Biomedical Engineering Reference
In-Depth Information
bR
570
~ 4 ps
~ 5 ms
O
640
K
590
~ 5 ms
~ 1
s
µ
N
560
L
550
~ 5 ms
~ 40
µ
s
M
412
H
+
H
+
(a)
bR
570
~ ns
K
~ s
~ ms
FIGURE 17.2
Photochemical cycles of the bR molecule: (a) in aqueous
phase, where the proton transfer begins with the release of a
proton to the extracellular side during the
L
~ 3
µ
s
M
M
transition
and ends with proton uptake from the cytoplasmic side dur-
ing the
M
L
N
transition (17). (b) In dried form, only the
K
,
L
, and
M
intermediates are involved in the photochemical
cycle; the protein returns to its ground state through several
paths, each with a different lifetime (18).
~ 4
µ
s
(b)
17.1.4
Bacteriorhodopsin Applications
Its unusual stability and high quantum efficiency, coupled with unique photochromic and
photoelectronic properties, make bR a promising biological material for technical applica-
tions. Various device designs based on bR have been explored since its discovery. Most
early works have applied its photochromic properties to optical applications. Recently,
emerging interests lie in the exploration of its photoelectric properties, which can be real-
ized within hybrid bioelectronic architectures.
Photochromic applications of bR range from all-optical logic gates and holographic
interferometry to optical volumetric memories and parallel associative processors
(23-26). These applications are based on bR's photochromic properties, where absorp-
tion of a photon leads to reversible photochemical and thermal cycles. bR can be
reversibly converted between different intermediates of the photocycle by exploiting
photoexcitation at different wavelengths. Genetically engineered bR can provide sig-
nificant spectral shifts and long lifetimes within certain intermediate states, showing
additional potential in such optical applications (27). Simple alignment of PM patches
is required; therefore, embedding PM in an appropriate matrix is sufficient to meet
application needs.
Photosensitive properties of bR provide a basis for its sensing and imaging applica-
tions. Immobilization of bR molecules to charge-sensitive substrates is required to utilize
such properties. The generated photoelectric response from immobilized bR on various
substrates has been investigated. Widely adopted substrates include silica glass coated by
conductive oxides such as tin oxide or indium tin oxide (ITO), and metal electrodes such
