Biomedical Engineering Reference
In-Depth Information
the basis of our new insight into human creativity, speculation and suggestions about
future biosensor design are also presented.
15.2
AC and DC Photoelectric Effects
Direct transplantation of classical electrophysiological methodology and methodology
used in solution-phase photochemistry has generated considerable controversy and
confusion in the early literature regarding the interpretation of measured photoelectric
signals. Additional consideration must be given to the presence of photoelectromotive
force (photoemf) as well as the interaction of signal-generating element, bR, and proper-
ties of inert membrane structures, resistance and capacitance. In addition, a novel capaci-
tance appears because the current source is either deep inside the membrane or at the
membrane-water interface(s). This is to be contrasted with classical electrophysiological
studies, in which all current sources are outside of the membrane.
Substantial clarification can be made by taking into account the presence of consecu-
tive electron (or proton) transfer reactions and the almost ubiquitous presence of reverse
reactions. The most common charge transfer reactions are redox (electron-transfer) reac-
tions and acid-base (proton-transfer) reactions. Consecutive electron-transfer reactions
are present in both photosynthetic membranes and in the inner membrane of mitochon-
dria, thus forming the electron-transport chain. Similarly, consecutive proton-transfer
reactions are present in the purple membrane of Halobacteria . A detailed description of
coupled consecutive charge transfer reactions will be presented in Section 15.5. When
these reactions take place in a highly organized and anisotropic medium, such as layered
membranes or thin films, the net-forward charge transfer results can be observed exter-
nally via a pair of electrodes as DC electric currents whereas reversible rapid charge
movements accompanying reverse reactions can be detected as AC electric currents. This
way of classifying photosignals greatly simplifies the interpretation and facilitates
designing photoelectric biosensors.
As will be demonstrated later, the transient photosignal generated by a brief light pulse
contains mainly a transient capacitative component, which reflects reversible charge trans-
fers with no net charge transport across the membrane. In contrast, the steady-state
photosignal reflects the net unidirectional charge transport (vectorial charge transport).
Under continuous illumination, the capacitative component sometimes appears only
briefly at the onset and again at the cessation of illumination. These transients may not be
detected under certain experimental conditions. It can further be shown analytically that
this descriptive classification can be replaced by a technically accurate classification: the
DC and the AC photoelectric signals [27-30].
The fast photovoltage mentioned earlier is therefore an AC photoelectric signal. That the
ERP is a capacitative signal was established by an experiment reported by Hagins and
McGaughy [31]. These investigators found that the time-integral of the measured current
associated with the ERP approaches zero after the signal decays, indicating no net charge
transport across the membrane (see Section 15.3 and Section 15.4).
It was widely recognized that the ERP is generated by rapid charge displacement rather
than by ionic diffusion [32,33]. The ERP are therefore different from other bioelectric signals
commonly encountered in biology, for example, action potentials and synaptic potentials.
The AC and the DC photoelectric signals are related to the underlying photochemistry and
are manifestations of interactions of the intrinsic photochemical processes and the inert
support structure (substrate). The latter feature is absent in solution-phase photochemistry
but these factors must be taken into consideration in smart sensor designs.
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