In scientific research, the search for one result often leads, fortuitously, to a different, even more important result. This is what happened to physicist José Antônio Souza when advising master’s student Cynthia Gómez, both at the Federal University of the ABC (UFABC). She developed coaxial microcables that can be used in telecommunications in order to understand how electric current flows through them. That was when she observed the formation of unexpected structures, namely microtubes covered by nanowires with a diameter equivalent to 1 millimeter divided by one million. The study led Souza to envision future technological applications for the discovery, from its use in nanoelectronics and microelectronics to drug delivery systems in which microtubes carry drugs to the location in the body where they should act. In order to ensure their rights to possible financial exploitation of the discovery, the pair filed for a patent related to the new material with the Brazilian Industrial Property Institute (INPI). Once protected, the results were published in the journal Applied Physics Letters, together with professors Jeroen Schoenmaker, Alejandro Zúñiga and Denise Criado, all at UFABC.
The coaxial microcables with which they worked consisted of metallic zinc (Zn) wires with a diameter between 30 and 120 micrometers, covered with a nanometric layer of zinc oxide (ZnO). “Our objective was to study the electrical resistance—or its inverse, conductivity—of these microwires, with increases in temperature and the presence of magnetic fields, and infer the formation of nanostructures on their surface,” says Souza. “The intention was to manufacture magnetic coaxial cables, because it is believed that all nanostructured materials exhibit magnetic properties.”
For the study, Souza and Gómez heated the metal microwires made of zinc in a special chamber with atmosphere and temperature control. Next, the nanowires grew on the microlayer of zinc oxide via a physical mechanism involving the diffusion of ions (electrically charged atoms) from the metal. In the next step, they passed an electric current through the coaxial microcables. What happened surprised the researchers. “The electricity had a colossal effect on the structure,” recounts the physicist from UFABC. “When the temperature reached 600°C, the metallic zinc nucleus evaporated completely, leaving only a hollow microwire, covered with nanowires.
The temperature at which this process occurred also surprised the researchers. Normally, zinc evaporates at 1,000°C. “The electric current greatly increased the vapor pressure, leading to total evaporation of the metal,” explains Souza. “Our discovery opens the way to obtaining microtubes of other materials such as tin and aluminum, for example.” Structures like that discovered by the UFABC researchers are called hierarchical, and are composed of two or more different shapes, such as wires and tubes. Sometimes they also have different sizes, such as in the case of the experiment by Souza and Gómez.
The technological possibilities of the discovery, according to the researcher, lie in the fact that one can combine the applications of nanowires with those of microtubes in a single device because they can be manufactured together in the same structure. Nanowires, alone, can be used in sensors and to build electron microscopes and in computer circuits smaller that current ones. Microtubes, in turn, are used to transport nanofluids, such as ferrofluids, that contain magnetic nanoparticles. “A combination of these features is very important in the field of microelectronics,” says Souza. “In addition, one can imagine, for example, a microtube filled with a drug and nanowires of a biocompatible material, which would allow development of devices for drug delivery.”
Physicist Fábio Coral Fonseca, of the Nuclear and Energy Research Institute (IPEN) in São Paulo, who studies materials for energy conversion and magnetism, mentions other possible applications of the discovery by the UFABC researchers. “These structures can be exploited in lab-on-a-chip devices [a sort of miniature laboratory that enables testing and biological analysis on a chip] if they demonstrate the necessary properties,” he says. “Another application that can be imagined is in catalysis [acceleration of chemical reactions] if, for example, nanowires can be obtained from metals or alloys with good catalytic properties.”
Fonseca believes the UFABC work is important because of these technological application possibilities. “I think that the processing and phenomena involved in manufacturing these structures are important,” he says. “It is interesting to note the simplicity of the method and the use of electric current in the process, which seems to play an important role in obtaining the structures.” He recalls, however, that discovering how to combine these more specific properties and applications of nanowires and microtubes still depends on future research.
Synthesis and characterization of physical properties of nanosized materials (nº 2013/16172-5); Grant Mechanism: Regular Research Grant; Principal Investigator: José Antônio Souza (UFABC); Investment: R$75,905.92 and $59,151.64 (FAPESP).
Gómez, C. M. R. et al. Microtubes decorated with nanowires. Applied Physics Letters. V. 106, May 2015 (on-line).