Numerous plant species permit AgNP production. The production of AgNPs utilizing bioorganic chemicals has been the subject of numerous papers in recent years. Plant extracts have emerged as a valuable resource because they are readily available, simple to obtain, and can be used to safely and affordably generate non-toxic nanoparticles [Hano et al., 2021]. Plants can decrease the amount of metal ions on their surface, take them in through their roots, and move them to different parts of their bodies where they can be reduced. By lowering Ag + ions, plant extracts can influence the formation of AgNP. The active component that reduces Ag + ions to Ag0 determines a plant extract's nanoparticle synthesis ability. The organism or plant extract determines how it changes [Jaison et al., 2023]. The ability to biosynthesize various antioxidants that can reduce oxidative damage from reactive oxygen species (ROS) is intrinsic in plants. Numerous chemical compounds, including proteins, lipids, carbohydrates, flavonoids, phenols, terpenes, alkaloids, enzymes, and coenzymes, are included in these reducing agents. AgNO3, or silver nitrate, can be reduced by [Alharbi et al. 2022] to create silver nanoparticles [20]. The choice of a solvent, a secure reducing agent, and a nontoxic component for NP stabilization must be considered during the NP synthesis process [Mahalingam et al., 2023]. Several additional parameters need to be improved to improve the efficacy, size, and morphology of NPs produced using a green technique. These include pH, salt content, incubation duration, temperature, centrifugation, and reaction time [Gupta et al. 2021, Liaqat et al. 2022, Ansari et al. 2023]. In a different study, AgNPs were produced efficiently at temperatures between 60 and 80°C [EuroMed PlantBase]. Several investigations have verified that AgNPs undergo morphological changes as reaction temperature rises, with decreasing AgNP dimensions [Fu et al. 2021].
Using an alkaline pH during the synthesis of AgNPs had many benefits, including a stable and high yield of nanoparticles, a fast growth rate, and an improved reduction process [Nocera et al. 2020]. Incubation time is also considered an essential parameter for enhancing the yield, stability, and size of synthesized AgNPs [Abdelmoneim et al. 2022].
Adjusting the concentration of bioactive chemicals can enhance the reaction rate. A few challenges need to be addressed, including the intricate task of pinpointing the specific plant bioactive components that play a role in the synthesis of NPs and how they impact their therapeutic effects [Liaqat et al. 2022; Lade and Shanware 2020]. Temperature plays a crucial role in nanoparticle synthesis, as highlighted by Akshaya et al. (2022) and Lade and Shanware (2020). In their study, Mountrichas et al. [2014] demonstrated that higher temperatures (60°C) resulted in a more rapid and consistent synthesis of nanoparticles. In a study conducted by Gontijo et al. [2020], the synthesis of Ag NPs was examined in terms of pH. The researchers observed a range of NPs with varying sizes (5–249 nm) across pH values ranging from 2 to 9. Additionally, the researchers noted a decrease in clustered nanoparticles at pH levels below 3 and above 7. Meanwhile, Traiwatcharanon et al. [41] employed UV–vis spectroscopy and transmission electron microscopy to investigate how pH affects the characteristics of Ag NPs. Smaller particles were observed when Ag NPs were synthesized in an acidic medium rather than a basic one, as noted by the authors.
The synthesis of extracellular silver nanoparticles using ethanolic aqueous leaf extract demonstrates a rapid, straightforward, cost-effective process that can be compared to chemical and microbial methods. These silver nanoparticles have antibacterial properties against various bacteria, including Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumonia, Enterococcus faecalis, and S. pseudintermedius.
Scientists have discovered that silver nanoparticles possess remarkable antimicrobial properties, making them effective against bacteria resistant to multiple drugs. This is due to their large surface-area-to-volume ratio and distinct chemical and physical characteristics. AgNPs exhibit a wide range of particle sizes, typically between 1 and 100 nm. As the particle size decreases, the surface area-to-volume ratio of AgNPs increases. A recent study by Bruna et al. [2021] found that AgNPs ranging from 10–100 nm in length exhibited strong antimicrobial properties against both Gram-positive bacteria and harmful bacteria. With their tiny size, AgNPs can easily attach to the cell wall and enter bacterial cells, enhancing their ability to fight against bacteria.
On the other hand, AgNPs have clear advantages over conventional chemical antimicrobial agents. One major issue with traditional chemical antimicrobial agents is the development of multidrug resistance. Understanding the efficacy of chemical antimicrobial agents relies on the precise interaction between microorganisms and the surface and metabolites of these agents. Nevertheless, the effectiveness of chemical antimicrobial agents is restricted, particularly in medicine, due to the development of multiple resistance traits by various microorganisms over generations. Therefore, developing AgNPs could be a viable solution to combat multidrug-resistant microorganisms. Metal nanoparticles, such as AgNPs, have shown promise in their ability to hinder the development of resistance in bacteria, unlike traditional antibiotics.
The diffraction peaks obtained in XRD corresponded to the FCC structure of metallic silver ions in its purest form, and the particle size ranged between 10 and 60 nm [Albarbi et al. 2022, Ali et al. 2023, Dilbar et al. 2023]. The characterization studies by scanning electron microscopy have provided more information on the green synthesized silver nanoparticles in the presence of Sempervivum tectorum L. leaf extract, such as the size and morphology of nanostructures at the scale of 200 nm, 1–2 µm.
The TEM studies have given further inputs on the morphology and size of green silver nanoparticles ranging between 10 and 60 nm with a scale of 100 nm and histogram showing sizes of particles with spherical morphology.
Given the characteristics of NPs, such as a higher surface area and multiple reactive active sites, their interaction in a suspension can lead to their accumulation. Based on the organic layers observed in the TEM images, it appears that the aggregation of the nanoparticles may have been influenced by secondary metabolites like phenolics, flavonoids, alkaloids, and so on. It was found that certain secondary metabolites produced by the plant extract remained firmly attached to NPs, even after undergoing consistent centrifugation during the initial step. The results obtained through TEM were consistent with the XRD observations. Observing the TEM micrographs of the AgNPs synthesized through a green method, one can see the presence of individual nanoparticles with a spherical shape. This particular characteristic is a defining feature of silver nanoparticles. Just like a microbiologist, it's important to consider the surface area and size of the silver NPs as they significantly impact their biological activities. Similar to a microbiologist's perspective, nanoparticles possess a larger surface area at the nanoscale, resulting in numerous active and reactive sites. This unique characteristic enables them to interact effectively with bacterial cell walls, enhancing their permeability and promoting their antibacterial activity.
The antibacterial studies with green synthesized AgNPs showed a significant antibacterial effect against S. pseudintermedius strains. The disc diffusion studies demonstrated larger inhibition zones, similar to norfloxacin (positive control).
The MIC and MBC studies showed varied concentrations of green synthesized AgNPs against S. pseudintermedius strains.