Microplasma Synthesis of Antibacterial Active Silver Nanoparticles in Sodium Polyacrylate Solutions

The great demand for functional, particularly biologically active, metal nanoparticles has led to the search for technologically effective, green, and controlled methods of synthesizing these metal nanoparticles. Plasma glow discharge is one of the most promising techniques in this direction. The results of studies based on the synthesis of colloidal solutions of stabilized silver nanoparticles (AgNPs) by the microplasma method in solutions of a nontoxic surfactant sodium polyacrylate (NaPA) are presented. It is shown that AgNPs with a size of 2–20 nm are formed in solutions of 0.05–0.2 mmol·L−1 AgNO3 + 5 g·L−1 NaPA at U = 250 V by tungsten cathode plasma glow discharge. At 20°C, the yellow solutions are formed with λmax ≈ 410 nm, which are stable during long-term storage. It was found that the process of AgNPs formation corresponds to a first-order reaction on the AgNO3 concentration. Its value has little effect on the geometry of nanoparticles, so the Ag(I) concentration in solution is one of the main factors influencing the rate of microplasma synthesis of AgNPs. The antimicrobial activity of synthesized AgNPs solutions against strains of Escherichia coli, Staphylococcus aureus, and Candida albicans was established.


Introduction
Metal nanoparticles, especially silver nanoparticles, demonstrate high efficiency in medicine [1][2][3], sensing [4], catalysis [5], and the agrifood sector [6]. Considering the dependence of the functional properties of MNPs on their shape and size, and in the case of M 1 M 2 NPs, also dependent on the content of components, significant attention has been paid in the last decade to synthesis methods. After all, they are decisive in forming the structure (composition) of nanoparticles and, accordingly, their properties. erefore, the controlled synthesis of nanoparticles is one of the main criteria in choosing a method. e defining criteria also include engineering and "green" technology. e latter is the most studied for the use of these three major biological sources: plant extract [5,7], bacteria [2,3,6,8], and fungi [9]. All of them contain components, which are necessary for the synthesis of colloidal solutions of stabilized metal nanoparticles with the help of reductants, which reduce metal ions (1) and surfactants, which after the formation of MNCs and MNPs (2), provide their stabilization (3).
Sonochemical and sonoelectrochemical production of nanoparticles [10] and plasma glow discharge [11][12][13] are effective for increasing the rate of processes similar to (1)- (3). Plasma electrolysis is one of the relatively new and promising methods for the synthesis of nanomaterials. Significant attention is paid to the microplasma, which is characterized by a relatively low temperature and, at the same time, high "density" of electrons.
is is especially important for synthesizing colloidal solutions of metal nanoparticles using surfactants. Plasma electrolysis allows the controlled formation of particle geometry, which is one of the main requirements of modern nanotechnology. It is also worth noting that plasma meets the requirements of green technologies. is is because plasma does not provide for the use of reducing reagents A feature of the microplasma is the generation of electrons (f − ) during the discharge, which are hydrated in aqueous solutions (4). e latter (f − aq ) reduce metal ions (5) and cause the formation of H· and OH radicals and new compounds, in particular, H 2 O 2 [14]. Some of them are reducing agents for many metal ions (6) and (7). Microplasma synthesis is characterized by its high rate and the absence of a chemical reductant, which minimizes the number of precursors. is method allows using surfactants as stabilizers of MNCs and MNPs, which allows a controlled influence on the formation of their geometry during the stages of nucleation and growth (8).
Surfactants are one of the main factors influencing the formation of MNCs and MNPs, as well as their functional properties and stability during long-term storage [33,[45][46][47][48][49][50][51][52]. e aim of the work is to study the regularities of microplasma synthesis of silver nanoparticles in sodium polyacrylate solutions and establish the conditions of their stable formation. Polyacrylate is a nontoxic synthetic polymeric anionic surfactant that is known to be an effective stabilizer for MNPs [50][51][52][53][54][55][56].

Experimental
Plasma synthesis of colloidal solutions of silver nanoparticles was performed in a thermostated reactor ( Figure 1) with a volume of 100 ml under continuous agitation at 20°C. e platinum plate was used as an anode and tungsten (99.9% purity) wire (∅ � 0.1 mm with a working length of 5 mm) as a cathode. e following electrolytes were used for research: 0.05-0.2 mmol·L −1 AgNO 3 and 5 g·L −1 NaPA (pH8). e acidity value of the solutions was adjusted with a 1 mol·L −1 solution of CH 3 COONa. e synthesis was performed at U � constant � 250 V. e values of current and voltage over time during the glow discharge were measured with a recorder (MTech ADC-UI18), as shown in Figure 2.
TEM images were obtained using a transmission electron microscope JEM-I230 (JEOL, Tokyo, Japan) with an acceleration voltage of 80 kV. e samples for TEM investigations were prepared by drying 0.05 μL of silver sol on the carbon grid at room temperature. e diameters of obtained AgNPs were determined using TEM images by comparing individual particle sizes with the scales presented on images.
eoretical calculations and processing of experimental data were carried out using the software Inconico Screen Calipers 4.0 and OriginPro 8.0. e statistical histograms were obtained using the Origin software pack with standard deviation values as nanoparticle size. In addition, NP size and density were determined by using the public domain Java image processing program ImageJ2. e antibacterial activity of AgNPs was evaluated against Gram-negative Escherichia coli (E. coli) and Gram-positive bacteria Staphylococcus aureus (S. aureus) and Candida albicans (C. albicans). To do this, the bacteria were inoculated into Petri dishes with a solid selective nutrient medium for each species of microorganisms: yellow-salt agar for the culture of S. aureus, Endo agar for the culture of E. coli, and Sabouraud Agar for Candida albicans. Inoculation was performed after 1, 6, 18, and 48 hours of contact of bacteria with 0.8 mm solution of AgNPs. All the biological material was incubated at 310 K for 24 hours in a bacteriological incubator. Antibacterial activity was indexed by counting the number of microorganisms (CFU/mL).

