Biomedical Engineering Reference
In-Depth Information
The uncoated silica surface on the tip then allowed for the binding of the pH-sensitive
fluorescent dye acrylofluoresceinamine via a variation of a common photopolymerization
process often used in the construction of larger chemical sensors.
38,39
However, since the
tips of these fibers were smaller than the wavelength of light used to initiate the pho-
topolymerization process, the crosslinking of the dye-doped polymer solution was
restricted to the near field of the fiber. This near-field polymerization process resulted in a
sensor with a sampling volume greater than six orders of magnitude smaller than the con-
ventional fiber-optic chemical sensors, making it ideal for subcellular measurements. This
reduced sampling volume resulted in a 100-fold faster response time than conventional
fiber-optic chemical sensors fabricated via similar processes. After the development of this
first fiber-optic-based chemical nanosensor, several additional sensors have been reported
for a wide variety of chemical species, including pH
40-43
and various other ions
44,45
as well
as nonionic species.
46
Application of these fiber-optic nanosensors to biological analyses was first reported for
the monitoring of pH in rat embryos.
12
In this study, nanosensors similar to the ones
described above were inserted into the extraembryonic space of a rat conceptus and pH
measurements were performed, resulting in a means of monitoring pH differences with
minimal damage to the surrounding visceral yolk sac. In a similar study using the same
type of sensor, indirect measurements of nitrite and chloride levels in the yolk sac of rat
conceptuses were also performed.
47
Such minimally invasive analyses provide a great deal
of promise for biological measurements and may aid in furthering our understanding of
the effect that environmental factors play in embryonic growth.
As the field of optical-fiber-based nanosensors has evolved, the size of the environment
capable of being probed has become smaller and smaller, allowing for the monitoring of
chemical species inside individual living cells. The first report of such single-cell analyses
using opto-chemical fiber-optic nanosensors employed nanosensors fabricated in a
process similar to that described above. These fiber-optic nanosensors were then
employed to measure sodium ion (Na
) concentrations in the cytoplasmic space inside a
single mouse oocyte.
1
In another application of fiber-optic nanosensors to biological meas-
urements, calcium-ion-sensitive nanosensors were used to measure calcium ion fluctua-
tions in vascular smooth muscle cells while the cells were being stimulated.
48
3.2.1.2 Fiber-Optic Nano-Biosensors
Owing to the complexity of biological systems and the number of possible interferences
present in cellular environments, added specificity is often required. Like their larger
counterparts, conventional fiber-optic biosensors, biological receptor molecules (i.e., anti-
bodies, enzymes, etc.) are often used in conjunction with tapered fiber-optic probes to pro-
vide added specificity. The different types of bioreceptor molecules that have been used
for the fabrication of fiber-optic nano-biosensors include antibodies, oligonucleotides, and
enzymes, thereby allowing for the detection of a wide array of analytes.
3.2.1.2.1 Antibody-Based Fiber-Optic Nano-Biosensors
The first such fiber-optic nano-biosensor was reported in 1996 by Vo-Dinh et al
.
33
In this
work, antibody-based nano-biosensors were developed for benzo[
a
]pyrene tetrol (BPT), a
DNA adduct of the carcinogen benzo[
a
]pyrene (BaP). As a biomarker for exposure to BaP,
the measurement of BPT is of great interest for health monitoring. These immuno-based
nano-biosensors were fabricated in a simple four-step process. In the first step, 600-
m-
diameter multimode fiber optics was tapered to 40 nm at the tip. Following tapering, the
fibers were coated with 200 nm of silver to prevent light leakage and enhance the delivery
of the excitation light to the end of the fiber. Once the metal was coated, the fibers were
