Formation and Antibacterial Activity of AlOOH/Ag Composite Coating on Macroporous α -Al 2 O 3 Ceramics

: In this study, the modiﬁcation of macroporous α -Al 2 O 3 ceramics with AlOOH nanostructures impregnated with silver particles is carried out using bicomponent Al/Ag nanoparticles obtained by the simultaneous electrical explosion of Al and Ag wires. Nanoparticle suspension impregnation of porous ceramics followed by oxidation with water is shown to lead to the formation of a continuous AlOOH nanosheet coating on the ceramic surface, with silver releasing on the surface of nanosheets in the form of individual particles sized 5–30 nm. Modiﬁed with AlOOH/Ag nanostructures, macroporous α -Al 2 O 3 pellets with a diameter of 11 mm and a thickness of 5 mm show 100% efﬁciency for water puriﬁcation from bacteria with a concentration of 10 5 CFU/mL for 7.5 min at a ﬂow rate of 6.7 mL/min. Author Contributions: Conceptualization, A.S.L.; methodology, E.I.S., A.S.L. and O.V.B.; software, A.S.B.; validation, M.G.K.; formal analysis, E.I.S. and S.O.K.; investigation, S.O.K. and E.I.S.; resources, A.S.L. and A.S.B.; data curation, M.G.K. and A.S.B.; writing—original draft preparation, A.S.L. O.V.B.; writing—review and editing, A.S.L. and O.V.B.; visualization, S.O.K. A.S.B.; supervision,

The most typical representative of macroporous ceramics is Al 2 O 3 (alumina). A variety of applications of alumina-based ceramic materials are available due to its high strength and stability resulting from strong ionic and covalent bonds, as well as a high surface area which allows the production of highly porous materials [11]. Alumina-based ceramics are used in the production of catalyst carriers [12][13][14], membrane materials [15][16][17][18][19][20], gas-liquid filters [21,22], and sensor materials [23].
However, even with all the above advantages, porous ceramics often need surface modification, which can be realized by adding various functional groups [24][25][26] or by the attachment of nanoparticles [27][28][29][30][31][32]. Surface modification can improve the sorption and selective efficiency and antimicrobial and photochemical properties, as well as the catalytic activity of the porous ceramic materials.
Ceramics surfaces can be modified with nanoparticles formed by physical or chemical vapor deposition, laser cladding, sol-gel method, etc. [33,34]. However, not all methods are suitable for the surface modification of porous ceramics. As a rule, wet chemistry methods are used for this purpose by performing chemical reactions on the ceramic surface [35]. This concerns both the reduction of silver salts on the ceramic surface [36,37] and the synthesis of complex nanostructures by sol-gel methods [38], chemical grafting [39], and immersion methods [40].
The water oxidation of Al/Ag nanoparticles, including those fixed on the macroparticle outermost surface, yields boehmite nanosheet structures with highly specific surface areas of up to 300 m 2 /g, decorated with silver nanoparticles of 5-30 nm in size [41,42]. This approach can be used to modify macroporous alumina ceramics by impregnation in the suspension of Al/Ag nanoparticles followed by the oxidation of the nanoparticles fixed in the pore space.
In the present work, for the first time, nanostructured AlOOH/Ag composite coatings are obtained on the surface of macroporous alumina ceramics using electroexplosive Al/Ag nanoparticles, which contribute to an increase in antimicrobial activity.

Preparation of Alumina Macroporous Ceramics
The initial ceramic composition was obtained by the mechanical mixing of Al 2 O 3 nanopowder (Advanced Powder Technologies LLC, Tomsk, Russia) with a porogenic agent as irregularly shaped rosin particles sized 300-500 µm, with the rosin particle content being 80 vol.%. The initial ceramic composition weight, about 0.5 g, was pressed in a steel mold under a pressure of 130 MPa. The removal of the porogen agent was carried out by annealing the pressed samples in a muffle furnace (Naberterm LHT 08/18/3310 (Naberterm, Lilienthal, Germany) at 1100 • C with isothermal dwelling for 1 h. To sinter the ceramic material the temperature was increased to 1600 • C and kept for 1 h.

