Antibacterial potential of nanocomposite-based materials – a short review

1 Introduction

The recent development of nanotechnology increases the number of potential applications of synthetic nano-scale materials (nanopowders, nanocomposites, nanofibers, etc.), the products characterized by sizes below 100 nm in at least one dimension [1]. Examples of practical applications [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] are presented in Table 1.

Among nanoparticle-based materials, antimicrobially active products have become of particular interest lately. The biocidal or bacteriostatic properties of nano-scaled materials such as fabrics, plastics and metal coated with nano-silver as well as nanocomponents based on titanium dioxide, magnesium oxide, copper and copper oxide, zinc oxide, cadmium selenide/telluride, chitosan and carbon nanotubes have been reported [15], [16]. The antibacterial activity of nanosized metal compounds has also been confirmed in both gram-positive (Staphylococcus aureus and Bacillus subtilis) and gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) [3].

Silver-based nanomaterials are those most frequently used as antibacterial agents. Jastrzębska et al. [17] described the antibacterial activity of nano forms of ionic silver, silver monoxide and metallic silver incorporated into alumina nanopowder. The differences in sensitivity of gram-positive and gram-negative bacteria to silver-doped nanocrystalline material was observed by Costescu et al. [18].

The antimicrobially active forms of nanomaterials include salts, oxides, complexes and elemental nanoparticles [19]. Their efficiency is related to the chemical toxicity, small size, and their characteristic shape, which allows them to damage a cell membrane [6]. Nanoparticles can negatively influence the structure of cell membranes including disturbances in its surface loading and permeability. The production of reactive oxygen species (ROS) is probably the most common mechanism of nanoparticles’ influence on bacteria [20]. The mechanisms responsible for antibacterial activity of nanomaterials at the molecular level are summarized in Figure 1.

Figure 1:

The main mechanisms of antimicrobial activity of nanoproducts [2], [21], [22], [23].

Antimicrobial properties of newly developed or modified nanoparticles are usually tested on “model” bacterial strains such as E. coli. The main procedures include a disc diffusion method, turbidity assay and a microdilution method. Several ISO regulations were developed to test the antimicrobial activity of some industrial products, for example, ISO 20743 for textile products and ISO 22196 for plastics and other non-porous materials [24].

2 Antibacterial nanoproducts in medicine

The antibacterial activity of nanoparticle-based materials is helpful in numerous medical applications including implants, wound and burn dressings, medical devices, filters and dental plaque reduction materials [6], [24]. Their potential influence on antibiotic-resistant bacteria, which pose a severe problem in current medical settings, is one of the most essential reasons for application of novel nanocomposites for clinical use [22], [25], [26].

Nanoproducts can be applied as components of coatings for medical devices, modification agents and impregnates for textiles, and “construction” elements for implants, cements and resins (Table 2) [14], [18], [24], [25], [26], [27], [28], [29], [30]. One example application of Ag-nanoparticles is in antibacterial coatings of external ventricular drains and venous catheters which diminish the threat of potential infections [14].

Numerous medical applications of nanoproducts are based on chitosan and its derivatives (chitosan-carbon dots, chitosan-i-cysteine quantum dots, chitosan-based biosensors and biomarkers) [31] since chitosan nanoproducts have been shown to have potential antimicrobial properties [32]. Chitin- and chitosan-based materials have been proposed for application in scaffolds and wound healing materials. Archana et al. [23] prepared a wound healing film based on chitosan and polyvinylpyrrolidone with an addition of silver oxide nanoparticles which showed significant antibacterial activity towards S. aureus (stronger effect) and E. coli (a less intensive impact). Some newly synthesized chitin/nanosilver composites were characterized by their influence on E. coli and S. aureus, revealing proper blood-clothing capacity [27], [29].

The chitosan-poly(N-vinylpyrrolidone)-TiO₂ nanocomposite was proposed as a wound dressing material due to its significant antibacterial impact against P. aeruginosa, E. coli, S. aureus and B. subtilis and its ability to intensify the wound treatment. The titanium dioxide nanocomponent was suggested to be responsible for the adsorption of bacteria and their inactivation [33].

Srivastava et al. [34] developed a nanocomposite based on chitosan and 4-(ethoxycarbonyl) phenyl-1-amino-oxobutanoic acid supplied with nano-Ag. The obtained nano-film was proposed as a material for application in medicine due to its effect on bacteria including S. aureus, E. coli and P. aeruginosa [34].

