Silver–Silica Nanocomposite Materials Incorporatedinto Textile Fabrics: Chemical and Biological Study

Alexander Timin & Evgeniy Rumyantsev

Published online: 25 September 2013# Springer Science+Business Media New York 2013

Abstract Silver nanocomposite materials were prepared byhydrolysis and condensation of tetraethyl orthosilicate (TEOS,Si(OC2H5)4) in the presence of diamminesilver(I) cation([Ag(NH3)2]

+). The obtained materials were investigated bytransmission electron microscopy (TEM) and X-ray diffractionanalysis (XRD). The morphology and size of silver nano-particles inside the silica gel matrix were identified. TEMmicrophotographs of silica with nanosilver have shown thatthe particle size of the silver nanocomposite materials dependson the amount of nanosilver in the silica gel matrix. Thesematerials were grafted on the surface of textile fabrics. Anti-microbial activity was studied by testing all obtained samplesin vitro: gram-positive prokaryotes of the genus Staphylococ-cus (Staphylococcus aureus associated with skin and mucousmembranes was used as test microbe) and gram-negativeprokaryotes of the genus Escherichia (Escherichia coli asso-ciated with gastrointestinal mucosa was used as test microbe).The textile fabrics with different contents of silver nanocom-posites have shown high antimicrobial activity against Staph-ylococcus and E . coli bacteria. The results of the present workhave proved the possibility of applying these materials in thetextile industry and engineering process.

Keywords Sol–gel . Silver nanoparticles . Antibacterialactivity . Nanocomposite materials . Textile fabrics

1 Introduction

Nanostructured and nanocomposite materials have attractedconsiderable attention in recent years because they exhibituseful and unique properties compared to conventional

polycrystalline materials. In particular, they have potential ap-plications in optics, optoelectronics, catalysis, and engineeringprocess [1–5]. These materials are often prepared as nanocom-posites to avoid the tendency of nanoparticles to aggregate.Nowadays, many scientists develop new methods for synthesisand stabilization of nanoparticles. Different metals, metal ox-ides, sulfides, polymers, core–shell, and compositenanoparticles can be prepared using a number of chemicaltechniques such as chemical vapor deposition, vapor-phasesynthesis, hydrothermal synthesis, sonochemical technique,microemulsion technique, etc. The sol–gel method is one ofthe simplest and effective methods for obtaining different nano-structured and nanocomposite materials which then can be usedin various branches of the engineering process [6–9]. The sol–gel-derived amorphous silica matrix is an excellent host forsupporting different types of guest nanoparticles like Ag. Theporous nature of the amorphous silica matrix provides nucle-ation sites for Ag particles and minimizes the aggregationphenomena imposing an upper limit to the size of the particles[10–13].

Nanosilver has chemical and biological properties that areappealing to the consumer products, food technology, textiles/fabrics, and medical industries. Nanosilver also has uniquephysical properties that are not present in bulk silver andwhich are claimed to have great potential for medical appli-cations and the textile industry [14]. Nanosilver is an effectivekilling agent against a broad spectrum of Gram-negative andGram-positive bacteria including antibiotic-resistant strains[15]. Small nanoparticles with a large surface area-to-volume ratio provide a more efficient means for antibacterialactivity even at very low concentration. Nanosilver has manymedical applications including diagnosis, treatment, drug de-livery, coating tools, and medical devices. It is also incorpo-rated in wound dressing, sterilization materials in hospitals,and medical textiles [14, 16].

The sol–gel method provides a practical way of achievinguniform distribution of silver particles in both pure and organ-ically modified silica matrices. Sol–gel also enables control of

A. Timin (*) : E. RumyantsevDepartment of Inorganic Chemistry, Ivanovo State University ofChemistry and Technology (ISUCT), 7, Sheremetevsky prosp.,Ivanovo, Russian Federatione-mail: a_timin@mail.ru

BioNanoSci. (2013) 3:415–422DOI 10.1007/s12668-013-0108-3

the size, distribution, and chemical state of silver, all of whichare critical for technical applications. Sol–gel, a low-temperatureprocess, also provides easy control of silver concentration andthe possibility to add reducing and oxidizing agents in smallconcentrations to modify the chemical state of silver.

