Nanotechnology and nanoscience

The world within the nanoscale scale

Konstantin Andreev, Post-scienceHow interestingly the atoms are grouped…

A compliment from a tipsy physicist

following the passing beauty

There's plenty of room down there!From a speech by Nobel Prize winner Richard Feynman

at the California Institute of Technology in 1959 –
about the world of ultra-small sizes

Unfortunately, for some reason, of all the others, the term "nanotechnology/ and" has managed to become the target of so many and equally unscientific speculations over the past decade, which, in turn, naturally turned into an inexhaustible source of inspiration for humor - subtle and not too much. Therefore, everything that is twisted with a wrench less than 24 will not be discussed here. Nor will it go about fiction, in which nanotechnology leapt forward much earlier, outstripping reality by a little over a hundred years (here it usually comes to mind not even Leskov's Lefty, but rather the talents of the Danish royal tailors who sewed the "New Dress of the King" from Andresen).

But jokes are jokes, but in fact, this field of science disposes to a certain seriousness of attitude towards oneself. Moreover, in biomedicine, the design and use of synthetic objects of such small size to overcome biological barriers in the body and purposefully affect the affected tissue, cell or intracellular apparatus is one of today's priorities. And the design of such ultra–small objects that, on top of everything else, would be resistant to protective mechanisms (to the immune response, or to the effects of proteases) and recyclable (if they are toxic) with minimal side effect is already the actual cutting edge of research. And no matter how banal it may sound, in general, the future belongs to him. Which has already come in some places.

TerminologyPerhaps, in this context, it would be appropriate to separate "nanotechnology" from "nanoscience" (nanoscience), which for some reason, unlike English, is unusually ear-jarring in Russian.

In July 2004, experts from the Royal Society and the Royal Academy of Engineering, at the request of the UK government, prepared a report entitled "Nanoscience and nanotechnologies: opportunities and uncertainties", in which they tried at that time to assess the possible benefits of the development of nanotechnology, as well as the problems associated with them. In this report, as far as I know, separate definitions for both terms were formulated for the first time.

• Nanoscience is the study of phenomena and objects at the atomic, molecular and macromolecular levels, the characteristics of which differ significantly from the properties of their macroanalogs.
• Nanotechnology is the design, characterization, production and application of structures, devices and systems whose properties are determined by their shape and size at the nanometer level.

As can be seen from the definition, these two concepts have the same relation to each other as, say, engineering to theoretical physics. However, one common detail is the size of the object being studied/constructed.

The range of operated sizesA nanometer (from the Greek nanos – dwarf) is a measure of length equivalent to one billionth of a meter (10-9 m).

In principle, the prefix "nano" when denoting the dimensions of a real object is quite formal, because if we are dealing with a two-dimensional plane, then a square with a side of 1 nm will have an area of 1 square attometer (10-18 ^m2). And if you calculate the volume of a cube with an edge of 1 nm, you will get an even more indigestible figure (10-27 ^m3). Solely for convenience, it is preferable to operate with numbers with fewer zeros. For a visual representation of how much 1 nanometer is less than one meter, you can speculatively compare the diameter of the Earth and the diameter of a ping-pong ball. In the same coordinate system, a single atom will have a size from a dime to a quarter (0.1-0.5 nm), depending, of course, on the location of the element in the Periodic Table. The DNA helix (1.8-2.3 nm) will be as thick as a good ship rope, the erythrocyte (7,000 nm) will be as large as the Colosseum, and the cross–sectional area of a human hair (80,000 nm) will be larger than the territory of the Principality of Monaco.

The range that nanoscience deals with is from 0.1 to 100 nm. One tenth of a nanometer, equivalent to 1 angstrom, is considered as a conditional boundary between the sphere of interests of biology and physics. For two reasons. Firstly, this value is approximately equal to the diameter of the orbit of an electron in an unexcited hydrogen atom, and at such distances all processes are already described by quantum mechanics, the principles of which are the same for organic and inorganic matter. Secondly, the maximum resolution of modern scanning devices is limited within the same 0.5-1 angstrom, and biology, unlike physics, is a slightly more empirical science, where it is preferable to be able to, if not touch, then at least see.

Visualization of ultra-small objects. Wave scatteringFor optical devices, the angular resolution of teta is determined by diffraction on the lens, and is calculated by the formula sin teta = 1.22*lambda/D, where lambda is the length of the emitted wave, and D is the aperture, which is also the diameter of the entrance pupil of the optical system (in the case of a microscope, it is the lens).

