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New materials

Molecular precision

Sensors on a nanometric scale detect and differentiate tastes, smells, and pollutants

eduardo cesar Adjustments for producing and controlling nanofilms with thicknesses of between 1 and 2 nanometerseduardo cesar

A new generation of ultrasensitive sensors, capable of detecting nuances in the composition of liquids and gases should invade the market in the near future, in the wake of the advances of nanoscience and nanotechnology. These are areas that investigate the properties of materials on the scale of 1 to tens of nanometers, equivalent to the atomic or molecular level (1 nanometer corresponds to 1 millimeter divided by 1 million). At the heart of these devices are extremely thin films, called nanostructured films, with just a few molecules of protein, for example.

They are products that are the focus of study of a group of researchers from the São Carlos Physics Institute of the University of São Paulo (IFSC-USP). “There are ample prospects for the use of these devices, ranging from analyzing tastes , gas, and liquids, to electroluminescent devices like computer and television screens, optical memories, holographic materials and nanoreactors, miniature equipment ideal for chemical reactions in highly controlled environments, with a few molecules, and which can be used, for example, in the production of cell phone batteries”, explains physicist Osvaldo Novais de Oliveira Júnior, the coordinator of the Polymers Group at IFSC, who has several projects in this area, many in cooperation with institutions from Brazil and from abroad.

The field for sensors fitted with these nanometric films is extensive, but basically the way they work involves immobilizing a given protein (on a solid material, without the losses of its properties) used for detecting substances that react to it specifically. The solid material in this case is a polymer called dendrimer, which has globular structures and pores which encapsulate the proteins with any loss of their activities.

With this technique, one of the researches of the group, in association with USP’s Chemistry Institute, led to the production of biosensors for detecting glucose in the blood – the study was accepted for publication in the Biosensors and Bioelectronics magazine, from the Dutch publishing house Elsevier. Another innovation from the team, carried out in conjunction with the Biophysics Group from the Physics Institute, may serve to be used in controlling pollutants. It is a sensor based on the Cl-catechol 1,2 dioxygenase, which can interact specifically with catechol, an organochloride substance frequently associated with insecticides, present in polluted waters.

The group has also made headway in the manufacture of a sensor for detecting paraoxon, a toxic substance that can be used in chemical weapons. “The relevance of this research lies in the high sensitivity of the sensor, which was only possible thanks to the successful immobilization of an enzyme called organophosphate hydrolase. The work enjoyed the participation of researchers from the University of Miami and from the Chemistry Department of the Federal University of São Carlos (UFSCar). An article about this study was published in February last year in the Journal of the American Chemical Society, one of the most important international chemistry magazines.

The sensors developed at São Carlos also work in another way, without the cellular recognition between the molecules of the film and of the substances to be detected. In this manner, the sensor works on the basis of a change in the properties of the film, following a physical interaction with the substance analyzed, and not on the specific interaction between given molecules. The properties that change with the interaction may be optical or electrical. This is the case of the electronic tongue, one of the most notable technological innovations to have come up from these studies. In this equipment, the molecules of the substance to be detected do not necessarily need to react with the molecules of the film.

All that is needed is to alter the electrical properties of the surface of the sensor, which is extremely sensitive, due to the ultrathin nature of the film. The electronic tongue is a sensor with a palate built with a nanostructured film with just one layer of polymeric molecules (see Pesquisa FAPESP Nos. 3 and 90). The equipment performs a function similar to the taste buds, but with a far greater degree of sensitivity than the human tongue. Produced by the Agricultural Instrumentation unit of the Brazilian Agricultural Research Corporation (Embrapa), in São Carlos, the invention received collaboration from the USP’s Polymers Group and is being tested at the Brazilian Coffee Industry Association (Abic) to differentiate flavors of this beverage.

If the electronic tongue is now in existence, the researchers are now planning nanofilms for a future electronic nose, capable of detecting and differentiating smells.Produced from organic materials, nanostructured films are not self sustainable, which means that they cannot be handled. This is why they are deposited on a solid substrate, like a polymer, or a glass, metal, or semiconductor slide. The thickness of these films is highly controlled and depends on the number of molecular layers that make it up, besides the size of each molecule – in general, they measure from 1 to 2 nanometers in thickness.

According to Oliveira Junior, the focus of his team is carrying out the development of these nanofilms, always with a concern for possible applications and technology transfers. “We have already made contact with companies interested in applying the methods of nanoscience and nanotechnology for perfecting the production of materials, but up until now we have not signed any agreement”, says the researcher. The quest for innovation, though, has already yielded good results. The group has a patent request under way on optical data storage, in which a nanometric film can be appliedto a credit card, for example, making robbery and fraud difficult.