A potential wound healing nano-based material composed of genipin-crosslinked chitosan, poly(ethylene glycol), zinc oxide and silver produced significant antibacterial activity in tests with E. coli, P. aeruginosa, S. aureus and B. subtilis [35]. Vimala et al. [36] achieved a significant antimicrobial impact for chitosan films with the addition of silver nanoparticles (12 nm), making them potentially useful in wound dressings also. Li et al. [37] described the antimicrobial effect of the mixture of silver nitrate and titanium dioxide nanoparticles applied on facemasks. During the tests they observed 100% reduction in viable E. coli cells after 48 h.

Singh et al. [38] dispersed magnetic nano-α-Fe₂O₃ in a chitosan matrix resulting in a magnetic nanocomposite film with potential applications in biosensors and tissue engineering. The antibacterial activity of chitosan-containing composites used for tissue engineering was then confirmed by Archana et al. [39].

Nowadays, it is stressed that nanoparticle production for medical purposes should avoid the usage of highly toxic chemical reagents, especially in the case of Ag-containing materials. Therefore, a “green synthesis approach” is considered. For example, starch-protected Ag nanoparticles can be obtained from aqueous solutions of AgNO₃, glucose and starch. These solutions allow the production of nano-Ag films (50–200 nm), Ag hydrosols (20–50 nm) and Ag colloids through the process of Ag(NH₃)₂⁺ reduction by carbohydrates. The Ag(NH₃)₂⁺ can also be reduced using n-hexadecyltrimethylammonium bromide. Irradiation of Ag⁺ ions is proposed as an alternative “green” method for the creation of Ag-nanoparticles for medicine [11].

Recently, metal nanoclusters have appeared as new functional materials. They consist of several to tens of atoms that result in a particle core size below 2 nm. Ultrasmall metal particles can differ in chemical and physical properties (geometric structure, molecular magnetism, molecular chirality and luminescence) from traditionally used nano-metals. They can also reveal significant antimicrobial properties, making them useful as disinfecting agents [40].

Nanoclusters based on Au, Ag and Cu can be widely applied in medicine and biosensing [41]. The mechanism for the elimination of pathogenic bacteria in the presence of Ag-nanoclusters most likely depends on the generation of ROS according to Zhang et al. [42], while the cellular toxicity of the nanoclusters can be related to the core surface speciation [43]. Their ultrasmall size allows them to achieve a significantly higher surface-to-volume ratio, increasing the antibacterial effect caused by ROS generation and interaction with intracellular components [40]. There is a possibility that combining Ag nanoclusters with drugs can intensify the therapeutic effects. Damage to microbial DNA was observed in the presence of combined daptomycin-Ag nanoclusters [44]. Zheng et al. [40] described thiolate-protected Ag ~1 nm nanoclusters which can be applied as antimicrobial agents. Yuan et al. [45] reported the synthesis of highly luminescent, stabile Ag nanoclusters that revealed remarkable antibacterial activity against a multi-resistant strain of P. aeruginosa. Some new perspectives appear in the area of “molecularly pure” metal nanoparticles of a high precision of atomic structure [46].

Nano-scale materials can be widely used in dentistry as well. Uses include not only nano-apatite liquids and pastes for teeth biofilm control but also products for remineralization of tooth enamel lesions, resin-based composites and implant materials [47], [48].

Acrylic resins are commonly used polymers for denture-based applications. Kassaee et al. [49] added 38 nm silver nanoparticles into a commercial acrylic resin. The obtained dental composite revealed improved mechanical properties accompanied by significant antimicrobial activity against E. coli. Aydin Sevinç and Hanley [48] showed that 10% (w/w) fraction of zinc oxide nanoparticles applied in dental composites limited the growth of one-species dental biofilm created by Streptococcus sobrinus ATCC 27352 strain reducing bacterial number by 80%. However, the effect was not so significant in the case of a three-species biofilm.

Application of polyethyleneimine nanoparticles resulted in antibacterial efficiency of composite resins against biofilm-producing Streptococcus mutans. The antibacterial properties of the material were observed even in 6-month-aged samples. The total growth inhibition of S. mutans was observed during direct contact tests, confirming the suitability of the material for clinical purposes [50]. A long-lasting antibacterial effect was also observed for dental composite resins supplied with 1% of immobilized quaternary ammonium polyethylenimine nanoparticles [51], [52].

