The extraction method was adopted from [21] with minor modifications. The Prunus africana stem bark was washed with tap water, rinsed with distilled water, and left to dry for 15 days at room temperature. A mechanical grinder was used to pulverize the dried sample into a powder. A total of 10 g of powdered plant sample was dissolved in 500 mL of distilled water in a 1000 mL conical flask and heated at 60°C for 2.5 hrs on a hot plate with a magnetic stirrer. The extract solution was filtered through filter paper (Whatman No. 1). The filtrate was refrigerated at 4°C for further use.

Prunus africana-capped silver nanoparticles (AgNPs@PA) were synthesized via various protocols with minor modifications. The effects of temperature, reaction duration, and concentration of AgNO₃ solution as well as plant extract on the synthesis of AgNPs were studied to determine their impact during the reaction process. Eighteen milliliters (18 mL) of 2% (w/v) aqueous bark extract was added to 1 mL of (0.01, 0.1, 1 M) AgNO₃ solution, and the formulation of AgNPs was observed via a UV–Vis spectrophotometer (Shimadzu model, UV–Vis 1800 series) in the 200–800 nm range [22]. In addition, the experiment was conducted at room temperature (35, 40, 45, 50, 60, 65, 70, 80, 90, and 100°C) for 40 minutes to assess the significance of temperature during synthesis [23]. The transition from a colorless solution to a dark brown color indicates that AgNO₃ has been bio-reduced to form AgNPs. The mixture was subsequently centrifuged for 25 minutes at 10,000 rpm [24]. The solid masses were then dried in a vacuum oven at 60°C overnight after being individually rinsed with distilled water (DI-H₂O) in triplicate to remove unreacted precursor salts and biological particles [25].

The absorption spectrum and SPR of the formed AgNPs were recorded via UV‒Vis (UV-1800 series, Shimadzu model). The functional groups in the AgNPs samples and Prunus africana extract were investigated via Fourier transform infrared (FTIR) spectroscopy (IRAffinity-1S, SHIMADZU model) via the ATR technique. Transmission electron microscopy (TEM) (JEOL-JEM-1011, Japan) was used to examine the size and morphology of the biosynthesized AgNPs.

The method was adopted from [26] with significant adjustments. Nutrient agar medium was prepared by dissolving 14 g of agar powder in 500 mL of distilled water and then autoclaving it. A total of 20 mL of prepared agar was poured into each Petri dish, which was left to stand for 15 min for the agar to solidify. Then, the plates were inoculated overnight with human pathogens, such as the gram-negative strains (Escherichia coli and Pseudomonas aeruginosa), gram-positive strains (Staphylococcus aureus and Bacillus subtilis), and one fungus (Candida albicans) obtained from MKU Laboratory, Kenya. All the organisms were tested simultaneously via the disc diffusion method. The green-synthesized AgNPs, positive control (ampicillin), and pure extract of Prunus africana were added steadily until the wells were confluent, followed by incubation at 37°C for 24 h. The diameter of the zone of inhibition was measured.

Currently, the green fabrication of silver nanoparticles is fascinating. An aqueous solution of silver nitrate precursor salt and aqueous Prunus africana stem bark extract was used for the synthesis of silver nanoparticles. The mixture was uniformly mixed via a mechanical shaker at regulated temperatures and times. The formation of AgNPs was indicated by color changes from red‒brown to dark-brown (Fig. 1), as also reported by [22].

UV–visible spectral analysis was performed via a UV‒visible spectrophotometer (UV-1800 series, Shimadzu model). The analysis was carried out in a scan range of 200–800 nm with a resolution of 1 nm. Using distilled water as a blank, 300 microliters of the biogenic AgNP sample mixture was pipetted out and scanned with dilution in 3 mL standard quartz cuvettes. The process for monitoring the absorption patterns of the extracts and mixtures of extracts with silver nitrate was monitored at various concentration ratios, synthesis temperatures, and contact times. The peak broadening decreases as the concentrations of the reducing and capping agents in the plant extract increase (Fig. 2). The reaction time of Ag⁺ ion reduction to Ag⁰ by biomolecules present in Prunus africana aqueous extracts is concentration-, temperature- and time-dependent (Figs. 2, 3 and 4); therefore, as reported by [15], increasing the reaction temperature to 60°C increases the reduction rate and shortens the reaction time necessary for AgNP synthesis (Fig. 3) [27]. Figure 4 shows that the maximum reaction time was 55 minutes since, at 65 minutes, peak broadening was noted, which can be a result of agglomeration [28]. A blueshift was revealed by the surface plasmon resonance (SPR) peak at 435–437.5 nm (Figs. 2, 3 and 4), which shifted to a lower wavelength (432.5 nm) (Fig. 6), indicating that the particle size decreased with optimized synthesis parameters, i.e., a temperature of 60°C, O.1 M AgNO₃ and a reaction time of 55 minutes, whereas the redshifts indicate the production of large-sized particles or the presence of agglomerates [29]. The tauc plot shows that the biosynthesized AgNPs possessed a bandgap energy of 4.16 eV (Fig. 6). Increasing the temperature above 60°C results in agglomeration, disappearance of peaks, and peak broadening of the nanoparticles (Fig. 3). The common peak located at 278.5 nm (Figs. 2, 4, and 6) was a result of the chemical constituents present in the Prunus africana aqueous extract.

