by Viktor AKSENOV, Dr. Sc. (Phys. & Math.), supervisor of studies at the Laboratory of Neutron Physics named after I. Frank, Joint Institute for Nuclear Research (Dubna, Moscow Region), Head of the Neutron Diffraction Chair, Lomonosov Moscow State University

Man was always attracted by the idea to explore the environment beyond the known. Scientists invented a microscope to observe microprocesses in a substance, but it was not enough to study the behavior of small particles such as molecules and atoms. Thus appeared other tools, for example, invisible neutron radiation that makes it possible to get much more useful information. In addition, it is essential for producing nanosize structures.

WHAT ARE NEUTRONS FOR?

Nanomaterials may be produced given two conditions: first of all, spatial limitation (confinement), which is explained by the fact that sizes of their components depend on the length of typical physical values (for example, free run of electrons). As a rule, it has an interval from a few to some hundred nanometers. This forms a common characteristic of all nanomaterials—a significant role of the surface limiting structural elements.

In the second place, self-organization, or self-assembly, resulting in "upwards" formation of structures from atoms and molecules (this is the main difference between natural technologies and those applied until recently by man). In the course of self-organization, there appear structures that change in the course of time, which gives rise to one of the main problems—stabilization of nanomaterials with strictly defined parameters.

Due to neutrons we can control the process of formation of these conditions, i.e. carry out nanodiagnostics and study the phenomena, that take place in small-scale objects. In some cases neutrons are absolutely indispensable, which is conditioned by special relations between these particles and the substance.

As they are neutral, neutrons interact with atomic nuclei, not with electron shells. Besides, scattering length of neutrons on isotopes of one and the same element may significantly vary, which makes it possible to "see" light nuclei against heavy ones, effectively apply the isotope contrast method. It is especially evident in the systems containing hydrogen—polymers, biological objects, organic and aqueous solutions.

Magnetic moment of the neutron allows to study magnetic structures and identify their microstructure. Interacting with the substance, neutrons do not destruct

even delicate biological structures and penetrate deep into the sample. In such conditions scientists can use auxiliary devices—high-pressure chambers, furnaces, complex cryostats and electromagnets.

Scientists actively use all types of scattering in neutron diagnostics: diffraction, small-angle scattering (implying only an insignificant deflection from the direction of incident beam), reflectometry (measuring of neutron beam parameters at sliding angle of incidence), as well as non-elastic scattering.

Neutron reflectometry deserves special attention: it was actively developed in the early 1980s after creation of the technology of production of layered nanostructures. In the last decade, in addition to a specular reflection technology that provides data on the indepth structure of the sample, experimenters apply methods of non-specular (diffused) scattering, diffraction and small-angle scattering near the sliding angle to collect data on the structure, properties of the sample and in a plane with two coordinates. As a result, neutron reflectometry enables to perform complex studies of small-sized systems at the nanolevel, including magnetic multilayer films, banded structures, quantum dots, polymers with inclusions of magnetic nanoparticles, multilamellar vesicles ("multilayer bubbles"), and magnetic liquids.

Let's consider some cases of application of neutron technologies for diagnostic and research purposes, we have developed on the IBR-2 research pulsed nuclear reactor at the Joint Institute for Nuclear Research*.

CARBON NANOMATERIALS

Many branches of science and technology, including biomedicine, have revealed interest in colloidal solutions (dispersions). The point is that carbon nanoparticles, in particular, fullerenes** and their clusters offer ample opportunities for functionalization—their attachment to the surface of biomacromolecules. Discovery of fulle-renes in 1985, one of three basic forms of plain carbon (other two are graphite and diamond), was one of the most memorable events of late 20th century. Right from the beginning the branch of science related to these chemically stable closed surface structures had an interdisciplinary nature: these structures were first predicted in quantum-chemical calculations, discovered in the course of studies of cosmic dust, synthesized for the first time in 1985 by an American Harold Kroto (Nobel Prize in 1996) via physical methods (evaporation of graphite in cross laser beams). In 1990 physicians Wolfgang Kretsch-mer (Germany) and Donald Huffman (USA) synthesized fullerene systems in macroscopic numbers and later many laboratories all over the world commenced studies of their unconventional physical and chemical properties. These scientific studies coincided in time with recognition of a specific value of nanomaterials and nanotechnologies, and that is why the fullerene is often used as their "symbol".