More generally, diffraction must satisfy the Wolf-Bragg condition, described as nlambda = 2d*sin teta, that is, in the case of ideal diffraction of the 1st order (n, sin teta, = 1), the minimum distance between the distinguishable points d will be equal to half the wavelength.

The resolution of the average human eye is 0.176 mm. The limit of a light microscope using wavelengths in the visible spectrum range (420-760 nm), but having a more powerful focusing lens and, as a result, a smaller aperture than the eye, is somewhere around 0.2-0.3 microns. But it is impossible to reduce the aperture indefinitely, since, at D, less than the wavelength, the equation loses its meaning. But you can reduce the wavelength. This is the basis of methods using X-ray scattering on electron clouds of atoms (as well as electron microscopy). The wavelength in X–ray radiation used in crystallography is about 1-2 angstroms, which corresponds to a photon energy of 5-15 keV. This is a relatively average energy, which makes it possible to achieve elastic (i.e., without loss of kinetic energy of particles) reflection at small beam angles and obtain a diffraction pattern with a resolution of half this length. To date, this is the highest level of resolution with which information about the structure of biomolecules can be obtained. The problem is that in order to observe X-ray diffraction, the structure of the sample must be very ordered, with absolutely identical distances between neighboring atoms. In other words, a kind of crystal lattice is required, which, for example, can be obtained for a protein only by growing a protein crystal by precipitation from a solution. And this is a long, delicate and, at times, very creative procedure for a researcher.

It is not possible to use even shorter–wavelength radiation than X-ray - gamma radiation, since it has an energy two orders of magnitude higher, and this is such a monstrous value that a photon of gamma radiation simply knocks an electron out of an atom, ionizing it, and instead of the necessary scattering of rays, their absorption will occur.

Perception "by touch". Scanning Tunneling MicroscopeSo, if the image detail using wave scattering has its limit, then, instead of "looking at" the object, why not fundamentally change the approach and try to study it tactically.

It's like reading a book for the blind written in Braille, only instead of finger tips – a needle with a monatomic tip. The idea is beautiful and elegant, but it involves a lot of difficulties, starting with the fact that it is necessary to completely exclude the possibility of vibrations of the sample, as well as its thermal deformations, in fact, to make such a needle and, moreover, learn how to move it with subatomic precision. This problem was solved in 1981 by Gerd Binnig and Heinrich Rohrer, employees of the Swiss branch of IBM, who designed the first "scanning tunneling microscope" (STM) and five years later received the Nobel Prize for this invention.

The principle of STM is as follows. A needle probe with a thickness of one atom moves in a vacuum above the surface of the sample at a distance of about one nanometer. In accordance with the laws of quantum mechanics, a tunneling effect occurs, and electrons overcome the vacuum barrier between the object and the needle, thereby closing the electrical circuit through which current begins to flow. The magnitude of the current is inversely proportional to the distance between the tip of the needle and the surface of the sample and is quite sensitive even to a slight change in it, which makes it possible, by monitoring the dynamics of the magnitude of the current when the needle moves along the surface, to obtain information about its relief.

"Constructor of cubes"A scanning tunneling microscope, in principle, even allows the manipulation of individual atoms.

The result of a well–known experiment conducted by IBM Almaden Research Center employees Donald Eigler and Erhard Schweitzer in 1990 was that they managed to lay out the company's logo of 35 xenon atoms on the surface of a nickel crystal (fortunately the abbreviation is short - only three letters). A year later, a similar work was done by a group of Joseph Stroszio from the National Institute of Standards and Technology in Maryland (NIST) already on caesium, platinum and cobalt atoms (here the researchers faced a slightly more difficult task, since they had one more letter in the name).

The focus of manipulation of single atoms using STM is based on the play of the magnitude of the electric potential between the needle and the contact surface. If it is increased, simultaneously bringing the sample closer to a distance of about several picometers (10-12 m), then it is possible to provoke the formation of an ionic bond between the selected atom and the atom at the end of the needle, thereby disconnecting the formed ion from the substrate. Then, by moving the needle, change the voltage applied to it and break the fragile ionic bond, returning the atom to its place. Obviously, it is possible to move it this way only for a very small distance – within the limits of the local negative charge formed by the left electron. Therefore, for long-distance "walks", the atom has to be moved as if by jumping between the needle and the surface of the substrate. At the same time, in order to avoid unwanted thermal movement, the sample temperature should be maintained close to absolute zero (i.e. ^about -270 °C)

This is truly a jeweler's work. In addition, many technical questions arise – for example, how is it possible to drag electrons from a much more electronegative Xe to a silicon atom, from which needles for STM are usually made? (In the case of metals, this is even more or less clear.) How to make sure for sure that the right atom is captured, and not an atom from the next impurity, if one got into the sample? How to achieve an absolutely smooth two-dimensional lattice of substrate atoms? When Donald Eigler gave a lecture at the University of Chicago a year ago, I asked him these questions, but the answer turned out to be too lengthy to give it here.

