PLASMA-CHEMICAL FORMATION OF SILVER NANODISPERSION IN WATER SOLUTIONS

The synthesis of stable concentrated aqueous dispersions with desired physicochemical properties on the basis of silver nanoparticles is a necessary step during the further creation of nanostructured materials. The latter are currently used in microelectronics, electrochemistry, synthesis of optoelectronic sensors, pigments, decontamination materials, etc. [1]. In addition, a wide range of antimicrobial action of silver nanoparticles allows creating various drugs and biomedical materials with prolonged antimicrobial action [2]. The constraint to the wide involvement of water dispersions of silver nanoparticles in various industries is the lack of a universal synthesis method. This method should allow synthesizing storage-stable silver dispersions with a controlled particle size using minimum reagents and technological operations. At present, there is a significant number of methods for the synthesis of silver nanodispersions, providing nanoparticles of different composition, morphology [3]. However, the variety of technological methods of synthesizing colloidal nanodispersions causes differences in properties. Moreover, with all the benefits, most of the existing methods require the use of reducing reagents, temperature regime maintenance, several technological stages, etc. These parameters narrow down further practical application and increase the cost of silver nanodispersions. In recent years, the use of plasma discharge for the synthesis of silver nanodispersions has become widespread. The processes based on plasma technologies using different plasma discharge generation units are considered promising and competitive. Most of them have not gone beyond laboratory studies, are multi-stage and power-intensive. Given this, the development of innovative high-performance plasma-chemical methods aimed at synthesizing nanosized silver particles and concentrated water dispersions on a basis is relevant.


Introduction
The synthesis of stable concentrated aqueous dispersions with desired physicochemical properties on the basis of silver nanoparticles is a necessary step during the further creation of nanostructured materials. The latter are currently used in microelectronics, electrochemistry, synthesis of optoelectronic sensors, pigments, decontamination materials, etc. [1]. In addition, a wide range of antimicrobial action of silver nanoparticles allows creating various drugs and biomedical materials with prolonged antimicrobial action [2]. The constraint to the wide involvement of water dispersions of silver nanoparticles in various industries is the lack of a universal synthesis method. This method should allow synthesizing storage-stable silver dispersions with a controlled particle size using minimum reagents and technological operations.
At present, there is a significant number of methods for the synthesis of silver nanodispersions, providing nanoparticles of different composition, morphology [3]. However, the variety of technological methods of synthesizing colloidal nanodispersions causes differences in properties. Moreover, with all the benefits, most of the existing methods require the use of reducing reagents, temperature regime maintenance, several technological stages, etc. These parameters narrow down further practical application and increase the cost of silver nanodispersions. In recent years, the use of plasma discharge for the synthesis of silver nanodispersions has become widespread. The processes based on plasma technologies using different plasma discharge generation units are considered promising and competitive. Most of them have not gone beyond laboratory studies, are multi-stage and power-intensive. Given this, the development of innovative high-performance plasma-chemical methods aimed at synthesizing nanosized silver particles and concentrated water dispersions on a basis is relevant.

Literature review and problem statement
Today, one of the most innovative, environmentally safe and promising methods of synthesis of nanosized compounds is the use of plasma discharges [4,5]. Discharges in which electrodes (one or both) are low-conductivity liquids (water spectrophotometer using quartz cuvettes in the wavelength range of 300-700 nm. Microphotographs of the nanoparticles were obtained on JEOL JSM-6510LV (France) and REM-106I (Ukraine) scanning microscopes. The disperse phase of the solution, obtained by plasma-chemical treatment, dried in the air at 25 °C was investigated using the Ultima IV Rigaku X-ray diffractometer (Japan).
The yield of silver nanoparticles was estimated by the difference between the silver ions in the initial solution and after plasma discharge treatment. For measurements, the "ELIS-131Ag" silver-selective electrode was used.
Infrared spectroscopy was performed on the (NICO-LET5700) spectrometer (USA) to form a KBr tablet, in the frequency range of 400-4,000 cm -1 .

