3.4 Summary and the Bigger Picture

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3.4 Summary and the Bigger Picture

The eyes are human photoreceptors; detection occurs, however, via a change in the

three-dimensional structure of a molecule. This change in the shape of the molecule

initiates a change in the shape of a protein, which initiates a signal cascade that am-

plifies the signal and activates an ion channel, which changes the membrane potential,

which initiates the signal transfer to the brain and with that the analysis of the signal.

Analysis here does not only imply the recognition of a specific shape such as a rose, but

it also means the creation of a connection with the rose to the feeling of love, also using

memory in the process.

Using the actual molecules in the process for nanosized photosensors is difficult,

since proteins denature and lose function easily in environments other than the natural

one. Nevertheless, bacteriorhodopsin has been successfully attached to an electrode,

which created a sensor that reacted not to the amount of light itself but the change in

the amount of light, as does the human vision system. Rhodopsin can also be genetically

modified to measure other things than light, such as voltage or chloride concentration.

Another function of the human system has been mimicked: the constant movement

of the eyes to create a more accurate picture and to assist with the analysis of this picture

(e. g., by identifying different distances for the various parts of the picture). The combi-

nation of both processes should create an even more powerful artificial photosensor.

When developing photosensors on the nanoscale that measure light, the options are

endless and a large variety of systems have been reported. They range from chemical

or electrical sensors that measure a change in light absorption or fluorescence to the

opposite case, where a change in a chemical or current is reported as a color change.

What is less common is the connection of sensing to amplification and analysis within

one system. Phototransistors have been developed that achieve this. Other examples

will be touched on in the following chapter on smell and taste.

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