Biomedical Engineering Reference
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protein in a number of different formats. We studied its optical properties incorporated
within a sol-gel matrix, interacting with a conducting polymer and oriented between par-
allel plate electrodes. In the study of bR incorporation into sol-gel matrices, the absorption
and fluorescence properties of bR were unaltered following incorporation (20). Interaction
of bR with a soluble anionic conducting polymer, poly(3-thienylacetic acid)—a thiophene
monomer-based system, was carried out and the complex was cast on either quartz win-
dows or interdigitated electrodes, dried, and then studied. Within the polymer matrix, the
bR was photoactive and the difference absorption spectra indicated that the M state of the
bR photocycle was preferentially stabilized. In fact, bR could be switched back and forth
between the photochromic M and bR states using photons of appropriate wavelength.
Through photocurrent studies upon interdigitated electrodes, bR was shown to increase
its charge transport in the presence of the conducting polymer, compared with bR alone
(20). This is in agreement with the expectation that the positive-charged defect structures
existing within the
electron conjugation system of the conducting polymer backbone
should facilitate charge transport from bR upon its photoexcitation. It also confirms the
potential for using bR-conducting polymer conjugates in biosensor designs where charge
transport is required.
We also investigated the potential of oriented bR films for use in two-photon 3-D erasable
optical memory systems (20,34). The bR protein exhibits an unusually large optical nonlinear-
ity that is due to a large change in the light-induced dipole moment. We studied a bR-based 3-
D erasable optical memory storage system based upon this nonlinear optical property to carry
out nondestructive reading. Specifically, a two-photon-induced photochromic transition is
used for writing and the second harmonic generation is used for reading. As indicated in
Figure 1.11, the writing is carried out by transitions between the bR and M states of the pho-
tocycle, representing 0 and 1 binary information states possible in the optical memory. The bR
protein system is a good material for this type of 3-D memory for a number of reasons. It pos-
sesses long-term thermal stability and has a high threshold to photochemical degradation.
Also, it exhibits large quantum efficiencies,
, for the photochromic transitions and has a large
bR [State 0]
M [State 1]
3.0
2.0
1.0
M
bR
0.0
300.0
400.0
500.0
600.0
700.0
800.0
Wavelength (nm)
FIGURE 1.11
Two-photon photochromic states bR [state 0] and M [state 1] of bR, the two-photon transition characteristics
(wavelength and quantum yield) between states, and the absorption spectra corresponding to those states.
Reprinted from Chen, Z., Lewis, A., Kumar, J., Tripathy, S.K., Marx, K.A., Akkara, J., Kaplan, D.L. (1994). Second
Harmonic Generation of Bacteriorhodopsin and Its Application for Three-Dimensional Optical Memory. In: M.
Alper, H. Bayley, D. Kaplan, M. Navia, eds. Biomolecular Materials by Design, Proc. Mat. Res. Soc. , 330:263-268.
With permission from the Materials Research Society.
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