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2 / Nanos on all fronts

Tomorrow, nanotechnology will certainly be part of our daily lives: a number of fields are already involved such as medicine, the environment, electronics, and optics. Here is a review of developments now underway.

 

HEALTH

“How you give is just as important as what you give,” claims Patrick Couvreur, director of the Physical Chemistry, Pharmaceutical Technology and Biopharmacy laboratory,1 which specializes in drug vectorization (the manner of administering drugs) and in designing nanocapsules and nanospheres that can bring an active molecule right up to its target. “When the medication is ingested orally,” Couvreur continues, “the active substance must cross the intestinal epithelial barrier in order to be absorbed. The medication then loses its effectiveness and can trigger side effects by affecting other tissues.” This is the case with anticancer medication or anti-AIDS therapy, for example. Ideally, we should be able to bring the active molecule to the core of its site of action. For several years, Couvreur's laboratory has been developing different vectors, all of nanometric size (70 times smaller than a red blood cell), that can be administered through the blood stream thus avoiding any thromboembolic phenomena. The vectors, such as liposomes or nanoparticles of biodegradable polymers, have the capacity to encapsulate and protect many small synthetic molecules or macromolecules (proteins and nucleic acids). Fatal hepatic diseases, such as liver cancer, are the first potential targets of nanomedications which, administered intravenously, necessarily travel through the liver. By adding plasma proteins on their surface, nanomedications could be specifically recognized by liver and spleen macrophages. Through the use of ligands specific to a particular cell type, the nanoparticles should be able to attach directly to the targeted cell or to “very precise cellular regions,” Couvreur explains.

 

ENVIRONMENT

Nanotechnologies could also play a role in the environment, especially in pollution cleanup. Jérôme Rose and Jean-Yves Bottero, CNRS researchers at Cerege,2 are designing water filtration membranes based on nanometric-size materials of oxy-aluminum hydroxide or iron oxide. These common and inexpensive materials, and the use of synthetic processes that avoid organic solvents and polymers, could be an ideal solution for certain southern countries where water is often unfit for consumption. “The porosity of these membranes can be controlled by the size of the nanoparticles, ranging from 10 to 500 nm,” explains Bottero. “They are then attached to substrates and processed to make them reactive so that they can oxidize organic molecules such as those from bacteria, viruses and pesticides. Furthermore, we have observed that these membranes are permeable to the protons present in water,” adds Bottero. The protons could then be recovered, thus opening up the possibility of making small fuel cells that could eventually provide enough electricity for the needs of a household.

Another major application for the environment is catalysis. This chemical phenomenon is used in automobile mufflers to transform highly polluting molecules like carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons, into “better” molecules, like carbon dioxide (CO2), molecular nitrogen (N2), and water vapor. To improve the performance of these catalysts, “pollutant molecules must be brought onto a reactive surface that transforms them by creating new molecular bonds,” explains Claude Henry, a physical chemist at the CNRS Center for Condensed Matter and Nanosciences Research (CRMCN)3 in Marseille. “For example, on such a reactive surface, a CO molecule can attach itself onto an oxygen atom, produced by breaking the oxygen molecules in air, and thus turn into CO2.” Today, the catalysts are noble and rare metals such as platinum and palladium. But their availability is shrinking and prices are skyrocketing. Furthermore, the current efficiency of mufflers is far from optimal. “Industrial companies know how to prepare particles of platinum or palladium in nanometer sizes to increase the reactivity of catalysts,” Henry continues. “But nanometer size is not sufficient to make them very efficient. That's why we're trying to come up with other solutions.”