The formation of AgNPs can be monitored by the increased intensity of the absorbance band with respect to time (Figs. 4 and 5). The particle characteristics, including size, shape, type of metal, and dielectric compound around the medium, affect the surface plasmon resonance band wavelength and intensity [16].

The phytochemical constituents present in the stem bark extracts of Prunus africana that are responsible for reducing, capping, and stabilizing AgNPs were determined via FT-IR spectroscopy in the 4000–400 cm^− 1 scan range [30]. The FT-IR spectra of the plant extracts and formulated AgNPs are shown in Figs. 7(a) and 7(b), respectively. The presence of a strong, broadband band (Fig. 7(a)) at 3209–3028 cm^− 1 in the Prunus africana stem bark extract can be attributed to hydrogen-linked O-H stretching vibrations of phenol, alcohol, carboxylic groups, and other compounds [31].

Vibrational bands at 3446 cm^− 1 and 1369 cm^− 1 were also observed for the synthesized AgNPs (Fig. 7(b)), which were assigned to the O‒H bands of carboxylic groups, phenolics, and alcohols [32][33]. The C–O stretching vibrations at 1060 cm^− 1 indicate the presence of carbohydrates, terpenoids, and flavones in the Prunus africana extract [34]. Compared with those of the spectra of Prunus africana stem bark extracts, the intensities of the AgNP spectra at 769 cm^− 1, 1060 cm^− 1, 1211 cm^− 1, 1369 cm^− 1, 1600 cm^− 1, and 3209 cm^− 1 are lower [35]. The reason for the reduction and alteration of the spectra is that phytochemicals, such as alcohols, amides, and carboxylic groups, are involved in redox reactions during the synthesis of AgNPs [36]. The absorption peak at 1211 cm^− 1 is assigned to the stretching of N–O [3].

A strong absorption spectrum for Prunus africana stem bark extract (Fig. 7(a)) was observed at 1600 cm^− 1, which may be attributed to C = O bands and the N–H groups of proteins and enzymes [37]. The spectra of the C = O bands and the N–H groups of proteins and enzymes for the biosynthesized AgNPs shifted to 1597 cm^− 1, and the intensity significantly decreased, confirming the role of enzymes, proteins, and other biomolecules in bio-reduction [38], stabilization, and capping of the silver nanoparticles (Fig. 7(b)) [31]. The presence of saturated aliphatic esters is attributed to the sharp absorption band at 1741 cm^− 1 corresponding to carbonyl C = O bonds [39]. The reduced stretching vibration of Ag-O with an absorption band at 769 cm^− 1 (Fig. 7(b)) confirms the production of silver nanoparticles (the stretching was attributed to the metal‒ligand frequency that formed due to the interaction between the biomolecules and the AgNP surfaces) [40].

The XRD pattern of the AgNPs prepared from Prunus africana is shown in Fig. 8. XRD analysis was conducted to determine the purity, size, and crystalline structure of the biosynthesized AgNPs. In the XRD spectrum, three prominent diffraction bands were observed at 2θ = 38.26^°, 44.45^°, and 64.76^°, which could be indexed to the (111), (200), and (220) diffraction planes, respectively [41]. All the diffraction peaks were attributed to the cubic structure of pure Bragg's reflections of the FCC (face-centered cubic) structure of the metallic silver powder phase. These planes confirmed the crystalline nature of the green-synthesized AgNPs. The highest peak intensity of the (111) plane with a narrow full width at half maximum (FWHM) illustrates the good crystalline nature of the synthesized AgNPs, as observed from the XRD patterns. The resulting peaks and their corresponding Bragg reflections strongly agreed with the Joint Committee on Powder Diffraction Standards (JCPDS, file no. 04–0783) [42]. The prominent characteristic peaks of the green synthesized silver nanoparticles indicate the purity of the synthesized nanoparticles without any additional diffraction peaks [43]. The average crystallite sizes of the particles were calculated via the Debye–Scherrer equation (Eq. (1)):

