# Green Synthesis of Silver Nanoparticles in the Presence of Polysaccharide: Optimization and Characterization

Authors: Margarita I. Skiba; Victoria I. Vorobyova; Alexander Pivovarov; Natalya P. Makarshenko
Journal: Journal of Nanomaterials
 (2020)

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## Abstract

The process of obtaining aqueous solutions of silver nanoparticles with the use of a low-temperature nonequilibrium contact plasma and stabilizing agent—polysaccharide (sodium alginate)—has been examined. The synthesized Ag NPs were characterized by using UV-Vis spectroscopy, dynamic light scattering (DLS), scanning electron microscope (SEM), and XRD analysis. The effect of concentration of Ag+, sodium alginate, duration of processing by plasma discharge, and pH of liquid on the production of silver nanoparticles has been studied. The results demonstrated that synthesis provides the formation of silver nanoparticles for investigated concentrations of Ag+ (0.3-3.0 mmol/l) and 5.0 g/l Na-Alg (pH=7–10) within 1–5 minutes. From the SEM images, the silver nanoparticles are found to be almost spherical. Powder XRD results reveal that Ag nanoparticles have a face-centered cubic crystal structure. Zeta potential of plasma-chemically obtained colloidal solutions at various concentrations of Ag+ ions and stabilizing agent varies from −32.8 to −39.3 mV, indicating the moderate stability of synthesized nanoparticles.


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## Body

## 1. Introduction

One of the most widely studied types of nanomaterials is silver nanoparticles (Ag NPs) [1–3]. Nowadays, world production of Ag NPs is estimated to be in the range of 360–450 tons per year [4]. And according to [4–6], by 2025, it will grow to 800 tons per year. The main producers of Ag NPs are the USA, China, Japan, and Europe. Recent reports from the International Project on Emerging Nanotechnologies (PEN, http://www.nanotechproject.org) program report 1814 consumer products from 622 companies in 32 countries involved in the production and marketing of Ag NPs. This demand is caused by multifunctional properties of Ag NPs [1–8]. It is now well established that Ag NPs have antimicrobial, antifungal, antiviral, catalytic, sensory, and other properties, which make it possible to apply them in various fields: water treatment, textile manufacture, chemical industry, medicine, pharmaceutical industry, etc. [7–9]. There are currently a number of traditional methods known to produce silver nanoparticles with specific physicochemical properties, such as physical, chemical, photochemical, and biological methods. According to ISO international standards, it is recommended to opt for technology with fewer reagents, that is more environmentally friendly, and with energy-saving resources [10]. Therefore, the development of new highly efficient and innovative technologies for obtaining Ag NPs and exploring their properties for further practical application is still relevant.One of the most innovative and environmentally friendly methods for producing nanoscale compounds is the use of low-temperature plasma discharges [11–14]. Currently, a large number of experimental installations for the formation of a plasma discharge above or in an aqueous medium are known. The following parameters can vary: the processing environment, the material of the electrodes, the configuration of the electrodes, and the source of electricity. Given the number of parameters that can be changed, a precise classification of plasma systems is unlikely to be possible. In general, there are three main types of plasma discharge formation. In the first, the discharge spreads above the surface of the solution (one of the electrodes is in the gas phase). In the second, the discharge is created directly in the solution (both electrodes are immersed in the solution). The third type of discharge (the so-called hybrid) is when the discharge is excited simultaneously in the gas and liquid phases. Among these types of plasma-chemical discharges, from the point of view of practical application, the most promising is the contact nonequilibrium low-temperature plasma (CNP) (first type). It is now established that by changing the composition of the liquid phase, it is possible to control the composition of the obtained products and, in particular, to obtain silver nanoparticles [12]. Studies have shown that plasma nanoparticles of silver in aqueous solutions are characterized by relative stability [12].Natural and synthetic stabilizers are currently used as the substances of stabilizers of the metal colloidal solutions of silver [14–17]. The nature of the stabilizer functional group defines the properties of obtained silver nanoparticles. At the present time, many natural polymers are involved in “green” production of nanoparticles as a reducing/stabilizing agent. Thus, a number of research papers indicate the effectiveness of the choice of natural polysaccharides to obtain stable silver nanoparticles (NPs) in situ or/and their base and materials which are characterized by antibacterial properties [14–17]. In such works [15, 16], silver nanoparticles (Ag NPs 25±2 nm) were synthesized by using sodium hyaluronate as a stabilizer and capping with different parameters (silver nitrate concentration, molecular weight, etc.). Silver and gold nanoparticles were synthesized using sodium alginate as the reducing and stabilizing agents to obtain Ag NP colloids and impregnated into nonwoven viscose fabrics by the authors in [17].The authors have demonstrated the effectiveness of using CNP for the synthesis of silver nanoparticles in the presence of different types of stabilizers: sodium alginate, sodium citrate, etc. [18, 19]. One of the most effective is the use of sodium alginate [18]. The use of a suitable amount of sodium alginate (1-5 g/l) to stabilize the nanoparticles was researched in this work. Study of the process of obtaining silver nanoparticles in the presence of a stabilizer reagent, sodium alginate, under the action of plasma discharge, is of scientific and practical interest. It is advisable to optimize the synthesis parameters: determination of the appropriate Ag+/Stab ratio, the duration of plasma discharge treatment, the effect of solution pH on the formation, and the characteristics of silver nanoparticles.The main goal is to determine the influence conditions of synthesis silver nanoparticles in the presence of sodium alginate using low-temperature nonequilibrium contact plasma and the properties obtained by the Ag nanoparticles.


