Biomedical Engineering Reference
In-Depth Information
15.6
Prototype Molecular Sensors Based on Photoelectric Effects
15.6.1
Two Ways of Configuring Photoelectric Sensors
The term “photoelectric sensors” suggests two ways to configure sensors. From the above
discussion, it is evident that bR is a
bifunctional
electronic material: it is sensitive to light as
well as to small ions, such as H
, Cl
, Ca
2
, and Mg
2
. The amplitude of a photoelectric
signal depends on the stimulating light intensity. The dependence is linear until it
approaches saturation [93]. The photoelectric effects can be exploited to design light sen-
sors. In addition, by exploiting the ion sensitivity of the photoelectric signals, sensors can
be configured to monitor specific ion concentrations. The variety of ions to be detected
depends on the type of pigment and its photochemistry. The design principle can be gen-
eralized to pigments other than rhodopsins or porphyrins. In view of the vast repertoire
of photochemical reactions accumulated in the literature, the variety and specificity of ion-
sensing photoelectric sensors can be rather impressive.
Here, I deliberately use the term “photoelectric effects” in the plural form instead of the
singular “photoelectric effect.” The intent is to remind us that there are two ways to
deliver light stimuli: pulsed light and continuous light, and there are two effects: AC and
DC photoelectric effects. Both types of illumination have been used in sensor designs (see
below). Note that photoelectric effects can also be exploited to design artificial solar
energy converters. But the design considerations are quite different. The essence of artifi-
cial solar energy conversion is the DC photoelectric effect. In this regard, the accompany-
ing AC photoelectric effect is an unwanted side effect because charge recombination
internally short-circuits the device and dissipates the partially converted photon energy as
heat. The designers of artificial solar energy converters are constrained to utilize continu-
ous light as the energy source and to focus on improving and maximizing the DC photo-
electric effect. As far as sensor designs are concerned, there is no such constraint. In
addition, the AC photoelectric effect is more advantageous than the DC photoelectric
effect. Pulsed light is more versatile than continuous light in eliciting a response from pho-
toelectric sensors.
Earlier, Trissl [94] and Rayfield [95] have entertained the idea of using a bR thin film as
a photodiode. Although, strictly speaking, bR is not a photodiode, bR thin films remain
one of the prototype biological thin films capable of delivering photon-switched electric
signals with an ultrafast risetime. Two specific examples will be described and discussed
in detail here. One was configured as a photon sensor whereas the other was configured
as a specific ion sensor.
15.6.2
A Light Sensor Based on the AC Photoelectric Effect
Miyasaka et al. [93,96] successfully constructed a motion detector using wild-type bR
(Figure 15.20). The detector was constructed as a square array of 8 by 8 patches of
oriented bR thin films deposited on transparent metal electrodes, using photomicrolitho-
graphy (Figure 15.20A and Figure 15.20B). Each bR electrode is connected by a pair of
metal circuit lines, which allow the sensor array to be interfaced to conventional micro-
electronic circuitry for further signal processing. A typical photocurrent response from a
patch of bR electrode is shown in Figure 15.20C. A transient photocurrent develops upon
the onset of illumination and another transient photocurrent of opposite polarity devel-
ops upon the cessation of illumination. The photocurrent subsides during steady illumi-
nation and during the dark after the initial transient. Such a waveform is the “signature”