Eq. 1

where x is the estimated crystal size in the nanometer (nm) from the XRD patterns, θ is Bragg’s angle (in radians), λ is the wavelength of the X-ray maximum of the diffraction peak (in radians), K is the shape factor or source used (CuKα = 1.5419 Ǻ), and β is the angular width at the half-Scherrer constant (0.9) of Debye–Scherrer's equation [34]. The estimated average crystalline size (x) of the synthesized silver nanoparticles was found to be 17.07 nm.

The TEM image of the AgNPs synthesized via Prunus africana revealed that the nanoparticles are predominantly spherical and of different sizes (Fig. 9 (a)). In addition to the spherical shape, a few other shapes, such as oval and triangular, were also observed. The aggregation of particles was also observed. A study by [44] also reported spherical AgNPs synthesized from Aloe vera gel extract. The clear boundaries observed around the nanoparticles indicate the occurrence of phytochemicals as capping agents that stabilize the silver nanoparticles. Figure 9 (b) shows the particle size distribution of the synthesized AgNPs. The synthesized nanoparticles are polydisperse and range in size from 15.95 nm to 43.04 nm, with an average size of 32.04 nm. Our findings followed those of previous reports, where plant extract was used as a reducing and capping agent in the synthesis of AgNPs, and similar results have been reported for AgNPs with sizes ranging from 18.23 to 53.68 nm [43].

The EDS spectrum reveals the purity and elemental composition of the biosynthesized AgNPs, as shown in Fig. 10. The strong signal for Ag, with higher atom percentages, was located at 3 keV, confirming the formation of silver nanoparticles biosynthesized with an aqueous extract of Prunus africana. Additionally, a few weaker signals of O and C were also obtained, indicating the existence of biomolecules capping and stabilizing the AgNPs [3].

The zeta potential, which provides important information about nanoparticle dispersion through the magnitude of the charge, reflects the mutual repulsion between particles. The particle sizes range from 100 to 1000 nm in diameter. The measured zeta potential of the AgNPs was − 12.44 mV (Fig. 11), preventing agglomeration and improving their stability in solution [35].

The in vitro antibacterial potential of the synthesized AgNPs was examined against two types of selected human pathogenic microbes, namely, fungus (Candida albicans), gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis), and gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) (Table 1 and Fig. 12). The clear zones of inhibition around the discs impregnated with the biosynthesized AgNPs, plant extract, and erythromycin (positive control) are shown in Fig. 12. Erythromycin is a commonly used antibacterial agent against superficial and deep infections caused by human pathogens [45]. AgNPs exhibited superior antibacterial properties against all the test microbes, as evidenced by the calculated zones of inhibition (Table 1), which may be due to the synergistic effects of the bioactive capped nanoparticles. The structures that make up bacterial cells include proteins, DNA, and cell membranes. These structures contain phosphorus and sulfur. Since silver is a Lewis acid and these substances are Lewis bases, sulfur proteins and silver ions are attracted to each other electrostatically, which could be the antibacterial action mechanism of silver nanoparticles [46]. Therefore, AgNPs can bind to the cell wall and penetrate bacterial cells. The internalization of silver nanostructures disrupts respiratory function, inactivating respiratory enzymes and generating reactive oxygen species (ROS). This overproduction of ROS damages intercellular components, including DNA, lipids, and proteins. The destruction of the cellular membrane causes loss of cytoplasm, leading to cell death [33]. In addition, cell wall thickness affects how effectively different bacteria respond to silver nanoparticles. Compared with gram-positive bacteria such as S. aureus, which have a thicker cell wall, gram-negative E. coli have a thinner cell wall, making it more vulnerable to silver nanoparticle penetration [47]. In general, nanoparticles have a high surface area-to-volume ratio, which enables them to interact more with microbes than larger particles do, resulting in improved microbial activity [48].

The acquired antibacterial and antifungal results were significant because the p value was less than 0.05 (Table 2).