Page 401 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN012C-562 July 26, 2001 15:30

8 Photoacoustic Spectroscopy

frequency and time domains, we found a clear difference tochemotherapeutic agents are irradiated in vivo, a rapid
in the diffusion behavior of the sunscreen compounds in photobleaching of the sensitizer is normally observed.
the skin. The combination of Fourier transform combined PAS has been used to monitor such photobleaching in a
with PPAS allowed us to determine the depth profile of highly scattering medium. It may also be possible to locate
the chromophores such as solar creams and other drugs. and delimit diseased areas in the case of external cancers
The frequency spectra helped to locate the chromophore fromthehematoporphyrinsignalsandtofollowtheregres-
diffusion with respect to the different layers of skin. sion of the tumor. Similarly, photophysical parameters of
the naphthalocyanines, merocyanine 540 having various
applications in photobiology, have been investigated us-
D. Oncology and Photosensitizers
ing fluorescence and PA spectroscopies. A significant im-
Investigations on the photosensitized reactions in provement in instrumentation made during the past decade
medicine,particularlyinphotodynamictherapy,havebeen has resulted in much wider interest in the applications of
of considerable interest in recent years. Intensive research PAS in dermatology.
efforts are being made currently to the study of photosensi-
tized reactions in medicine, particularly in photodynamic E. Ophthalmology
therapy of tumors. Photodynamic therapy is a common
treatment for cancer and various pathologies such as pso- The visual process is a series of reactions beginning with
riasis. These treatments rely on the administration of pho- the absorption of a quantum of light energy. There are
tosensitizer and the irradiation of the zone treated. In the several temperature-dependent reactions during the con-
photosensitized reactions, the photosensitizer is excited version of the visual pigment rhodopsin to retinol. Sev-
by absorbed light and undergoes intersystem crossing to eral intermediates are formed during this process. Using
triplet state from which it either reacts with the nearby the depth profile method, a three-dimensional image of
molecules or produces cytotoxic oxygen derivatives, such the whole retina is recorded at low temperatures by vary-
ing the frequency. At high frequencies the rhodopsin–
as singlet oxygen or superoxide anion. The drugs used
for photosensitization processes must show very low fluo- bathorhodopsin photostationary mixture appears to be the
rescent quantum yield and nonradiative decays with very main absorbing species. At low frequencies, deeper lay-
high efficiency. In this sense, PAS is an excellent tool for ers are probed, and the PA spectra reveal the increasing
studying the photophysical properties of the drugs and amplitude of the PA band at 412 nm, which is due to
their interaction with biological molecules. blood hemoglobin. The PA data suggest that in the 25-µm
The photophysical properties of the photosensitizers are layer which is probed between 50 and 60 Hz, the first half
usually monitored to determine the efficiency of the treat- (0–13 µm) essentially contains the rhodopsin (outer rod
ments.Parameterssuchasthelifetimeofexcitedstates,the segments).
1
intersystem crossing yield, and singlet oxygen O 2 ( g ) The bovine retinal pigment epithelium (RPE) is a single
production yield are known only in solution form. As the layer of cells underlying the photoreceptor outer segments
PA and photothermal spectroscopic techniques are least and is supported by the highly vascularized choroid. Upon
sensitive to light scattering and are known to be sensi- dissection and removal of the retina, the RPE appears as
tive to relaxation processes, several attempts have been an area of different colors, which are known as amelan-
made to use PAS to detect the photosensitizers in solu- otic and melanotic RPE. Using PAS, these amelanotic and
tion, in tissues, in excised organs, and in skin in situ and melanotic RPE areas (Fig. 7A, B) are detected in the RPE.
in vivo. Porphyrin derivatives are being actively investi- In healthy bovine eye lenses, PAS revealed the presence
gated as photosensitizers for use in photodynamic therapy of the 340- to 360-nm band, which increases in magnitude
of tumors. The aggregation tendency of many porphyrins with age and is particularly pronounced in brown nuclear
may be related to their selective retention in neoplastic tis- cataracts. UV irradiation effects on eye lenses of dogfish
sues. Triplet state energy levels and intersystem crossing sharks and rats have also been investigated using PAS.
quantum yields of porphyrin derivatives have been ob- Other biomedical applications of PAS include the study
of the different states of blood hemoglobin, cytochromes,
tained by measuring the phase angle difference between
the PA signals of an unquenched sample and of the sample pharmaceutical powders, and diary products.
with triplet quenched. For molecules whose photophysi-
cal properties cannot easily be obtained by purely opti- F. Plant Biology
cal means, such an approach can prove to be a powerful
1. Biomass Determination
method. Similarly it is also possible to study the reaction
kinetics in environments that are not possible to monitor PAS was used to determine the number of cells in the
via absorbance. For example, when porphyrin based pho- fermentation of cyanobacteria, as this technique is not