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Physical chemistry

Power games

Nanotubes of biological molecules may turn into sensors and energy cells

THIAGO CIPRIANO / UFABC / LME-LNLSNanoforest: standing tubes in an electrode THIAGO CIPRIANO / UFABC / LME-LNLS

Erecting minuscule tubes no thicker than a strand of hair divided dozens of thousands of times, piece by piece, is the game that keeps the team of the chemist Wendel Alves, from the Federal University of ABC (UFABC), busy. This enables him to uncover the ideal conditions for producing these nanotubes and for controlling their properties. This may sound like science fiction, but it is true. Furthermore, it is far from being a useless toy: the idea is to make miniature power generators and biosensors in the future.

Part of the National Institute of Science and Technology (INCT) for Bioanalysis, coordinated by the chemist Lauro Kubota, from the State University of Campinas (Unicamp), the newly set up laboratory, which Alves’s group shares with the teams of three other professors, is ready to face up to this challenge. Nonetheless, some molecules are too small for actual manipulation, even in an advanced chemistry laboratory. In this case, the theoretical studies are conducted by the physicist Alexandre Reily Rocha, who also teaches at UFABC, a university that is still being set up in the city of Santo André, in the São Paulo metropolitan area. “With a computer, I can put one molecule at a time into the nanotube and study what happens with simulation models,” he explains.

Alves’s nanotubes are formed by aminoacids, the units that make up proteins and their sub-parts, the peptides. Unlike carbon nanotubes, which are now fairly common in laboratories around the world and require specific conditions to be formed, such as very high temperatures and a strong electric current, those made of peptides are still new in Brazil, according to the UFABC chemist. Moreover, they are a promising innovation.

“Peptide nanotubes are cheaper and faster to make,” he sums up. Actually, they form spontaneously: it is enough to dissolve the peptides in a test tube, in which they reorganize and form countless tubes. The São Paulo state team showed, in an article due to be published in  the September issue of Journal of Materials Science, that to form nanotubes such as those they obtained one must use, under the right conditions, a series of parts made with a specific type of aminoacid: phenylalanine, which bonds into sets of two, forming the peptide diphenylalanine.

Once a suspension of this material has been formed, a white solid appears in the test tube. To the naked eye, that is all it seems to be, but high-tech resources such as X-ray diffraction, spectroscopy and scanning electronic microscopy help scientists to characterize the nanotubes, at first arranged randomly. Luckily, a nano-comb is not needed to disentangle them: it is enough to suspend the solid in water for the nanotubes to self-organize. Their arrangement depends on the pressure, temperature, pH and solvent. They may stand vertically, like a nanoforest, or be disposed horizontally, like spaghetti on a plate.

Alves is studying how the parameters affect the arrangement of nanotubes in order to be able, in the future, to control their organization – which influences their properties – according to how they are to be used. Overall, the idea is to imprison enzymes within nanotubes stuck to an electrode to use them as biosensors. An enzyme that binds with glucose molecules, for example, may lie at the core of a minuscule sensor for diabetics. When they adhere to the enzymes, the glucose molecules change the variation of the current over time, allowing one to measure the sample’s glucose content.

The UFABC group of chemists has already imprisoned in nanotubes the active site of microperoxidase-11, an enzyme that contains iron, which makes the white nanotube powder red, when examined by the naked eye. The study, reported the Journal of Materials Science article, is part of the master’s degree of Thiago Cipriano and shows that these nanotubes give up electrons to the hydrogen peroxide, turning it into water, a type of reaction called a reduction.

Alves is now trying to produce nanotube systems based on biological models that can remove electrons from oxygen at ambient temperature, generating electricity. “And this is what we look for in fuel cells,” he explains. Along with the master’s degree student Iorquirene Matos, he assembled peptide nanotubes with a structure of four copper ions that, according to an article in the July issue of Electrochimica Acta, actually manage to conduct such a reduction.

Virtual reality
In simulations, Alexandre Rocha studies how to improve electricity conduction by peptide nanotubes, which are isolating by their very nature. The work indicates that particles of gold and copper can be stuck onto the tubes, producing more precise sensors – an effect that the chemists have already confirmed.

Another aspect dissected by the physicist is the influence of water upon the properties of the nanotubes. Although it is virtually omnipresent all over the Earth, water still holds many secrets from science. The UFABC duo has found, based both on simulations and on X-ray images, that the water molecules adhere to the inside of the tube forming a helix, albeit an imperfect, irregular one.

By adding one molecule at a time within the virtual environment, Rocha was able to characterize how the water molecules are connected by hydrogen bonds to the aminoacids that make up the tubes. The structure is so stable that even close to ambient temperature researchers refer to this water as “ice”. For it to evaporate, more than 100 degrees Celsius (oC) are required: 150 oC are needed to remove the water from the nanotubes. The next step consists of physicochemical analyses, to determine how the water affects the properties of the nanotubes.

The chemist brings these findings together to assemble in his mind a system that, he admits, still lies in the realm of science fiction: a nanometric biosensor attached to a nanometric electricity biocell, to enable diabetics to implant in their pancreas a device to measure glucose levels and release insulin as necessary, all of it fueled by a biocell. In theory, it should also be possible to use such biocells to feed pacemakers, which are at present implanted in cardiac patients with small batteries.

The projects
1. Synthesis, characterization and study of the electric properties of titanium oxide and peptide nanotubes (nº 2008/53576-9); Type Young Researcher; Coordinator Wendel Andrade Alves – UFABC; Investment R$ 377,263.69
2. Ab initio simulations of electronic transport in non-ordered nanostructure systems (nº 2009/15129-3); Type Regular Research Awards; Coordinator Alexandre Reily Rocha – UFABC; Investment R$ 127,795.55

Scientific articles
CIPRIANO, T. C. et al. Spatial organization of peptide nanotubes for electrochemical devicesJournal of Materials Science. v. 45, n. 18, p. 5.101-08. 2010.
MATOS, I. O. et al. Approaches for multicopper oxidases in the design of electrochemical sensors for analytical applications. Electrochimica Acta. v. 55, n. 18, p. 5.223-29. 2010.