Results and Discussion
At pH >6, the structural elements in the polymer chain of polyacrylate transform from the form of (-CH 2 -CH-COOO) n into the anionic form of (-CH 2 -CH-COO -) n . e fraction of the anionic form increases with an increase in the concentration of PO − ions [52]. In sodium polyacrylate solutions, Ag + cations form soluble [Ag m PA] (n-m)complexes [50,51,53]. e latter are transformed into stabilized nanoclusters and nanoparticles (9) during microplasma glow discharge due to the reduction of Ag (I) to Ag (0) by hydrated electrons and generated radicals and compounds. Stabilization occurs due to the formation of surface complexes between the [−SPP − ] polymer chain of polyacrylate and silver atoms of AgNCs and AgNPs with the formation of a continuous protective layer of polymer surfactant [52].
During microplasma glow discharge, yellow solutions are formed with an absorption maximum of ∼410 nm, the value of which practically does not change during synthesis and long-term storage (Figure 3). e UV-Vis spectra do not show intermediate absorption bands at 310 and 345 nm belonging to "magic" silver clusters of special stability [55,56]. eir formation is characteristic during the reduction of Ag(I) in NaPA solutions, and they are converted into nanoparticles with the formation of "blue silver" that is characterized by wide absorption bands at 490-530 nm [54][55][56].
is difference can be explained by the features of glow discharge synthesis, which causes a high concentration of active reducing agents (mf − aq , mO·, and mH 2 O 2 ) in the solution volume and, accordingly, a high rate of Ag(I) reduction process and the formation of AgNCs and AgNPs (9). Under such conditions, the stability of the "magic" silver clusters, which are charged and contain a small number of Ag atoms (Ag 8 2+ , Ag 9 + ) [57], is complicated. e nature of the UV-Vis spectra with the absorption maximum of ∼410 nm does not change in a wide concentration range of AgNO 3 (Figures 4(a)-4(c)).
Increasing concentration of AgNO 3 increases the increment of optical density value at λ max � 410 nm over time ( Figure 5), which is identical to an increase in the formation rate of AgNPs ( Figure 6).
Analysis of the obtained kinetic curves showed that the formation process of AgNPs during microplasma synthesis in the AgNO 3 −NaPA solutions corresponds to the following first-order reaction: e values of the rate constants of AgNPs formation process (k G ) at initial AgNO 3 concentrations of 0.05, 0.10, and 0.20 mol L −1 are close to -0.212, 0.207, and 0.193 min −1 , respectively. e sizes of nanoparticles do not exceed 30 nm (Figure 7) and depend little on the concentration of Ag(I) ions. So, the latter can be considered one of the main factors influencing the rate of microplasma synthesis of stabilized silver nanoparticles.
As already mentioned, the value of the absorption maximum (∼410 nm) of the synthesized AgNPs solutions practically does not change during long-term storage (Figures 7(a) and 7(c) and 8).
All this indicates the stability of the geometry of the synthesized nanoparticles during long-term storage.   (Tables 2 and 3 and Figure 9). Two different mechanisms can cause antibacterial and fungicidal properties of the synthesized AgNPs solutions: (1) fixation of silver nanoparticles on cell membranes and their penetration into the cell, followed by damage to the membrane and release of the cell contents; (2) release of Ag + ions, which have bactericidal and fungicidal properties [58,59]. e authors [60] consider that silver nanoparticles have the maximum     antimicrobial action with a size of less than 10 nm, as shown in Figure 8(b), and 58% of the silver nanoparticles obtained by microplasma synthesis have sizes less than 10 nm.

Conclusions
In solutions of the nontoxic surfactant sodium polyacrylate (NaPA) and AgNO 3 by tungsten cathode plasma glow discharge at 250 V, the stabilized silver nanoparticles are formed. At the precursor's concentration of 0.05-0.2 mmol·L −1 AgNO 3 + 5 g·L −1 NaPA, AgNPs are formed with a size of 2-20 nm. At 20°C, the yellow solutions are formed with λ max ≈ 410 nm, which are stable during long-term storage. e process of AgNP formation corresponds to a first-order reaction in the AgNO 3 concentration. e latter value has little effect on the geometry of nanoparticles, so the Ag(I) concentration in solution is one of the main factors influencing the rate of microplasma synthesis of AgNPs. e antimicrobial activity of synthesized AgNPs solutions against strains of Escherichia coli, Staphylococcus aureus, and Candida albicans was established.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.  Bioinorganic Chemistry and Applications 9