In the data storage industry, the researchers from IFSC want to go further. They are committed to the development of a polymer capable of storing digital data in three dimensions. The work is being done with the cooperation of the Photonics Group of USP’s São Carlos Physics Institute. The researchers’ idea is to develop a block that receives data in different layers, expanding storage capacity. “This is the great dream of the scientists. The main advantage of this technology over what exists today is that it increases the capacity of the memory”, says Oliveira Júnior. “Today, this storage is done in just two dimensions. We now know how to produce this polymer, and we are ready to submit an article to an international magazine.”

In spite of this breadth of application, nanostructured films are still not employed on a large scale. “The films are very expensive, and, for the time being, there is no system of industrial production at an accessible cost.” In part, this problem is due to the large number of raw materials used in the production of these films. Oliveira Júnior’s group is working mainly with electronic and luminescent conducting polymers and photoreactive molecules like azo-polymers (substances synthesized from benzene, from aniline, and from other compounds derived from petroleum). It is also worth pointing out the importance of the group’s studies with chitosan, a substance extracted from the shells of crustaceans, like shrimps and crabs. These molecules have a high fungicidal and bactericidal power, and a great capacity for binding to metals and other substances, which potentializes their use as a sensor.

Techniques for manufacturing
There are two main techniques for producing nanofilms: the Langmuir-Blodgett technique, known simply as LB, and self-assembly, of layer-by-layer (LBL). The former was developed in the 1930’s, and it bears this name in honor of two American scientists who worked at General Electric, in the United States, at the beginning of the beginning of the 20th century, Irving Langmuir and Katharine Blodgett. The LB films are produced inside an appropriate container, called a Langmuir Trough. At the beginning of the process, the material that is to originate the film is dissolved in a volatile solvent, like chloroform, and spread in the Langmuir trough, which contains water.

After the solvent evaporates, movable barriers are used to form, on the surface of the water, a molecular layer called Langmuir film. It is compressed until reaching a condensed state. Next, the film is transferred to a solid substrate, forming the Langmuir-Blodgett film.The self-assembly (layer-by-layer) technique, proposed by German researcher Gero Decher at the beginning of the last decade, uses the principle of physical adsorption (sticking), in which the molecules stick to the substrate by the attraction of opposite electrical charges.

The main difference between the LB films and the self-assembled ones is that the latter are produced from materials that are soluble in water, while the LB films are made with materials that are insoluble in water. “The advantage of the self-assembly technique over LB is its experimental simplicity, since it does not require any sophisticated equipment to produce the films”, the physicist from USP explains.

Network of collaboration
The researcher makes a point of stressing that the advances obtained by his team are due, in great measure, to cooperative work with other institutions. “Our researches are carried out within a network of collaboration in Brazil and abroad, which makes it possible for us to take advantage of the experience of other researchers, besides using several experimental techniques for making and studying nanofilms”, he says. Amongst the institutions that are working with the Polymers Group, the chief ones are the São Paulo State University (Unesp), Unicamp, Coppe (UFRJ), the Federal University of Uberlândia, and the State University of Ponta Grossa, amongst others. On the international plane, the researches enjoy the support of the universities of Leipzig, in Germany, of Bangor, in the United Kingdom, of Krakow, in Poland, of Windsor, in Canada, New Lisbon, in Portugal, and of Miami and of Massachusetts, in the United States.

According to Oliveira Júnior, FAPESP financed a good part of the structure for manufacturing the films, including a clean room (an environment with a minimal quantity of unwanted particles in the air), of the laboratory where they are processed. “My estimate is that since 1991, FAPESP has invested around US$ 500,000 in our group, specifically for nanostructured films”, says the physicist. The support has been rewarded with the training of specialized professionals and with a vast scientific production. The Polymers Group has already formed over 20 researchers in nanofilms, and, in the last four years, about a hundred articles have been produced for publication in indexed international magazines.

The projects
1. Ultrathin Langmuir-Blodgett and Self-Assembled Films (nº 98/10026-5); Modality Regular Line of Research Grants; Coordinator Osvaldo Novais de Oliveira Júnior – IFSC-USP; Investment R$ 49,310.00 and US$ 56,050.00
2. Langmuir-Blodgett and Self–Assembled Films (nº 01/03734-8); Modality Regular Line of Research Grants; Coordinator Osvaldo Novais de Oliveira Júnior – IFSC-USP; Investment R$ 82,621.70 and US$ 70,822.75