Green silver: Unveiling the role of biomolecules in plant-based nanoparticle synthesis with SIRIUS

Metal nanoparticles from gold, silver, iron, copper, and others, range in size from 1 to 100 nanometers and have a broad variety of applications in computing, optics, cosmetics, food industry, medicine, and water treatment. Silver nanoparticles, known for their antimicrobial properties, are effective for remediating contaminated waters. Plant extracts are used as reducing agents for the environmentally friendly synthesis of silver nanoparticles. To improve this synthesis, identifying the biomolecules involved in the process is crucial. UHPLC-QTOF-MS and SIRIUS identified the key phenolic compounds involved in the silver reduction.
Silver nanoparticles, known for their antimicrobial properties, are effective for remediating contaminated waters. (Photo by Luis Tosta on Unsplash)

Cleaning water with silver

Silver nanoparticles have unique physical, chemical, and biological properties compared to their bulk parent materials, due to their high surface-to-volume ratio and the nanometer scale. Silver nanoparticles are toxic to microorganisms and the antibacterial effects​1​ (even against antibiotic-resistant strains​2​) and antiviral effects​3​ have been used to control pathogenic microorganisms to provide safe drinking water from contaminated sources​4,5​. Traditional disinfection methods, such as chlorine, can produce harmful by-products with high geno- and cytotoxicity​6​. However, it is important to also address the potential risks associated with exposure to silver nanoparticles in suspension, both to human health and the environment​7​. Immobilizing silver nanoparticles on supports, like cellulose​8​ or cotton fabric​9​, minimizes risks and enhances antibacterial activity. These support materials prevent agglomerate formation, enable nanoparticle recovery for reuse, and reduce synthesis and application costs​10​.

How to make silver cotton with Eucalyptus leaves

Nanoparticle synthesis can be achieved through different methods, utilizing chemical and physical pathways. These conventional methods are not considered environmentally friendly, as they consume high amounts of energy and use toxic and expensive solvents and reagents​11​. One approach to greener synthesis is the utilization of microorganisms or plant extracts​12​, which eliminates the requirement for high pressures, high temperatures, and toxic chemical reagents​13​. While the use of microorganisms is often challenging​11​, using plant biomass for nanoparticle synthesis provides an alternative that is effective and simple​14​. For silver nanoparticles, the first successful synthesis using leaf extracts was reported in 2003​15​. Since then, researchers have explored a wide variety of plant extracts for this purpose​16​. Southern blue gum leaves, which are typically considered waste products in cellulose production​17,18​, contain phenolic compounds which makes them an excellent candidate for the production of silver nanoparticles.

Eucalyptus globulus leaves are waste products in cellulose production but contain phenolic compounds for the production of silver nanoparticles. (Photo by Liliana Eira on Unsplash)

Advancing synthesis by understanding the chemical processes

To improve green synthesis it is necessary to identify biomolecules or phytochemicals and understand their roles in the synthesis processes. Phytochemicals present in plant extracts play a crucial role in the synthesis of nanoparticles by acting as reducing and capping agents, ensuring their stability during formation​19​. Among these phytochemicals, phenolic compounds are primarily responsible for nanoparticle formation​20​, while carbohydrates, proteins, terpenoids, and vitamins also contribute to the process​19,21​. However, the specific roles of these phytochemicals in terms of nanoparticle stability, aggregation, morphology, and reactivity remain unclear​16​. By gaining a better understanding of these components, it becomes possible to enhance the efficiency and effectiveness of green synthesis methods. 

Researchers from several Chilean universities have investigated the phytochemicals that are involved in the formation of silver nanoparticles on cellulose paper and cotton fabric supports using Eucalyptus extracts to develop an efficient and eco-friendly protocol​22​. The Eucalyptus leaves are washed, dried, crumbled, heated in water, centrifuged, and filtered to get a leaf extract. The paper and fabric is inserted into silver nitrate solution, then removed and washed and inserted into the plant extract to reduce silver ions. Also they test sodium hydroxide to modify the pH of the system and test the effect on the structure and properties of the resulting nanoparticle. Different from previous studies that focused only on qualitative spectrophotometric analyses, this research group used UHPLC-QTOF-MS to analyse the phytochemical content in the leaf extracts before and after the nanoparticles are produced.

Unveiling the role of biomolecules in silver nanoparticle synthesis

In the quest to understand the intricate process of silver nanoparticle synthesis, the identification of biomolecules involved in the transformation becomes paramount. Characterizing Eucalyptus leaf extracts before and after nanoparticle formation provides valuable insights. FTIR spectroscopy analysis indicated that phenolic compounds, reducing sugars, and proteins are involved in the synthesis process of silver nanoparticles. Spectrophotometric analysis further revealed that phenolic compounds decrease in concentration after silver nanoparticle formation, indicating their possible role as reducing agents or stabilizers on the surface of the nanoparticles. Concentration of reducing sugars decreased only slightly, implying a potential stabilizing role. The protein content did not exhibit considerable changes. The researchers used UHPLC-QTOF-MS to identify the key phenolic compounds involved in the silver reduction. Most of the phenolic compounds were reported in Eucalyptus leaves before​23–25​. The remaining features were annotated using SIRIUS CSI:FingerID​26​. The main phenolic compounds that act as reducing agents are those derived from gallic acid, mainly due to the large number of -OH groups in these compounds. 

Overall they found that cotton fabric as support generates a more notable decrease in phenolic compounds in the leaf extracts and a higher proportion of nanoparticles on the fabric. Moreover, the silver nanoparticles on cotton fabric show excellent photocatalytic activity and antibacterial against E. coli and could be an interesting application for remediation of contaminated waters. 


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