The practical prospects of meticulously laying out mosaics of atoms in the form of names and titles, except to demonstrate the technology, are still quite vague, though tempting. But in any case, this experiment is probably no less beautiful than the creation of miniature (8-15 microns wide) copies of the Mona Lisa using anodic-oxidative nanolithography, which amused many research groups (for the first time – S. Yamamoto, Ryukoku University). Scientists are no strangers to either a sense of humor or a sense of beauty.

Atomic force microscopyFor the study of organic structures, including biological objects, STM, however, is not the best way.

Due to two significant limitations. The first thing that has already been mentioned in passing above is the need to place the sample in vacuum conditions so that foreign molecules (gas or solution, depending on the type of experiment) do not settle on its surface. And vacuum– alas, is not a biologically relevant environment for the system. The second and more significant limitation is that the surface resistance of the sample should not be more than 20 mOhm/^cm2; in other words, it can only be a metal or a semiconductor. To study dielectrics (for example, diamond), a tunneling microscope is completely useless. Not all organic compounds have good electrical conductivity.

Both of these limitations are circumvented using an atomic force microscope (AFM). Created in the mid-80s, as a modification of the STM, it already allowed working with any surfaces and in conditions of both air and aqueous solutions. In the case of AFM, the measured parameter is not the magnitude of the tunneling current, but the van der Waals interactions between the probe and sample atoms. They also depend on the distance between the needle and the surface, but not linearly, but as a kind of function, where the initial force of interatomic attraction, having reached the point of overlap of their electron clouds, is replaced by the opposite force of repulsion. The design of the scanning needle in an atomic force microscope is different than in the STM - here it is fixed on a flexible cantilever console, on the outer surface of which a laser beam is directed and, reflecting, hits the photodetector. The force acting on the probe from the surface causes the console to bend. When scanning, any unevenness under the tip, whether it is an elevation or a depression, leads to a change in this force and, as a consequence, to a change in the bending size of the cantilever. This is fixed by the position of the reflected laser beam on the photodetector, and based on its movement, the surface relief can be analyzed. Unlike a scanning tunneling microscope, there is no need to use a needle with a thickness of one atom in the AFM, and even more, the needle should not be so sharp, otherwise the signal recorded from it will be too weak.

By changing the working distance and, accordingly, the nature of the force action between the cantilever and the sample surface, the atomic force microscope can be configured to operate in three modes - contact ("contact mode"), semi–contact ("tapping mode") and non-contact ("non-contact mode"). In the contact mode, the tip of the cantilever is directly in contact (blocking the electron clouds) with the surface of the sample and, as it were, "scrapes" on it, and with an unchanging force (that is, with a constant amount of bending of the console). Of the advantages here: high scanning speed, noise immunity and, perhaps, the AFM contact mode is the only one that allows you to achieve atomic image resolution. However, it is unsuitable for working with materials having low mechanical rigidity, which is inherent in almost all biological macromolecules.

When operating in non-contact mode, the probe is constantly located at some distance (preventing the overlap of electronic orbitals) from the object under study and, in the absence of extraneous influences, oscillates at a certain frequency. In the case of approaching the surface atoms, a change in the force of attraction acting on the probe from their side leads to a shift in the amplitude and phase of its oscillations, which will be recorded by the sensors. Nevertheless, there is the same drawback here as in the case of STM – there should be nothing extraneous between the probe and the sample. In other words, an atomic force microscope in contactless mode can function, again, only in vacuum conditions. Therefore, the "tapping mode" is the most optimal for working with biological samples, which is, as it were, an intermediate option between the first two (that is, a certain oscillation frequency is set to the cantilever in the same way, but in such a way that it touches the surface in their lower half-period). This is how today they get the most detailed picture of the biological world.

Physical laws in the microcosmNow, in fact, about how nanoobjects are arranged, how they work, how they are modified for a particular task, and also what application they find in practical medicine or pharmaceuticals.

