Microwave-assisted synthesis of silver nanoparticles and their application in catalytic, antibacterial and antioxidant activities

Eco-friendly silver nanoparticles (AgNPs) were synthesized using a carboxymethylated gum kondagogu (CMGK) as capping and reducing agent by microwave irradiation method. Synthesized AgNPs were analyzed by TEM, showed spherical in shape with an average particle size distribution of 9 ± 2 nm. XRD results showed that the cubic-structured AgNPs with crystallite size of 8.6 nm. The synthesized CMGK capped AgNPs was tested for catalytic activity using hexacyanoferrate(III) in the presence of NaBH4 and the impacts of catalyst dose and temperature were examined. The synthesized AgNPs demonstrated a huge antibacterial action on both Gram-positive and negative groups of microorganisms with inhibition zone of 23, 25 and 28 mm, respectively, for Bacillus subtilis, Bacillus cereus and Escherichia coli, they showed moderate antibacterial action (zone of inhibition 15 mm) with Pseudomonas aeruginosa. In addition, the ability of the AgNPs scavenging activity with different concentrations was studied using the DPPH radical scavenging.


Introduction
Nanotechnology has the ability to recommend solutions to the society in diverse fields [1,2] such as sustainable production of chemicals, water treatment, and medicine, solar energy conversion, etc. [2,3]. Researchers have great interest towards the green-production of metal nanoparticles owing to their inimitable characteristics and extensive aptness in a various field like chemistry, physics, biomedical science and material science [4,5]. The metal nanoparticles showing notable physical, optical, chemical and thermal properties [6], which might be owing to the combination of the nanometer scale and a substantial proportion of high energy surface atoms relative to the bulk solid, mean free path of an electron changing its conductivity and mobility [7][8][9]. Particularly, silver nanoparticles (AgNPs) show significant applications in bio imaging, catalysis, colorimetric sensors, drug delivery, surface-enhanced Raman scattering, etc. [10][11][12][13]. The AgNPs in the colloidal state have widely been used to research the catalytic action in the model reaction is electron exchange reaction between hexacyanoferrate(III) to hexacyanoferrate(II) [14,15]. Antibacterial agents have importantly benefited the health-related quality of human life towards fighting with bacterial infections [14,[16][17][18]. AgNPs has been used for antibacterial agent incident times, due to smaller size, low toxicity easily synthesize.

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Antioxidants play a role in the oxidative process of aging in human and animals [19,20].
Several methods are reported [1,4,7] for AgNPs synthesis, among them, most conventional and comfortable method is the reduction of the metal salt solution by means of reducing agents like N,N-dimethyl form amide, when the above chemicals used as reducing agent, it causes environmental toxicity or hazards. Alternatively, researchers are interested towards the green synthesis of AgNPs to avoid the negative impact on the environment [21,22]. AgNPs synthesis using polysaccharides [14][15][16][23][24][25][26][27][28] is an alternative resolution for the aforementioned complications. The synthesis of AgNPs by different plant parts such as leaf, fruit, bark, latex, stem, gums and various biological systems has been reported [16,17,19,[29][30][31][32]. Although the synthesis of AgNPs using polysaccharides has some disadvantages, it needs more time for the synthesis. Similarly, the as-synthesized AgNPs showed lower catalytic and biological activity. To activate the AgNPs, carboxymethylation is one of the systems utilized for the functionalization of natural polymers [33]. Carboxymethylation in general expands the hydrophilicity and solution clearness of the polysaccharides and improves it dissolvable in an aqueous system. It is a broadly utilized alteration technique in light of its simplicity of handling, bring down cost of chemicals and versatility of the product obtained. Many natural gums such as Cassia tora gum, gellan, guar gum, and gum karaya were investigated using the carboxymethylation process [34][35][36][37].
Synthesis of AgNPs using carboxymethylation gum kondagogu enhances the stability of nanoparticles, as well as the as-synthesized AgNPs showed good catalytic and biological activities. In this research investigation, a modified gum (carboxymethylated gum kondagogu) was utilized for the development of AgNPs, and the AgNPs were characterized by UV, FTIR, XRD and TEM. The as-synthesized AgNPs examined for potential catalytic, antioxidant and antibacterial activities. The antimicrobial action of the AgNPs was inspected using Gram-negative and Gram-positive bacterial strains.