It was already said that nanosilver is an effective killingagent of broad-spectrum Gram-negative and Gram-positivebacteria, but little attention has been paid to test the antimi-crobial activity of nanocomposite materials containing silvernanoparticles with subsequent incorporation of these nano-composites on the surface of textile materials. This informa-tion is highly essential for engineering applications of thesematerials in the textile industry and medicine becauseantibacterial properties are directly related to the silver sizeand diffusion capacity of the nanocomposite materials.

The aim of the present study is to prepare silver nanocom-posite materials by the sol–gel technique with subsequentincorporation of the silver nanocomposite on the surface oftextile fabrics. Also, we present physicochemical and biolog-ical characterizations of the obtained materials. Transmissionelectron microscopy (TEM) was performed to understand thephysics and chemistry of the surface and interfaces of bioma-terials. X-ray laser diffraction was carried out in order to provethe structural changes of our nanocomposite materials com-pared to individual amorphous silica. The synthesis sampleswere tested in vitro.

2 Materials and Methods

2.1 Reagents and Materials

Powder samples with different silver concentrations were pre-pared via sol–gel technique. The starting solution used to

prepare the samples consisted of mixtures of tetraethylorthosilicate (TEOS, Si(OC2H5)4, “Ecos-1”), silver nitrate (Ag-NO3, “ChemMed,” Russia), ethanol (C2H5OH, ChemMed,Russia), and deonized water. The diamminesilver(I) cation([Ag(NH3)2]

+) was prepared from silver nitrate by addingammonia solution (NH4OH). In synthesis, only ammonia so-lution (NH4OH) was used as base catalyst.

The textile samples containing silver nanocomposite mate-rials were made by Ivanovo Textile Company, “Ivkogh”(Russia). Escherichia coli (ATCC 25922), Staphylococcusaureus , and nutrient agar were provided by the Laboratoryfor Microbiology—Ivanovo State Medical Academy, Russia.

2.2 Synthesis of Silver Nanocomposite Materials

In the first step, the required amount of AgNO3 and H2O isstirred at room temperature for 20 min to form solution A.Then, NH4OH was added to form diamminesilver(I) cation([Ag(NH3)2]

+) in solution A. In another beaker, TEOS and therequired portion of ethanol were mixed for 30 min to obtainsolution B. Then solution Awas slowly added into solution Bunder stirring. After complete addition, the mixture was stirredfor 125 min at room temperature. NH4OH solution was addedevery 25 min as a base catalyst.

Then the glucose solution was added into the reactivemixture under stirring. The obtained mixture was stirred for60 min at room temperature. Finally, sol–gel mixtures werepoured into special test tubes and sealed with parafilm. Thegelation occurred in air after 1–2 days. The images of finalsilver nanocomposite materials are shown in Fig. 1.

All concentrations are presented in molar ratio. Sol–gelmixtures as listed in Table 1 with low concentration, i.e.,[Ag(NH3)2]

+/[TEOS]=0.008, and highest concentration,[Ag(NH3)2]

+/[TEOS]=0.038, were prepared.

Fig. 1 The images of the silvernanocomposite materials withdifferent concentrations of silvernanoparticles: [Ag(NH3)2]

+/[TEOS]=0.008 (a), [Ag(NH3)2]

+/[TEOS]=0.012 (b),[Ag(NH3)2]

+/[TEOS]=0.015 (c),[Ag(NH3)2]

+/[TEOS]=0.023 (d),and [Ag(NH3)2]

+/[TEOS]=0.038(e)

416 BioNanoSci. (2013) 3:415–422

2.3 Treatment of Fabrics

In a previous work [17], it was proved that all obtainedsamples are characterized by high antimicrobial activityagainst E . coli and Staphylococcus bacteria. Because of this,silver nanocomposites with the lowest concentrations ofnanosilver ([Ag(NH3)2]

+/[TEOS]=0.008 and [Ag(NH3)2]+/

[TEOS]=0.012) were used. Samples of textile materials withimmobilization of our silver nanocomposites were obtainedusing special engineering equipments. The percentage of sil-ver nanocomposite materials incorporated into textile fabricsranged from 1 to 3. Figure 2 shows the obtained textilematerials with different amounts of silver nanocomposite.

It can be seen that the colors of the textile materials dependon the content of immobilized silver nanocomposite materials.Moreover, it is clear that the silver nanocomposite materialswere well dispersed on the surface of the textile fabrics.

