are obtained by the oxidative polymerization of 5,6-dihydroxyindoles and behave

as amorphous semiconductors. Figure 2.4 shows the structure of eumelanins monomers.

According to their structure at a biological level, these molecules have functionality as

protective agents against UV radiation. Although their exact charge transport mechanism

is unknown, some studies have shown that it occurs by cations mobility and depends on the

system hydration. Thin films of eumelanins may be used for the fabrication of electronic

devices such as photovoltaic devices, light-emitting diodes, organic electrochemical tran­

sistors, and sensing devices [46]. Wu and Hong developed a humidity sensor that uses

dopamine-melanin thin films; the sensor showed response times around 0.45 s and a re­

covery time of 0.46 s, outperforming devices made of inorganic materials. Natural melanin

is insoluble in practically all solvents, so they proposed a preparation method of dopamine-

melanin oligomers by dopamine autoxidation induced polymerization, which at basic

pH (>11) allowed the formation of water-soluble aggregates [47].

On the other hand, carotenoids are molecules of natural origin, low molecular weight,

and high conjugation, which perform functions of antioxidant, pigmentation, and light

capture in photosynthetic processes. Some carotenoids, such as β carotenes and bixin,

have shown remarkable semiconductor properties. Although the use of carotenoids in

bioelectronics is limited by their size, these may be employed as a template for

the manufacture of efficient semiconductors [48]. Finally, indigo is a natural pigment

found in some plants such as Indigofera tinctoria and snails of the Hexaplex trunculus

family, but that is currently synthetically obtained. The electrical properties of indigo

are given by its high aromaticity and have allowed its use in the design of bioelec­

tronics devices [45,49].

In addition to semiconductors, there are also insulators of natural origin; for example,

polypeptides and proteins that could be used for the formation of the dielectric film due

to their monodisperse nature. Chang et al. used chicken egg albumin to make insulating

films and networks, for thin-film transistors whose active phase was pentacene layers;

these devices showed comparable electrical properties to the synthetic materials [50].

Deoxyribonucleic acid (DNA) can also be used for the fabrication of dielectric thin films,

as the work of Yumusak et al. showed, who synthesized highly resistant thin films of

DNA, from the cross-link of poly(phenyl isocyanate-co-formaldehyde) and DNA mix

with a cationic surfactant [51].

FIGURE 2.4

Structures of eumelanin monomers.

Materials and Their Classifications

29