Phytochemical-assisted synthesis, optimization, and characterization of silver nanoparticles for antimicrobial activity

Abstract

The increasing prevalence of antimicrobial resistance (AMR) bacteria poses a major global health threat, compounded by the limited development of new antibiotics. To address this challenge, alternative strategies, including nanoparticle-based therapies, are being explored. This study investigates the antimicrobial properties of green-synthesized silver nanoparticles (AgNPs) derived from leaf extracts of Ocimum gratissimum (OG), Apium graveolens (AG), and Aloe arborescens (AA). These plant extracts act as reducing, capping, and stabilizing agents during the synthesis process. By controlling the reaction parameters, the synthesized AgNPs displayed surface plasmon resonance (SPR) peaks at 434, 427, and 435 nm for OG, AG, and AA, respectively, indicating successful nanoparticle formation. The particles were predominantly spherical, with average sizes of 28.5 ± 6.3 nm (AgNPs-OG), 15.07 ± 3.8 nm (AgNPs-AA), and 20.2 ± 2.5 nm (AgNPs-AG), although some particles exhibited triangular and cylindrical shapes. X-ray diffraction (XRD) confirmed the formation of crystalline, face-centered cubic (FCC) metallic silver, while Fourier Transformation Infrared (FTIR) identified functional groups such as alcohols, amines, amides, carboxyl, and esters capping the surface of AgNPs. Energy dispersive spectroscopy (EDS) further confirmed the purity of the AgNPs. The antimicrobial activity of the synthesized AgNPs was tested against Gram-negative E. coli and Gram-positive S. aureus bacteria. Notably, AgNPs demonstrated high antimicrobial efficacy, particularly with smaller-sized, spherical particles showing superior performance. The minimum inhibitory concentration was as low as 1.016 μg mL^-1, highlighting the strong antimicrobial potential of AgNPs, whereas the minimum bactericidal concentration was recorded for E. coli, indicating greater susceptibility of Gram-negative bacteria to AgNPs and a concentration-dependent bactericidal effect. A comparison analysis showed that the antimicrobial effectiveness of the aqueous extract was significantly enhanced when AgNPs were incorporated, whereas higher antimicrobial performance was observed for green-synthesized AgNPs compared with wet chemically synthesized AgNPs reported in the literature. This is attributed to enhanced biocompatibility and a synergistic effect between the nanoparticles and plant-derived bioactive compounds. The mechanism of action of AgNPs involves silver ion (Ag⁺) release and reactive oxygen species (ROS) generation via surface oxidation and photoactivation. These findings underscore the potential of green-synthesized AgNPs as an alternative strategy in mitigating AMR.

Fig. 1. Photographic representations of (a) OG, (b) AG, and (c) AA, plant materials.

Fig. 2. UV-vis spectra of AgNPs synthesized using OG plant extract under varying conditions: (a) reaction temperature (b) time, (c) AgNO₃ concentration, and (d) plant extract to silver ion volume ratio.

Fig. 3. Photographic representation of sequential color changes observed during the synthesis of AgNPs using OG extract at a temperature of 75 °C and time (a) 15 min, (b) 30 min and (c) 60 min.

Fig. 4. FTIR spectra showing the functional groups capping the surfaces of the three synthesized AgNPs.

Fig. 5. XRD crystallographic patterns showing the phase structures of the synthesized AgNPs.

Fig. 6. STEM-EDS elemental mapping of (a) AgNPs-OG, (b) AgNPs-AA, (c) AgNPs-AG, and the corresponding EDS spectra showing the elemental composition of the synthesized samples.

Fig. 7. TEM micrographs of (a) AgNPs-OG, (b) AgNPs-AA, and (c) AgNPs-AG, illustrating the particle size, shape, and distribution at high (top) and low (bottom) magnifications, respectively.

Fig. 8. Absorption spectra of (a) plant extract and (b) AgNPs synthesized from the extracts of the three plants.

Fig. 9. Visible inhibitory Zones generated by AgNPs-AA, AgNPs-AG, and AgNPs-OG against (a) E. coli and (b) S. aureus.

Fig. 10. Schematic illustration of the antibacterial mechanism of action of AgNPs.