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3 Vision

Figure 3.6: (a) Current output for an organic phototransistor that can react to two different wavelengths

(365 nm and 450 nm) differently. As with all transistors, this can be used as a switch, but with the different

outputs, this one transistor can be used as an “or” or “and” logic gate, dependent on the wavelength that

is used (b). (c) shows the truth table for the logic gates (adapted from [11]).

An optical mimic of rods was developed not from a phototransistor but from a mod-

ified pyrrole N’-1,N’-6-bis(3-(1-pyrrolyl) propanoyl) hexanedihydrazide (DPH) that self-

assembles into rod-like, long structures that are a few nm in diameter [12] (Figure 3.7).

These rods are photoluminescent; their signal was enhanced in the presence of a sensi-

tizer. These rods have been effectively used to selectively measure the concentration of

pyrrole. So, here the detector is the photoluminescent rod. Detection occurs via chang-

ing the optical properties (i. e., quenching the fluorescence) of these rods by changing

the excited state energy of the molecule, instead of by changing its three-dimensional

structure. The light energy-level is then detected and analyzed.

An example of a chemical structure change coupled with a signal is the use of

fluorescence-labeled liposomes as intracellular biosensors [13] (Figure 3.8). Here, a lipid

is labeled with the fluorescent dye Nile-Blue and incorporated into liposomes. The li-

posomes can easily be taken up into cells. A different pH in different cells creates a

different protonation state of the dye, which results in a color change (Figure 3.7). Ana-

lyzing the specific color results in a measurement of the internal cell pH, and thus acts as

an indicator of various diseases (e. g., cancerous cells often have a lower pH). Therefore,

this sensor does not measure light, but rather uses light (fluorescence) to measure pH.

This is different from the human visual system, where light is detected and the signal is

transduced and analyzed as an electrical signal.

These few examples already show that there are endless possibilities to create

biomimetic photosensors. Most of them are based on specific compounds binding to a

detector surface, thus measuring the amount of a compound in solution, with a light or

color output. Only two examples will be highlighted here.