3.2 Photosensors Using Biological Molecules and Methods

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One group successfully used bacteriorhodopsin, a rhodopsin that can be found in pho-

toactive bacteria called Halobacterium salinarum, for a photosensor [4] (Figure 3.2).

The sensor uses a photocell with two ITO electrodes, one of them coated with bacteri-

orhodopsin. An electrolyte solution maintains the native structure of the protein. A laser

beam was used to activate the rhodopsin; the laser beam was expanded so that the com-

plete sensing area was irradiated. The amount of light the laser beam emitted was inde-

pendently monitored via a photodiode. The photocurrent from the bacteriorhodopsin

was measured by an oscilloscope and amplified electronically.

Figure 3.2: Set-up and signal of a photosensor that uses bacteriorhodopsin on an electrode (adapted from

[4]). (a) Set-up of the photocell; (b) generated photocurrent based on the amount of light irradiation.

As with the human visual system, this sensor reacts to changes in the amount of light,

instead of measuring continuously the amount of light coming from the source. This

makes this sensor specifically good at reacting to movement (similar to human vision).

When the bacteriorhodopsin film is patterned correctly, the direction of movement can

be determined as well. The authors are developing this optical sensor for robots; with

this sensor, robots could react to sudden changes of light, and thus dangers [4].

The above sensor used the biological sensing molecule and built it into a common

digital photocell. Photocells are used, e. g., in the sensors that turn streetlights on auto-

matically when it is getting dark. But with a biological sensing material functionality

can be expanded to detect sudden directional changes and movements, mimicking the

function of human eyes instead.