The rise of nanoscience and nanotechnology has driven significant advances in the fabrication of nanoparticles, particularly silver nanoparticles (AgNPs), which have attracted increasing interest due to their remarkable physicochemical properties and wide-ranging applications in medicine, biotechnology, environmental science, and the pharmaceutical industry. Traditionally, their synthesis has relied on physicochemical processes involving toxic reagents, organic solvents, and energy-intensive production conditions, raising major concerns regarding environmental safety and biological compatibility.

To overcome these limitations, green synthesis has emerged as an innovative, sustainable, and environmentally friendly alternative. This approach uses plant extracts, microorganisms (bacteria, fungi, microalgae), or natural biomolecules that act simultaneously as reducing and stabilizing agents. Green synthesis allows the production of clean, biocompatible, and stable nanoparticles, while offering improved control over size, shape, and distribution [1] - [5] . Moreover, the natural richness of plant extracts in bioactive compounds (phenols, flavonoids, enzymes, tannins, etc.) enhances the biological functionalization of nanoparticles, thereby opening new prospects in diagnostics, targeted therapy, tissue engineering, and antimicrobial coatings [6] - [9] .

Several plant species have demonstrated strong potential for the biosynthesis of homogeneous AgNPs with sizes ranging from 10 to 50 nm, characterized by excellent colloidal stability and pronounced antibacterial and photocatalytic properties [1] . Numerous studies have also highlighted the crucial influence of synthesis parameters such as pH, temperature, and reaction time on the size, distribution, and functional properties of the resulting nanoparticles [2] [3] . For instance, AgNPs synthesized from Ficus carica have shown notable antibacterial, antibiofilm, and anticancer effects, with an IC₅₀ value of 8.4 µg/mL on certain cell lines [4] [10] .

Similarly, Eugenia uniflora fruit extract has been reported to enable efficient green synthesis of silver nanoparticles with homogeneous morphology, good colloidal stability, and excellent antibacterial properties, highlighting the value of phytochemical-rich plant sources for the eco-responsible production of functional nanomaterials [11] . In another study, AgNPs biosynthesized from Sida acuta leaves were compared to nanocomposites combining these nanoparticles with reduced graphene oxide (rGO). The results revealed strong antibacterial activity, particularly against MRSA and Proteus mirabilis, as well as promising anticancer potential. These green nanoparticles, which were very small (−5 nm) and well dispersed, also exhibited notable antioxidant, anti-inflammatory, and anti-diabetic properties, underscoring their therapeutic potential [12] .

In parallel, the use of microalgae such as Graesiella emersonii represents a promising strategy: extracellular metabolites produced by these microorganisms facilitate the reduction of metal ions and the stabilization of AgNPs in a bioinspired and finely regulated environment [5] . Recent developments have further enabled the incorporation of AgNPs into polymeric nanocomposites, opening new avenues for applications in regenerative medicine, drug delivery, and the design of smart antimicrobial materials [6] - [9] .

In this context, the present study aims to investigate the potential of green synthesis of AgNPs from specific biological resources, to characterize the resulting nanoparticles, and to evaluate their colloidal stability. More specifically, the objective is to synthesize silver nanoparticles from Vachellia seyal extracts via a chemical reduction process using ascorbic acid, with sodium borohydride serving as a reference, and to characterize them using UV-Visible spectrophotometry, dynamic light scattering, and electron microscopy.

The synthesis was carried out by chemical reduction in three steps. First, a control synthesis was performed using sodium borohydride (NaBH₄). Next, a second synthesis was conducted using ascorbic acid as the reducing agent. Finally, the last synthesis was carried out with the Vachellia seyal extract. Each resulting solution was characterized by UV-Visible spectrophotometry, dynamic light scattering, and electron microscopy (performed for the synthesis using the V. seyal extract) to analyze their optical and structural properties.

