Biomedical Engineering Reference
In-Depth Information
for the purpose of imaging the external world (photon energy sensor). However, the
primary events in both types of devices are similar: light-induced charge separation.
Because of the asymmetric orientation of the photopigments in these membranes, light-
induced charge separation results in a unidirectional charge movement. This vectorial
charge movement can be detected as a photovoltage or a photocurrent via a pair of
electrodes placed across the membrane. The structure of photosynthetic and visual
membranes has inspired useful structural configurations of molecular optoelectronic
devices. Prototype molecular and molecular electronic devices were often constructed as
layered thin films. Using the Langmuir-Blodgett (LB) technique to configure thin organic
films with embedded organic dyes and other functional molecules on a solid support
(substrate), Kuhn and coworkers [1-3] pioneered the study of supramolecular
photochemistry and have been able to demonstrate fundamental processes mimicking
photosynthetic membranes.
The electrical phenomena associated with light-induced vectorial charge movement
in membranes or thin films are collectively referred to as the
photoelectric effect
. If brief
light pulses are used to excite the membrane, the response is known as the fast photo-
voltage or the fast photoelectric effect. This particular effect is relevant for biosensor
designs. The fast photoelectric effect was first discovered by Brown and Murakami [4],
in monkey retina, about four decades ago; the signal was named the
early receptor poten-
tial
(ERP). Rigorous analysis of the ERP was not feasible prior to the advent of the
method of reconstituting artificial membranes. Visual membranes with complex geom-
etry contribute to shaping (and distortion) of the signal waveforms. Too many extrane-
ous electric parameters made it difficult to construct mathematical models with
satisfactory precision.
In 1962, Mueller et al. [5-7] discovered a method to form planar bilayer lipid mem-
branes (BLM) in vitro. Rigorous mathematical modeling became a serious possibility.
However, the necessity of using organic solvents in the original method hampered the
incorporating of proteins and other biomolecules into the artificial membrane.
Subsequently, Takagi and coworkers [8] succeeded in reconstituting a protein—the visual
pigment rhodopsin—into a BLM free of organic solvent. Tien [9] succeeded in recording
a fast photovoltage in a BLM reconstituted from a chloroplast extract. Using a modified
method similar to that of Takagi et al., Montal and coworkers [10,11] observed an ERP-
like fast photosignal in both a reconstituted rhodopsin membrane and a reconstituted
bacteriorhodopsin (bR) membrane. bR is a protein pigment similar to rhodopsin in chem-
ical structure, but it is found in the purple membrane of
Halobacterium salinarum
(for-
merly
Halobacterium halobium
) [12-19]. bR has a chemical structure similar to that of
rhodopsin but is considerably more stable than rhodopsin. Vsevolodov and coworkers
[20,21] pioneered the use of bR as an advanced functional material (“Biochrom-BR”).
Subsequently, a variety of prototype devices were reported by a number of laboratories
(for reviews, see [22-26]).
The present chapter focuses on the fundamentals of the photoelectric effect and illus-
trates smart sensor designs with several representative prototype examples reported in the
literature. An equivalent circuit simulating the photoelectric effect is developed from first
principles, based on realistic molecular models. Experimental photoelectric data are inter-
preted in mechanistic and molecular terms. The pitfalls that have led to misinterpretation
and confusion in the earlier literature are also pointed out. We shall first consider a mini-
malist experimental prototype in which the underlying chemistry reflects a single-reaction
step—an artificial light-driven electron pump. We shall then consider several reconsti-
tuted bR model membranes, which exhibit the next higher level of complexity. The tech-
nical issues related to molecular sensor designs will be discussed, using several prototypes
reported in the literature. Anticipated features of future smart sensors are speculated. On
