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Considering the importance of biosynthesized silver nanoparticles (AgNPs) using plant phytochemicals, the aim of this work was to evaluate the effect of pH in the formulation of AgNPs, bioreduced with thyme essential oil. Biosynthesized AgNPs were prepared under different pH (7, 8, 9 and 10), and the physicochemical stability was evaluated during 90 days at 6°C and 25°C. Using GC/MS technique, 17 different compounds were observed in the thyme essential oil; however, borneol and αterpineol were the majoritarian compounds, probably responsible to the formation of AgNPs. UV-vis spectroscopy with absorbance between 415 and 440 nm and Transmission Electron Microscopy (TEM) confirmed the synthesis of nanoparticles, with diameters of 40 nm and spherical shapes to pH 7, 8 and 9. Through DLS approach, was observed that the average particle diameter was around 90 nm for all pH tested. AgNPs presented homogeneous size distribution, and zeta potential values very close to the theoretical stability range. Moreover, biosynthesized AgNPs presented high antibacterial activity against Escherichia coli and Staphylococcus aureus. Through x-ray Photoelectron Spectroscopic (XPS) analysis was possible confirm the presence of crystalline silver nanoparticles. Thus, silver nanoparticles eco-friendly prepared using thyme essential oil can be considered as an alternative way to produce this nanomaterial with great stability and high antibacterial activity.

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The present investigation details a green synthesis of silver nanoparticles (AgNP) using the essential oil of Syzygium aromaticum L. (clove) as reducing agent, which is a matrix with a high content of eugenol, an important compound for the reducing action of silver nitrate. The synthesis of AgNP was performed at different pH conditions (pH 7, 8, 9 and 10), and was monitored by UV-Vis Spectroscopy, Dynamic Light Scattering (DLS) and Transmission Electron Microscopy. The synthesized nanoparticles presented characteristic Surface Plasmon Resonance bands with maximum absorbance between 405 and 460 nm. The DLS analysis revealed particle sizes from 31 to 72 nm and zeta potential between −30.1 and −50.8 mV indicating good stability against the agglomeration of the particles in solution. The micrographs obtained by TEM showed different particle shapes and a predominance of spherical-shaped nanoparticles, and average size ranging from 27 to 94 nm. The clove-based silver nanoparticles were efficient in controlling the growth of Escherichia coli and Staphylococcus aureus bacteria, and the minimum inhibitory concentration ranged from 60 to 100 μL/mL. This study highlights the feasibility of clove essential oil as an alternative for the synthesis of silver nanoparticles by a simple, inexpensive and eco-friendly method.

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