Upon addition of the NaBH₄ solution at 5.0 × 10⁻³ mol·L⁻¹, the reaction mixture immediately changed color: yellow after one minute, followed by orange, then brown, gray, and finally blue after approximately three minutes. The corresponding absorption spectrum is shown in Figure 1 .

After the addition of the ascorbic acid solution at 1.0 × 10⁻² mol·L⁻¹, the reaction mixture changed color, turning light blue after approximately three minutes. The corresponding absorption spectrum is shown in Figure 2 .

After the addition of the Vachellia seyal extract solution at 1.7612 g·L⁻¹, the reaction mixture rapidly developed an orange color, which remained stable after several minutes of stirring. The corresponding absorption spectrum is shown in Figure 3 .

The absorption spectrum of the NaBH₄-synthesized sample exhibits a broad band between 350 and 500 nm, confirming the presence of nanoparticles with a highly polydisperse size distribution [22] . In contrast, the sample synthesized with ascorbic acid shows a much narrower band with an absorption maximum at 430 nm, indicating a lower degree of polydispersity and nanoparticles ranging in size from 60 to 70 nm. For the sample synthesized with ascorbic acid, which is a weaker reducing agent than NaBH₄, the spectrum displays a very broad band, suggesting a very high polydispersity index, with an absorption maximum at 430 nm.

For the Vachellia seyal extract, the absorption spectra of the resulting nanoparticles display moderately narrow bands, suggesting a relatively low to moderate polydispersity. Silver nanoparticles exhibit unique optical properties that enable strong interactions with specific wavelengths of light. Numerous studies have shown that, in general, absorption bands in the 200 - 800 nm range are optimal for characterizing nanoparticles with sizes between 2 and 100 m. In silver nanoparticles, the conduction and valence bands are very close, allowing electrons to move freely. These free electrons give rise to a surface plasmon resonance (SPR) absorption band, resulting from the collective oscillation of electrons in resonance with incident light. The SPR band depends on particle size, the dielectric medium, and the chemical environment. The presence of a surface plasmon peak is well documented for metallic nanoparticles in the 2 - 100 nm range [20] [21] [23] .

Characterization by DLS confirmed the presence of silver nanoparticles. NaBH₄-synthesized nanoparticles had an average size of 18.69 ± 0.04 nm with a polydispersity index (PDI) of 0.639. Using ascorbic acid, the average nanoparticle size was 42.44 ± 5.37 nm with a PDI of 0.234 ± 0.074. The use of V. seyal extract significantly increased particle size to 129.53 ± 0.57 nm, with a PDI of 0.092 ± 0.016. These results indicate an inverse relationship between nanoparticle size and polydispersity index and confirm that nanoparticle size correlates with the reducing strength of the agent [23] - [25] .

Overall, these findings demonstrate the feasibility of green synthesis using plant extracts to produce silver nanoparticles with controlled sizes suitable for pharmaceutical applications. Morphological characterization by electron microscopy further confirmed the successful biosynthesis of these nanoparticles.

This study demonstrates the successful synthesis of silver nanoparticles (AgNPs) using both conventional (NaBH₄, ascorbic acid) and green (Vachellia seyal extract) reducing agents. Spectroscopic and microscopic analyses confirmed the formation of AgNPs in all cases, with notable differences in size and polydispersity depending on the reducing agent used. In particular, nanoparticles synthesized with V. seyal extract exhibited larger sizes but a lower polydispersity index, indicating good colloidal stability and uniformity. These results highlight the critical role of the reducing agent’s antioxidant capacity in determining nanoparticle characteristics. The green synthesis approach using V. seyal not only provides an environmentally sustainable alternative but also offers promising potential for biomedical and pharmaceutical applications, owing to its simplicity, eco-compatibility, and the biofunctional properties of the plant-derived compounds. Future studies will focus on optimizing synthesis parameters and evaluating the biological activities of these nanoparticles for potential therapeutic applications.