2.4 Characterization of Silver Nanocomposite Materials

X-ray diffraction analysis of the powder samples wasperformed using a Bruker X-ray diffractometer D8 Advance.The X-ray source was Ni-filtered MoKα radiation. The X-raysource operating voltage was 40 kV, and the scan rate was 2°/min. The silver particle size was estimated by the Scherrerequation by analyzing the half-width broadening of the dif-fraction peak [18].

TEM (electron microscope EMW-100L) was used to studythe shape and size of the powder samples. Firstly, a suspensionof the samples was obtained by using a special ultrasonicdisperser. Then this suspension was deposited onto a specialcoal film.

2.5 Antimicrobial Tests

Antimicrobial activity of the textile fabrics containing silvernanocomposites was evaluated using cultures of Staphylococ-cus genus (S . aureus associated with skin and mucous mem-branes was used as test microbe) and gram-negative prokaryotes

of the Escherichia genus (E . coli associated with the gastroin-testinal mucosa was used as test microbe). The experimentswere carried out in solid and liquid nutrient media. In order toprepare the solid nutrient media, a microwave-safe containerwas used for mixing and heating the agar with water. Thesemixing proportions make enough nutrient agar to prepare twohalves of the Petri dish with the following boiling for 1 min tocompletely dissolve the agar. Then the obtained textile materialswere put inside the Petri dish and incubated for 24 h at 37 °C.

The liquid nutrient medium bacterial suspensions wereprepared as it was described in [19]. Bacterial concentrationswere measured bymeans of optical density at 600 nm using anultraviolet–visible spectrophotometer (“Aquilon”, Russia).The percentage of microbial reduction (R , in percent) wascalculated using the following equation:

R ¼ Co−CCo

⋅100

Fig. 2 Images of the obtained textile materials containing silver nanocom-posites: sample (a ) containing 1 % of silver nanocomposite(ТЕОС:[Ag(NH3)2]

+=1:0.008), sample (b) containing 1 % of silver nano-composite (ТЕОС:[Ag(NH3)2]

+=1:0.012), sample (c) containing 3 % ofsilver nanocomposite (ТЕОС:[Ag(NH3)2]

+=1:0.008), and sample (d) con-taining 3 % of silver nanocomposite (ТЕОС:[Ag(NH3)2]

+=1:0.012)

Table 1 Formulations and ap-proximate gelation times for silicagels with silver nanoparticlesobtained by base catalysis route

Base catalysis Formulation (molar ratio) Gelation time

[Ag(NH3)2]+/[TEOS]=0.008 TEOS:[Ag(NH3)2]

+:C2H5OH:H2O 1–2 days1:0.008:10:4.5

[Ag(NH3)2]+/[TEOS]=0.012 TEOS:[Ag(NH3)2]

+:C2H5OH:H2O 1–2 days1:0.012:10:4.5

[Ag(NH3)2]+/[TEOS]=0.015 TEOS:[Ag(NH3)2]

+:C2H5OH:H2O 1–2 days1:0.015:10:4.5

[Ag(NH3)2]+/[TEOS]=0.023 TEOS:[Ag(NH3)2]

+:C2H5OH:H2O 1–2 days1:0.023:10:4.5

[Ag(NH3)2]+/[TEOS]=0.038 TEOS:[Ag(NH3)2]

+:C2H5OH:H2O 1–2 days1:0.038:10:4.5

BioNanoSci. (2013) 3:415–422 417

where Co is the number of microbial colonies in liquid nutri-ent medium together with the control textile fabric withoutsilver nanocomposite materials, and C is the number of mi-crobial colonies in liquid nutrient medium together with thecontrol textile fabric in the presence of silver nanocompositematerials.

3 Results and Discussion

3.1 Characterization of the Obtained Materials

Figure 3 shows the X-ray diffraction analysis (XRD) patternsof the obtained samples of five different concentrations after

Fig. 3 X-ray diffraction patterns of silver-containing silica gels preparedby base catalysis process with five different concentrations: [Ag(NH3)2]

+/[TEOS]=0.008 (a ), [Ag(NH3)2]

+/[TEOS]=0.012 (b ), [Ag(NH3)2]+/

[TEOS]=0.015 (c), [Ag(NH3)2]+/[TEOS]=0.023 (d), and [Ag(NH3)2]

+/[TEOS]=0.038 (e)

418 BioNanoSci. (2013) 3:415–422

heat vacuum treatment at 45 °C. The insets show the XRDpattern of sol–gel-derived pure silica powder for comparison.

