Biomedical Engineering Reference
In-Depth Information
other words, the two interfacial redox reactions are
decoupled chemically
during the brief
transient but are coupled in the steady state. It is observed that the amplitude of tran-
sient photocurrent exceeds that of the steady-state photocurrent by several orders of
magnitude. This means that the transient photocurrent is preferentially amplified. The
reason for the preferential amplification can be understood in terms of AC-coupling of
the photocurrent source as indicated by the zero time-integral condition above. AC-cou-
pling can be implemented with a high-pass
RC
filter. Thus, the simplest electrical equiv-
alent circuit that can account for all these observations is shown in the bottom half of
Figure 15.1. Since the transient photoelectric signals were DC-coupled to the measuring
circuitry and there were no external high-pass filters, the phenomenological high-pass
RC filter might be intrinsic.
In the equivalent circuit, the photocurrent (or photovoltage) source is light-dependent
(and therefore time-dependent) if a brief light pulse, instead of continuous light, is used
to illuminate the membrane. This time-dependent electromotive force is designated by
E
p
(
t
) in Figure 15.1. The combination of a capacitance
C
p
and a resistance
R
p
constitutes
the high-pass filter mentioned above. A transmembrane resistance
R
s
is added to provide
the pathway for the steady-state (DC) photocurrent. These circuit elements represent the
process responsible for the generation of a light-induced transient signal (AC current
driven by
E
p
through
R
p
and
C
p
) and a steady-state signal (DC current driven by
E
p
through
R
p
and
R
s
). It is however necessary to connect this
RC
circuit to another circuit
formed by
R
m
(transmembrane ionic resistance) and
C
m
(ordinary membrane capaci-
tance). Here
C
p
and
C
m
are regarded as two separate elements. Whether
C
p
is indeed a
distinct physical entity has been a topic of much debate. Contrasting views using alter-
native approaches can be found in several old articles [41-51]. Detailed experimental and
theoretical basis of chemical capacitance has been published [35,52,53]. The equivalent
circuit in Figure 15.1 is equally applicable to other types of pigment-containing mem-
branes. In brief, the molecular origin of
C
p
is light-induced charge separation and subse-
quent recombination.
C
p
has thus been named
chemical capacitance
because of its
physicochemical origin [34]. Contrary to a recent claim [51], it can be shown analytically
and experimentally that chemical capacitance is an intrinsic property of pigment-con-
taining membranes with built-in asymmetry.
The equivalent circuit shown in Figure 15.1 predicts that the time course of the observed
photoelectric signal is strongly dependent on the access impedance,
R
e
, which usually
comprised the input impedance of the instrument, the electrode resistance (or impedance),
and the resistance of the intervening electrolyte solution (between the tip of electrodes and
the membrane surfaces). Short-circuit conditions are achieved by rendering
R
e
much
smaller than the source impedance of the membrane. The entire photocurrent thus pro-
ceeds to the measuring device, which is usually a current amplifier. Upon pulsed light illu-
mination, the photocurrent first rises as a sharp spike, and then reverses its polarity and
decays in a single exponential time constant of
p
, which is a function of
R
p
,
C
p
, and
R
s
p
is equal to
R
p
C
p
(see below). Under continuous
illumination, the photocurrents exhibit transient spikes when the light source is turned on
(“on” overshoot) and when it is turned off (“off” undershoot). The steady-state signal
between the two transients during continuous illumination represents the DC photocur-
rent (Figure 15.4). The two spikes, which decay with the time constant
(Figure 15.4). To a good approximation,
p
, reflect the AC
photoelectric effect.
If, however,
R
e
is rendered much greater than the source impedance, almost the entire
photocurrent proceeds to charge
C
m
and only a trace fraction reaches the measuring
device—usually an electrometer (open-circuit condition). The photosignal relaxation will
then be dominated by the
RC
relaxation with a decay constant of
R
m
C
m
(
l
in Figure 15.4 is
approximately equal to
R
m
C
m
).