Preparation of Al and Al/Ag Nanoparticles
Al and Al/Ag nanoparticles were obtained by the electrical explosion of Al wires and twisted Al and Ag wires, respectively, in an argon atmosphere according to the methods reported previously [43,44]. The Al/Ag ratio of the bi-metallic nanoparticles was determined by the ratio of metals in the exploded wires. In this work, a twist of two Al wires with a diameter of 0.35 mm and an Ag wire with a diameter of 0.15 mm was used, corresponding to the Ag content in the prepared nanoparticles of about 8.4 at. %.

Formation of a Composite AlOOH/Ag Coating on the Surface of α-Al 2 O 3 Ceramics
A suspension with Al or Al/Ag nanoparticle content of 1 wt.% in ethanol was prepared using an ultrasonic bath. After that, alumina macroporous ceramic disks weighing about 0.5 g were immersed in the suspension and impregnated for 30 min under constant ultrasonic irradiation. Then, the samples prepared were removed from the suspension and dried for 30 min at 60 • C. The dried samples were placed in a reactor with 10 mL of deionized water preheated to 60 • C and held for 1 h. During the interaction of aluminum and water, the amount of hydrogen released was recorded from the displaced volume of liquid in the communicating vessel system. After the reaction was complete, the samples were removed and dried at 120 • C for 2 h. The amount of immobilized nanoparticles on the alumina ceramics was determined by the sample mass gain and the amount of released hydrogen.

Characterization of Research Objects
Nanoparticles and porous composites were characterized using an XRD-6000 diffractometer (Shimadzu, Kyoto, Japan) with CuKα radiation. X-ray diffraction was carried out using the PCPDFWIN database as well as the PowderCell 2.4 software package (W. Kraus& G.Nolze, Berlin, Germany). The morphology and elemental composition of the nanoparticles and nanostructures were characterized by transmission electron microscopy (TEM) using a JEM-2100 microscope (JEOL, Tokyo, Japan) with an integrated energy-dispersive X-Ray spectroscopy (EDS) system X-Max (Oxford Instruments, Abingdon, UK). The average nanoparticle size was determined from the TEM data, and histograms of the nanoparticle size distribution were constructed using the sizes of at least 1500 particles measured.
The surface morphology of the composites before and after modification was studied by scanning electron microscopy (SEM) using an LEO EVO 50 electron microscope (Carl Zeiss AG, Jena, Germany) equipped with an INCA-Energy 450 EDS analyzer (Oxford Instruments, Abingdon, UK).

Antibacterial Assay
Staphylococcus aureus ATCC 6538-R bacterial culture was used to determine the antimicrobial activity of the porous ceramic samples. Using a single-channel peristaltic pump (JSC LOIP, Saint-Petersburg, Russia), 100 mL of deionized water containing 10 5 CFU/mL of bacteria was pumped through the fixed sample of 11 mm in diameter and 5 mm in thickness at a rate of 6.7 mL/min. Filtrate samples were taken in sterile 10 mL tubes; then, 20 µL was taken from each sample, diluted 10-fold, and 20 µL was inoculated on nutrient agar into Petri dishes using the track method. Incubation was performed at 37 • C for 24 h in a Binder APT line™ series BF 115 thermostat (Binder, Tuttlingen, Germany). Colonies were counted using the Scienceware ® colony counting system (SP Scienceware, Pequannock, NJ, USA). The bacterial concentration in the filtrate was determined by taking into account the dilutions used in the experiment.

Results and Discussion
Al and Al/Ag nanopowders were used to form AlOOH and AlOOH/Ag nanostructured coatings on the surface of macroporous alumina ceramics. Figure 1 shows TEM images of the nanoparticles used and their size distribution. The particles had a spherical shape and were covered with an oxide layer 4-5 nm thick. The nanopowders were characterized by a lognormal particle size distribution with the average size of Al nanoparticles being 89 ± 1.4 nm and Al/Ag nanoparticles being 96 ± 0.7 nm (Figure 1c). histograms of the nanoparticle size distribution were constructed using the sizes of at least 1500 particles measured. The surface morphology of the composites before and after modification was studied by scanning electron microscopy (SEM) using an LEO EVO 50 electron microscope (Carl Zeiss AG, Jena, Germany) equipped with an INCA-Energy 450 EDS analyzer (Oxford Instruments, Abingdon, UK).