Results of the research on the efficiency of plasma-chemical synthesis of silver nanoparticles in comparison with conventional methods
The efficiency of the plasma-chemical method of synthesizing silver nanoparticles in comparison with the conventional method of chemical reduction in solutions and photochemical deposition is investigated.
According to the Mie-Drude theory, optical properties of colloidal solutions of metal nanoparticles are characterized by the presence of a pronounced resonance absorption band of surface plasmon resonance (SPR) in the visible spectrum. The positions of SPR peaks serve as the characteristics of both the form of existence of colloidal silver in the aqueous medium, and the size [17,18]. It is now well established that the maximum absorption of 10-80 nm silver nanoparticles is characterized by the maximum absorption in the range of 395-465 nm [19].
The analysis of the data obtained ( Fig. 2, curve 2) shows that treatment of the silver nitrate solution by low-temperature plasma discharge on the spectrum results in the SPR absorption peak (λ max ) at 420 nm (D=0.83), characterizing the formation of silver nanoparticles. Application of plasma discharge in the presence of sodium alginate stabilizer ( Fig. 2, curve 1) contributes to a sharp increase in the SPR peak intensity to D=2.7. A slight shift to the short-wave region to 414 nm, corresponding to the nanoparticles is observed. Such data can be explained by the formation of a larger amount of silver nanoparticles and/or size reduction.
The possible reason is that an increase in the number of carboxyl and hydroxyl groups of sodium alginate facilitates the complex formation between Ag + and the polymer molecular matrix. Moreover, a greater number of hydroxyl groups, as is known [20], contributes to Ag + reduction.
Chemical synthesis of silver nanodispersions was carried out by the reduction of 0.5 g/l silver nitrate by ascorbic acid (1.0 g/l) in the ratio of 1:2, respectively, without/in the presence of sodium alginate stabilizer while stirring for 15 min.
Photochemical synthesis of silver nanodispersions was carried out by treatment of 0.5 g/l silver nitrate solution on the UV installation, the irradiation range 230-400 nm, DKB-7 lamp (7 W) for 15 min.
Application of conventional methods, as expected, also allows synthesizing silver nanoparticles with a characteristic absorption peak (λ max ) at 414-430 nm, but lower absorption intensity (D=0.35-0.43), compared to the plasma-chemical method. This is known to indicate a lower concentration of the formed silver nanoparticles [16][17][18]. To confirm the preliminary data and efficiency of the investigated methods for synthesizing silver nanoparticles, the quantitative yield of silver nanoparticles in dispersions was estimated. Table 1 shows the yield of silver nanoparticles depending on the synthesis method. Photochemical deposition 20.0 The data obtained agree with the previous results. CNP discharge gives a highly effective yield of silver nanoparticles and provides the yield of Ag 0 silver particles of 95.1 % without the stabilizer and 97.17 % under plasma discharge in the presence of sodium alginate stabilizer.
The yield of silver particles, when synthesized by the conventional chemical method, is 93.9 %, by the photochemical -20 %. Thus, application of CNP for synthesizing silver nanoparticles, both using the AlgNa stabilizer and without it is an effective way compared to the known conventional methods. In addition, it should be noted that the duration of plasma discharge treatment for the formation of silver nanodispersions is 3-5 min, compared with the duration of synthesis by conventional methods (15 min).
In addition to the yield efficiency of the particles, description of the properties of the synthesized dispersions is necessary in the study of new synthesis methods. One of the main parameters that characterize silver dispersions and determines the further practical application is aggregate stability. The level of aggregation of metal nanoparticles can be effectively estimated by changing the absorption characteristics: the shift of the SPR peak in the spectrum and its intensity. Fig. 3 shows the spectra of silver nanodispersions synthesized by different methods when stored for 4 days. The analysis of the data obtained shows a high aggregate stability of plasma-chemically synthesized nanodispersions compared to the methods of chemical reduction in solutions and photochemical deposition. In the plasma-chemical synthesis, in the spectra of silver dispersions (Fig. 3, a, b), there are no significant changes in the SPR band intensity and structure in 4 days after the synthesis. These results indicate the absence of rapid aggregation processes in silver dispersions. In the spectra corresponding to the chemical and photochemical synthesis methods, a significant decrease in the absorption intensity, indicating the coagulation of nanoparticles and possible recrystallization, is observed in 4 days due to the SPR peak shift.
To further evaluate changes in the properties and stability of silver dispersions synthesized by different methods, changes in the acidity of freshly synthesized silver solutions during storage were investigated as a pointer to dimensional changes (Fig. 4). Unlike other methods, the plasma-chemical synthesis of silver nanodispersions is characterized by the absence of a clear change in pH during storage.
Characteristics of dimensional and morphological parameters of silver nanodispersions synthesized by chemical and photochemical methods are sufficiently investigated today and some are given in [20,21]. It was of scientific interest to determine the dimensional characteristics of plasma-chemically synthesized silver nanoparticles. According to the electron microscopy data, a dispersed phase of particles up to 100 nm is formed under plasma discharge without and in the presence of AlgNa (Fig. 5). The solid phase of both samples of plasma-chemically synthesized silver nanodispersions was investigated by the X-ray diffraction analysis.
In both cases, the data obtained (Fig. 5)  corresponding to silver metal particles in accordance with [22]. As shown by the data on the quantitative yield of nanoparticles during the plasma-chemical synthesis, sodium alginate, probably, acts as both the stabilizer of the formed nanoparticles, and additional reducer.