## 2. Materials and Methods

### 2.1. Materials

Silver nitrate (99.8%, Kishida), sodium alginate, and sodium hydroxide were obtained. Aqueous solutions of a precursor were prepared using ultrapure water (Direct-Q UV, Millipore) and were utilized as starting materials without further purification.


### 2.2. Synthesis of Silver Nanoparticles (Ag NPs)

Sodium alginate was dissolved in distilled water in a glass of 100 ml with the use of a magnetic stir bar. Na-Alg was added to the aqueous solution of AgNO3 in a flask to obtain the complex of the silver ion with the substrate [Ag(Na-Alg)]+ with fixed ratios of reagents. The obtained solutions were distributed into five cuvettes under stirring. The resulting reaction mixture was treated in the reactor with the discharge of contact nonequilibrium low-temperature plasma. Installation parameters: pressure was 80 kPa, current strength was 120 mA, distance from the electrode to the surface was 5-7 mm, and the material of the electrodes was X18H10T. The color change of the mixture of “AgNO3-Na-Alg” to brown indicates the formation of [Ag(Alg)] nanoparticles. The strong SPR band at 400-450 nm in UV-Vis spectra additionally confirms the formation of Ag NPs. The Ag NPs obtained by plasma-chemical synthesis were centrifuged at 5000 rpm for 5 min. The dried powders were used for further characterization.


### 2.3. Characterization Techniques

The spectra of colloidal solutions were obtained on a UV-5800PC spectrophotometer using a quartz cell in the wavelength range of 300-700 nm in 5-10 nm increments. The degree of conversion of Ag+ to NP was also estimated by the difference of the argon ions in the initial solution and after treatment with the plasma discharge. The ion-selective electrode of Ag+ ions, the “ELIS-131Ag,” was used for measurements. The particle size distribution, ζ potential, was determined using a Zetasizer Nano ZS laser correlation spectrometer (LCS) (Malvern Instruments Ltd., United Kingdom). The value of the hydrogen index of the original solutions and the obtained sols was measured using the pH meter pH-150 MI (relative measurement error of 0.5%). Transmission electron microscopy (TEM) analysis was performed using the JEOL TEM (Model 100 CX II; Tokyo, Japan). Nanoparticles were suspended in distilled water and homogenized, and then a drop was placed on a copper grid which was air-dried and observed in TEM.


## 2.1. Materials

Silver nitrate (99.8%, Kishida), sodium alginate, and sodium hydroxide were obtained. Aqueous solutions of a precursor were prepared using ultrapure water (Direct-Q UV, Millipore) and were utilized as starting materials without further purification.