Amorphous calcium phosphate nanocompounds with a quaternary ammonium dimethacrylate and nano-silver caused a 10-fold reduction in the number of S. mutans cells in biofilms as well as a diminution of the production of lactic acid and metabolic activity [53].

3 Fibers and fabric

The control and prevention of bacterial proliferation is essential not only for medical materials like bandages and dressings but also in case of clothes and domestic-use textile products. Textiles can easily serve as the substratum for microbial growth. Therefore, numerous projects are being undertaken to develop fibers and textiles with “self-cleaning properties”, allowing them to limit microbial contamination.

Nanoparticle-based coatings and impregnates are also of large interest nowadays. Nanocompounds like TiO₂, silver, ZnO, copper, gold and gallium-based nanoparticles, as well as some nano-clays, are applied due to their antimicrobial properties and their potential activity against microbial pathogens [3], [7], [54].

Silver-based nanoproducts are those most recommended for fabric modifications. The antimicrobial influence of nanosilver-based colloidal solutions applied on textile fabrics was observed by Lee et al. [55]. The cellulose/cotton fiber can be easily modified by the nano-additives (Table 3).

Escherichia coliand S. aureus were sensitive to nano-Ag-containing poly(methyl methacrylate) nanofiber described by Kong and Jang [64]. Rujitanaroj et al. [65] developed ultrafine gelatin fiber mats with nano-Ag (11–20 nm). Smooth fibers of the material, with average diameter of 230 and 280 nm, revealed activity against bacterial strains typical for wound infections, with the efficiency as follows: P. aeruginosa >S. aureus >E. coli > methicillin-resistant S. aureus.

Antibacterial properties can also be achieved in synthetic fibers such as nano-Ag modified nylon and polyester [57]. Yeo et al. [66] developed nano-Ag/polypropylene nanocomposite fibers characterized by permanent antibacterial activity against S. aureus and Klebsiela pneumoniae. Perkas et al. [67] achieved significant antimicrobial effect for the nano-Ag/nylon 6,6 composite with 1% of metallic silver in the form of 50–100 nm nanocrystals. Polyamide 6 with 2% of silver nanoparticles revealed the antibacterial effect against E. coli, even after 100 days of immersion in water [68].

Some silver-containing dressings are available commercially. Applying the tests with bacteria (S. aureus, E. coli) and yeasts (Candida albicans), Thomas and McCubbin [69] confirmed the antimicrobial activity of three commercial dressings: Acticoat (Smith and Nephew), Actisorb Silver 220 (Johnson and Johnson) and Contreet-H (Coloplast).

Not only silver but the other metals in nano form can also be applied to imbue fibers and fabrics with antibacterial properties. The positive effect can be obtained for nano-Cu-modified fabrics [19], [70]. The impregnation of cotton and polyester fibers with cationic copper (3–10%) resulted in broad-spectrum activity towards viruses, bacteria and fungi [71]. Vigneshwaran et al. [72] observed the activity of cotton fabric, impregnated with zinc oxide/soluble starch nanocomposite, against S. aureus and Klebsiella sp.

Antibacterial properties can also be achieved by application of non-metallic nanoparticles. Wang et al. [73] investigated the activity of nano-SiO₂ grafted on wool. The best results were obtained for wool fibers with 4.3% of the nanocompound.

4 Paper

Paper characterized by antimicrobial properties can be very useful in different applications, including packaging and filtering.

Gottesman et al. [15] described a method for the production of colloidal silver-coated paper, with a uniform coating of 90–150 nm, which is very stable due to the silver penetration into the paper. The material inhibited the growth of E. coli and S. aureus. Cellulose-based filter paper supplied with the entrapped silver nanoparticles can be obtained following the methods found in Tankhiwale and Bajpai [74]. Ghule et al. [75] applied ultrasound in the process of production of nano-ZnO (20 nm)-coated paper. Both the cellulose-based filter paper and nano ZnO-coated paper revealed excellent antibacterial properties in tests with E. coli.

A paper-like ceramic matrix characterized by antimicrobial properties was developed by Koga et al. [76]. The authors used ceramic fibers as a framework for the spontaneous formation of silver nanoparticles on ZnO whiskers. The disc diffusion test with E. coli showed that the material had a higher antimicrobial efficiency with the silver-modified paper.

