RU2794900C1 - METHOD FOR PRODUCING ANTIMICROBIAL COMPOSITE NANOSTRUCTURE BOEMITE-SILVER OR BAYERITE-SILVER AND METHOD FOR PRODUCING ANTIMICROBIAL COMPOSITE NANOSTRUCTURE γ-ALUMINUM-SILVER OXIDE - Google Patents

Abstract

FIELD: chemical technology.

SUBSTANCE: group of inventions relates to chemical technology and can be used in the production of aluminum oxide/hydroxide composite nanostructures intended for use as components of sorption-antimicrobial materials for water purification and disinfection, treatment of wound infections, as well as in catalysis processes. A method for obtaining an antimicrobial composite nanostructure of the chemical formula AlOOH/Ag or Al(OH)₃/Ag is disclosed, including the preparation of Al/Ag bimetallic nanoparticles with a given Ag content by electric explosion of aluminum and silver wires twisted together and aqueous oxidation of the resulting Al/Ag bimetallic nanoparticles, at in this case, the mentioned Al/Ag nanoparticles with an Ag content of 9 at.% is obtained by the method of electric explosion of two aluminum wires with a diameter of 0.35±0.01 mm and one silver wire with a diameter of 0.15±0.01 mm twisted together. Also a method for producing an antimicrobial composite nanostructure of gamma alumina is disclosed.

EFFECT: group of inventions provides obtaining a low-toxic sorption-antimicrobial material with improved antimicrobial properties.

8 cl, 5 dwg, 2 tbl, 6 ex

Description

FIELD OF TECHNOLOGY

SUBSTANCE: group of inventions relates to chemical technology and can be used in the production of aluminum oxide/hydroxide composite nanostructures intended for use as components of sorption-antimicrobial materials for water purification and disinfection, treatment of wound infections, as well as in catalysis processes.

PRIOR ART

Due to their low toxicity, positive surface charge, bioinertness, and developed surface, aluminum oxides are widely used as adsorbents. They have a low cost and, depending on the production conditions, are capable of acquiring a different morphology, composition, and a given set of surface centers.

Known product in the form of agglomerates of metal oxyhydroxides selected from the group consisting of Al, Fe, Mg, Ti or mixtures thereof [RU2560432C2, publ. 20.08.2015]. EFFECT: invention provides obtaining agglomerates of oxyhydrates, which can be used as sorbents or as an agent having wound healing and antibacterial activity, as well as for inhibiting the proliferative activity of tumor cells. The product is also disclosed in International Application [WO2014189412 (A1) - 2014-11-27].

The disadvantages of the above product include the lack of a pronounced bactericidal effect due to the absence of antimicrobial agents in the composition. As a result of the adsorption of bacteria by these materials, biofouling is possible.

The solution could be the use of silver nanoparticles as an antimicrobial agent, which are currently considered the best antibacterial nanoparticles used in dressings, glasses, ceramic coatings, coatings on implants and filter membranes [Xu, Y., Zhou, F., Zhang, T., Lin, L., Wu, J., Zhang, M., & Chen, H. (2019). Advances in Preparation and Application of Supported Nano-Silver Composites. Nanoscience and Nanotechnology Letters, 11(11), 1477-1488.].

A variant of a filter material for a gaseous medium is known [WO 2009031944 A2, 2009], which contains as a base a non-woven polymeric fibrous material, on the fibers of which particles of aluminum oxide hydrate are fixed. The material additionally contains an antimicrobial additive represented by silver nitrate. The disadvantages of this material include the use of ionic silver.

The disadvantages of the above material and the method of its production include the use of silver nitrate as an antimicrobial additive. Silver nitrate is a highly soluble compound, and therefore, upon contact with water, it can release a large amount of Ag ⁺ ions, which have a toxic effect. Also, due to the high solubility of silver nitrate, there is no prolonged antimicrobial action in aqueous renewing media.