## 2.2. Synthesis of Silver Nanoparticles (Ag NPs)

Sodium alginate was dissolved in distilled water in a glass of 100 ml with the use of a magnetic stir bar. Na-Alg was added to the aqueous solution of AgNO3 in a flask to obtain the complex of the silver ion with the substrate [Ag(Na-Alg)]+ with fixed ratios of reagents. The obtained solutions were distributed into five cuvettes under stirring. The resulting reaction mixture was treated in the reactor with the discharge of contact nonequilibrium low-temperature plasma. Installation parameters: pressure was 80 kPa, current strength was 120 mA, distance from the electrode to the surface was 5-7 mm, and the material of the electrodes was X18H10T. The color change of the mixture of “AgNO3-Na-Alg” to brown indicates the formation of [Ag(Alg)] nanoparticles. The strong SPR band at 400-450 nm in UV-Vis spectra additionally confirms the formation of Ag NPs. The Ag NPs obtained by plasma-chemical synthesis were centrifuged at 5000 rpm for 5 min. The dried powders were used for further characterization.


## 2.3. Characterization Techniques

The spectra of colloidal solutions were obtained on a UV-5800PC spectrophotometer using a quartz cell in the wavelength range of 300-700 nm in 5-10 nm increments. The degree of conversion of Ag+ to NP was also estimated by the difference of the argon ions in the initial solution and after treatment with the plasma discharge. The ion-selective electrode of Ag+ ions, the “ELIS-131Ag,” was used for measurements. The particle size distribution, ζ potential, was determined using a Zetasizer Nano ZS laser correlation spectrometer (LCS) (Malvern Instruments Ltd., United Kingdom). The value of the hydrogen index of the original solutions and the obtained sols was measured using the pH meter pH-150 MI (relative measurement error of 0.5%). Transmission electron microscopy (TEM) analysis was performed using the JEOL TEM (Model 100 CX II; Tokyo, Japan). Nanoparticles were suspended in distilled water and homogenized, and then a drop was placed on a copper grid which was air-dried and observed in TEM.


