Biomedical Engineering Reference
In-Depth Information
profile, Stokes shift and response time, which can be varied slightly when embedded in
other media such as a polymer. In addition, the surrounding lipid membrane allows par-
ticular analytes to pass into the interior of the sensor and react with the indicator dye, while
preventing the dye from exiting and being degraded by the cell. Presently, liposome-based
nanosensors have been developed and applied to the measurement of pH
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and molecular
oxygen
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in various cellular environments, demonstrating the high degree of sensitivity as
well as specificity of such sensors.
A recent variation to these liposome-encapsulated nanosensors has been to first par-
tially immobilize the fluorescent indicator dye of interest in a polystyrene nanoparticle,
prior to encapsulation in a phospholipid membrane.
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This modification in the
nanosensor fabrication allows the dyes to be partially embedded in the polystyrene bead,
providing a much more stable sensor that is less susceptible to biological degradation,
while still retaining much of the solution-based characteristics of the dye. This second sub-
class of phospholipid-based nanosensors, known as lipobead nanosensors, also allows the
fluorescent indicator dye to be partially embedded in the phospholipid membrane,
increasing the accessibility of the dye to the analyte species. This dramatically increases
the potential number of analytes that can be investigated over other encapsulated
nanosensors such as the PEEBLEs that require the analyte to travel through tiny pores to
interact with the indicator dye.
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3.2.2.4 SERS-Based Nanosensors
In addition to the luminescent-based nanosensors and nano-biosensors that have been
developed over the past decade, a fourth class of implantable nanosensors and nano-
biosensors have been developed that rely on SERS to provide information about the envi-
ronment being probed. Unlike their luminescent counterparts, these nanosensors take
advantage of the rich molecular information provided by Raman spectroscopy and the
localized sensitivity provided by the roughened metal surface necessary for SERS to pro-
vide a sensitive and selective detection scheme. In addition, the narrow linewidths associ-
ated with Raman and SERS spectroscopies allow for the multiplexed monitoring of many
different species simultaneously.
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While metallic colloidal nanoparticles have been
used for SERS analyses for several decades, the first true nanoparticle-based SERS
nanosensors and nano-biosensors have been around for a much shorter period of time.
Some of the earliest reported SERS nano-biosensors developed were reported by Mirkin
and coworkers,
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in which oligonucleotide sequences were attached to metallic
nanoparticles. These oligonucleotide-labeled nanoparticles were then used to monitor the
binding of complementary strands of dye-labeled oligonucleotides, resulting in an SERS
signal from the particular dye used for labeling. Since a different dye was attached to each
different oligonucleotide sequence being investigated and the SERS spectra of each type
of dye molecule is unique, this resulted in a form of molecular “barcoding” that could be
used to interrogate individual strands of DNA or RNA. By taking advantage of the nar-
row linewidths, these nanosensors were capable of probing many different oligonu-
cleotide sequences simultaneously.
More recently, antibody-labeled SERS nanoparticles that are capable of being
implanted into individual cell for the direct detection of specific protein species have
been developed by Cullum and coworkers. Unlike previous nanoparticle-based SERS
biosensors, these sensors do not require a washing step or dye label to identify the pres-
ence of the analyte. These nano-biosensors are fabricated by coating a thin layer of a
SERS-active metal such as silver or gold onto a silica nanoparticle followed by attachment
of the antibody of interest through a thiol linkage.
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Once bound, the sensors are
implanted into a cell by either nano-injection or a pressured-injection technique such as