Known medical structure [US 2008026041 A1, 2008], which includes, along with nanofibers of aluminum oxide, the second fibers mixed with nanofibers of aluminum oxide, particles distributed on the nanofibers of aluminum oxide. The above particles may be a bactericidal agent - ions silver.

The disadvantages of the above also include the use of silver nitrate as an antimicrobial additive.

Known sorption-bactericidal material, medical sorbent based on it and a method for its production [EN 2426557 C1, publ. 08/20/2011], which includes applying particles of aluminum-based material in the form of an aqueous or water-alcohol suspension to a base layer of non-woven polymeric fibrous material, followed by hydrolysis of particles of aluminum-based material, and additional processing of the resulting material - fibers with particles deposited on them aluminum oxide hydrate - an inorganic bactericidal component and particles of colloidal silver for sorbing the latter on particles of aluminum oxide hydrate, and the treatment is carried out by impregnation with a solution of an inorganic bactericidal component or by spraying the latter onto the material; one of the figures shows an agglomerate of particles of aluminum oxide hydrate with sorbed particles of colloidal silver, which are visible along the periphery of the agglomerate in the form of denser rounded inclusions (the image was taken by transmission electron microscopy).

The disadvantages of the above include a method for obtaining and applying colloidal silver, which consists in the reduction of Ag solution of silver nitrate with tannin in a borate buffer. As a result, the colloidal silver solution sprayed onto the material is a mixture of colloidal silver particles, tannin, and a buffer solution based on sodium tetraborate. Tannin molecules are large organic molecules that are well adsorbed on the surface of alumina hydrate particles and are able to shield the positive surface charge necessary for electrokinetic trapping of bacteria [Lozhkomoev, AS, Savel'ev, GG, Svarovskaya, NV, & Lerner, MI (2009). Adsorption of negative eosin ions, Tannin molecules, and latex spheres on aluminum oxohydroxide nanofibers. Russian Journal of Applied Chemistry, 82(4), 581-586]. Sodium tetraborate salts can lead to a decrease in the thickness of the electric double layer of aluminum oxide hydrate particles due to the addition of additional counterions to the system, which will also affect the deterioration of the electrokinetic forces of interaction between the oxide and bacterial cells.

A known method for producing antimicrobial composite Al ₂ O ₃ ×H ₂ O/Ag nanostructures from bimetallic Al/Ag nanoparticles obtained by a joint electric explosion of Al and Ag wires, disclosed by the authors Lozhkomoev Aleksandr, Pervikov Alexander, a Bakina Olga, Kazantsev Sergeya and Gotman Irena in article Synthesis of antimicrobial AlOOH–Ag composite nanostructures by water oxidation of bimetallic Al–Ag nanoparticles [RSC Adv., 2018, 8, 36239–36244]; in which bimetallic Ag–Al nanoparticles are preliminarily synthesized by the electric explosion of twisted wires in an argon atmosphere at a pressure of 2 × 10 ⁵ Pa: silver wire with a diameter of d ¼ 0.15 mm and aluminum wire with a diameter of d ¼ 0.35 mm, both 80 mm; The atomic ratio of Al to Ag in the wires was about 85 at. % Al and 15 at % Ag. Then, water oxidation of the obtained Ag–Al bimetallic nanoparticles is carried out; The reaction of bimetallic Ag–Al nanoparticles is carried out in a dilute aqueous suspension: 1 g of Ag–Al nanopowder, weighed in 100 ml of deionized water, was placed in a heat-insulated glass reactor equipped with a combined pH electrode (ESK-10601) and a temperature sensor (DTS-4- 01); the suspension was heated from 23 to 67°C at a heating rate of 1.0 C per minute, with continuous stirring.

The disadvantages of the above include the fact that Al/Ag particles with an atomic content of Ag 15 at. %, which contain an excess amount of Ag, which in turn affects the specific surface area of nanostructures synthesized from them, their toxicity and cost.

