Biomedical Engineering Reference
In-Depth Information
nanosensors, developed by Kopelman et al
.
over the last decade, are comprised of fluo-
rescent indicator dyes that are embedded in 20-200 nm diameter polymer or sol-gel
spheres.
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By encapsulating fluorescent indicator dyes that are sensitive to particular
ionic species (i.e., H
, and Ca
2
, etc.) in a polymer matrix, the dye is protected from cellu-
lar degradation by proteins or enzymes, while still allowing ions, which are much smaller
in size, to pass into the matrix and react with the dye. Additionally, the polymer coating
protects the cell from any potential toxic effects of the dye.
Since they were first developed in 1996, PEBBLEs have been used to monitor fluxes in the
concentration of many different species within individual cells, including pH, Ca
2
, NO, O
2
,
and Zn
2
.
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Upon fabrication of the particular type of PEBBLE sensor desired for a
measurement, they are inserted into a cell typically via nano-injection or gene-gun insertion.
Owing to the small size of the particles, hundreds to thousands of these PEBBLE sensors can
be inserted into individual cells without affecting routine cellular function. This allows for
sensors to be present throughout the cell and various subcellular compartments, providing
a means for imaging cellular response at many locations within the cell simultaneously.
Once the sensors have been injected into the cell, the entire cell is illuminated and the result-
ing fluorescence signal from the indicator dye is measured over the autofluorescent
background and correlated to the amount of analyte species present.
Owing to the potential for intense autofluorescence from cellular materials following
excitation of the implanted nanosensors with ultraviolet or blue light, detection limits for
the analyte of interest from such analyses can be relatively high. Because of this potential
problem, Kopelman and coworkers have recently developed a variation of these PEBBLE
nanosensors that can be magnetically modulated at a known frequency. These magneti-
cally modulated nanosensors, known as magnetically modulated optical nanoprobes
(MagMOONs), are fabricated by either creating the polymer-encapsulated sensor around
an aspheric magnetic nanoparticle, or coating a spherical nanoparticle on one side with a
thin layer of a magnetic material.
87,88
This aspheric magnetic particle is then inserted into
the cell of interest, and a magnetic field is applied. As the magnetic field is applied, the
particles begin to rotate at a rate that is related to the strength and location of the applied
field. When these MagMOONs are optically excited, the resulting fluorescence emission,
which can be related to the presence of the analyte of interest, is modulated. This modu-
lated luminescent signal can then be recovered from the continuous autoluminescence sig-
nals of the cell via conventional techniques such as lock-in amplification, dramatically
improving the detection limit of the MagMOON over the PEBBLE sensors that require
energetic ultraviolet or blue excitation wavelengths.
3.2.2.3 Phospholipid-Based Nanosensors
A third class of implantable nanosensor that has focused on the need for biocompatibility
is the phospholipid-based nanosensors. These nanoparticle-based sensors, developed by
Rosenzweig and coworkers,
89-95
rely on the concept of encapsulation of the indicator dye,
similar to PEBBLEs; however, they employ biological components for the encapsulation
instead of polymers or silica. Encapsulating fluorescent indicator dyes inside biological
materials not only reduces any toxic effects to the cell and biodegradative effects to the dye
but it also allows for common biological mechanisms to be employed for cellular uptake
of the sensors, as compared with potentially intrusive methods such as nano-injection.
95
Within the class of phospholipid-based nanosensors, two distinct subclasses exist: (1)
liposome sensors
93,94
and (2) lipobead sensors.
89,91
The former of these two subclasses, lipo-
some-based nanosensors, employ fluorescent indicator dyes encapsulated in the internal
aqueous compartment of a liposome. By encapsulating the dye in the aqueous media of the
liposome, it is allowed to retain its solution-based characteristics such as spectral emission
