DOI: 10.14457/tu.the.2020.99
title: Synthesis of Ag/Au core-shell nanoparticles using skim natural rubber latex after coagulation with chitosan

description: This research focused on a green synthesis of Ag/Au core-shell nanoparticles (Ag@AuNPs) by using the serum from the skim natural rubber latex (SNRL), which is simple and low cost. The chitosan in acetic acid solution was used as a coagulant for removing rubber particles from SNRL. This leftover serum was diluted with the same amount of water and used as a source of reducing agents in the syntheses of AgNPs, Ag@AuNPs, and AuNPs. The characteristics of AgNPs, Ag@AuNPs, and AuNPs were examined by UV-visible spectrophotometry, transmission electron microscopy (TEM), and EDX. This study examined conditions affecting the synthesis of silver cores, including the NaOH concentration in the range of 0.1 and 2 M and the AgNO3 concentration at 0.5, 1 and 3 mM. It was found that when the NaOH concentration increased from 0.1 to 1 M, resulting in the AgNPs of smaller sizes and when the AgNO3 concentration increased, the nanoparticles became larger. After that, the HAuCl4 solution was added to synthesize the gold shell covering the as-synthesized silver cores. The effect of NaOH concentration in the range of 0.25 to 1 M was studied and it was found that for the small silver cores (using 0.5 mM AgNO3 solution), the increase in the NaOH. concentration yielded the Ag@AuNPs of smaller sizes. But for the larger silver cores, there was no clear trend. As for the effect of varying HAuCl4 concentrations at 3, 5, and 9 mM, it was found that at 3 and 5 mM, the maximum wavelength (λmax) and absorbance value did not differ much. However, when the concentration was increased to 9 mM, the λmax increased and the absorbance value increased. The Ag@AuNPs colloids were kept for 4 days. It was found that on Day 1 to Day 3, the absorbance value greatly increased until on Day 4, it increased only slightly. In addition, the synthesis of AuNPs was studied and compared with Ag@AuNPs. In this study, The NaOH concentration was varied in the range of 0.1 and 2 M and the HAuCl4 concentration at 3, 5 and 9 mM. The average size of nanoparticles prepared by using 5 mM HAuCl4 and 1 M NaOH solutions (denoted as 5Au1Na) was 11.218 nm. The average size of Ag@AuNPs synthesized at the condition; 1 mM silver cores, 5 mM HAuCl4 and 1 M NaOH solutions (denoted as 1Ag5Au1Na) was 15.676 nm and for 3 mM silver cores (denoted as 3Ag5Au1Na) was 16.30 nm, so it is possible to use AgNPs as the silver cores to synthesize Ag@AuNPs. The applications of the synthesized nanoparticles were also studied through H2O2 sensing ability and antibacterial activity. The obtained AgNPs were first reacted with H2O2 at 0.0625 to 0.5 mM for 30 min. The AgNP colloids used had λmax at 405, 417, and 424 nm corresponding to the average sizes of 7.942, 8.464, and 9.284 nm, respectively. The 5% wt/v polyethylene glycol solution was necessary to modify AgNPs to increase their stability. It was found that the AgNPs had a good H2O2 sensing ability. Later, the same conditions were applied to study the H2O2 sensing ability of 0.5Ag5Au1Na core-shell NPs. The reduction in absorbance was found insignificant, but for one sample, the λmax shifted to the shorter wavelength, indicating the obvious change back to the transition state. The three nanoparticles, which were 5Au1Na, 0.5Ag5Au1Na at the transition state, and 0.5Ag5Au1Na, were chosen to be tested for their antibacterial ability. Escherichia coli was used as an agent for gram-negative bacteria, where as Staphylococcus aureus for Gram-positive bacteria. It was found that 5Au1Na was able to inhibit Escherichia coli with a clear zone of 7.33 mm while 0.5Ag5Au1Na at the transition state was unable to inhibit the infection at all. It is expected that due to the transition of the nanoparticles, they have low stability. Thus, it lost its ability to inhibit bacteria while 0.5Ag5Au1Na inhibited Escherichia coli with a clear zone of 7.67 mm, which was comparable to 70% alcohol solution.