Biomedical Engineering Reference
In-Depth Information
BLM is required to reconstitute bR. But even if a genuine BLM is used to reconstitute bR,
a high-bandwidth current amplifier will allow the AC photosignal to become so promi-
nent as to overwhelm the concurrent DC signal for reasons presented earlier. Detection of
DC photocurrents requires substantial amplification of the signal and the use of a low-pass
filter to suppress the concurrent AC component. Illumination with continuous light allows
the DC current to emerge after the initial AC transient, if any, subsides (Figure 15.4).
Instead of the TVC method, we used a null-current method [68,69] to analyze the DC
photoelectric effect.
There are two important attributes of the DC photoelectric effect: the DC
photoconductive
effect and the DC
photovoltaic
effect. The photovoltaic effect is the manifestation of
photoemf. Photoconductivity is the manifestation of a specific current pathway activated
by light. In bR, the photoconductive pathway is thought to be a series of protonation sites
extending from one membrane surface to the other [18,19]. This pathway is different from
an ionic conduction pathway provided by ion channels or by leakage through the mem-
brane. Several pertinent questions can be raised here. Does the specific proton pathway
stay open in the dark or only during illumination? Is it possible to reverse the proton cur-
rent by applying an opposing transmembrane potential? If the answer to the latter ques-
tion is yes, then it ought to be possible to cancel the light-induced DC current by
concurrently applying a transmembrane voltage with an appropriate magnitude in the
opposite direction. This reasoning forms the basis of the null-current method, which is
based on the potentiometric principle in electrochemistry: the photoemf is equal but oppo-
site to the applied voltage that cancels the DC photocurrent. Detailed discussion explain-
ing why the photoemf must be measured with a potentiometric method and how
photoconductivity can exist without a concurrent photovoltaic effect can be found in [69].
It suffices to say that measurements under conditions other than null diminishes the meas-
ured emf values by virtue of a voltage drop caused by internal resistance.
The equivalent circuit in Figure 15.1 can be used for the DC photoelectric with proper
identification of terminology as follows. The resistance encountered by the DC photocur-
rent is equal to (
R
s
R
p
), or approximately
R
s
since
R
p
is much smaller than
R
s
. The chem-
ical capacitance
C
p
can be ignored in the DC photoelectric effect for obvious reason. Here,
we define the photoconductance
G
p
as the reciprocal of (
R
s
R
p
). Note that
G
p
is actually
closer to 1/
R
s
than to 1/
R
p
even though the notation suggests that the erroneous relation:
G
p
1/
R
p
. Also we define
G
m
as the reciprocal of
R
m
.
We used a conventional voltage clamp amplifier to measure the DC photocurrent. The
procedure of a null-current measurement is illustrated by the schematic diagram in Figure
15.12A and by an actual example in Figure 15.12B. For the sake of simplicity, let us assume
that the conductance
G
p
is much greater than
G
m
so that the shunting effect of the parallel
current path through
G
m
can be ignored. We further assume that the conductance
G
p
has
the same value in the dark as in the light. These restrictions will be subsequently removed
after an intuitive picture is presented.
At the beginning of a null-current measurement, the clamping voltage is set to 0 and the
illuminating light source is also turned off (current level 1). While the light source remains
off, the clamping voltage is then set to a preselected value
V
c
, and the current responds by
settling at a new value (level 2) after a brief capacitative transient (shown in Figure 15.12B
but not in Figure 15.12A). While the clamping voltage is maintained at
V
c
, the light is then
turned on. The current responds, again after a brief AC photoelectric transient, by settling
at yet another new value (level 3). The difference, level 3 minus level 2, gives the DC pho-
tocurrent,
I
p
(corresponding to the clamping voltage
V
c
). While the illumination continues
at a fixed intensity (power), the clamping voltage is then adjusted to a new value so that
the measured current is “tuned” back to level 2. That is, the end point of tuning (
V
c
) is the
new voltage that is required to cancel (“nullify”) the DC photocurrent. In other words, by