As shown in Fig. 3, the chosenmethod of synthesis leads tocrystallization of amorphous silica to crystabalite. In addition

to the crystabalite peaks, one peak assigned to metallic silverat around 2θ ≈17.348o is present. The diffraction pattern ofsamples with [Ag(NH3)2]

+/[TEOS]=0.038 shows an increasein the intensity of the diffraction line of silver compared toother samples. Another distinction between these sets withvarying silver concentrations is the difference in the crys-tallization trend of the silica matrices which is promotedby an increase in the silver amount. The Scherrer equationwas used to calculate the diameter of silver nanoparticlesin silica gel.

d ¼ K⋅λβ⋅cosθ

Table 2 Average diameter of silver particles in silica gel

Sol–gel formulation Silver particle size (nm)

[Ag(NH3)2]+/[TEOS]=0.008 61±2

[Ag(NH3)2]+/[TEOS]=0.012 62±2

[Ag(NH3)2]+/[TEOS]=0.015 65±2

[Ag(NH3)2]+/[TEOS]=0.023 64±2

[Ag(NH3)2]+/[TEOS]=0.038 67±2

Fig. 4 Transmission electron microphotographs of the nanocomposite materials prepared with different concentration of silver: [Ag(NH3)2]+/[TEOS]=

0.008 (a), [Ag(NH3)2]+/[TEOS]=0.012 (b), [Ag(NH3)2]

+/[TEOS]=0.015 (c), [Ag(NH3)2]+/[TEOS]=0.023 (d), and [Ag(NH3)2]

+/[TEOS]=0.038 (e)

BioNanoSci. (2013) 3:415–422 419

where d is the average diameter of the nanoparticle, λ is theX-ray wavelength (forMoKα, it is equal to 0.71 nm), and K isthe shape factor. The dimensionless shape factor has a typicalvalue of about 0.9; β is the line broadening at half themaximum intensity (FWHM) in radians, and θ is the Braggangle.

The average particle size of the silver is shown in Table 2.In low-size-containing gel sample ([Ag(NH3)2]

+/[TEOS]=0.008), the average size of the silver particles was 61±2 nm.In high-silver-containing gel sample ([Ag(NH3)2]

+/[TEOS]=0.038), the average particle sizes were 67±2 nm.

Figure 4 shows TEM images of the silver nanocompositematerials with different concentrations of silver nanoparticles.It is clear that all investigated samples are spherical particles.The particle size distribution (PSD) is shown in Table 3.

TEM images of the obtained nanocomposites demonstratethat the increase of silver amount leads to agglomeration ofnanoparticles. In the low-silver-containing gel sample([Ag(NH3)2]

+/[TEOS]=0.008), the particle size distributionwas 62.5÷125 nm. In the high-silver-containing gel sample([Ag(NH3)2]

+/[TEOS]=0.038), the particle size distribu-tionwas128÷961.5 nm.

Nowadays, different silver-modified textile materials havebeen synthesized and already applied in our life [20]. How-ever, most of them have some disadvantages concerning theirsynthetic procedure, especially the procedure of obtainingsilver nanoparticles with subsequent incorporation onto thefabric surface. Basically [21], the electrochemical process ofsynthesis used, which requires expensive equipment and alsoelectrochemical process, suffers from mass transport limita-tions and the size of the specific electrode. It often used thephysical approaches of synthesis of AgNPs. One of them isuse of a tube furnace at atmospheric pressure which also hassome disadvantages: the tube furnace occupies a large space;this process requires a great amount of energy and a lot of timeto achieve thermal stability [22].

In our case, we used a chemical method for synthesis ofsilver nanoparticles. The synthetic procedure is simple andfavorable. Such type of synthesis provides development ofcost-effective antimicrobial textile materials because the syn-thetic route does not require any complicated equipments,

expensive reagents, and special initial preparation for synthe-sis. In addition, it can be observed that our hybrid materials arewell dispersed onto the surface of textile materials (Fig. 2).Also, stable silver nanoparticles (the average size equals64 nm) inside the silica matrix were obtained which savestheir stability for a long period of time.