Materials and chemicals
Purchased gum kondagogu from Girijan Cooperative Society Corporation, Telangana, India. AgNO 3 was purchased from the Sigma-Aldrich. Potassium hexacyanoferrate(III), monochloroacetic acid; sodium borohydride, sodium hydroxide, nitric acid, and hydrochloric acid were purchased from Merck. Agar-agar, tryptophan bacterial grade and yeast extract were acquired from Hi Media Laboratories, Mumbai, India. Purchased all test strains from IMTECH, Chandigarh, India. All the chemicals utilized were of analytical grade.

Preparation of gum solution
One gram of gum kondagogu was dissolved in 200 ml of distilled water and this was stirred for 5 h at room temperature. This gum solution was centrifuged to remove the insoluble materials and later the supernatant was lyophilized and used for analysis. The lyophilized gum powder was used for further process.

Synthesis of carboxymethyl gum kondagogu
The carboxymethyl gum kondagogu was synthesized using the previously described method adapted from [38] with slight modifications [15,[39][40][41][42][43][44]. In the typical preparation, first, 0.5 g of gum kondagogu powder was dispersed in 40 ml of ice-cold NaOH (40%, w/w) with the aid of 60 min, and 5 ml of monochloroacetic acid (60% w/v) was added to the reaction mixture over a period of 10 min under constant stirring. The whole mixture was heated to 75 °C and stirred for 60 min. After the reaction, mixture was cooled naturally. Then it suspended into an 80% (v/v) methanol, a precipitate was obtained, which was filtered out. The precipitate was neutralized using glacial acetic acid. The precipitate was washed three times with 50 ml of 80% (v/v) methanol, sifted and dried in a hot air oven at 45 °C.

Synthesis of silver nanoparticles
All glassware utilized were washed with water regia (3:1 HCl-HNO 3 ) and afterward thoroughly flushed with twofold refined water. All the working solutions were prepared with double distilled water. The CMGK solution (5 ml) was mixed with 2 ml of AgNO 3 (0.5%) solution in a bubbling tube and the blend was subjected to microwave irradiation for 50-90 s at 750 W power. The subsequent solution was of yellowish-orange in color, evidence of the development of AgNPs. After formation, the AgNPs were centrifuged at 20,000 rpm for 30 min and then the filtrate was re-dispersed in Milli-Q water. The centrifugation and scattering process were repeated twice to expel any unreacted AgNO 3 and CMGK solution from the end product.

Characterization of AgNPs
The developed CGMK capped AgNPs were characterized by UV-Vis spectrophotometer (UV-3600, Schimadzu) in the wavelength range of 200-700 nm against the blank gum solution as a reference. The FTIR (JASCO, Model FT/IR-4600/4700) studies were performed to appraise the functional groups of CMGK potentially associated with the reduction and stabilization of synthesized NPs. The crystallinity of CMGK capped AgNPs was considered by XRD (Rigaku, Miniflex) strategy with Cuk α (λ = 1.5418 Å) radiation). The morphology and size dispersion of the CMGK capped AgNPs dispersion was studied by TEM. TECNAI G2 F20 S-TWIN instrument was used for recording TEM images.

Catalytic activity
The formed AgNPs potential catalytic activity was examined utilizing an electron exchange reaction between hexacyanoferrate(III) and NaBH 4 . The reaction methodology was as per the following; 0.7 ml of 10 mM hexacyanoferrate(III), freshly prepared 1.3 ml of 36 mM NaBH 4 in 0.1 M NaOH solution were mixed with 1 ml of Milli-Q water and the reaction mixture was transferred into a 5 ml quartz cuvette. To this mixture, varying concentrations of (50-250 µl) of AgNPs solutions were added. Thus, the absorbance intensity of ferric cyanide for the initial reaction was recorded at 420 nm. After the addition of AgNPs, the intensity decrease was monitored using UV-Vis spectrophotometer. The electron transfer reaction was performed at different temperatures between 30 and 70 °C [36,45].