Antibacterial Assay
Staphylococcus aureus ATCC 6538-R bacterial culture was used to determine the antimicrobial activity of the porous ceramic samples. Using a single-channel peristaltic pump (JSC LOIP, Saint-Petersburg, Russia), 100 mL of deionized water containing 10 5 CFU/mL of bacteria was pumped through the fixed sample of 11 mm in diameter and 5 mm in thickness at a rate of 6.7 mL/min. Filtrate samples were taken in sterile 10 mL tubes; then, 20 µL was taken from each sample, diluted 10-fold, and 20 µL was inoculated on nutrient agar into Petri dishes using the track method. Incubation was performed at 37 °C for 24 h in a Binder APT line™ series BF 115 thermostat (Binder, Tuttlingen, Germany). Colonies were counted using the Scienceware ® colony counting system (SP Scienceware, Pequannock, NJ, USA). The bacterial concentration in the filtrate was determined by taking into account the dilutions used in the experiment.

Results and Discussion
Al and Al/Ag nanopowders were used to form AlOOH and AlOOH/Ag nanostructured coatings on the surface of macroporous alumina ceramics. Figure 1 shows TEM images of the nanoparticles used and their size distribution. The particles had a spherical shape and were covered with an oxide layer 4-5 nm thick. The nanopowders were characterized by a lognormal particle size distribution with the average size of Al nanoparticles being 89 ± 1.4 nm and Al/Ag nanoparticles being 96 ± 0.7 nm (Figure 1c). The peaks of the diffraction pattern ( Figure 2a) correspond to planes 111, 200, 220, 311 and 222 in accordance with a powder diffraction file of the International Centre for Diffraction Data (ICDD), card No. 00-004-0787. In both cases, the lattice parameters are standard for aluminum and are 4.049 Å. According to the conventional Williamson−Hall plot, the crystallite size for Al/Ag nanoparticles is smaller than that for Al nanoparticles and is 40 ± 2 nm versus 55 ± 2 nm. The absence of Ag peaks in the diffraction pattern of Al/Ag nanoparticles may be due to its distribution in the particles in the form of X-ray amorphous clusters (Guinier−Preston zones). This phenomenon is associated with the formation of an oversaturated Al-based solid solution [45] and is characterized by the extremely small size of Ag clusters [46][47][48][49][50]. The peaks of the diffraction pattern ( Figure 2a) correspond to planes 111, 200, 220, 311 and 222 in accordance with a powder diffraction file of the International Centre for Diffraction Data (ICDD), card No. 00-004-0787. In both cases, the lattice parameters are standard for aluminum and are 4.049 Å. According to the conventional Williamson−Hall plot, the crystallite size for Al/Ag nanoparticles is smaller than that for Al nanoparticles and is 40 ± 2 nm versus 55 ± 2 nm. The absence of Ag peaks in the diffraction pattern of Al/Ag nanoparticles may be due to its distribution in the particles in the form of X-ray amorphous clusters (Guinier−Preston zones). This phenomenon is associated with the formation of an oversaturated Al-based solid solution [45] and is characterized by the extremely small size of Ag clusters [46][47][48][49][50]. The presence of silver in the nanoparticles was confirmed by TEM-EDS analysis (Figure 2b). In the TEM-EDS images obtained in the mapping mode, Al and Ag were distributed within the nanoparticles. The weight ratio of Al to Ag in the sample corresponded to that of metals in the wires exploded, being 73 and 27 wt.%, respectively. The presence of Au and C in the spectrum (Figure 2b) was due to the use of formvar/carbon-coated Au TEM grids.
The properties of Al and Al/Ag nanoparticles, as well as the features of the nanoparticle−water interaction, have been described in detail previously [51,52].
The macroporous ceramic samples were prepared in the form of 11 mm diameter discs with a thickness of 5 mm (Figure 3a). To obtain AlOOH and AlOOH/Ag coatings on the surface of the macroporous ceramics, they were pre-impregnated in 1 wt.% Al or Al/Ag nanopowder suspension in order to fix the nanoparticles on the surface (Figure 3b). An open pore system of high porosity and pore size are important for the complete impregnation of porous ceramics. In this case, the porosity of the alumina ceramic samples was 75% with an average pore size of 187.2 ± 2.5 µm (Figure 3c).