Of course, for the most part, they have nothing to do with any nanorobots, so beloved by the screenwriters of fantastic movies and depicted by them in the form of miniature crabs with spider legs and a needle for intracellular injections or something similar. Although the so-called "nanomotors", working on the principle of the rotor or crankshaft of the engine, perfectly exist. They do not need to be invented – they have already been invented and implemented in nature: in the electron transport chain of mitochondria and chloroplasts (ATP-aza /ATP-synthase), in transcription and protein synthesis apparatus (RNA polymerase, reverse HIV transcriptase, ribosome, and so on), in the cytoskeleton (myosin and other motor proteins), in cellular locomotion (basal body of the bacterial flagellum). And many other places. However, due to the complexity of the assembly, the created synthetic analogues of such nanomotors are still rare.

A standard nanoparticle, potentially suitable for medical purposes, usually has a very simple design and is a cluster of, say, several thousand atoms. The whole trick lies not in the complexity of the structure at all, but in the fact that at the atomic level matter exhibits slightly different properties – different from those that matter possesses in the usual macrocosm. Therefore, with a decrease in the particle size, starting from some point it begins to work radically differently than just a miniature copy of a macro object. There are several reasons for this.

Firstly, according to the Galilean square-cube law, if a physical body is reduced in size, then its volume will decrease in proportion to the third degree of the reduction coefficient, while its surface area is proportional only to the second degree. Thus, the smaller the particle, the higher the proportion of atoms located on its surface, which means that their contribution to its properties becomes decisive. Since the forces of interaction between the atoms composing the object are not compensated on its surface, the properties of surface atoms differ from the properties of the same atoms in the volume. At the macro level, this difference manifests itself in well–known phenomena - surface tension, capillary effect, wetting, adsorption, and so on. For a single nanoparticle, a change in the ratio of the total number of atoms to the number of atoms on the surface can lead, for example, to other adhesive properties. If we are dealing with a larger structure consisting of many nanoparticles, it will also mean that the total surface area of its surface will be many times larger than that of a single piece of matter of the same volume. And, accordingly, due to the larger contact surface, the chemical reaction occurring between small particles and the medium will be more intense, which has been used in colloidal chemistry since the beginning of the XX century.

The second reason directly follows from the same law and consists in the fact that the magnitude of the physical forces affecting the object also undergoes changes along with its decrease. Since its mass, while maintaining the same density of the material, will decrease similarly to the volume, and therefore faster than the diameter of its cross-section, the mechanical load on the reduced copy will be much weaker. In other words, any nanoparticle has a truly huge margin of safety.

And finally, the ultra-small size of the object imposes certain restrictions on its movement in space. The main hydrodynamic characteristic describing the mechanics of the movement of an object in aqueous solutions is the Reynolds number, calculated as Re = roUL/mu – that is, inversely proportional to the viscosity of the medium (mu) and directly proportional to its size (L). If for a person floating in water (ro = 1 g / ^cm3), the Reynolds number will be somewhere 10 ⁵, then for a micrometer bacterium floating in the same place - ten orders of magnitude lower. This means that for her, the aqueous medium subjectively feels as viscous as glycerin or concentrated sugar syrup for us. In fact, the bacterium does not float, but is pulled up with the help of saws, like a climber on hooks, since there is no inertia force in its microcosm. For nanoscale objects, the Reynolds number is still tens of thousands of times smaller, so they usually face an almost insoluble problem – how to make movement more controllable and faster than by simple diffusion.

Nanoparticles in oncodiagnostics and molecular therapyBy itself, any nanoparticle is not functional.

Its main role, like any transport carrier, is just to deliver the right "cargo" to the right address. And this "load" in the face of biologically active molecules must either be fixed on its surface or enclosed inside (if the nanoparticle has a cavity). For a more targeted action, in addition to the contents, it can also carry ligand molecules (for example, antibodies) that specifically interact with receptors on the surface of target cells. A kind of analogue of a postage stamp or an address on an envelope. After that, such a functionalized nanoparticle is already called a nanovector.

Depending on the "load" carried, nanovectors in medicine can be used either for the purpose of diagnosis or for selective delivery of drugs to affected tissues. The first is convenient for the timely detection of cancers, in particular, for the visualization of a tumor at the initial stage of oncogenesis. And also in the future – to track the pathways of metastases. Moreover, both a fluorescent protein and a magnetized iron-containing component or a radioactive isotope can serve as a marker. In the second case, the use of nanovectors should theoretically reduce the side effects of chemotherapy on adjacent untransformed cells.