Thus, there is still a need to obtain nanostructures based on aluminum oxide Al ₂ O ₃ with comparable antimicrobial efficiency, lower silver content, lower toxicity and lower cost.

DISCLOSURE OF THE INVENTION

The objective of the invention is to develop a method for obtaining composite nanostructures of the composition Al ₂ O ₃ ⋅ xH ₂ O/Ag, where x=1.5-3.2 and γ-Al ₂ O ₃ /Ag with a reduced number of silver particles on the surface of the nanostructure.

EFFECT: low-toxic sorption-antimicrobial material with improved antimicrobial properties (better antimicrobial effect at lower silver concentration in nanostructures).

An additional technical result is the production of structures without additional chemical reagents (only nanoparticles and water participate in the reaction) and the absence in the resulting product of impurities characteristic of materials obtained by modification with colloidal silver solutions.

Another technical result is the possibility of varying the morphology of Al ₂ O ₃ ⋅ xH ₂ O/Ag nanostructures by changing the conditions for the reaction of Al/Ag nanoparticles with water (ratio of water and reacting nanoparticles, temperature, pressure), which ensures controlled antibacterial activity of the resulting product .

The task is achieved by the fact that, like the well-known proposed method for obtaining nanostructures based on the Al ₂ O ₃ ⋅ xH ₂ O/Ag composite, where x = 1.5-3.2, includes:

– production of Al/Ag bimetallic nanoparticles by electroexplosion of aluminum and silver wires with the required Ag content;

– aqueous oxidation of the resulting Al/Ag nanoparticles to form the mentioned nanostructures.

What is new is that in order to obtain bimetallic Al/Ag nanoparticles with an Ag content of about 9 at %. they are obtained by electric explosion of two aluminum wires with a diameter of 0.35 ± 0.01 mm and one silver wire with a diameter of 0.15 ± 0.01 mm twisted together.

At the same time, to obtain nanostructures based on the Al ₂ O ₃ ⋅ xH ₂ O/Ag composite with a given morphology and characteristics, aqueous oxidation of the mentioned Al/Ag nanoparticles is carried out:

or in excess water with a concentration of Al/Ag nanoparticles of 1±0.1 wt. % containing 9 at. % Ag, at a temperature of 40-90 °C, preferably 60 °C, and holding at this temperature for 30-90 minutes, preferably 60 minutes, to obtain a composite nanostructure in the form of agglomerates of fine-crystalline boehmite nanosheets of the chemical formula AlOOH / Ag with silver nanoparticles with an average up to 17 nm in size, predominantly located in the cavity formed after the dissolution of Al in the particle;

or in moist air at 40-80 °C, preferably 60 °C, and relative humidity of 60-90%, preferably 80 °C, to form a composite nanostructure in the form of hexagonal bayerite rods of the chemical formula Al(OH) ₃ /Ag and accumulation of nanoparticles silver on the surface of rods with an average size of up to 19 nm , stabilized with aluminum hydroxide;

or under hydrothermal conditions of Al/Ag nanoparticles at a temperature of 150-250 °C, preferably 200 °C, and holding time in an autoclave for at least 3 hours, preferably 3 hours to form a composite nanostructure in the form of boehmite nanoplatelets with silver nanoparticles with an average size of up to 22 nm , chemical formula AlOOH/Ag.

The task is also achieved by the fact that in order to obtain a composite nanostructure γ-Al ₂ O ₃ /Ag, subsequent heat treatment of the above described products of oxidation of Al/Ag nanoparticles is carried out - synthesized composite nanostructures Al ₂ O ₃ ⋅ xH ₂ O/Ag with two different crystalline modifications described above - boehmite and bayerite and three types of morphologies before the formation of gamma-aluminum oxide and the migration of silver particles to the surface of nanostructures and a decrease in their average size, which leads to an increase in the surface area of silver nanoparticles and, accordingly, to an increase in the concentration of Ag ⁺ ions .