5 Ceramics, glass and fiberglass

Antibacterial ceramic or fiberglass can be successfully applied for the removal of bacteria, especially in drinking water treatment. Lv et al. [77] developed a cheap and effective ceramic material modified with 3-aminopropyltriethoxysilane and supplied with nano-silver that allowed for 100% removal of E. coli from water at a 0.01 l/min flow rate. No significant release of the nanoparticles from the material was observed during tests including a 15 min ultrasound treatment followed by storage for several weeks. Its high stability makes it potentially applicable for drinking water treatment. Kallman et al. [78] described the results of tests using nano-Ag-modified ceramic filters. The filters were used by people living in a peri-urban community. The presence of nano-silver additives increased the efficiency of E. coli removal by up to 92% and 87% after 12 and 23 months, respectively.

A novel bactericidal fiberglass impregnated with silver nanoparticles (below 1.8%) was described by Nangmenyi et al. [79]. The 50 mg/l of the tested material revealed a 7 log reduction of the number of E. coli cells after 5 min. The fiberglass can be regenerated by thermal treatment at 350°C. A similar material supplied with nano-Ag and Fe₂O₃ revealed both antibacterial and antiviral properties, with a 100-fold decrease in bacterial number after 1 min (from 10⁶ cfu/ml to 10⁴ cfu/ml) [80].

6 Membranes

Antibacterial membranes are very useful in water and wastewater treatment as well as in some branches of industry (biotechnology and food and drug production). It was revealed that the antimicrobial activity of nanoparticle-modified membranes makes them less sensitive to biological fouling [81], [82]. Membrane modification with TiO₂, SiO₂, ZnO and Ag nanoparticles all limited the microbial growth [83]. Therefore, it is possible to apply many of them in environmental applications [84].

Fibrous membranes composed of nano-Ag and a poly-l-lactic acid (Ag/PLLA) with a silver content of 5% (w/w) were proposed as a scaffold for tissue engineering. The membranes revealed antibacterial activity towards E. coli and S. aureus [85]. A strong effect on both gram-positive and gram-negative bacteria was noticed for nano-Ag/cellulose composite membranes [86]. Jain and Pradeep [87] obtained nano-Ag-coated polyurethane foam which lowered the number of E. coli in water from 10⁵ cfu/ml to 0 cfu/ml when used at a flow rate of 0.5 l/min. The authors stated that nano-silver is not washed away from the material, which, therefore, recommends its use as a potential antibacterial drinking water filter. Nanoporous alumina membranes coated with titania nanocrystals were also recommended for water purification systems. Their antibacterial activity was confirmed in tests with E. coli [88]. Jame and Zhou [89] described electrochemically active carbon nanotube filters also suitable for effective wastewater and water treatment. They showed the ability to remove biological agents, including viruses, and to limit the biofouling of the filter.

Liu et al. [90] obtained a water treatment membrane, which was highly and uniformly loaded with Ag nanoparticles, demonstrating antibacterial resistance towards E. coli up to 14 days under laboratory conditions. Brady-Estévez et al. [91] reported significant antibacterial and antiviral activity of a filter membrane coated with single-walled carbon nanotubes.

The properties of polyethersulfone-based products, commonly used in the food processing industry and medicine, can be improved by addition of silver nanoparticles. Cao et al. [92] obtained polyethersulfone membranes coated with nano-Ag with antibacterial effects towards S. aureus, Staphylococcus albus and E. coli. The antibacterial features were also observed for a poly(vinylidene fluoride)/sulfonated polyethersulfone blend membrane modified with TiO₂ nano-particles and activated by UV irradiation [93].

7 Food packaging

Food packaging should support a comparatively long shelf life for products, providing proper food quality and microbiological safety. Packages characterized by antimicrobial properties limit the growth of microorganisms responsible for food spoilage and pathogenic effects. Nanotechnology allows the development of new kinds of food packaging materials that combine good barrier properties and antimicrobial activity [94], [95]. Gold, silver or copper nanoparticles, nano-metal oxides and organic nano-compounds (for example starch-clay nanopreparations) are usually applied inside of packages [5], [74], [96], [97].

Cárdenas et al. [98] proposed a composite food packaging material obtained by supplementation of a chitosan matrix with colloidal nano-Cu. The material revealed good barrier properties (reducing oxygen and water vapor permeability and providing UV light protection) and antibacterial activity against Salmonella enterica and S. aureus. It was confirmed that chitosan provides a significant antimicrobial effect towards bacteria and fungi responsible for food decay as well as potentially pathogenic microorganisms [99]. It makes chitosan-based materials suitable for food protection and packaging [100], [101], [102]. The food packaging film developed by Tripathi et al. [103], which utilized a solution casting method to obtain a chitosan-silver oxide nanomaterial, revealed an outstanding antimicrobial effect against E. coli, P. aeruginosa, S. aureus and B. subtilis.