Dubna specialists began system studies of fullerenes soon after their discovery in conjunction with the Institute of Therapy, Ukrainian Academy of Medical Sciences, where an original method of their solution in water was developed. In addition to specific issues associated with this problem, scientists focused on the cluster state of the substance-one of the key problems of modern chemistry and physics. Fundamentals of this problem were developed together with our counterparts from the Laboratory of Theoretical Physics named after N. Bogolyubov***.

Creation of stable water solutions of carbon nanomaterials promising from medical and biological point of view (they have antiviral characteristics, have no cancerogenic and mutagenic properties, do not suppress cell respiration processes and do not affect the blood-dotting sequence) is an ambitious task. We are tackling it through a small-angle neutron scattering and measure many

* See: V. Aksenov, "Pulsed Nuclear Reactor", Science in Russia, No. 6, 2002.-Ed.

** See: "Fullerenes", Science in Russia, No. 6, 2000.-Ed.

*** See: D. Shirkov, "Bogolyubov Lessons", Science in Russia, No. 4, 2009.-Еd.

Evolution of the cluster structure of nanodiamond dispersions: initial powder with 40 nm clusters with a fractal dimension of about 2.5 (I); dispersed ultrananociystalline diamond (II); the same in the solution of low (~ 1%) (III) and high concentrations of diamond nanocrystals (~ 10%) with a fractal dimension of about 2.3 (IV); ultrananociystalline diamond after evaporation (V).

Schematic diagram of the structure of magnetic liquids.

important physical parameters in liquid dispersions of fullerenes and other nanocarbon particles. Resolution of this method given modern neutron sources is about 1 nm, accuracy of measurement is less than 0.1 nm.

This method has more advantages as compared with the X-ray scattering. The latter is practically insensitive to the atoms of hydrogen (H) and its isotope deuterium (D) in the structure of the substance, while their nuclei act as strong scatterers of neutron radiation. It means that we can more accurately position H and its thermal vibrations in the crystal structure. Moreover, lengths of neutron scattering of hydrogen and deuterium have opposite characters, which allows to apply a "contrast variation" method, i.e. changing isotope composition of the sample (varying the quantity of H and D), the researcher is able to change the level of scattering of different components of the studied sample.

MAGNETIC LIQUIDS

In the physics of magnetic phenomena, all substances are divided into weak and strong magnetic substances, depending on their magnetic susceptibility. Liquid strong magnetic substances do not occur in nature, but their artificial analogues have been studied and used for more than 40 years. These systems (also called ferrofluids), representing an artificially synthesized colloidal suspension of magnetic nanoparticles about 10 nm in size, surface-active substance preventing their sticking together, and liquid media are used in different technical devices. They were applied for the first time in the USA in the 1960s to replace bearings in space helmets. These materials turned out to be promising in biomedicine (local delivery of drugs to the organism, diagnostics, tumor therapy). To control stabilization mechanisms of ferrofluids, it is necessary to understand their structure, magnetic properties and how they change in different conditions, which in its turn means a possibility of practical application of ferrofluids to solve specific tasks.

Diagnostics of such structures is also complicated by the fact that nanoparticles making part of these structures are highly polydisperse, multicomponent and actively interact in the magnetic field, which makes it difficult to apply conventional methods and provokes their further development. Neutron radiation, in particular small-angle scattering, is another thing altogether. It allows to analyze concurrently atomic and magnetic structure of ferrofluids in a wide spectrum of sizes (1-100 nm) via iso-topic substitution of hydrogen with deuterium and scattering of polarized neutrons. By the way, the X-ray radiation has no such characteristics due to poor interaction with hydrogen and low contrast of components in organic molecules of surface-active substances used to stabilize

Reflectometry with the use of polarized neutrons: geometry of the experiment-in the top left part, map of scattering intensity of neutron with set polarization-in the top right part; below-peculiarities of magnetic structure of the sample.

Distribution of magnetic nanoparticles Fe₃O₄ (red circles) in terms of changing concentration in the laminated films on the basis of copolymers-polyestherine-block-polymethacrylate (deuterated) after annealing at a temperature of 160°C in the course of 3 hours: nanoparticles are put in order in the layers of one of the copolymers-polyestherine (marked in green).

such systems. The same is true of electron microscopy and magnetometry—a macroscopic method sensitive only to the magnetic component of ferrofluids.

Small-angle scattering for diagnostic purposes is also very effective as it allows to study volumetric samples without any essential modification of such samples. When applying technologies of electron microscopy and magnetometry, we always take into account an impact on the microstructure of the sample of interaction with the measuring cell. In case of studies of ferrofluids by neutron methods, this problem practically does not exist.