## 3. Results and Discussion

It is known that one of the main factors influencing the formation of NP is the concentration of the precursor, the stabilizing reagent in the reaction medium, and the duration of the process of the dispersion formation of silver nanoparticles [1–10]. In the preparation of nanoparticles under the plasma exposure without a stabilizer, compounds formed in a liquid aqueous medium are the reducing agents [13, 18–20]. Therefore, Figure 1 shows the results of the study of the dependence of the intensity of absorption (A) and the change of silver ions on the duration of the plasma discharge exposure on the original solution without the use of a stabilizer.Figure 1
Dependence of absorption intensity (A) and change of Ag+ concentration on the duration exposure to plasma discharge of the original aqueous solution of silver nitrate without stabilizer: 0.3 mmol/l (a), 0.5 mmol/l (b), 1.0 mmol/l (c), and 3.0 mmol/l (d).
(a)
(b)
(c)
(d)The main conclusions from the experimental data obtained are as follows:(i)
It has been found that at all initial concentrations of AgNO3 (0.3–3.0 mmol/l) as a result of the silver nitrate solution being exposed to plasma discharge for up to 60 min, silver nanoparticles were formed and then characterized by the presence of maximum SPR absorption (λmax) at 390-450 nm, which corresponds to the nanoparticles of silver with mainly spherical form
(ii)
For all initial concentrations of silver ions, increase of the duration of the plasma discharge from 10 sec to 40-60 sec is accompanied by an extreme increase in the intensity of absorption, which indicates an increase in the concentration of formed nanoparticles in the solution
(iii)
Plasma treatment from 1 to 6 min helps reduce the intensity of absorption while maintaining a gradual decrease in the concentration of silver ions in solution. A similar phenomenon is likely due to the transition of silver nanoparticles into silver oxide nanoparticles in the absence of a stabilizing reagent. Nanoparticles of silver oxide are characterized by an absorption peak at 885 nm. A number of papers [25, 26] have shown the possibility of the formation of silver oxide nanoparticles of different Ag2O and AgO structures (confirmation of this assumption is further studies of the phase composition of the treatment duration).Figure2 shows the dependence of the spectrum of the obtained silver nanodispersions on the duration of action of the plasma discharge on the initial solution of the silver nitrate with the addition of a stabilizer—sodium alginate. It was found that when sodium alginate is added to the solution, at all tested initial concentrations of argentum nitrate (0.3-3.0 mmol/l) as a result of the exposure to plasma discharge from 10 seconds to 6 min, the silver nanoparticles are formed. The nanoparticles are characterized by the presence of a maximum SPR absorption (λmax) in the range of 400-425 nm, which corresponds to silver nanoparticles of mainly spherical shape up to 50 nm in size.Figure 2
The spectra of silver nanodispersions obtained depending on the duration of a plasma discharge to the stock solution with the addition of a stabilizer—sodium alginate—at different concentrations of Ag+: 0.3 mmol/l (a), 0.5 mmol/l (b), 1.0 mmol/l (с), and 3.0 mmol/l (d).
(a)
(b)
(c)
(d)In Figure3, the results of the study of the dependence of the absorption intensity (A) and the change of silver ions on the duration of the plasma discharge on the original solution with the addition of the stabilizer sodium alginate are presented. The main conclusions from the experimental data obtained are as follows:
(i)
In contrast to nanoparticles obtained without the stabilizer, the increase of the duration of the plasma discharge to 5-6 min for all initial concentrations of silver ions is accompanied by an increase in the intensity of absorption, which indicates an increase in the concentration of formed nanoparticles in solution
(ii)
A significant increase in the intensity of the peaks, compared to the data without the introduction of sodium alginate, indicates that it acts as an effective stabilizer in the formation of nanoparticlesFigure 3
The intensity of the absorption (A) and Ag+ concentration changes depending on the duration of the plasma discharge in the presence of the sodium alginate stabilizer: 0.3 mmol/l (a), 0.5 mmol/l (b), 1.0 mmol/l (с), and 3.0 mmol/l (d).
(a)
(b)
(c)
(d)X-ray structural analysis obtained with the help of plasma discharge (without stabilizer) of nanoparticles was performed at different intervals of time after 50 s (a) and 5 min (b) of exposure to plasma discharge in silver nitrate solution and in the presence of stabilizer (c). The experimental conditions were as follows: plasma parametersI=120 mA and P=0.08 MPa (with Ag+=3.0 mmol/l). Figure 4(a) shows the X-ray diffraction phase after 50 sec of plasma discharge treatment. Peaks at 2θ 38, 1, 44.9, and 77.5° can be attributed to the (111), (200), and (311) crystalline planes of the face-centered cubic crystalline structure of metallic silver. Such data are in agreement with data presented in works [20, 21]. A radiograph of the dispersed phase after 5 minutes of treatment with a plasma discharge without stabilizer is presented. The X-ray diffraction shows two intense peaks at 27.94 and 32.27, which correspond to (110) and (111) of Ag2O. Apart from this, diffraction peaks at 46.34, 54.92, and 67.48 can be indexed to (211), (220), and (222) planes of face-center cubic silver, respectively. These peaks corroborate with the standard Ag2O (JCPDS 76-1393) [21, 22]. The possibility of forming Ag2O structures is also reported. Figure 4(с) shows a typical XRD pattern of silver nanoparticles obtained in the aqueous solution: peaks at 2θ values of 38.1, 44.9, and 77.5 deg can be attributed to the (111), (200), and (311) crystalline planes of the face-centered cubic crystalline structure of metallic silver. The diffraction peaks are broad, which indicates that the crystallite size is minimal.Figure 4