(a) (b) (c) Figure 3. Images of the macroporous ceramic samples before (а) and after (b) impregnation with Al or Al/Ag nanoparticle suspension and pore size distribution curve (c). The disc diameter is 11 mm.
The amount of hydrogen released as a result of the aluminum−water interaction was used as an estimate of the amount of fixed nanoparticles on the ceramics (Figure 4). The The presence of silver in the nanoparticles was confirmed by TEM-EDS analysis (Figure 2b). In the TEM-EDS images obtained in the mapping mode, Al and Ag were distributed within the nanoparticles. The weight ratio of Al to Ag in the sample corresponded to that of metals in the wires exploded, being 73 and 27 wt.%, respectively. The presence of Au and C in the spectrum (Figure 2b) was due to the use of formvar/carbon-coated Au TEM grids.
The properties of Al and Al/Ag nanoparticles, as well as the features of the nanoparticlewater interaction, have been described in detail previously [51,52].
The macroporous ceramic samples were prepared in the form of 11 mm diameter discs with a thickness of 5 mm (Figure 3a). To obtain AlOOH and AlOOH/Ag coatings on the surface of the macroporous ceramics, they were pre-impregnated in 1 wt.% Al or Al/Ag nanopowder suspension in order to fix the nanoparticles on the surface (Figure 3b). An open pore system of high porosity and pore size are important for the complete impregnation of porous ceramics. In this case, the porosity of the alumina ceramic samples was 75% with an average pore size of 187.2 ± 2.5 µm (Figure 3c). The presence of silver in the nanoparticles was confirmed by TEM-EDS analysis (Figure 2b). In the TEM-EDS images obtained in the mapping mode, Al and Ag were distributed within the nanoparticles. The weight ratio of Al to Ag in the sample corresponded to that of metals in the wires exploded, being 73 and 27 wt.%, respectively. The presence of Au and C in the spectrum (Figure 2b) was due to the use of formvar/carbon-coated Au TEM grids.
The properties of Al and Al/Ag nanoparticles, as well as the features of the nanoparticle−water interaction, have been described in detail previously [51,52].
The macroporous ceramic samples were prepared in the form of 11 mm diameter discs with a thickness of 5 mm (Figure 3a). To obtain AlOOH and AlOOH/Ag coatings on the surface of the macroporous ceramics, they were pre-impregnated in 1 wt.% Al or Al/Ag nanopowder suspension in order to fix the nanoparticles on the surface (Figure 3b). An open pore system of high porosity and pore size are important for the complete impregnation of porous ceramics. In this case, the porosity of the alumina ceramic samples was 75% with an average pore size of 187.2 ± 2.5 µm (Figure 3c).
(a) (b) (c) Figure 3. Images of the macroporous ceramic samples before (а) and after (b) impregnation with Al or Al/Ag nanoparticle suspension and pore size distribution curve (c). The disc diameter is 11 mm.
The amount of hydrogen released as a result of the aluminum−water interaction was used as an estimate of the amount of fixed nanoparticles on the ceramics (Figure 4). The The amount of hydrogen released as a result of the aluminum−water interaction was used as an estimate of the amount of fixed nanoparticles on the ceramics (Figure 4). The obtained kinetic dependences had an S-shape form typical for hydrogen release during the water oxidation of Al powders [53]. Hydrogen release began after an about 10 min delay and was completed after 30 min both in the case of the oxidation of Al nanoparticles and in the case of the oxidation of Al/Ag nanoparticles. The reaction of the fixed Al nanoparticles with water resulted in the release of 9.4 mL of hydrogen, while the oxidation of Al/Ag nanoparticles resulted in the release of 5.3 mL. These amounts of hydrogen were released upon the complete oxidation of 7.5 mg and 4.2 mg of aluminum, respectively.   Figure 5 shows SEM images of the ceramic surface before and after modification. As can be seen, the pristine ceramic surface had multiple pores, and the ceramic structure was represented by grains with an average size of 5.1 ± 0.6 µm (Figure 5a). After ceramic modification by the water oxidation of Al and Al/Ag nanoparticles fixed on its surface, the formation of continuous porous coating and separate agglomerates of about 1 µm size was observed (Figure 5b,c). The agglomerates had a nanosheet structure similar to that formed as a result of the water oxidation of Al and Al/Ag nanopowder [41,52] (Figure 5b,c inset).