LAYERED MAGNETIC NANOSTRUCTURES

Layered magnetic nanostructures belong to small-sized systems and are characterized by unique properties: exchange interaction of different types between magnetic layers, strongly developed fluctuations, proximity effects, etc. These structures have become well known thanks to iron-chrome "ligament": at the end of 1987, German physicist Peter Grünberg together with his colleague from France Albert Fert discovered an effect of giant magnetoresistance in layered nanostructures (in 2007 both scientists were awarded Nobel Prizes). These systems are often used as elements of magnetic memory. Moreover, the scientists believe that they will form the basis of nanoelectronics of the future.

Reflectometry of polarized neutrons gave an answer to the problem of interrelation between the discovered effect and magnetic domains. As it turned out, they do not affect electric properties. Scientists also managed to bring out a new arrangement of magnetic domains, appearing in case of even number of iron and chrome layers.

Layered heterosystems of ferromagnetic material-supperconductor are of high interest lately as they are very promising in terms of spinotronics*. For example, creation of similar structures such as iron-niobium and iron-vanadium made it possible to solve the problem put in the 1950s by a prominent Soviet physicist, Nobel Prizewinner of 2003, Academician Vitaly Ginzburg, and the famous American specialist Philip Anderson, dealing with the influence of superconductivity on the magnetic

* Spinotronics (from English "spin electronics")-a branch of science, which studies interaction between own magnetic moments of electrons (spins) and electromagnetic fields, and developes magneto-electronic devices and mechanisms on the basis of discovered phenomena and effects.-Ed.

Self-assembly of a lipid membrane in the mixture of dimyristoilphosphatidilcholine-sodium cholate in terms of thermal changes from 20 to 40°C at a rate of 0.2°C/min.

state of ferromagnet resulting from proximity effects. Scientists who tried to solve it by methods of nuclear magnetic resonance and Mossbauer spectroscopy* on synchrotron radiation sources have found out: the influence exists but questions concerning its mechanism and magnetic structure of a new state, called cryptoferro-magnetic, are still open. We used the method of reflec-tometry of polarized neutrons and experimentally have proved: it has a cluster structure of nanometer dimensions. At the present moment, we are studying their parameters and nature of origin. For this purpose we developed a method of layer by layer neutron magne-tometry using reinforced standing neutron waves. We are likely to achieve a record spatial resolution of the magnetic structure of about 0.1 nm.

POLYMEROUS NANOCOMPOSITES

These materials in the form of a thin layer structure polymerous film have become more and more popular in bioengineering, electronics and other industries. The "pep" of these materials are nanoparticles with different properties randomly built in the polymerous matrix in the course of preparation. We can produce new functional materials with different nanoproperties depending on the conditions of this process.

We studied polymerous magnetic nanocomposites. They are produced through layer by layer mixing of components by rotation. Base matrix of lamellar structure is produced through annealing. Magnetic nanoparticles put into a double block-copolymer form nanoplates, the size of which depends on the concentration of added admixture. Our task is to explore resistance of this composite film. When we determine conditions allowing to stabilize new material, we'll proceed to studies of its magnetic characteristics.

Experiments conducted at our Laboratory have shown: magnetic nanoparticles accumulate in layers of one polymer and thus avoid interaction with each other. It is a newly observed phenomenon. As early as in 1907, the English chemist Spencer Pickering discovered: mixtures are stabilized with nanoparticles located at junctions of components (Pickering stabilization). In our case they accumulate in nanoplates inside the layers of copolymerous multilayer film.

Admixtures trigger a number of structural changes: at the expense of the growth of each bilayer, there increases thickness of the composite, increases roughness factor, which means weakening of stabilization of the material. It is also proved by the behavior of correlation length (domain size in plane). Its shortening points to changes of junction parameters between the layers and decrease of elasticity between two polymers. These results obtained through methods of neutron nanodiagnostics are of high relevance for production engineers.

NANOTECHNOLOGIES AND LIFE

The 21st century is considered a heyday for life sciences, but physicists should pay special attention to them not only for this reason. Strictly speaking, nanotech-nologies are neither more nor less than use of life tech-

* Mossbauer spectroscopy—a method of nuclear gamma-ray resonance based on the effect of Mossbauer (after the name of its discoverer-German physicist Rudolf Mossbauer),-resonance absorption without output of monochromatic y-radiation, emitted by a radioactive source, by the atomic nucleus.-Ed.

nologies in the scientific and technical field. Scientists are mastering them through wide use of physical methods in biology, medicine and pharmacology. Here we shall discuss one of the problems of molecular biology related to studies of the cell structure of living organisms.

