Unformatted text preview: -v 1 7.4.5 Micrometer and Nanometer Biosensing Applications \J‘IHr I :n II DIUMEUILAL Ul’l R.) AND LASERS each patient so that not only the path length but alignment remains ﬁxed each time a.
reading is taken. In most tissues, including the eye, the change in rotation due to other"
chiral molecules such as proteins needs to be accommodated in any ﬁnal instrument:
In addition, most other tissues also have a birefringence associated with them
would need to be accounted for in a ﬁnal polarimetric glucose sensor. The birefringence and retardation of the polarized light and polarized scatte ' ii»-
of the tissue is the signal rather than the noise when using polarized light
tissue characterization. For example, a scanning laser polarimeter has been .-"
to measure changes in retardation of the polarized light impinging on the re ' w“
nerve ﬁber layer. It has been shown that scanning laser polarimetry provides statistic
ally signiﬁcant higher retardation for normal eyes in certain regions over glauco... eyes. Images generated from the scattering of various forms of polarized light ha .2
also been shown to be able to differentiate between cancer versus normal ﬁbrob V Developments in microtechnology and, in particular, nanotechnology are transfo IT?
mg the ﬁelds of biosensors, prosthesis and implants, and medical diagnostics. In t -"
of medical diagnostics, these devices are being used for external, lab-on-a-chip . I
throughput screening for analyzing blood and other samples. Many researchei's
companies are developing nanotechnology applications for use inside the body :
anticancer drugs, insulin pumps, and gene therapy. Others are working on pros .
devices that include nanostructured materials. One nanotechnology that has come to the forefront is that of quantum - '
Quantum dots are devices capable of conﬁning electrons in three dimensions i
space small enough that their quantum (wavelike) behavior dominates over,r“'
classical (particle-like) behavior. At room temperature, conﬁnement spaces of V 30 nm or smaller are typically required. Once the electrons are conﬁned, they
one another and no two electrons can have the same quantum state. Thus, 1-
electrons in a quantum dot will form shells and orbitals highly reminiscent of :
ones in an atom, and will exhibit many of the optical, electrical, thermal, and ch ..
properties of an atom. Quantum dots can be grown chemically as nanoparticles
semiconductor surrounded by an insulating layer nearly colloidal in nature.
particles can also be deposited onto a substrate, such as a semiconductor
patterned with metal electrodes, or they can be crystalized into bulk solids by a V of methods. Either substance can be stimulated with electricity or light (e.g., ,
order to change its properties. area has been to probe living cells in full color over extended periods of
Such a technique could reveal the complex processes that take place in all I
organisms in unprecedented detail, such as the development of embryos. ’7
imaging techniques use natural molecules that ﬂuoresce, as discussed
such as organic dyes and proteins that are found in jellyﬁsh and ﬁreﬂies. H 1/.3 I-UNUAMtN IAL) ur FHUIUI I'IIZI‘MﬂL Il'IEMI'IZU I ix. erncu Ur mac-u each dye emits light over a wide range of wavelengths, which means that their spectra
overlap and this makes it difﬁcult to use more than three dyes at a time to tag and
image different biological molecules simultaneously. The ﬂuorescence of dyes also
tends to fade away quickly over time, whereas semiconductor nanocrystals—quantum dots—can get around these problems. In addition to being brighter and living longer
than organic ﬂuorophores, quantum dots have a broader excitation spectrum. This
means that a mixture of quantum dots of different sizes can be excited by a light
source with a single wavelength, allowing simultaneous detection and imaging in
color. Although the preceding example has focused on biomedical sensing using ﬂuores-
cent quantum dots, these micro- and nanoparticles can also be made of various
materials and used with all of the light propagation methods. For instance, metal
nanoparticles such as gold or silver can be used with Raman spectroscopy to produce
an effect known as surface-enhanced Raman spectroscopy (SERS) which gives rise to signals a million times more sensitive than regular Raman signals. These same types of
metal nano- or microparticles can be injected into cancerous tissue and used as
absorbers that when hit with infrared light will absorb the energy, cook the cancerous
tumor, and kill it. Since nanoparticle development is still in its infancy, it remains to be
seen what other biomedical applications are to come from the combination of these particles with light. 17.5 FUNDAMENTALS OF PHOTOTHERMAL THERAPEUTIC EFFECTS
OF LASERS Therapeutic application of a laser is mediated by conversion of photonic energy to
absorbed energy within the material phase of the tissue. The primary mode of this
energy conversion manifests itself as a nonuniform temperature rise which leads to a
series of thermodynamic processes. These thermodynamic processes can then be
exploited as a means to affect therapeutic actions such as photothermal coagulation
and ablation of tissue. Another mode of interaction is the utilization of the absorbed
energy in activation of endogeneous or exogenous photosensitizing agents in a photo—
chemical process known as photodynamic therapy. Laser interaction with biological tissue is composed of a combination of optical
and thermodynamic processes. An overview of these processes is shown in Figure
17.13. Once laser light is irradiated on tissue, the photons penetrate into the tissue
and—depending on the tissue optical properties such as scattering coefﬁcient, absorp—
tion coefﬁcient, and refractive index—the energy is distributed within the tissue.
A portion of this energy is absorbed by the tissue and is converted into thermal energy,
making the laser act as a distributed heat source. This laser-induced heat source in
turn initiates a nonequilibrium process of heat transfer manifesting itself by a tem-
perature rise in tissue. The combined mechanisms of conduction, convection, and
emissive radiation distribute the thermal energy in the tissue, resulting in a time- and
space-dependent temperature distribution in the tissue. The temperature distribution ...
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- Spring '08