Antimicrobial properties were also confirmed for TiO₂-based nanopowders, nano-xerogels and nano-aerogels used as additives to different packaging materials [104]. Antibacterial effects against Alicyclobacillus acidoterrestris were observed for a Ag-containing polyethyleneoxide-like coating layered on polyethylene [105]. A significant reduction of the number of Lactobacillus plantarum bacteria in orange juice was observed in the case of nanocomposite-based packaging supplied with Ag and ZnO nanoparticles [106].

Silver nanoparticles present in absorbent pads were able to prevent growth of E. coli and S. aureus [107]. Fernandez et al. [108] developed a silver-based antibacterial hybrid material that was able to significantly reduce the total number of Pseudomonas spp. and Enterobacteriaceae bacteria in absorbent pads in meat storage containers.

It was observed that polypropylene films supplied with TiO₂ were able to inhibit growth of E. coli [109]. The promising results of zone inhibition tests and growth kinetics analysis on the same bacterial species confirmed the antibacterial properties of a starch-coated polyethylene film loaded with ZnO nanoparticles [110]. Díaz-Visurraga et al. [111] produced semitransparent composite films made of TiO₂-modified chitosan revealing the activity against S. enterica, E. coli and S. aureus. Rhim et al. [112] developed four types of chitosan-based antibacterial composites embedded with nano-Ag, Ag-zeolite, as well as modified and not modified montmorillonite.

8 Household products and cosmetics

Antibacterial properties related to the presence of nano-compounds can be provided to some household products like paints and cleaners also. Antimicrobial activity of Ag nanoparticles makes them useful for production of foot powders, soaps and sprays [28]. Metallic nano-silver and titanium nanoparticles are applied in paints [7], [113]. Kumar et al. [114] reported a synthesis method of applying an oxidative drying process in order to reduce metal salts and disperse metal nanoparticles in oil which allowed the creation of a metal-nanoparticle-embedded paint. The nanoparticle-in-oil mixture may be used directly as paint on wood, metal, glass and polymer surfaces. The bactericidal properties of the paints were tested against S. aureus and E. coli.

Antimicrobial features of Ag nanoparticles allow their application as preservative agents in cosmetics. Nano-Ag-based products are used in deodorants to reduce the number of bacteria responsible for the appearance of odors. Ag-zeolite powder spray revealed an antibacterial activity in the case of skin-resident bacteria [115]. The results of the research of Kokura et al. [116] suggested that antimicrobial Ag nanoparticles did not penetrate into human skin.

Aoshima et al. [13] proposed a material which can be potentially applied in the production of cosmetics. They observed that hydroxylated derivatives of fullerens (fullernenols) can inhibit bacterial growth. One of them, C₆₀ (OH)₄₄, was characterized by significant antibacterial activity against such microorganisms as Propionibacterium acnes, Staphylococcus epidermidis, C. albicans and Malassezia furfur.

9 Sensors

Antimicrobial applications of nanoproducts cover not only bactericidal and bacteriostatic activities but also the possibility to detect potentially pathogenic microorganisms in medicine and food. For this purpose, some nanoparticle-based sensors were developed. This application of nanoproducts allows the achievement of higher sensitivity, stability, and selectivity of biosensors [4]. The nanoproducts, including bio-conjugated materials, can be effectively used in selective bacterial sensing, biological imaging and therapy without traditional drugs [117], [118], [119].

Wang et al. [117] developed a kind of glyconanoparticle useful in biosensing by means of combining gold and iron oxide nanoparticles with underivatized carbohydrates. The function of such biosensors covers both ligand-receptor biological interactions and specific physicochemical features of the nanomaterial.

Carbon nanotubes, metal nanoparticles, nanowires and other nanocomposite structures are used to design sensors and biosensors applicable to the food industry. Nanoparticle-based biosensors are effectively used in food analyses to detect such microorganisms as Salmonella spp., Salmonella typhi (Au) and E. coli (Au, Cu/Au and Silica/Ag) [4]. Nano-sensors facilitate the control and monitoring of food quality, especially during storage and transport [95].

