Biomedical Engineering Reference
In-Depth Information
properties of industrial interest: temperature stability, catalytic function, reaction rate, and
substrate specificity. All of the just-listed characteristics are variations on the given pro-
tein's natural abilities, that is, the enhancements are of preexisting naturally occurring
functions. However, careful selection of the screening process, coupled with selective
improvements conferred by successive generations of random progeny, makes possible
the optimization of nonnatural properties custom-tailored for specific applications.
Directed evolution therefore provides the applied researcher with a powerful tool for mak-
ing full use of the advantages inherent to biological molecules. A full discussion of these
novel techniques is beyond the scope of this chapter; many reviews exist in the literature
for the interested reader [60,61,67-75].
14.3
Bacteriorhodopsin as a Sensor Element
Biological molecules can offer distinct advantages and unique functionalities to many
device applications—introduction of biological molecules into inorganic environments
often poses many challenges, but once accomplished, the resulting enhancements impart
unique capabilities not previously possible. For sensors, these improvements take the
form of better sensitivity, specificity, and often lower detection limits. The discussion
below will focus on the use of genetically engineered BRs as integrated sensor elements,
including the rationale for using BR for sensor technology, an overview of the architecture,
technical issues that need to be resolved, and interpretation of the sensor's response.
In many ways, BR is already a sensor by natural design. It detects light with extreme
efficiency, and shares a common chromophore and structural motif with the two other reti-
nal proteins in
H. salinarum
, which have known sensory activities: sensory rhodopsins I
and II. The sensory rhodopsins function as sensors that facilitate positive and negative
phototaxis in the native organism [76]. Unlike these proteins, however, BR is produced by
the organism in much larger quantities (~30-50:1). The key question is whether the
researcher can take advantage of the protein's natural properties and apply them to new
sensor architectures.
14.3.1
Bacteriorhodopsin as an Integrated Element in Microelectronics
Bacteriorhodopsin arguably has received more attention than any other protein for bio-
molecular electronic research, and with good reason; BR has a unique set of properties—
both photochromic and photoelectronic—that are amenable to signal transduction. As
expected, however, several challenges exist when integrating any biological molecule into
a nonbiological environment; the two are not always compatible. This section will exam-
ine a number of these challenges, as well as some of their solutions. It should be noted that
these are issues that will be critical to the success of integrating any protein into a synthetic
environment.
It is worth reiterating the qualities that make BR an appealing candidate for device appli-
cations, and for sensors in particular. Many of these characteristics have been discussed
above: (1) the protein is inherently rugged and robust, and is resistant to both thermal and
photochemical damage; (2) the cyclicity is >10
6
, considerably higher than most synthetic
photochromic materials; (3) the protein is inherently radiation hardened and protected
from free radical degradation; (4) it is inherently resistant to radiation damage; and (5) the
primary photochemical event, that is, photon absorption and formation of the K state, pro-
ceeds with a very high quantum efficiency (~65%) [36]. In addition, the semicrystalline
