optical microscopes or similar microscopic imaging systems
with CCD detectors, this integrated biochip system fit onto a single integrated circuit
microchip, creating a device with a small footprint that could be mass produced
inexpensively. Based upon the concept of a fully integrated and disposable biochip
microarray and the various bioreceptor molecules that have been used for biochip
fabrication, many different commercial variations of this CMOS-based chip design have
been fabricated and are commercially available, ranging from low- to high-density arrays
for many different applications.
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