Antibacterial studies
The synthesized silver nanoparticles were tried for antibacterial action by agar well diffusion technique against the following microorganism, i.e., Pseudomonas aeruginosa, Escherichia coli, Bacillus cereus and Bacillus subtilis. The formation of clearing zones was measured in mm using the ruler scale method and compared with standard Streptomycin (positive control). In brief, 100 µl of a log phase cultures were seeded in the nutrient agar medium (beef extract, peptone and agar) for bacteria's. After solidification of all agars Petri plates 8 mm diameter five wells were formed by punching with sterile borer. In four wells 100 µl of various concentrations of AgNPs samples (0.2, 0.4, 0.6 and 0.8 µg/ ml) and in one well streptomycin (400 µg/ml) as positive control were loaded. At 37 °C in incubator the petri plates were incubated for 24 h. The experiment was performed in triplicate for each pathogenic bacterium's and compared with the standard. Zone of inhibitions was measured by means of ruler method.

Antioxidant activity
The antioxidant activity for green synthesized AgNPs was evaluated using DPPH assay. This assay was followed by formerly reported method with few modifications [28]. Different volumes (3,7,10,15,20, and 25 μl) of AgNPs is made up to 60 μl with dimethyl sulfoxide and 205 ml DPPH (0.15 mM) solution was added. The above mixture was shaken vigorously and kept in the black paper (dark) for 40 min at room temperature. The absorbance was recorded at 517 nm against a blank of DPPH and ascorbic acid was used as Ref. [19]. The free radical scavenging activity was estimated using the following expression: A 0 is the absorbance of the control and A t is the absorbance of the sample.

UV-visible spectroscopy
Electronic spectroscopy is one of the simplest methods to characterize AgNPs. AgNPs show a strong absorption band due to surface plasmon resonance in the visible region at 415-430 nm. The development of AgNPs were easily identified by alter in shade of the reaction blend from light yellow to dark yellow. In request to contemplate the impact of CMGK concentration on the development of AgNPs, different trails were performed out by varying the amount of CMGK from 0.1 to 0.5% keeping the concentration of AgNO 3 at constant. The corresponding changes in the SPR peak are shown in Fig. 1a. With the increase of CMGK concentration from 0.1 to 0.5, the SPR peak intensity was increased due the decrease of particle size. This indicates that increased CMGK concentration could increase the formation of AgNPs by reducing more Ag + ions. Similarly, it controlled the size of AgNPs by effectively capping the nanoparticles. We also examined the effect of concentration of AgNO 3 for the synthesis of AgNPs by keeping the concentration of CMGK (0.5%) constant. Figure 1b clearly suggested that with increase of AgNO 3 concentration, more and more number of Ag + ions become available in the solution, which further reduced and effectively capped by CMGK molecules to produce more number of AgNPs [36,39,46,47].