To improve the reactivity of catalysts, the CRMCN researchers analyzed the size and shape of the nanoparticles, their density and spatial distribution on the substrate, the nature of the metals and the chemical composition of the alloys, the homogeneity of the substrate and its interactions with the catalysts. The objective was to find a combination of parameters that could lead to the best catalyst. “We have shown that palladium particles around 7 nm are the most active for eliminating nitrogen oxides,” Henry announces. “And that a hexagonal arrangement of atoms on the surface of the cluster is more efficient.” The researchers also showed that the flat surfaces of clusters are better than their sharp edges, as the latter bind strongly to undesirable molecules that block the catalytic reaction. Another problem must be resolved: Finding a replacement for metals that become too rare. “Gold is a good candidate, because it has very good catalytic properties below 5 nm, even at room temperature (unlike the metals currently used) and is also fairly common,” Henry offers. The inconvenience is that it is soft and not very resistant mechanically. “So we are thinking about associating it with another metal and creating an alloy to make it more mechanically resistant.”

 

ELECtrONICS

Get ready for it: The “nano” wave could entirely change the face of electronics. “In fact, we could completely rethink the architecture of circuits,” says Jean-Michel Lourtioz, director of the Institute for Fundamental Electronics (IEF)4 in Orsay. “One idea is to reproduce three-dimensional circuitry modeled on the brain, in which each neuron is connected to several thousands of its neighbors.” This would lead to considerable gains in speed and moreover would also generate new functions. “We may also think of circuits capable of self-arrangement, self-connection to each other and self-configuration to adapt to a given task,” adds Daniel Bouchier, senior researcher at IEF. Several projects launched in CNRS laboratories lay claim to the title of “the most promising” nano solution. One of these is the MOS transistor, a sort of silicon channel controlled by a gate that modulates the electric current passing through the channel.

This component, emblematic of modern electronics, is the building block of digital processors and RAM memories, for example. Several avenues for miniaturizing this transistor are currently under exploration. In one of these, researchers expect to use nanocrystals in which each bit of information (zero or one) would be reduced to a single electron that would control the transistor's channel. For this to happen, the technological limit, estimated today at 22 nm for the gate length, should be further lowered. Various ways of lowering this technological limit are currently being investigated, especially within the framework of the European NanoCMOS project.5

Other favorites include nanowires and nanotubes. “Using these new nano-objects, we may rethink all semiconductor electronics,” states Didier Stievenard, deputy director of IEMN6 in Lille, who fabricates and characterizes certain types of nanowires. These nano-objects may, in fact, open up a vital path for the successors of diodes and transistors. IEMN also works on one of the most promising areas of the bottom-up approach: molecular electronics. Starting from a molecule, selected for its specific electronic properties, “we take advantage of its capacities of self-organization and self-assembly to fabricate electronic systems,” details Dominique Vuillaume, CNRS senior researcher at IEMN. “Finally, we build components made of one or a few molecules capable of processing information, grafted onto a silicon substrate.” Take for example a memory register in which the values of “zero” and “one” are represented by two distinct forms of the same molecule: “The passage from one state to the other is done by shining light on the molecule,” says Vuillaume. “Furthermore, this process is almost infinitely reversible.” Another possibility is the use of molecules capable of storing several electrons each, which would amount to encoding several bits at the same time. This could form the basis of a new form of computer logic that could replace the present binary system. “But we need to be cautious in this field, as everything must still be demonstrated,” warns Vuillaume. So, could molecular electronics be the technology of tomorrow, offering a ten-fold increase in integration density and low-cost fabrication processes? “Generally speaking, in electronics, the future belongs to hybrid systems mixing different technologies,” responds Vuillaume.

This “mixing” could be useful in overcoming certain obstacles, such as the problems associated with a memory register, the storage element that can take the values of “zero” or “one.” “Stored charges always end up disappearing: that's why a small electric current is sent regularly to refresh the memory,” explains Bouchier. “But smaller and denser systems require more energy.” One possible solution is to use not only the charge of electrons to store information, but also their spin, basically the fact that electrons are actually tiny magnets. This approach has given rise to a whole new field called “spin electronics” or “spintronics,” pioneered by Albert Fert, director of the CNRS/Thales joint laboratory, who was awarded the 2003 CNRS Gold Medal for initiating this new field. One important advantage of using the spin is that the orientation of the magnetization of a nanoelement can be kept much longer than the electric charge in classic memory. And we already take advantage of this to store information on our hard drives.