Moreover, the heat treatment of the synthesized nanostructures is carried out at a temperature of 500 ± 30 °C for at least 2 hours, which does not lead to significant changes in the morphology of the above-described synthesized nanostructures of the Al ₂ O ₃ ⋅ xH ₂ O/Ag composition, while the silver nanoparticles migrate to the surface of the particles and there is a phase transition of boehmite and bayerite to gamma-Al ₂ O ₃ .

At the same time, antimicrobial composite nanostructures are obtained, which are γ-Al ₂ O ₃ nanostructures with silver particles on their surface with an average size of not more than 15-17 nm, a specific surface area of nanostructures of 200-225 m ² /g, with an average pore size of 4- 5 nm, pore volume 0.241-0.605 cm ³ /g, zeta potential at pH 7.2 - 5-14 mV, with an average nanostructure size of 192-740 nm.

The electric explosion conditions proposed in the present invention made it possible to obtain a bicomponent Al/Ag nanopowder with a lower silver content relative to the prototype, the amount of which in the powder was about 9 at. %.

Preparation of Al/Ag nanoparticles with an Ag content of 9 at. % of two wires, as in the prototype, would suggest the use of an aluminum conductor with a diameter of ~0.5 mm. The use of a conductor with such a thickness leads to surface breakdown and a decrease in the input energy. As a result, a large number of large micron-sized particles are formed, and the yield of nanopowder is significantly reduced. Reducing the diameter of the silver conductor leads to a decrease in the yield of nanopowder and frequent wire breaks, due to the peculiarities of the technology of winding the wire feed into the explosion chamber.

Proposed in the present invention, the conditions for the oxidation of Al/Ag nanoparticles (Ag about 9 at. %) with water made it possible to obtain antimicrobial nanostructures of the Al ₂ O ₃ ⋅ xH ₂ O/Ag composition, where x = 1.5-3.2, with three types morphologies:

– nanosheet agglomerates, which can be used as sorption antimicrobial agents in dynamic filtration modes and as components of dressings due to their high specific surface area and accessible pore system for various adsorbates;

– hexagonal rods that can be used as desiccants, catalyst carriers, where antimicrobial activity is required;

– nanoplates that can be used as polymer fillers to give them antimicrobial properties (paints, ointments, gels), where requirements are placed on the sedimentation stability of the components.

In addition, the proposed modes of subsequent heat treatment (calcination at temperatures of 500 ± 30 °C) of all three types of the above nanostructures does not lead to a significant change in their morphology in the final product, but promotes the migration of silver particles to their surface, the phase transition of boehmite and bayerite to γ- Al ₂ O ₃ and a decrease in the average particle size of silver:

– in nanosheets up to 15 nm,

– in nanorods – up to 17 nm,

– in nanoplates – up to 17 nm.

The migration of silver particles to the periphery of Al ₂ O ₃ particles leads to a decrease in the minimum inhibitory concentration of nanostructures in the gamma modification by a factor of 3-7 in relation to the initial nanostructures on the example of E.coli and S.aureus bacteria.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 - The images obtained by transmission electron microscopy (a, b, c) and the results of X-ray phase analysis (d, e, f) for agglomerates of fine-crystalline boehmite nanosheets with silver nanoparticles (a, d), hexagonal bayerite rods with accumulations of silver nanoparticles are shown. on their surface (b, e) and boehmite nanoplatelets with silver nanoparticles (c, f).

Fig. 2 - The images obtained by transmission electron microscopy (a, b, c) and the results of X-ray phase analysis (d, e, f) after heat treatment at 500 °C of fine-crystalline boehmite nanosheets with silver nanoparticles (a, d), hexagonal bayerite rods and accumulations of silver nanoparticles (b, e) and boehmite nanoplatelets with silver nanoparticles (c, f).

Fig. 3 - The dependence of the minimum inhibitory concentration of Al ₂ O ₃ ⋅ xH ₂ O/Ag and γ-Al ₂ O ₃ /Ag nanostructures on the calcination temperature with respect to E. coli and MRSA is shown.

