Biomedical Engineering Reference
In-Depth Information
a gene gun, and positioned at the location of interest via optical tweezers. Once in place
within the cellular compartment of interest, antigens bind to the antibodies on the sensor
either bringing the antigen close enough to the metal surface for direct SERS monitoring,
or causing conformational changes in the antibody that can be monitored via the result-
ing SERS spectrum. By optimizing the surface coverage of the antibody on the nano-
biosensor, exposure of the metal surface to other potential species within the cellular
environment is dramatically reduced, resulting in minimal interferences.
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In addition to
SERS-based nano-biosensors such as those described above, similar nanosensors have
recently been developed for real-time monitoring of pH in microscopic environments, in
which organic molecules are attached to the surface of a SERS-active nanoparticle and the
protonation of those organic molecules is monitored via SERS spectroscopy.
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Based
upon the sensitivity and selectivity of SERS-based nanosensors and nano-biosensors and
their ability to monitor many different species simultaneously, they should see a great
deal of application to cellular analyses in the future.
3.3
Biochip and Chip-Based Biosensor Arrays
While fiber-optic and implantable nanosensor and nano-biosensor technologies have pro-
vided the scientific community with unprecedented methodologies for quantitatively deter-
mining the presence and location of cellular species ranging from ions to oligonucleotides
(e.g., DNA and RNA) to proteins, knowledge of the intracellular location of these species is
not always necessary. In addition, it is sometimes necessary to monitor thousands of differ-
ent analytes from a cell or tissue sample simultaneously. Analyses such as these have resulted
in the advent of biosensor arrays, with individual receptor sites having micrometer- or
nanometer-scale dimensions. Such arrays, are often referred to as “gene chips,” “DNA chips,”
or “protein chips,” depending upon the analyte being probed, or the more general terms of
“biochip” or “microchip array.”
17,104-106
The underlying technologies necessary for biochips
were developed in the early 1980s and since that time these devices have found a large num-
ber of applications,
11,105-107
spawning the fields of genomics and proteomics. Owing to the
rapid growth and widespread application of biochip technologies, several good reviews have
been written on the subject and their application to various fields over the years.
6-11
In general, biochips can be described as consisting of a highly ordered array of micro-
scopic- or submicroscopic-sized spots of biological receptor molecules on a substrate of one
form or another (see Figure 3.8) and an array transducer for location-specific measurements.
Although transduction can occur through many different means (e.g., optical and electro-
chemical), this chapter focuses solely on optical transduction schemes, which account for the
vast majority of biochip-based arrays. Fabrication of high-density bioreceptor arrays used
for biochip analyses is typically performed using robotic spotting systems, capable of deliv-
ering microliter to nanoliter volumes of the bioreceptor of interest to the appropriate sub-
strate. The most common substrates employed for such analyses are microscope slides;
however, membrane materials (e.g., zeta-probe membrane) have also been used depending
upon the sample that is being analyzed and the degree of nonspecific binding of the sample
to the array substrate.
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Once an array has been produced with each spot corresponding to
a different particular analyte, the sample solution of interest is placed on the surface of the
array, and the different bioreceptors are allowed to bind their corresponding analyte. After
binding, the array is typically washed off to remove any unbound sample materials from the
analysis area, followed by reaction with an optically measurable marker, such as a
fluorescent dye, that binds solely to the bioreceptor spots that have reacted with analyte.