3.2 Antimicrobial Property of Silver NanocompositeMaterials

The antimicrobial activity of the silver nanocomposite mate-rials was investigated against E . coli and Staphylococcus insolid nutrient media. After 300 min (only after incubationtime), the experimental activity results are more evident. It isclear that all tested samples were shown to have high antimi-crobial activity suppressing test culture growth. The zone ofgrowth retardation of Staphylococcus on solid nutrient medi-um was more than 31 mm (Fig. 5). After that, the experimentwas repeated, and the same results were obtained. Figure 5demonstrates high antimicrobial activity of silver nanocom-posite materials against E . coli and Staphylococcus bacteria,proving the potential application of all samples in the forma-tion of antimicrobial surface in the textile industry.

3.3 Antimicrobial Properties of Textile Fabrics in the Presenceof Silver Nanocomposite Materials

Antimicrobial activity against E . coli and Staphylococcusbacteria of the obtained textile materials resulted from thepresence of silver nanocomposites grafted on their surface inliquid nutrient medium. In our previous experiments, it wasproved that pure SiO2 cannot impact on antimicrobial activityagainst E . coli and Staphylococcus . The results of antimicro-bial activity of textile materials were shown in Table 4. It isclearly seen that the percentage reduction of bacteria increasedwith the content of silver nanocomposite materials on textilefabrics.

Fig. 5 Antimicrobial properties of silver nanocomposite material in solidnutrient media against S . aureus and E. coli in solid nutrient medium:[Ag(NH3)2]

+/[TEOS]=0.008 (1 ), [Ag(NH3)2]+/[TEOS]=0.012 (2 ),

[Ag(NH3)2]+/[TEOS]=0.015 (3 ), [Ag(NH3)2]

+/[TEOS]=0.023 (4 ),[Ag(NH3)2]

+/[TEOS]=0.038 (5), k pure silica without silver nanoparticles

Table 3 The average particle distribution of the silver nanocompositematerials

Sol–gel formulation The particle size distribution (nm)

[Ag(NH3)2]+/[TEOS]=0.008 62.5÷125

[Ag(NH3)2]+/[TEOS]=0.012 63.4÷171.4

[Ag(NH3)2]+/[TEOS]=0.015 64.1÷184.3

[Ag(NH3)2]+/[TEOS]=0.023 65.5÷365.8

[Ag(NH3)2]+/[TEOS]=0.038 128÷961.5

420 BioNanoSci. (2013) 3:415–422

The excellent antimicrobial activity showed only the sam-ple containing 3 % of [Ag(NH3)2]

+/[TEOS]=0.012. The tex-tile materials containing silver nanocomposites were washedin water and kept stirred for a given time. Antimicrobialactivity was tested after the immersion. It was proved thatonly the last sample with the highest amount of silvernanoparticles saved the excellent antimicrobial activity afterten washing cycles. This fact might be associated with theweak physical bonding between the silver nanocomposite andtextile surface [14]. Therefore, only the last sample (with thehighest amount of nanosilver) might be used for many wash-ing cycles with the following preservation of antimicrobialproperties.

However, according to the literature [23], nanoparticles arehighly toxic to mammalian cells, brain cells, liver cells, andstem cells in addition to their antimicrobial effect on a widerange of microorganisms. Besides, silver also causes harm notonly in the human body but also in soil microbial communitiesand in aquatic systems. Despite the antimicrobial properties ofsilver nanoparticles, the toxicology and its effect on humanbody also play a very important role. The recent studies(in vivo and in vitro) show that nanosilver is absorbed viathe skin and may cause a negative health effect. So, the use ofmedical devices containing silver nanoparticles may increasethe risk of chronic diseases [24].

But it is well known that the hazardous effect of silvernanoparticles depends on their size, shape of particles, andconcentration of nanosilver. The smallest particles are charac-terized by the most toxicological effect [25]. Because of thesize, nanosilver can be easily absorbed by air, water, or soil. Inour case, we used silica matrix as a host for silver nano-particles. The silver nanoparticles were adsorbed inside thesilica gel matrix during sol–gel synthesis and interacted withsilica oxide via noncovalent van der Waals bonds. Thus, usingsuch synthetic method, we can reduce the release of silvernanoparticles in our natural environment. Our hypothesis isbased on scientific work [26] where the authors developed themethod of how to reduce the toxic effect of silvernanoparticles. They achieved this by applying the inert silicafilm where the role of silica was a shell which reduced therelease of toxic AgNPs. They proved that the toxic effect of

silver nanoparticles with saving of its biological activity canbe reduced by varying the molar ratio of silica and silvernanoparticles.