Fig. 4 – Kinetic curves of silver ion migration from Al ₂ O ₃ ⋅ x H ₂ O/Ag nanostructures (solid line) and γ-Al ₂ O ₃ /Ag (dashed line).

Fig. 5 – Viability of the 3T3 cell line upon incubation with the obtained nanostructures of the composition Al ₂ O ₃ ⋅ xH ₂ O/Ag and γ-Al ₂ O ₃ /Ag for 24 hours.

Table 1. Physicochemical characteristics of nanostructures obtained by water oxidation of Al/Ag nanopowder containing 9 at. % Ag, under various conditions.

Table 2. Physicochemical characteristics of nanostructures after heat treatment at 500°C.

IMPLEMENTATION OF THE INVENTION

Preliminary received Al/Ag with a reduced content of silver, the amount of which in the powder should be about 9 at. %. For this purpose, 3 wires were twisted: 2 aluminum wires with a diameter of 0.35 mm and one silver wire with a diameter of 0.15 mm. Next, the wire was fed into the electroexplosive chamber of the UDP-2M setup [http://www.nanosized-powders.com/technology/technologies/equipment.php], pre-filled with argon to a pressure of 0.2 MPa, in the direction from the grounded electrode of the setup to its high voltage electrode. The length of the dispersed conductor was 60 mm. When the electrical circuit was closed, a current pulse with a density of 5×10 ⁷ A/cm ² flowed through the wires for a time of about 1.5×10 ^-6 s. The ratio of the introduced energy E to the sublimation energy of the dispersible metals E _s was maintained at 1.9±0.05.

Below are examples 1-3 of obtaining the claimed nanostructures composition of Al₂O₃ ⋅xH₂O/Ag, where x = 1.5-3.2. Such structures have less cytotoxicity, but also less antimicrobial activity. After calcination (example 4) antimicrobial activity increases, but cytotoxicity also increases.

Example 1 . Obtaining agglomerates of AlOOH/Ag nanosheets was carried out by heating 1 wt. % aqueous suspension of Al/Ag nanopowder containing about 9 at. % Ag, to a temperature of 60 °C and holding at this temperature for 60 minutes. The oxidation products were filtered off and dried at 120°C for 2 hours. The morphology and composition of the oxidation products is shown in Fig. 1 a, d.

Example 2. Obtaining hexagonal rods Al(OH) ₃ /Ag was carried out by oxidation of Al/Ag nanopowder containing 9 at. % Ag, in a desiccator at 60°C and 80% relative humidity for 72 hours. The oxidation products were dried at 120°C for 2 hours. The morphology and composition of the oxidation products is shown in Fig. 1 b, e.

Example 3 AlOOH/Ag nanoplates were prepared in a hydrothermal synthesis autoclave with a Teflon insert. 1 wt. % aqueous suspension of Al/Ag nanopowder containing 9 at. % Ag, 200 ml volume was placed in a 500 ml autoclave. The autoclave was tightly closed and heated to 200°C, after which the nanopowder suspension was kept under these conditions for 6 hours. After cooling, the reaction products were filtered off and dried at 120°C for 2 hours. The morphology and composition of the oxidation products is shown in Fig. 1 c, e.

For the synthesized nanostructures, physicochemical characteristics were determined, such as: the value of the specific surface areaS _BET , pore sizeD _max , pore volumeV,average size of nanostructures, number of structural water molecules (X) and zeta potential. The results are presented in Table 1. The synthesized nanostructures have different values of the specific surface lying in the range from 80 to 245 m²/g, have a close average pore size of 4-5 nm, while the pore volume varies from 0.115 to 0.665 cm³/g, have a high zeta potential at pH 7.2 - 21-30 mV, the average size of nanostructures is from 206 to 860 nm.