Yang et al. [120] described conjugated biosensors consisting of biomolecules and nanomaterials with applications as bio-detectors for pathogenic microflora in food. The authors stated that an application of these biosensors causes a significant reduction in pathogen detection time and improves the sensitivity all the way up to a single bacterial cell.

10 Nanoparticle-based materials as an environmental challenge

Besides all of the benefits, the application of nanoparticle-based materials should be considered as a potential source of emission of substances that are potentially harmful to the natural environment [8], [54], [121], [122]. The production and application of newly synthesized nanoproducts results in the appearance of new types of wastes with special properties and reactivities [7], [122], [123]. Therefore, their specific disposal and landfilling requirements need to be considered [124]. Numerous literature confirm the negative interactions of engineered nanoparticles with the environment including ecotoxicological effects in living organisms [9], [125], [126]. The extensive application of nanomaterials results in their appearance in aquatic ecosystems. The ecotoxicity of silver nanoparticles has been confirmed in the case of microorganisms (E. coli, P. aeruginosa and nitrifying bacteria), invertebrates, vertebrates, algae (Thalassiosira pseudonana and Synechococcus sp.) and higher aquatic plants [42]. Al₂O₃ nanopowders inhibited growth of algae such as Scenedesmus quadricauda [127]. ZnO nanoparticles were shown to be harmful to tadpoles of Xenopus laevis, the algae Pseudokirchneriella subcapitata and the fish Danio rerio [126]. High concentrations of Al₂O₃, TiO₂ and ZnO were toxic to the crustacean Daphnia magna [128]. TiO₂/Ag, TiO₂/Au and TiO₂/SiO₂ nanocomposites inhibited the germination and early growth of higher plants (Sorghum saccharatum and Lepidium sativum) in a Phytotoxkit test (Figure 2) [129].

Figure 2:

The influence of TiO₂/Ag and TiO₂/Au nanocomposites (1000 mg/kg of soil) on plant germination and early growth. From the left: control culture, TiO₂/Ag (1 g/kg) and TiO₂/Au (1 g/kg) (photograph by E. Karwowska).

Examples of the potentially negative influence of nanoparticles based on silver, titanium, zinc oxide, carbon nanotubes and fullerenes (C₆₀) on the wastewater treatment process have also been shown [130]. They are widespread in domestic and industrial wastewater. However, at the moment only very low cytotoxicity of ZnO, Ag and TiO₂ nanoparticles have been reported in the case of the human gastrointestinal system [1].

In the environment, organisms can be exposed to nanoparticles via ingestion, inhalation and penetration through the skin surface. The inflammatory cell response can be activated even from a low concentration of nanoparticles. The induction of genes encoding pro-inflammatory cytokines was observed for a nanoparticle concentration of only 10⁻⁷ g/ml [131].

It was estimated that up to 14% of commonly used Ag-based nanoproducts can be a source of nanoparticle emission into the air, making them potentially harmful via inhalation [132]. Therefore, the fate of nanoproducts should be thoroughly investigated at the molecular, cellular and community level including their effect on environmental conditions such as pH, UV radiation, salinity and interactions with organic matter [6], [125], [133], [134]. Interestingly, nanoclusters appear to be less toxic compared with traditional nanoparticles. For example, the ecotoxicity of Ag-nanoclusters is rarely reported. Moreover, the investigation on the impact of strongly red-emitting fluorescent silver nanoclusters on green algae, Scenedesmus obliquus, revealed that the algal growth inhibition was not related to ROS production [42].

11 Perspectives and conclusions

Current literature shows that the most promising areas of nanotechnology applications is in the development of new antibacterial agents. There is evidence that they can be used in numerous environment protection technologies. The “molecularly pure nanoparticles” appear to be the best perspective due to their high precision of atomic structure. Nanocluster engineering can broaden the application of Ag- and Au-based antimicrobial preparations, although there is still a need for more research on the stability, distribution in biological systems, and potential toxicity and ecotoxicity of the nanoclusters.

For many potential applications, the key problem is in trying to obtain a product which is selectively active against potential pathogens yet harmless to other organisms. The investigated concentrations of nanoproducts should be relevant to those present in consumer products. The commercial application of nanoproducts should also be carefully monitored because of their potential negative environmental effects. It should be considered that the development of nanosciences can appear not only as a technological advantage but also as a kind of threat and new challenge in environment protection.