Biomedical Engineering Reference
In-Depth Information
role in earlier efforts of prototype device research, presumably because of its exceptional
thermal and chemical stability. bR exhibits two properties that could be exploited for sen-
sor constructions: photochromic and photoelectric effects. This chapter focuses on the pho-
toelectric side of bR-based sensor designs.
Electrical events are common control processes in living systems. Many such events take
place in the solution phase; these events are only indirectly observable because the direc-
tion of charge movements is somewhat random. A particular class of electrical phenomena
involves biological membranes and has been the subject of electrophysiological investiga-
tions. Biological membranes provide a spatially anisotropic medium for electrical events to
take place; the direction of charge movements is, therefore, largely perpendicular to the
membrane (vectorial charge movement), thus greatly facilitating electrical measurements
and permitting unequivocal interpretations. Bioelectric events such as action potentials and
synaptic potentials have their origin in passive electrodiffusion of small inorganic ions
through water-filled transmembrane ion channels. The photo-induced phenomena in bio-
membranes call for somewhat different analytical methodology because these phenomena,
just like those caused by electrogenic pumps, require movement of electrons or ions against
a transmembrane electrochemical gradient. These charge movements reflect an active
process of charge transport fueled either by metabolic energy (in the case of mitochondria)
or by absorbed photons (in the case of photosynthesis and purple membranes). The pres-
ence of additional electric parameters (such as photoemf) renders the application of Ohm's
law alone insufficient to analyze the electric events. Visual photoreceptor membranes do
not perform active charge transport but are rather equipped with mechanisms of tranduc-
ing absorbed photon signals into electric signals suitable for processing by the nervous sys-
tem. However, visual membranes and photosynthetic membranes share a common feature
due to similarity in the primary photophysical event upon illumination. Such primary
events are reflected in changes of absorption spectra and in charge movements far too rapid
to be accounted for in terms of electrodiffusion. Rather, they reflect rapid charge displace-
ments either at the membrane surfaces or in the interior of functional membrane-bound
proteins. Such events occur at a considerable faster time scale than electrodiffusion. This
fast photoelectric effect is highly suitable for biosensor designs. However, some prototype
photoelectric sensors reported in the literature utilized the steady-state photoelectric effect
rather than the fast photoelectric effect. We have shown that these two types of effects
correspond to the DC photoelectric effect and the AC photoelectric effect, respectively.
Direct transplantation of electrophysiological methodology appeared to be inadequate.
Many years ago, we introduced the method of null-current measurement and TVC meas-
urement, for handling the DC and the AC photoelectric effects, respectively. We demon-
strated how a combined electrophysiological and electrochemical approach could provide
a coherent description and analysis of the molecular and mechanistic events associated
with the fast photoelectric effect. This approach was initially adopted to better our under-
standing of Nature's general scheme for light-sensitive membranes, and to gain a deeper
insight into the process of electric signal generation. However, we got more than what we
had bargained for, because of its potential impact on bR-based sensor applications.
While the DC photoelectric effect is highly relevant for the construction of artificial solar
cells, rapid light-induced responses to the chemical environment offer a venue for sensor
construction. Mining of sensor properties needs not be an exercise of trial and error
because understanding of molecular mechanisms provides a road map. Most sensor appli-
cations were based on “differential responsivity.” However, elucidation of the underlying
mechanisms has never been straightforward. Even to date, alternative rival interpretations
thrive among different persuasions of bR researchers. This concern brings us to the topic
of mathematical modeling. Recently, Gauch [118] complained about biomedical
researchers' complacency in the lack of pursuing mathematical modeling, while pointing
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