The results on the antimicrobial activity of textile fabricsmodified by AgNPs were compared with the results of otherscientists who work in the same area. In another work [27],fabrics (62 % cotton and 38 % polyester) impregnated withdimethyltetradecyl (3-(trimethoxysilyl)propyl) ammoniumchloride were synthesized and its antimicrobial activityagainst S. aureus was studied. It has been shown that treatedunwashed fabrics had a good effect against S . aureus . How-ever, after 10–20 washing cycles, the antimicrobial activity offabrics was insufficient (Table 5). This fact proves that ourhybrid materials based on silica gel and silver nanoparticlesare firmly bonded to the textile surface, and as a result, thesemodified textile materials save their antimicrobial activitylonger than materials modified by dimethyltetradecyl(3-(trimethoxysilyl)propyl) ammonium chloride

Also, the obtained results of antimicrobial activity of fab-rics modified by SiO2/Ag NPs are consistent with a scientificwork [14] where the authors also used silver nanoparticles, butinstead of silica oxide, they used TiO2. They also showed thatafter ten washing cycles, the fabrics modified by TiO2/AgNPssaved their antimicrobial activity against E . coli and S .aureus .

In another work [28], a potential application of ZnOnanoparticles as an antimicrobial agent which was successful-ly coated onto the fabric surface was shown. The cottonfabrics coated with ZnO nanoparticles showed a very goodantimicrobial activity against Staphylococcus and E . coli(Table 6). However, according to the most sensitive organismof the test battery crustaceans–algae–fish, ZnO nanoparticlesare classified as “very toxic” (more than AgNPs and CuO) toaquatic organisms [29]. As a result, it would not be recom-mended to use ZnO nanoparticles for application in the textileindustry.

Table 6 The antimicro-bial activity of fabricscoated with ZnOnanoparticles after eightwashing cycles

Test organism Reduction (%)

Staphylococcus 97.24

E . coli 86.62

Table 5 The antimicrobial activity of fabrics (62 % cotton and 38 %polyester) impregnated with dimethyltetradecyl (3-(trimethoxysilyl)propyl)ammonium chloride

Fabrics S. aureus ATCC 6538 reduction (%)

After 5 washes 98

After 10 washes 20.37

After 20 washes 14.39

Table 4 The antimicrobial activity of textile material treated with differ-ent amounts of silver nanocomposite materials

The percentage of silver nanocompositematerial on surface of textile fabrics (%)

E . coli S . aureusBacterialreduction (%)

Bacterialreduction (%)

1 % of [Ag(NH3)2]+/[TEOS]=0.008 36.31 23.19

1 % of [Ag(NH3)2]+/[TEOS]=0.012 48.96 36.54

3 % of [Ag(NH3)2]+/[TEOS]=0.008 64.78 57.91

3 % of [Ag(NH3)2]+/[TEOS]=0.012 99.97 99.96

BioNanoSci. (2013) 3:415–422 421

4 Conclusion

The silver nanocomposite materials with different amounts ofnanosilver were synthesized via the sol–gel process. The sizeof nanoparticles in the silica gel matrix was determined. Also,it was proved that spherical particles can be obtained usingthis synthesis route, and the size distribution depends on theconcentration of nanosilver. The obtained silver nanocompos-ite materials were grafted and well dispersed on the surface ofthe textile fabrics. All obtained samples were tested for anti-microbial activity against E . coli and Staphylococcus bacteriaand showed excellent antimicrobial properties in solid nutrientmedium. However, only the sample with the highest contentof silver nanoparticles saved the antimicrobial propertieswhen the textile materials were tested in liquid nutrient medi-um. In addition, a good durability of the antimicrobial textilefabrics was obtained after washing ten times.

Acknowledgments We thank U.S. Marfin for help in synthesis of ourtextile materials and E.V. Garasko, Department of Virusology and Mi-crobiology (Ivanovo State Medical Academy, Russia), for testing oursamples for antimicrobial activity. The work is supported by the grantof the RFBR (project no. 12-03-31309).

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