Biomedical Engineering Reference
In-Depth Information
3.2
Submicron and Nanoscale Sensors and Biosensors
3.2.1
Fiber-Optic Nanosensors
Shortly after the development of tapered optical fibers for NSOM, they began to be
applied to the field of chemical sensing, allowing for the probing of specific chemicals in
highly localized areas (
50 nm from the fiber tip). Owing to this high degree of localiza-
tion, monitoring of concentration gradients of chemicals and their spatial inhomogeneity
in submicroscopic environments via various spectroscopic techniques became possible. In
particular, these submicron sensors or nanosensors provide a means of obtaining highly
localized analyses of chemical species within subcellular compartments of individual cells
and multicell organisms.
3.2.1.1 Fiber-Optic Chemical Nanosensors
The first such optical nanosensor was reported by Kopelman and coworkers
12,13
in 1992 for
the measurement of pH. In this work, the heated pulling process was used to taper multi-
mode and single-mode fibers to diameters of 100-1000 nm at the tip. Following the tapering
process, the walls of the fiber were coated with aluminum in a vacuum evaporator to ensure
total internal reflection over the pulled region of the fiber. During aluminum deposition, the
fibers were placed with their tips facing away from the metal source, thereby shadowing
them from the evaporating metal and leaving the tips uncoated (see Figure 3.4).
Metal free
Tip
Evaporating
metal
Rotating
fibers
Evaporating
metal
+
−
FIGURE 3.4
Schematic diagram depicting the shadowing of a tapered fiber-optic probe tip from the evaporating metal used
to coat the walls of the fiber.