Example 4. Obtaining nanostructures γ-Al ₂ O ₃ /Ag was carried out by thermal treatment of nanostructures obtained according to examples 1-3, at 500 °C for 2 hours in a muffle furnace SNOL 7.2/1300 (SNOL, Latvia). Using transmission electron microscopy, it was found that heat treatment does not lead to significant changes in their morphology, while silver nanoparticles migrate to the particle surface and a phase transition of boehmite and bayerite to γ-Al ₂ O ₃ occurs (Fig. 2). There is also a change in the textural and electrokinetic characteristics of materials (table 2). The specific surface area of nanosheet agglomerates slightly decreases to 214 ^m2 /g, while the specific surface area of nanoplates increases significantly up to 220 ^m2 /g and hexagonal rods up to 200 ^m2 /g. The value of the zeta potential of nanostructures at pH 7.2 decreases noticeably to 3-16 mV. An increase in the specific surface of nanoplates and hexagonal rods is due to the appearance of additional micro and mesopores as a result of the transformation of the structure of boehmite and bayerite into gamma-Al ₂ O ₃ . The decrease in the specific surface of nanosheets is associated with the processes of particle sintering. The decrease in the zeta potential of nanostructures is caused by the redistribution of Ag on the surface of particles and their participation in the formation of a double electric layer.

Example 5 Using the example of E. coli and MRSA bacteria, it was shown that the synthesized nanostructures have antimicrobial activity (Fig. 3). In this case, the maximum antimicrobial effect is achieved by calcining the nanostructures at 500°C. An increase in the calcination temperature leads to an increase in MIC, which may be due to the coarsening of silver nanoparticles and sintering of porous particles of gamma-Al ₂ O ₃ . Thermal treatment of nanostructures at 500°C promotes an increase in antimicrobial activity by a factor of 3–7.

Agglomerates of nanosheets calcined at 500°C have the highest antimicrobial activity; % Ag in [RSC Adv., 2018, 8, 36239–36244]. An increase in the antimicrobial activity of γ-Al ₂ O ₃ /Ag nanostructures is due to the migration of silver particles to the surface of the nanostructures and a decrease in their average size by 2–5 nm. This leads to an increase in the migration of silver ions from the surface of the nanostructures (Fig. 4).

Example 6 Cytotoxicity.

Using the 3T3 cell line (mouse fibroblasts) as an example, we evaluated the cytotoxicity of the obtained nanostructures at concentrations that have an inhibitory antimicrobial effect,

for AlOOH/Ag nanosheet agglomerates – 1.25 mg/ml,

for hexagonal rods Al(OH) ₃ /Ag - 5 mg / ml,

for AlOOH/Ag nanoplates – 2.5 mg/ml;

for gamma-Al ₂ O ₃ /Ag nanosheet agglomerates - 0.2 mg/ml, for gamma-Al ₂ O ₃ /Ag hexagonal rods - 0.3 mg/ml, for gamma-Al ₂ O ₃ /Ag nanoplates - 0 .5 mg/ml.

After 24 hours of cell incubation in a CO2 incubator at 37°C with nanostructures, cells were counted using the MTT assay.

The results are shown in FIG. 5.

All samples at the studied concentrations have a weak cytotoxic effect, the decrease in viable cells is reduced by less than 25% relative to the control.

At the same time, for gamma-Al ₂ O _3/ Ag nanostructures in concentrations that have an inhibitory effect on bacteria, a decrease in the cytotoxic effect is observed.

Table 1.