Elemental analysis of the modified samples' surfaces indicated the presence of silver in the case of ceramic modification by the oxidation products of Al/Ag nanoparticles (Figure 5e).   Figure 5 shows SEM images of the ceramic surface before and after modification. As can be seen, the pristine ceramic surface had multiple pores, and the ceramic structure was represented by grains with an average size of 5.1 ± 0.6 µm (Figure 5a). After ceramic modification by the water oxidation of Al and Al/Ag nanoparticles fixed on its surface, the formation of continuous porous coating and separate agglomerates of about 1 µm size was observed (Figure 5b,c). The agglomerates had a nanosheet structure similar to that formed as a result of the water oxidation of Al and Al/Ag nanopowder [41,52] (Figure 5b,c inset).
released upon the complete oxidation of 7.5 mg and 4.2 mg of aluminum, resp  Figure 5 shows SEM images of the ceramic surface before and after modif can be seen, the pristine ceramic surface had multiple pores, and the cerami was represented by grains with an average size of 5.1 ± 0.6 µm (Figure 5a). Aft modification by the water oxidation of Al and Al/Ag nanoparticles fixed on the formation of continuous porous coating and separate agglomerates of abou was observed (Figure 5b,c). The agglomerates had a nanosheet structure sim formed as a result of the water oxidation of Al and Al/Ag nanopowder [41,52] (F inset).
Elemental analysis of the modified samples' surfaces indicated the presen in the case of ceramic modification by the oxidation products of Al/Ag nanopar ure 5e). As a result of the modification of porous ceramics by AlOOH/Ag nanostructures, silver was distributed over the entire surface, and its content according to SEM-EDS analysis relative to Al and O was 1.5 ± 0.4 wt.% ( Figure 6). According to our previous works, the oxidation of Al/Ag nanoparticles by water yields boehmite nanosheet structures with silver particles 5-30 nm in size fixed on the nanosheet surface [41]. It should be expected that similar regularities are preserved during the oxidation of Al/Ag nanoparticles on the surface of alumina ceramics. As a result of the modification of porous ceramics by AlOOH/Ag nanostructures, silver was distributed over the entire surface, and its content according to SEM-EDS analysis relative to Al and O was 1.5 ± 0.4 wt.% ( Figure 6). According to our previous works, the oxidation of Al/Ag nanoparticles by water yields boehmite nanosheet structures with silver particles 5-30 nm in size fixed on the nanosheet surface [41]. It should be expected that similar regularities are preserved during the oxidation of Al/Ag nanoparticles on the surface of alumina ceramics.
According to the XRD phase analysis (Figure 7), the main diffraction peaks in the patterns of the studied samples corresponded to α-Al 2 O 3 -corundum (ICDD card No. 00-004-0787). The lattice parameters of the pristine α-Al 2 O 3 ceramics slightly differed from those of the ceramics modified with AlOOH and AlOOH/Ag nanostructures, at 4.7587 Å, 4.7562 Å and 4.7559 Å, respectively, which agrees with the values reported [54]. The value of the coherent scattering regions (d CSR ) determined by the the Williamson−Hall method for all samples was about 80 nm. The absence of the diffraction peaks corresponding to AlOOH was due to its low crystallinity [55] and its low content in the sample. The absence of silver diffraction peaks was due to the low content of noble metal particles in the sample. According to the XRD phase analysis (Figure 7), the main diffraction peaks in the patterns of the studied samples corresponded to α-Al2O3-corundum (ICDD card No. 00-004-0787). The lattice parameters of the pristine α-Al2O3 ceramics slightly differed from those of the ceramics modified with AlOOH and AlOOH/Ag nanostructures, at 4.7587 Å, 4.7562 Å and 4.7559 Å, respectively, which agrees with the values reported [54]. The value of the coherent scattering regions (dCSR) determined by the the Williamson−Hall method for all samples was about 80 nm. The absence of the diffraction peaks corresponding to AlOOH was due to its low crystallinity [55] and its low content in the sample. The absence of silver diffraction peaks was due to the low content of noble metal particles in the sample.  