From the point of view of origin, as well as their structure, nanovectors can be classified into modified natural ones, which include:

• Liposomes and apolipoproteins (lipid-based nanoparticles) – in vivo are most often responsible for the transport of cholesterol and its derivatives through the circulatory system. They have the structure of microbubbles (from 20 nm in diameter), the walls of which consist of a double layer of phospholipid molecules. Ligands to target cell receptors in this case are usually embedded in the walls of the vesicle. The "cargo" is placed inside and when the target is reached, it is released with the destruction of the shell. They are good because they do not require the removal of the remnants of the destroyed nanoparticle from the body, since all its components are easily used in further metabolic reactions.
• Nanoparticles of viral nature are basically an empty protein capsid of the virus with the genome extracted from it and replaced with the necessary "cargo".
• Ferritin is a protein complex that performs the role of an intracellular iron depot in nature. In some cases, the iron atoms in it can be artificially replaced with atoms of other metals.

and synthetic:

• Quantum dots are nanocrystals of a conductor or semiconductor, so small that their electronic and optical properties occupy an intermediate position between a bulk semiconductor and a discrete molecule. They can be covered with a layer of adsorbed "cargo" molecules.
• Larger metal-based nanoparticles (most often gold, molybdenum, or iron oxide).
• Carbon nanotubes are cylindrical structures consisting of monatomic hexagonal graphite sheets rolled into a tube.
• Dendrimers are macromolecules having a tree-like branching structure.
• Hydrogels - strictly speaking, they are not nanoparticles, but rather a container for them: a jelly-like porous substance of considerable volume, but with nanoscale cavities inside, in which the "cargo" is placed. The rate of its release from the gel and, as a consequence, the achievement of the target can be regulated due to different pore configurations.

With all the seemingly diverse variety of all kinds of nanovectors, working with any of them inside the human body will first require solving at least three serious tasks. First of all, it is the disposal of waste material. Especially if we are talking about nanoparticles with high toxicity (for example, built on the basis of heavy metals). Then – tissue specificity. Since healthy cells can often also carry receptors characteristic of tumor cells (just not in those quantities), then a letter even with a correctly signed address will sometimes come to the wrong address. And there are no ideal surface markers that distinguish a cancer cell from a normal one with absolute accuracy. And finally – suppression of the undesirable inflammatory response of the immune system to the introduction of foreign bodies.

There are many ways to solve these problems, and since the anthracycline antibiotic Doxorubicin encapsulated in liposomes (Doxil ^TM) was first used as a nanovector for the treatment of Kaposi's sarcoma in 1995, several dozen others have now been successfully created, tested and approved for use against various diseases by the Federal Quality Control Administration U.S. Food and Drug Administration (FDA). About four hundred more are currently at the stage of clinical trials. Therefore, nanotechnology in biomedicine is not even tomorrow; it is already today.

Literature:Richard Feynman’s classic 1959 talk: There’s Plenty of Room at the Bottom

• Eigler D, Schweizer E. Positioning single atoms with a scanning tunneling microscope. Nature, 1990 Apr 5; 344: 524–526.
• Stroscio J, Eigler D. Atomic and Molecular Manipulation with the Scanning Tunneling Microscope. Science, 1991; 254(5036), 1319-1326.
• Stroscio J, Tavazza F, Crain J, Celotta R, Chaka A. Electronically induced atom motion in engineered CoCu nanostructures. Science, 2006; 313, 948-951.
• Morita S. Atom world based on nano-forces: 25 years of atomic force microscopy. J Electron Microsc (Tokyo), 2011 Aug 1; S199-S211.
• Hu Y, Fine D, Tasciotti E, Bouamrani A, Ferrari M. Nanodevices in diagnostics. Wiley Interdiscip Rev Nanomed Nanobiotechnol. 2011 January; 3(1): 11–32.
• Nune S, Gunda P, Thallapally P, Lin Y, Forrest M, Berkland C. Nanoparticles for biomedical imaging. Expert Opin Drug Deliv. 2009 November; 6(11):1175-94.
• Cormode D, Jarzyna P, Mulder W, Fayad Z. Modified natural nanoparticles as contrast agents for medical imaging. Adv Drug Deliv Rev. 2010 Mar 8; 62(3): 329–338.
• De Jong W, Borm P. Drug delivery and nanoparticles: Applications and hazards. Int J Nanomedicine. 2008 June; 3(2): 133–149.
• Shi J, Votruba A, Farokhzad O, Langer R. Nanotechnology in Drug Delivery and Tissue Engineering: From Discovery to Applications. Nano Lett. 2010 Sep 8; 10(9): 3223–3230.
   
  The author is a post–graduate student of the Department of Biological and Chemical Sciences, a researcher at the Laboratory of Cell Membrane Biophysics at the Illinois Institute of Technology (Chicago, USA)

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