The interaction of AgNP S with sodium alginate is confirmed by the analysis of IR spectroscopy (Fig. 7). In the AlgNa alginate spectra, the peak within 3,200-3,600 cm -1 corresponds to deformation oscillations of the hydroxyl group bond. Peaks at 1,412 cm -1 and 1,591 cm -1 characterize -COO-symmetric and asymmetric deformation oscillations, respectively. The small and expanded peak within 1,327-1,370 cm -1 can be explained by the C-O bond. Two peaks at wavelengths of 1016.5 cm -1 and 1,078 cm -1 are characterized by deformation oscillations of C=O and C-O-C. Absorption peaks within 1,030-1,200 cm -1 are characteristic of natural polysaccharide.
When comparing the alginate spectrum with AgNP S /Alg, one can observe a significant shift of the wave position from 1591 cm -1 for alginate to 1,640 cm -1 for AgNP S , with a decrease in intensity. This shift confirms the interaction of oxygen carboxyl groups in the alginate structure with AgNP S . The peak shift within 3,200-3,500 cm -1 was negligible, but the decrease in intensity was sharp. Changes in position and intensity are related to the probable interaction of AgNP S and hydroxyl groups of alginate. The single peak expanded from 1,017 to 1,084 cm -1 can be attributed to the chemical transformation during the reduction of Ag + to Ag 0 [23]. Fig. 3. Dependence of the absorption intensity (D) on the wavelength (λ) of silver nanodispersions synthesized by different methods during storage: a, b -plasmachemical synthesis without (AgNO 3 =0.5 g/l, τ=5 min, I=120 mA, P=0.8 MPa) and in the presence of AlgNa (AgNO 3 =0.5 g/L, AlgNa=5.0 g/l, τ=5 min, I=120 mA, P=0.8 MPa); c, d -chemical deposition without and in the presence of AlgNa (AgNO 3 =0.5 g/l, ascorbic acid 1.0 g/L, AlgNa=5.0 g/l, τ=15 min); d -photochemical deposition of AgNO 3 =0.5 g/l, τ=15 min) sodium alginate solution during irradiation by the CNP discharge was proposed (Fig. 8).
For the formation of AgNP S , the standard mechanism involves two stages, namely, formation and polymerization of atoms. In the first stage, a part of metal ions in the solution is reduced by available reduction groups. The atoms formed in this way act as nucleation centers and catalyze the reduction of residual metal ions in the solution. Compared with other water-soluble polymers, alginate is an anionic polymer with high charge density. Negatively charged alginate facilitates the attraction of positively charged silver ions to polymer chains, which are then reduced by means of existing groups. Preformed silver atoms adsorb Ag + on the surface through dimerization or association with excess ions due to the binding energy between metal atoms. Surface ions are decreased again, then the atoms merge, leading to the formation of metal clusters. Thus, the aggregation process does not stop and leads to the formation of larger particles. The process is stabilized in the presence of sodium alginate, preventing further coalescence. Probably, the metal clusters will be fixed through strong bonds between the AgNP S surface and the "O" atom of the (СOO-and OH) functional group of sodium alginate [23]. So, the negative surface charge of alginate carboxyl groups stabilizes silver nanoparticles against aggregation due to electrostatic and steric effects.
Thus, the complex effect of active compounds of plasma discharge provides an almost complete reduction of silver ions in the solution to metallic silver. In this case, the simultaneous presence of the sodium alginate stabilizing reagent in the solution prevents aggregation of the formed particles. As a result, it is possible to synthesize silver dispersions with a particle size up to 100 nm. The synthesized dispersions are environmentally friendly and storage stable. This will allow expanding the areas of their application and increasing the antimicrobial functionality of materials on their basis, as well as producing materials on their basis with multifunctional properties and increasing the competitive capacity of known materials.
The research was conducted using a wide range of analysis methods. The possibility and efficiency of plasma discharge application for the controlled formation of silver nanoparticles are also confirmed by quantitative particle yields. The dimensional and morphological characteristics are given.
For a more detailed description of the plasma-chemically synthesized silver nanoparticles, determination of the size distribution and characterization of antimicrobial properties of dispersions were expedient.
Optimizing factors of plasma-chemical effects on water solutions are discharge current strength, reactor pressure, plasma effect duration, solution concentration, temperature and acidity. Further research may be aimed at determining the influence of these factors on the formation of silver nanodispersions. The study of the effect of the specified parameters on antimicrobial properties of the synthesized silver dispersions can also be a probable development. The main difficulty of further research is the transition from the discrete-type laboratory reactor to the flow industrial one.

Conclusions
1. The possibility of applying the discharge of contact nonequilibrium low-temperature plasma for synthesizing silver nanodispersions from water AgNO 3 solutions is established. The efficiency of plasma-chemical synthesis without the use of additional reducing reagents and in the presence of sodium alginate stabilizing reagent is shown. The process is more efficient in the presence of the stabilizer.
2. It is found that the yield of silver nanoparticles in the plasma-chemical synthesis is higher by 1.5-2 % and 75-78 %, in comparison with the conventional method of chemical reduction in solutions and photochemical precipitation, respectively. At the same time, the duration of the synthesis process is reduced almost 3 times. It is found that the introduction of sodium alginate stabilizer contributes to the increase of the yield of silver nanoparticles and allows synthesizing stable colloidal silver solutions.
3. The formation of silver nanodispersions under plasma discharge is characterized by the presence of the peak λ max =400-420 nm. The formation of silver nanoparticles was confirmed by the X-ray diffraction analysis. Microscopic examination (SEM) indicates that the size of the formed silver particles is up to 100 nm. The scheme of synthesizing silver nanoparticles in the sodium alginate solution under plasma discharge is proposed.