The cell is a principal structural and functional unit of life with its own metabolic activity, can independently exist, reproduce and develop, as well as keep, process and use genetic data. Its average size is in a range from 100 nm to 5 mm, but a common size of prokaryotic cells is about 1-10 µm and about 10-15 µm for eukaryotic cells. These live forms comprise also a multitude of structural units of smaller size called organelles. They have some specific functions, for example, produce energy and put the cell in motion. Since the 1970s, specialists of our Laboratory together with the RAS Institute of Protein (town of Pushchino, Moscow Region) on the initiative of Academician Alexander Spirin and Professor Igor Serdyuk have been studying one of such intracellular structures, namely, ribosome, the main function of which is to construct protein molecules of amino acids delivered by transport DNA. We managed to identify the functional model of this biological "molecular machine" of about 20 nm in size. Spirin is one of the founders of ribosomology, nowadays actively developing thanks to financial support and hundreds of research teams, working in this field of knowledge. In 2009, Venkatraman Ramakrishnan (Great Britain), Thomas Steitz (USA) and Ada Yonath (Israel) were awarded Nobel Prize for the studies of the structure and functions of ribosomes.

On the initiative of the Institute of Physico-Chemical Biology named after A. Belozersky, Lomonosov Moscow State University, we also carried out studies of one more organelle—mitochondrion—which participates in the processes of breathing and transforms released energy into the form suitable for use by other cell structures. Results obtained by using neutron methods, made it possible to understand operating principles of this "power station".

Organelles are "swimming" in the cytoplasm, and the cell itself is limited by a lipid-protein cytolemma—cell or plasmatic membrane. Smaller structures, mitochondria for example, also have such membrane. But the structure of membranes due to its diversity, complexity and a mul-ticomponent nature, is not still studied to the end.

Specialists of our Laboratory got interested in this problem. As you know, cells of the epidermis (Corneo-cytes) together with ceramides, fatty acids, cholesterol and its derivatives form the matrix. The matrix determines dermal absorption of water and acts as the main obstacle to transportation of medicines. That is why studies of the ceramide and cholesterol effect on nanos-tructures of lipid membranes is of high scientific and applied value. At present such works are carried out in almost all big scientific centers, in the leading pharmaceutical and cosmetic companies. Among industry leaders are L'Oreal, Christain Dior, Lancôme, etc.-they use phospholipids and ceramides as the main components of their products, as well as monolayer and multilayer vesicles (liposomes).

The structure of a lipid membrane is a liquid crystal nanoobject, which theoretically can be studied by the X-ray sounding method. But scientists could not diagnose the nanostructure of monolayer vesicles using standard methods until recently. We succeeded in this field only a while ago using the method of separate form factors. It turned out that aggregation of form factors in clusters is a negative factor that affects effectiveness of vesicular carriers of drugs. We studied physico-chemical fundamentals of the process through the method of small-angle scattering of neutrons using concurrently isotopic contrast in deuteration. The obtained results are widely used by nanobioproduct engineers.

LEADERS FOREVER

In 1984, the Joint Institute for Nuclear Research got the most high-flux pulsed neutron source in the world-IBR-2 reactor. In 2006, 22 years later, it completely exhausted its resource. In 1996, the Institute launched a program of its modernization* in order to replace the basic equipment, increase its safety and operational reliability, improve basic characteristics. Upon completion of these works, scientists will get a new modernized reactor IBR-2M with a neutron flux in a pulse up to 2 · 10¹⁶n/cm²/s. This will allow us to be the world leader in the field of neutron sources for 20-25 years. In November 2008, engineers began to assemble the reactor; first experiments are planned for the end of 2010.

Scientists are planning traditional research works in physics and chemistry of condensed media (solid bodies and liquids), polymers, molecular biology, colloidal chemistry, materials science, geophysics and results to be used in biomedicine, pharmacology, engineering, technologies of creation of new materials and devices. To a considerable extent they relate to nanosciences and may be used in nanotechnologies.

The IBR-2M reactor has another important function to train young specialists, carried out at the Chair of Neutronography, Lomonosov Moscow State University, Educational and Scientific Center of the Joint Institute for Nuclear Research. The training program has an interdisciplinary  character.   Thus,   the   reactor will become a solid base for implementation of present-day requirements to science and education.

* See: A. Sissakian, "Dubna's Worldwide Glory", Science in Russia, No. 2, 2006; A. Sissakian, "Frame Projects—Breakthrough to Future", Science in Russia, No. 6, 2008.—Ed.