FTIR analysis of AgNPs
CMGK and CMGK-stabilized AgNPs were recorded by FTIR spectrophotometer (Fig. 2). The major peaks [curve (a) of Fig. 2 Figure 2(curve b) shows the FTIR spectrum of CMGK-capped AgNPs. This FTIR spectrum shows the characteristic frequencies at 3375, 1727, 1591, 1406, 1220, and 1026 cm −1 . The shifts in the peaks of the CMGK-capped AgNPs FTIR spectrum were watched from 3430 to 3375, 1733-1727, 1615-1591, 1415-1410 cm −1 and other peaks observed were to remain unchanged. These shifts in the IR spectrum suggested that the linking of AgNPs with hydroxyl and carboxylate groups of carboxymethylated gum kondagogu. The peak shifts in the hydroxyl and carboxyl groups, it can be presumed that both hydroxyl and carbonyl groups of CMGK are involved [15,24,35,47] in the synthesis and stabilization of AgNPs. Figure 3a shows the XRD criterion of the developed AgNPs. The characteristic diffraction peaks were observed at 2θ values at 38.32, 44.50, 64.75 and 77.05, which were indexed to (111), (201), (222), and (312), planes of silver, respectively. In XRD pattern, all the peaks were readily indexed to a face-centered cubic structure of silver (JCPDS, File No. 04-0783) [48]. From Fig. 3a, the (111) peak is the most intense compared to the other peaks in the spectrum, which indicates the preferred growth of AgNPs along (111) direction [25][26][27]49]. The calculated lattice constant was observed to be a = 0.4085 nm, which is consistent with the standard value of a = 0.4086 nm. The synthesized AgNPs crystalline size was calculated using Scherer's formula:

XRD analysis
where D is the crystalline size, β is the full width at half maximum, λ is the wavelength of X-ray used (1.5406 Å) and θ is the Bragg's angle. The crystallite size was calculated to be 8.6 nm, which is in good agreement with the TEM results.

TEM studies
The TEM pictures of the synthesized AgNPs are shown in Fig. 4a. Synthesized silver nanoparticles have spherical shape with smooth surface and well dispersed. The resultant AgNPs completely revealed that the CMGK can protect silver nanoparticles from aggregation efficiently. The histogram (Fig. 4b) was developed by considering 100 AgNPs, it clearly recommends that the mean size dispersion of AgNPs is 9 ± 2 nm. The corresponding SAED criterion is shown in Fig. 3b. A clear and uniform lattice fringes are showed by AgNPs, which confirmed that the spherical AgNPs particles have highly crystalline nature [16,28]. Lattice space of 0.232 nm corresponds to (111) plane of silver, which indicates that the preferred face of AgNPs is along (111).

Stability of AgNPs
The stability of AgNPs in solution is an essential prerequisite for their chemical and biological applications. Stability of synthesized CMGK-capped AgNPs was studied by monitoring the SPR peak and zeta potential. In Fig. 5, under neutral conditions, the AgNPs exhibited a zeta potential value of − 18.7 mV, which indicates the surface of AgNPs is covered with negatively charged groups like hydroxyl and carboxyl groups. Therefore, the AgNPs are highly stable due to the strong electrostatic repulsions between the particles.

Proposed mechanism involved in the development of AgNPs
In this work, we have utilized CMGK as a reducing and stabilizing specialist for the development of AgNPs. The method which was totally nontoxic, low cost and ecofriendly. The primary structure of CMGK is made up of polysaccharides, which contains the carbonyl and hydroxyl groups in abundance. The large number of carbonyl and hydroxyl groups helps in the complexation of Ag + ions. These Ag + ions oxidize the hydroxyl to carbonyl groups, during which the Ag + ions are reduced to elemental Ag. In this oxidation, the dissolved air is also a source of oxidation of the hydroxyl groups to carbonyl groups such as carboxylates and aldehydes.