This approach could give rise to ultra rapid MRAM, magnetic memories that are already marketed for space applications and are likely to have a great career in computers. This link between magnetic and electronic recording started in 1988 with the emergence of spin valves. “Thanks to these valves, used for read heads in hard disk drives, the density of recording has gone from 0.15 to 80 Gb/cm2 in just a few years,” confirms Claude Chappert, a senior researcher at IEF. The information stored is neither volatile nor sensitive to radiation, which is important for aeronautic and space applications. In parallel, spin electronics is now at the forefront of a potential breakthrough that could make it possible, for example, to write magnetic information without using an external magnetic field, but rather through a process called “spin moment transfer.” In this process, when an electric current passes through the nanodevice, the spin of its electrons will directly interact with the spins of the magnetic layers, thus orienting their magnetization... like a “domino effect,” in which the orientation of one spin causes the next one to re-orient and so on.

 

OPTICS

Another promising field of research is the use of photons to transmit information, with optical connections replacing electrical ones. “This would avoid the high dissipation of energy that occurs when we connect distant components in an electronic circuit,” explains Lourtioz. “An additional challenge is splitting an optical signal many times as it travels, without losing too much energy.” In 2003, researchers from IEF and CEA-Leti in Grenoble were among the first to be able to split an optical signal into 16 equivalent signals in an optical circuit. Three years later, they are still leading the field, trying to split a signal several thousand times through complex systems of mirrors, close to a nanometer in size.

Photonic crystals, also called “light cages,” are capable of “taking control of light, by blocking it from certain paths in order to favor a chosen direction,” states Henri Benisty, a researcher at the Charles Fabry Laboratory of the Institute of Optics (LCFIO).7 In fact, like systems designed to reduce noise disturbances, these structures block the propagation of specific wavelengths of light (i.e., specific colors), thanks to a three-dimensional periodically-repeated pattern of nanometric dimensions. And there is a lot of interest in these structures: “Light bulbs today return only 3 to 4% of the energy they consume as direct light, the rest being dispersed in all directions and especially dissipated as heat,” explains Claude Weisbuch, CNRS senior researcher at the Laboratory of Condensed Matter Physics in Palaiseau.8 Using optical micro-cavities based on periodic structures, his team succeeded in returning 28% of the energy as direct light, a world record. This dazzling number could even reach 70% in the next 10 to 20 years, when LEDs (light emitting diodes) begin to replace classic light bulbs. With 15 to 20% of all electricity today being used for lighting, energy savings will be enormous. The ultimate goal? “To make the smallest 'light cage' possible,” summarizes Benisty. Other applications focus on imaging: Nanosources of light open the way to experiments at the scale of a single molecule in living environments.9

 

Fabrice Impériali and Matthieu Ravaud

 

 

 

sang doigt

© N. Rowe/Guetty

A “lab on chip” makes it possible to quickly perform biological analysis on a drop of blood or saliva.


LOTUS-INSPIRED LABORATORIES ON A CHIP

 

Nanometric scales also have a place in the field of medical analysis. In the spotlight, the already famous “labs-on-chip,” which miniaturize to an extreme degree circuits for biological analysis. “It's a whole laboratory reduced to a chip,” specifies Christophe Ybert, a researcher at the Laboratory for Condensed Matter and Nanostructure Physics (LPMCN) in Lyon.1 “All you need to do is place a drop of saliva or blood on it, and the analysis is done immediately.” To improve the flow of fluids at the micron scale, the scientists were inspired by the lotus leaf, on which a drop of water keeps its form and cannot spread out. Instead, it slides off and carries with it all the dirt. The explanation for this phenomenon is that “the hydrophobic surface of the leaf is naturally carpeted with minuscule micrometer and nanometer-sized bumps,” explains Cécile Cottin-Bizonne, a researcher on the team. The drop slides on the surface of the leaf by surfing on the spike tops. The idea is to apply this sliding phenomenon to improve flow in the chip's micro-tubes. “In fact, the smaller the tube, the more difficult flow becomes, since the fluid cannot slide on the tube walls,” explains Cottin-Bizonne.

