Biomedical Engineering Reference
In-Depth Information
FIGURE 3.7
(a) Microscopic image of a tapered fiber optic imaging bundle, demonstrating the flat edge produced from the
heated pulling process. (b) Scanning electron micrograph of a tapered fiber-optic imaging bundle that has been
etched via hydrofluoric acid.
fibers can be accurately and reproducibly tapered to diameters ranging from the initial 4
m diameter to 150 nm before cross talk is observed.
59
The untapered end of these fiber
probes can then be imaged onto a charge-coupled device (CCD), allowing images of the
tapered end of the fiber and any sample in contact with it to be imaged with resolutions
of approximately 150 nm.
To provide chemical information about the species imaged using fiber-optic nano-imag-
ing probes fabricated via the heated pulling process, a roughened layer of silver has been
deposited on these tapered bundles to allow surface-enhanced Raman scattering (SERS)
images to be obtained.
59
To produce this roughened metal surface, tapered imaging bun-
dles were placed in a solution of HF, thereby etching the core of each individual fiber ele-
ment faster than the cladding surrounding it. This can be seen in the SEM shown in Figure
3.7b. Once the fiber elements were etched, silver was deposited on the surrounding
claddings, via vacuum evaporation, to produce an efficient SERS substrate with a highly
ordered and controllable spacing. Using such SERS imaging substrates, images of homog-
enous SERS active substances with variability of less than 2% across the surface of the
probe have been produced, providing a probe capable of obtaining chemical images of
samples with spatial resolutions of 150 nm.
With the two different types of fiber-optic imaging probes that have been developed, it
is possible to obtain quantitative measurements of chemical species at many different loca-
tions simultaneously. Although these fiber-optic nano-imaging probes are too large to be
inserted into individual living cells, they can be used to monitor cellular membranes with-
out damage.
3.2.2
Implantable Nanosensors
While fiber-optic nanosensors and fiber-optic nano-imaging probes have had and are having
large impacts on the fields of cellular biology and biochemistry, significant advances and
variations in optical nanosensors are constantly being made. These advances range from the
