FACTORS AFFECTING SYNTHESIS OF SILVER-NANOPARTICLES AND ANTIMICROBIAL APPLICATIONS

Silver nanoparticles were synthesized from silver sulfate by using the chemical reduction method with dextran as both a reducing agent and a protective agent. The influence of reaction temperature, time, and initial pH on the synthesis was investigated. The formation of Ag nano-particles (AgNPs) and their morphology were characterized with UV-Vis spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and Fourier transform-infrared spectroscopy. The antifungal and antibacterial effects of AgNPs/dextran on Xanthomonas oryzae and Pyricularia oryzae were tested.


Introduction
Nowadays, metal nanoparticles have attracted tremendous attention from both domestic and international scientists because of their outstanding properties, such as high-performance catalysis and unique electronic and optical properties [1]. Nanoparticles with noble metals such as Au, Ag, and Pt have widely been studied and applied to various fields, such as environment, catalysis, and nanomedicine. Among them, silver nanoparticles (AgNPs) with special, physical, and chemical properties and relatively low cost [2] have extensively been studied and applied in numerous fields. Under normal conditions, silver nanoparticles are not usually stable in solution. To avoid agglomeration of silver nanoparticles, several polymers, such as polyethylene glycol, ethylenediaminetetraacetic acid, polyvinyl pyrrolidone, and polyvinyl alcohol, are used as stabilizing agents [3]. Among these polymers, dextran has received much scientific attention due to its great properties, such as biocompatibility, low toxicity, and slow degradation in the human body in comparison with other polymers [4].
Dextran is a biocompatible polysaccharide composed of D-glucose units and a substantial number of consecutive α-(1→6) glycosidic linkages in the main chain and α-(1→2), α-(1→3) or α-(1→4) branch glycosidic linkages. Dextran is an environmental-friendly biodegradable polymer and has applications in food and medicine as an emulsifier, a carrier, and a stabilizer [5,6]. In this work, we have propose a simple, fast, and effective chemical reduction method to synthesize silver nanoparticles by using dextran as a reducing and protecting agent.

Preparation of silver nanoparticles
In a typical process, the glassware was cleaned in a bath of freshly prepared aqua regia solution

Characterisation
X-ray diffraction patterns were recorded on a Bruker D8 Advance X-Ray diffractometer. UV-Vis spectra measurements were carried out with a Jasco V-550 UV-vis spectrophotometer, within the range of 300-650 nm. Scanning electron microscopy (SEM), elemental analysis and energy dispersive X-ray spectroscopy were analyzed by using FESEM HITACHI S-4800 instrument.
Transmission electron microscopy (TEM) images were acquired by using a JEOL JEM-2100F.

Antifungal test
The inhibition activity of AgNPs on dextran (AgNPs/dextran) against X. oryae and P. oryzae was studied. For the antibacterial test, the modified

Effect of reaction time
The UV-Vis spectra show that the maximum absorbance increases with reaction time (Fig. 2).
However, the maximum absorbance increases rapidly during the first stage of the reaction, but, in the next 15 minutes, it increases slightly, and, after 30 minutes, no further increase is observed. At this time, the reduction reaction is almost completed, and this is entirely consistent with the law of reaction rate. Besides, the UV-Vis spectrum of the 30-minute sample is sharp, and this proves that the particles are relatively more uniform. Therefore, 30 minutes is a suitable reaction time for this reduction process. Besides, the TEM image (Fig. 5b) shows that the nanoparticles are spherical. The particle size of the silver nanoparticles on dextran is in the range of 3-18.40 nm, with an average particle diameter of around 8.7 nm (Fig. 5c).
The energy dispersive X-ray analysis (EDX) of AgNPs shows a strong signal in the silver region and thus confirms the formation of silver nanoparticles (Fig. 6). Besides, the EDX spectrum shows the elemental composition of dextran (carbon, chlorine). An EDX spectrum is ineffective with light elements such as H, so it does not appear in the spectrum. Therefore, the AgNPs synthesized in this study are pure.  position of glucose, respectively [13]. Besides, the peak at 916 cm -1 is related to the -glycosidic bond.
When it comes to the spectrum of AgNPs/dextran, a similarity pattern is found. However, the intensity of the peak at 2929 cm -1 decreases considerably compared with that of dextran, and the peak at 1355-1345 cm -1 corresponding to the C-OH groups also shifts, indicating that some hydroxyl groups are oxidized to aldehyde, leading to the reduction of Ag(I) to Ag(0) [2].

Antibacterial and antifungal results
The antibacterial properties of AgNPs/dextran are tested against X. oryae. The optical images of X. The evidence suggests that AgNPs/dextran is an effective antimicrobial material against the growth of X. oryae and P. oryzae.

Conclusion
In this study, the silver nanoparticles were successfully synthesized by using dextran as a reducing and protecting agent, with an average particle diameter of around 8.7 nm. The reaction parameters are as follows: silver sulfate solution 0.1 mM, dextran solution 0.5%, w/v, reaction temperature 100 °C, initial pH 9, and reaction time