Morphology Phase composition S _BET , m ² /g D _max , nm V, cm ³ /g ζ (pH 7.2), mV Average nanostructure size H, nm Number of structural water molecules, x nanosheet agglomerates

Ag
AlOOH
Al _0.89 Ag _0.11 245±4 ~4 0.665 27±3 740±17 2.1±0.1 nanoplates

Ag
AlOOH 100±6 ~4 0.235 21±2 192±10 1.5±0.05 rods

Ag
Al(OH) ₃
Al _0.78 Ag _0.22 80±2 ~4 0.115 30±1 491±14 3.1±0.1

table 2

Morphology Phase composition S _BET , m ² /g D _max , nm V, cm ³ /g ζ (pH 7.2), mV Average nanostructure size H, nm nanosheet agglomerates

Ag
γ-Al ₂ O ₃
Ag ₂ Al 214±2 ~5 0.605 14±2 860±21 nanoplates

Ag
γ-Al ₂ O ₃ 220±5 ~4 0.241 5±2 206±11 rods

Ag
γ-Al ₂ O ₃
Ag _0.88 Al _0.12
Ag ₂ Al 200±6 ~5 0.241 11±1 514±16

Claims (11)

1. A method for producing an antimicrobial composite nanostructure with the chemical formula AlOOH/Ag or Al(OH) ₃ /Ag, including: – production of Al/Ag bimetallic nanoparticles with a given Ag content by electroexplosion of aluminum and silver wires twisted together; – water oxidation of obtained Al/Ag bimetallic nanoparticles, characterized in that said Al/Ag nanoparticles containing Ag 9 at. % is obtained by the method of electric explosion of two aluminum wires with a diameter of 0.35 ± 0.01 mm and one silver wire with a diameter of 0.15 ± 0.01 mm twisted together. 2. The method according to p. 1, characterized in that the aqueous oxidation of Al / Ag nanoparticles is carried out at a temperature of 40-90 ° C, preferably 60 ° C and holding at this temperature for 30-90 minutes, preferably 60 minutes, to obtain composite nanostructures in the form of agglomerates of fine-crystalline boehmite nanosheets with silver nanoparticles of the chemical formula AlOOH/Ag. 3. The method according to p. 1, characterized in that the aqueous oxidation of Al / Ag nanoparticles is carried out in moist air at a temperature of 40-80 ° C, preferably 60 ° C, and a relative humidity of 60-90%, preferably 80%, for 72 hours to obtain said composite nanostructure in the form of hexagonal bayerite rods of the chemical formula Al(OH) ₃ /Ag. 4. The method according to p. 1, characterized in that the aqueous oxidation of Al / Ag nanoparticles is carried out under hydrothermal conditions at a temperature of 150-250 ° C, preferably 200 ° C, and the exposure time in an autoclave is at least 3 hours, preferably 3 hours, for obtaining the said composite nanostructures in the form of nanoplatelets of the chemical formula AlOOH/Ag. 5. A method for producing an antimicrobial composite nanostructure of gamma alumina γ-Al ₂ O ₃ /Ag, characterized in that it includes thermal treatment of the oxidation products of Al/Ag nanoparticles - nanostructures obtained according to any of paragraphs. 1-4 before the formation of gamma-aluminum oxide - γ-Al ₂ O ₃ /Ag and the migration of silver particles to the surface of nanostructures. 6. The method according to p. 5, characterized in that the heat treatment of the synthesized nanostructures is carried out at a temperature of 500 ± 30 ° C for at least 2 hours. 7. The method according to claim 1, or 2, or 3, characterized in that antimicrobial composite nanostructures of boehmite or bayerite are obtained containing silver particles with an average size of 17-22 nm, the specific surface area of nanostructures is 80-245 m ² /g, with an average pore size of 4-5 nm, a pore volume of 0.115-0.665 cm ³ /g, a zeta potential at pH 7.2 - 21-30 mV, with an average nanostructure size of 206-860 nm. 8. The method according to claim 5, characterized in that antimicrobial composite nanostructures are obtained, which are γ-Al ₂ O ₃ nanostructures with silver particles on their surface with an average size of not more than 15-17 nm, the specific surface area of the nanostructures is 200-225 m ² /g, with an average pore size of 4-5 nm, a pore volume of 0.241-0.605 cm ³ /g, a zeta potential at pH 7.2 - 5-14 mV, with an average nanostructure size of 192-740 nm.

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