XRD patterns of Ag NPs obtained after 50 sec (a) and 5 min (b) of plasma discharge (without stabilizer) and after 5 min of treatment in the presence of a stabilizer (c).
(a)
(b)
(c)Figure5 summarizes the elemental and X-ray phase analysis of the composition of silver dispersions at different time intervals obtained without a stabilizer. In the absence of a stabilizer under the action of plasma discharge on an aqueous solution of silver nitrate for more than 1 min, nanoparticles of silver oxide are formed. Increasing the duration of a plasma discharge increases their content. Thus, the data obtained are consistent with the above spectrophotometric studies. In the presence of the stabilizer, only silver nanoparticles are formed at any time.Figure 5
Change of the phase composition of silver dispersions depending on the duration of the plasma discharge without stabilizer.To determine the reaction order, the dependence of the concentration of the Ag+ ions on the duration of the plasma discharge treatment (CAg+−τ) was researched, which is shown in Figures 1 and 3 in the coordinates C, lnC, 1/C, and 1/C2=fτ. They characterize the zero, first, second, and third orders of the reaction. The results of the data processing are presented in Table 1. It has been determined that the value of the rate constant of the plasma-chemical formation of silver nanoparticles with and without sodium alginate application is a first-order reaction (Figure 6). The velocity constant in both cases is equal to 0.4 min-1.Table 1
Value constants of the formation rate of silver nanoparticles (T=373K).
CAgNO3 (mmol/l)
C
lnC
1/C
1/C2
k
R2
k
R2
k
R2
k
R2
Without stabilizer
0,3
0,221
0,443
0,409
0,993
1,445
0,819
1,992
0,744
0,5
0,496
0,673
0,408
0,959
0,398
0,886
1,237
0,676
1,0
0,961
0,690
0,409
0,963
0,322
0,783
0,877
0,544
3,0
3,203
0,713
0,394
0,962
0,0723
0,958
0,039
0,812
With stabilizer (sodium alginate)
0,3
0,357
0,765
0,435
0,9875
1,377
0,712
1,462
0,744
0,5
0,552
0,756
0,400
0,9855
0,588
0,681
1,107
0,676
1,0
1,093
0,703
0,430
0,988
0,623
0,663
0,767
0,634
3,0
3,24
0,761
0,409
0,981
0,0923
0,758
0,0305
0,712Figure 6
Dependence of the logarithm of equilibrium Ag+ concentration as a function of the duration of plasma discharge at different initial concentrations of silver ions in solution in the presence (b) and without the addition of sodium alginate (a).
(a)
(b)The size of the particles formed in the aqueous solution under the conditions of plasma discharge upon addition of sodium alginate has been determined. For this purpose, the particle size distribution has been investigated, and the average particle size (Table2) of the plasma-chemically obtained silver dispersions at different ratios of the reagents has been determined. The zeta potential of plasma-chemically obtained silver nanoparticles in the presence of sodium alginate has been established (Table 2). The dimensional characteristics of the obtained solutions, established by the method of dynamic light scattering, have indicated that the average particle size is 22.5–26.0 nm. It is seen that Ag NPs of spherical shapes were obtained in the case of different Ag+ concentrations being used.Table 2
The size of silver nanoparticles obtained under the conditions of plasma treatment and sodium alginate.
CAgNO3 (mmol/l)
CNa‐Alg (g/l)
ζ potential (mV)
Average particle size, dAgNPs (nm)
Polydispersity index
0.3
5.0
-31.5
22.5
0.30
0.5
-36.6
25.1
0.33
1.0
-37.1
26.1
0.36
3.0
-39.1
26.0
0.42Depending on the Ag+/Na-Alg ratio, the solutions obtained are characterized by an average zeta potential within the range of −35.5 to −39.1 mV, which is a characteristic of stable colloidal systems. The degree of aggregation of the metal nanoparticles also can be effectively estimated by changing the absorption characteristics as follows: displacement of the SPR peak in the spectrum and its intensity. The analysis of spectra of silver colloids after aging during 4 weeks passing from the date of obtaining showed no significant changes in the structure and intensity of SPR bands, thus evidencing the absence of noticeable (quick) aggregative processes in silver dispersions. In order to evaluate the stability of colloidal systems, the pH value of freshly obtained solutions of silver was investigated during storage. Colloidal solutions are characterized by the absence of explicit pH change during storage.Figure7 shows the micrographs of the NPs obtained in the presence of a stabilizer. The data collected are consistent with the results of Table 2. The nanoparticles have a spherical shape. However, the image shows that the average size is slightly smaller than the measurement method (DLS) has demonstrated and is 25-35 nm. However, a considerable number of nanoparticles with a size up to 15 nm have been observed.Figure 7
