Abstract
Silver nanoparticles (AgNPs) have been widely employed or incorporated into different materials in biological application, due to their antibacterial properties. Therefore, antimicrobial capacity and cytotoxicity have been highly studied. However, most of these reports do not consider the possible corrosion of the nanomaterials during their exposure to atmospheric conditions since AgNPs undergo a transformation when they come in contact with a particular environment. Derived from this, the functionality and properties of the nanoparticles could decrease noticeably. The most common silver corrosion process occurs by the interaction of AgNPs with sulfur species (H2S) present in the atmospheric air, forming a corrosion layer of silver sulfide around the AgNPs, thus inhibiting the release of the ions responsible for the antimicrobial activity. In this work, AgNPs were synthesized using two different methods: one of them was based on a plant extract (Brickellia cavanillesii), and the other one is the well-known method using sodium borohydride (NaBH4). Chemical stability, corrosion, antibacterial activity, and toxic activity were evaluated for both sets of prepared samples, before and after exposition to atmospheric air for three months. The structural characterization of the samples, in terms of crystallinity, chemical composition, and morphology, evidenced the formation of link structures with nanobridges of Ag2S for non- “green” AgNPs after the air exposition and the intact preservation of silver core for the “green” sample. The antibacterial activity showed a clear improvement in the antimicrobial properties of silver in relation to the “green” functionalization, particle size control, and size reduction, as well as the preservation of the properties after air exposition by the effective “green” protection. The cytotoxicity effect of the different AgNPs against mononuclear cells showed a notable increment in the cell viability by the “green” functionalization.
Highlights
IntroductionRecent studies have extensively studied the toxicity of AgNPs [1], where the biological effects of these type of NPs have been analyzed using microorganisms, various cell lines, and nonvertebrate and vertebrate model organisms [2]
In the G-AgNPs “green” synthesis method reported in this work using leaves extract of Brickellia cavanillesii, the reduction of Ag+ to Ag0 occurred by the action of polyphenols, mainly tannins groups, by the extract acting as a bioreducer agents for the silver ions, in response to the pH change; the –OH groups contained in the tannins suffered a hydrolysis releasing a hydrogen atoms and electrons that subsequently reduce the Ag+ ions, initiating the process of formation of primary particles
The particles functionalization occurs simultaneously to the nucleation process by the bidentate coordination of the ligand group carboxyl with the Ag-atoms contained in the NPs surface, preventing the interaction, agglomeration, and growth of the particles. e biosynthesis process was evidenced by the solution color change to brown, indicating the formation of silver nanoparticles; the “green” synthesis of NPs was monitored by UV-Vis spectroscopy, observing the characteristic band of Ag at 429 nm after synthesis conclusion (Figure 1(a)). e increase in color of the solutions is directly proportional to the reaction time, in response to the excitation of the surface plasmon resonance effect (SPR) and the reduction of AgNO3 [17]
Summary
Recent studies have extensively studied the toxicity of AgNPs [1], where the biological effects of these type of NPs have been analyzed using microorganisms, various cell lines, and nonvertebrate and vertebrate model organisms [2] It is well known in materials science that in the application of nanomaterials formed of pure metals, chemical stability plays an important role, due to the high reactivity of that kind of materials, as a consequence of their large surface area, they exhibit kinetics of corrosion more accelerated and a high instability to the air exposition. It has been reported that silver atmospheric corrosion effects after several weeks of exposure are represented mainly by its interaction with reduced sulfur ligands presents in the surrounding environment, forming the passivation of the particles surface represented by a coreshell composed of a thin layer of silver sulfide (Ag2S), which hinders the reaction and interaction of the Ag0 core, modifying the their transport, reactivity, and toxicity [3, 4, 8]
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