“To overcome the friction on the walls, very high pressures are required to force the flow.” With her colleague Ybert, she studied the properties of flow and the behavior of an extremely confined liquid. Taking inspiration from the structure of the lotus leaf, the researchers created tubes with nanostructured walls by using nanotubes. “This considerably reduced friction between the liquid and the wall,” indicates Ybert. This enormous advantage will help extend the “lab-on-chip” technique, already used by diabetics, to more complex analysis applicable to other illnesses.

 

F. I.

 

1.CNRS / University Lyon-I Joint lab.

 

Contacts:

> Christophe Ybert,

christophe.ybert@lpmcn.univ-lyon1.fr

> Cécile Cottin-Bizonne,

cecile.cottin-bizonne@lpmcn.univ-lyon1.fr

 

 

 

 

Glossary

 

>> Liposomes

Artificial biocompatible vesicles whose membrane is made up of one or more layers of lipids.

 

>> Ligands

Small molecules attaching to a receptor.

 

>> Fuel cells

Generators of electric current which continuously transform energy from a chemical reaction into electricity.

 

>> Nanocrystals

A crystal is a solid made up of atoms, ions or molecules, arranged in a periodic and regular structure. A nanocrystal has overall dimensions of the order of a few nanometers and is composed of only a few thousand atoms.

 

>> Spin

Magnetic moment of an elementary particle, such as an electron or a proton. The spin of an electron turns it into a tiny magnet that can point in one of two directions (e.g., North/South).

 

>> Spin valves

Devices made up of two ferromagnetic layers separated by a third layer. If the first two layers are magnetized in the same direction (spins oriented in the same direction), the current passes more easily. This makes it possible to code binary information.

Notes :

1. CNRS / Université Paris-XI joint lab.
2. European Center for Research and Teaching in Environmental Geosciences (CNRS / IRD / Université de Provence / Université Paul Cezanne joint lab).
3. CRMCN: Centre de Recherche en matière condensée et nanosciences (CNRS lab). View web site
4. IEF: Institut d'Electronique fondamentale (CNRS / Université Paris-XI joint lab).
View web site
5. Project coordinated by ST Microelectronics with the participation of CNRS. View web site
6. Institute for Electronics, Microelectronics and Nanotechnology (CNRS / Université Lille-I / Université de Valenciennes / Isen Recherche joint lab).
7. CNRS / Institute of Theoretical and Applied Optics at Orsay / Université Paris-XI joint lab.
8. CNRS / École polytechnique joint lab.
9. Work done by the team of Maxine Dahan at Kastler Brossel Laboratory (CNRS / ENS Paris / Université Paris-VI joint lab).



Contacts :

> Patrick Couvreur
patrick.couvreur@ccp.u-psud.fr

> Jean-Yves Bottero
bottero@cerege.fr

> Claude Henry
henry@crmcn.univ-mrs.fr

> Jean-Michel Lourtioz
jean-michel.lourtioz@ief.u-psud.fr

> Daniel Bouchier
daniel.bouchier@ief.u-psud.fr

> Didier Stievenard
didier.stievenard@isen.iemn.univ-lille1.fr

> Dominique Vuillaume
dominique.vuillaume@iemn.univ-lille1.fr

> Claude Chappert
claude.chappert@ief.u-psud.fr

> Henri Benisty
henri.benisty@iota.u-psud.fr

> Claude Weisbuch
claude.weisbuch@polytechnique.fr


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