Regular ArticleMetal and bimetallic nanoparticles: Flow synthesis, bioactivity and toxicity
Graphical abstract
Introduction
As the demand for materials with antimicrobial properties increases, new methods of obtaining metal nanoparticles are constantly increasing [1]. Silver and copper nanoparticles are used as additives to enhance the biocidal activity of medical materials, textiles, paints and varnishes, plastics and other materials [2]. Due to the unexplored mechanisms of interaction of nanoparticles on cells, continuous testing for the potential harmful effects on organisms is required, including the determination of their phytotoxic, cytotoxic and genotoxic properties [3], [4].
The most broadly described nanomaterials are silver nanoparticles (nAg) [5], [6], which are highly effective against bacteria and can be found in many commercial products [7]. Despite the many advantages of nanosilver, such as the low concentrations that are sufficient to limit bacterial proliferation, a wide range of activities and simple methods to produce stable suspensions, materials are also sought that would work well as a biocide whilst limiting the negative effects of nanosilver [8], [9]. Examples of particles with similar effects to nAg are copper nanoparticles (nCu), which also have a high antimicrobial activity, especially antifungal [10]. In addition, nCu is more affordable and more accessible than nAg [11]. The main disadvantage of using nCu is the difficulty in obtaining a stable suspension with a concentration of nanoparticles that ensures sufficient biocidal activity. The process of obtaining nCu is time-consuming, and the nanoparticles themselves usually have a larger size compared to nAg, which may also decrease the biocidal activity of nCu [12].
A combination of the antibacterial properties of nAg and the antifungal properties of nCu allows the generation of a material with a wide spectrum of action against microorganisms [3], [4]. By synthesizing a product consisting of both components, it is possible to reduce the concentrations of individual metals while maintaining similar antimicrobial activity. Such materials may be obtained in single-stage or multi-stage processes, giving bimetallic particles or multi-stage core-shell particles [13], [14]. The sequence of ion reduction affects the biological activity of the final material. In addition, the contribution of individual metals to the product and the particle morphology are crucial for the biocidal properties of the nanoparticles [15]. Hikmah et al. studied the effect of the molar contribution of Ag to Cu on the microstructure and morphology of silver-copper core-shell nanoparticles [16]. Depending on the share of metals, nanoparticles from about 25 to 50 nm were obtained. The increase in the share of copper in the material caused a significant increase in the size of nCu. This may indicate the lower stability of nCu, while nAg were resistant to the influence of process parameters and their size remained unchanged. According to Nazeruddin et al., bimetallic nanoparticles showed higher antimicrobial activity compared to single metals of the same concentration [17]. Antimicrobial activity was tested on Gram-positive bacteria Bacillus subtilis. The growth inhibition zone for nAg and nCu was 15 mm and 8 mm, respectively; it was 18 mm for Ag-Cu nanoparticles, with a 1:1 wt ratio of Ag to Cu, which confirms their synergic effect against Gram-positive bacteria.
The application of silver, copper and bimetallic nanoparticles causes a gradual release of ions into the system, which play a significant role in the case of antimicrobial action. The release of metal ions generates reactive oxygen species (ROS), which inhibit the activity of cell respiratory enzymes, among others. The presence of thiol groups -SH favours the reaction with silver ions, which also increases ROS production. The nAg themselves interact with the bacterial membrane of the cells, causing damage to it, on the one hand, and contributing to the penetration of silver ions into the cell and deactivating it, on the other hand [18].
This paper presents properties of the metal nanoparticles nAg and nCu and the bimetallic nanoparticles nAg-nCu and nCu-nAg, which were synthesised in a flow microwave reactor. The nanoparticle suspensions obtained were characterized in terms of their stability, morphology and particle size. Selected materials were tested for their antimicrobial activity and antiviral properties. Additionally, their cytotoxic, genotoxic and phytotoxic properties were determined to verify potential harmful effects of nanoparticles.
Section snippets
2.1 Materials
Suspensions of metal nanoparticles were synthesised using silver nitrate(V) (), copper(II) sulphate(VI) (, tannic acid ( and sodium hydroxide (NaOH). All reagents were purchased from Sigma Aldrich. The initial salt concentration was constant and selected so that the total concentration of nanoparticles in the final solution was equal to 500 mg/dm3.
Preparation of nMe and nMe-nMe suspensions
Suspensions of metal nanoparticle were obtained in a flow system in a microwave reactor. The schematic diagram of the
Physicochemical characteristics of metal and bimetallic nanoparticles
On the basis of the performed analyses, three types of nAg, nCu, nAg-nCu and nCu-nAg were selected. In the case of the metal nanoparticles, depending on the initial pH parameters, reducer concentration, microwave power and mixture residence time in the reactor, suspensions of nanoparticles were obtained with average particle sizes of 27.4, 53.0, 96.7 nm and 37.7, 52.3, 76.2 nm, for nAg and nCu, respectively (Fig. 1). The distribution of the size of nCu was the highest, as a result of obtaining
Conclusion
This paper presents the antimicrobial properties of suspensions of metal (nAg, nCu) and bimetallic (nAg-nCu and nCu-nAg) nanoparticles produced in a continuous microwave reactor. The suspensions obtained were tested for their antibacterial and antifungal activity (against E. coli, S. aureus and C. albicans) and antiviral activity (on HHV-1 strains: Human alphaherpesvirus 1 and HHV-2: Human alphaherpesvirus 2). Selected systems were tested for their potential cytotoxic, genotoxic and phytotoxic
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sector.
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