SEM of silver nanoparticles obtained under the conditions of plasma treatment and sodium alginate (CAgNO3 3.0 mmol/l).In works [23–25], information was presented that a silver nanoparticle core-shell structure can form at different synthesis conditions and capping agents. The band at max 410 nm can be assigned to the plasmon resonance of core-Ag0 nanoparticles. Figure 8 shows the TEM images of the size of prepared nanoparticles. Transmission electron microscopy (TEM) images clearly illustrate that the synthesized silver nanoparticles were spherical in shape with an average size of 28-33 nm with an unstructured core shell.Figure 8
ТEM image of silver nanoparticles obtained under the conditions of plasma treatment and sodium alginate (CAgNO3 3.0 mmol/l).It has been shown in work [21] that, when a NP is obtained under the action of a plasma discharge without a stabilizer, the pH of the initial solution of silver nitrate decreases from 6.6 to 2.5. In Figure 9, the change in the pH of the solution in the presence of sodium alginate during the formation of silver nanoparticles for two concentrations (0.3 mmol/l and 3.0 mmol/l Ag+) is shown. Therefore, the initial value of the solutions is 6.03-7.05, depending on the initial C of silver ions and Na-Alg. The nature of the dependence coincides with one another, and a sharp decrease in pH from 6.3-7.0 to 3.0-3.6 is seen after 40 seconds of the effect of the discharge on the solution. The pH is then stabilized. For the samples with the addition of sodium alginate, the effect of the solution pH on the formation of silver nanoparticles under the action of plasma discharge has been investigated.Figure 9
The change in the pH of the solution in the presence of sodium alginate during the formation of silver nanoparticles for two concentrations (0.3 mmol/l and 3.0 mmol/l Ag+).It has been found that increase of the initial pH from 7 to 10 contributes, albeit slightly, to an increase in the optical density of the obtained silver dispersions and to a narrowing of the peak (Figure10).Figure 10
Absorption spectrum of obtained colloidal solutions of silver chemical using the plasma-chemical discharge in different water solutions of Ag+/Na-Alg.This indicates a decrease in particle size (Table3). However, at pH=10, there is some shift of the peak of surface plasmon resonance (SPR) in the short wavelength from 425 to 410 nm, which indicates a probable decrease in the size of the formed NPs. Obtained data are consistent with the results published by the author in [26]. In this work, with increasing the pH from 7 to 9, a new absorbance band appeared at about 400–420 nm, suggesting the formation of Ag nanoparticles. The intensity of the band increased proportionally to the pH within this interval. Therefore, it is likely that at higher pH, the anionic forms of the carboxyl groups prevail and the negative charges repel each other, giving a longer distance of particles from the alginate particles. Repulsion prevents the interaction of NPs, resulting in higher electrostatic stabilization and, as a consequence, smaller particle size. Increasing the initial pH to 11–12 is not desirable since, according to the data obtained, it contributes to a significant peak expansion of 500–600 nm, and this indicates an increase in the size of the nanoparticles (Table 3).Table 3
The size of silver nanoparticles obtained under the conditions of plasma treatment and sodium alginate at different pH values.
pH
Average particle size, dAgNPs (nm)
Polydispersity index
7
26.0
0.42
10
18.2
0.48
11
95.0
0.58
12
97.0
0.60The possible mechanism of formation of silver nanoparticles under the action of CNP in the presence of sodium alginate has been considered. After the dispersion of silver ions in the aqueous solution of the Na-Alg matrix (equation (1)), Na-Alg reacts with the Ag+ to form a Na-Alg complex [Ag(Na-Alg)]+.
(1)Ag+aq+Na‐Algaq⟶AgNa‐Alg+aqThe work [27] describes the successful restoration of the complex [[Ag(Na-Alg)]+(aq)] by OH ions by adding alkali as a result of the reaction (equation (2)):
(2)2AgNa‐Alg++2OH−⟶2AgAlg↓+H2O+1/2O2From [12, 13], it is known that under the action of the plasma discharge on water, as a result of chemical transformations, a wide list of reactive compounds is formed. Apart from the abovementioned products, reduction of water molecules and hydrogen ions may proceed in the plasma-chemical reactor (equation (3)):
(3)2H2O+2e−⟶H2↑+2OH−,2H++2e−⟶H2↑Thus, it is possible that under the action of CNP in the reactor, the formation of nanoparticles is caused by the reduction of silver ions through the regeneration of alginic acid (Alg) from sodium alginate (Na-Alg) (equation (2)).In fact, plasma-chemical processes that occur in the plasma-chemical synthesis of Alg-Ag NPs are complex, and we are only in the very beginning of understanding them.


## 4. Conclusions

Obtaining silver nanodispersions using plasma discharge and the polysaccharide sodium alginate is considered in the paper. It has been established that sodium alginate plays the role of a nanoparticle stabilizer. The effect of concentration of Ag+, sodium alginate, duration of processing by plasma discharge, and pH of liquid on the production of silver nanoparticles has been studied. The results demonstrated that synthesis provides the formation of silver nanoparticles for investigated concentrations of Ag+ (0.3-3.0 mmol/l) and 5.0 g/l Na-Alg (pH=7–10) within 1–5 minutes. The silver nanoparticles are found to be almost spherical.



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