Figure 8 shows the efficiency of S. aureus removal from water as a function of the water volume passed through the macroporous ceramic samples. When passing 100 mL of water with a bacterial concentration of 10 5 CFU/mL through the pristine macroporous ceramics, the removal efficiency was 76% (Figure 8, curve 1). The modification of the ceramics with AlOOH nanostructures led to an increase in removal efficiency of up to 89% 10 Figure 8 shows the efficiency of S. aureus removal from water as a function of the water volume passed through the macroporous ceramic samples. When passing 100 mL of water with a bacterial concentration of 10 5 CFU/mL through the pristine macroporous ceramics, the removal efficiency was 76% (Figure 8, curve 1). The modification of the ceramics with AlOOH nanostructures led to an increase in removal efficiency of up to 89% (Figure 8, curve 2), and when modified with AlOOH/Ag (Figure 8, curve 3) nanostructures, the efficiency was 98%. At the same time, for the sample containing silver, the removal efficiency after passing 50 mL of contaminated water was 100%. Figure 7. XRD pattern of pristine α-Al2O3 ceramic (1), α-Al2O3 ceramic modified with AlOOH (2) and AlOOH/Ag (3) nanostructures. Figure 8 shows the efficiency of S. aureus removal from water as a function of the water volume passed through the macroporous ceramic samples. When passing 100 mL of water with a bacterial concentration of 10 5 CFU/mL through the pristine macroporous ceramics, the removal efficiency was 76% (Figure 8, curve 1). The modification of the ceramics with AlOOH nanostructures led to an increase in removal efficiency of up to 89% (Figure 8, curve 2), and when modified with AlOOH/Ag (Figure 8, curve 3) nanostructures, the efficiency was 98%. At the same time, for the sample containing silver, the removal efficiency after passing 50 mL of contaminated water was 100%.  The results obtained indicate a significant increase in the bacterial removal (inactivation) efficiency from water by macroporous α-Al 2 O 3 ceramic samples after modification with AlOOH/Ag nanostructures. This increase in efficiency was caused by the presence of Ag in the composition of the modified ceramics. Ag is known to release Ag + ions, which cause the inhibition of certain oxidative enzymes, protein denaturation, or interference with DNA replication [56].
As for efficiency compared to other materials, it should be noted that this will strongly depend on many factors, such as the initial bacterial load on the material, the flow rate, and the area and thickness of the filter. In our work, we adsorbed (inactivated) 5 × 10 6 bacterial cells per 1 cm 2 of material while ensuring 100% purification efficiency. For example, it was possible to retain about 10 5 bacterial cells per 1 cm 2 of material on Ti3C2/Al2O3/Ag/Cumodified polypropylene fibers with a purification efficiency of 99.6% [57].
It should also be noted that, unlike previously obtained coatings on polymeric microfibers [42], ceramic composite filters can be regenerated by heat treatment and re-used.

Conclusions
Macroporous α-Al 2 O 3 ceramics with a porosity of 75% and an average pore size of 187 microns were obtained. The impregnation of the ceramics with Al and Al/Ag nanoparticle suspensions, followed by the water oxidation of the nanoparticles fixed on the ceramic surface, resulted in the formation of nanostructured coatings which comprised boehmite nanosheets and boehmite nanosheets doped with silver nanoparticles, respectively. According to the hydrogen release kinetics in the process of nanoparticle oxidation, the amount of the boehmite nanosheets fixed in macroporous ceramics was determined as 1.1-1.5 wt.%. It was found that, for the filtration dynamic mode of water via macroporous ceramics modified with nanostructures, an increase in the efficiency of S. aureus removal from 76 to 98% was achieved.