Catalytic reduction of hexacyanoferrate(III)
The silver nanoparticles were used as a catalytic in the electron exchange reaction between hexacyanoferrate(III) and sodium borohydride, bringing about the arrangement of hexacyanoferrate(II) ions and dihydrogen borate ions. The redox reaction is described as: The advantage of hexacyanoferrate ion for this redox study is that oxidation states of iron (+ 2 and + 3) are quite stable with reference to dissociation and hydrolysis. The redox potential corresponding to the reaction being studied, E o (Fe 2+/ Fe 3+) is + 0.44 V versus a normal hydrogen electrode. The standard reduction potential of the borate ion is E o = − 1.24 V. Hence, there is a huge free energy change associated with the reaction [14,15]. Notwithstanding, this reaction even though advance without a catalyst, however, it is accounted as moderate reaction with zero-order kinetics.
The progress of the reduced reaction of Potassium hexacyanoferrate(III) to Potassium hexacyanoferrate(II) was monitored during changes in the UV-visible spectrum. The characteristic absorption peak of hexacyanoferrate(III) was observed at 420 nm. By addition of nanoparticles, the magnitude of the absorption point was continuously decreased with time revealing the occurrence of catalyzed reduction (Fig. 6). The redox reaction is a pseudo first-order reaction with respect to hexacyanoferrate(III) [45]. As the centralization of NaBH 4 was more noteworthy than the hexacyanoferrate(III), the reduction rate can be thought to be autonomous of NaBH 4 quantity. The kinetics of reduction reaction of the hexacyanoferrate(III) catalyzed by AgNPs were investigated at different temperatures (30-70 °C) and different amounts of catalyst. The reaction was carried out keeping all experimental conditions such as initial concentration of hexacyanoferrate(III), borohydride concentration and temperature at constant. The rate constants were calculated by differing the amount of AgNPs (50-250 µl). Rate constant values (Table 1) were plotted against different quantities of catalyst as appeared in Fig. 7a. The response rate is found to increment straightly with the expansion in the measure of AgNPs, because of an improvement in the quantity of reaction surface sites. There is a linear relationship between the observed rate constant and the amount of catalyst.
The rate constant of catalytic reaction was studied at different temperatures (33,42,51, 62 and 73 °C) and at fixed concentrations of hexacyanoferrate(III), borohydride and   Fig. 7b. The activation energy was figured from the slope (− E a /R). It is watched that the increase in temperature expands the rate of reaction ( Table 2). The activation energy was computed from the (− E a /R) of the straight line and was observed to be 7.2 kcal/mol.

Antibacterial action of AgNPs
Antibacterial action of CMGK-capped silver nanoparticles was tested against Gram-negative (P. aeruginosa and E. coli) and Gram-positive (B. subtilis and B. cereus) bacterial strains and showed in Fig. 8 ( Table 3). The outcomes demonstrated that the as-synthesized AgNPs have discrete antibacterial action against pathogenic microorganisms [14,16,25,27]

Anti-oxidation studies
The antioxidant activity of the synthesized AgNPs is evaluated using DPPH assay using ascorbic acid as reference. This assay is a simple and less expensive. The antioxidant activity of synthesized AgNPs was assessed in terms of percentage inhibition of DPPH radicals in the presence of ascorbic acid. The DPPH is considered more stable nitrogen-centered free radical due to exhibiting a higher degree of accepting hydrogen atoms or electrons from antioxidant materials [19]. The DPPH solution color change was observed on the addition of AgNPs, which is due to the scavenging action of DPPH by addition of hydrogen to form the yellow-colored DPPH. Figure 9 shows the inhibition activity of AgNPs in comparison to the ascorbic acid (standard) and it is observed that the DPPH radical inhibition activity was enhanced with the quantity of silver nanoparticles.

Conclusion
The synthesis was completed in an aqueous medium by a microwave irradiation method using CMGK. The results of XRD showed that highly crystalline face-centered cubic structure to AgNPs. TEM analysis indicated that the synthesized AgNPs were spherical in shape with an average size distribution of 9 ± 2 nm. The catalytic activity application of green-synthesized AgNPs was examined by the reduction of potassium hexacyanoferrate(III) to potassium hexacyanoferrate(II) and activation energy was found to be 7.2 kcal/mol. The synthesized AgNPs showed good antibacterial activity with inhibition zone of 28, 25 and 23 mm, respectively, for E. coli, B. cereus and B. subtilis, whereas it showed moderate antibacterial activity (zone of inhibition 15 mm) with P. aeruginosa. The prepared AgNPs have excellent antioxidant characteristics against   DPPH assay. Our study indicates that the CMGK-capped AgNPs showed significant catalytic activity, antibacterial and antioxidant activity, which suggest their possible application in pharmaceutical industry.