Antibacterial, Antifungal, and Antioxidant Activities of Silver Nanoparticles Biosynthesized from Bauhinia tomentosa Linn

The biogenic synthesis of silver nanoparticles (AgNPs) has a wide range of applications in the pharmaceutical industry. Here, we synthesized AgNPs using the aqueous flower extract of Bauhinia tomentosa Linn. Formation of AgNPs was observed using ultraviolet-visible light spectrophotometry at different time intervals. Maximum absorption was observed after 4 h at 420 nm due to the reduction of Ag+ to Ag0. The stabilizing activity of functional groups was identified by Fourier-transform infrared spectroscopy. Size and surface morphology were also analyzed using scanning electron microscopy. The present study revealed the AgNPs were spherical in form with a diameter of 32 nm. The face-centered cubic structure of AgNPs was indexed using X-ray powder diffraction with peaks at 2θ = 37°, 49°, 63°, and 76° (corresponding to the planes of silver 111, 200, 220, 311), respectively. Energy-dispersive X-ray spectroscopy revealed that pure reduced silver (Ag0) was the major constituent (59.08%). Antimicrobial analyses showed that the biosynthesized AgNPs possess increased antibacterial activity (against Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative), with larger zone formation against S. aureus (9.25 mm) compared with that of E. coli (6.75 mm)) and antifungal activity (against Aspergillus flavus and Candida albican (with superior inhibition against A. flavus (zone of inhibition: 7 mm) compared with C. albicans (zone of inhibition: 5.75 mm)). Inhibition of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity was found to be dose-dependent with half-maximal inhibitory concentration (IC50) values of 56.77 μg/mL and 43.03 μg/mL for AgNPs and ascorbic acid (control), respectively, thus confirming that silver nanoparticles have greater antioxidant activity than ascorbic acid. Molecular docking was used to determine the mode of antimicrobial interaction of our biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs. The biogenic AgNPs preferred hydrophobic contacts to inhibit bacterial and fungal sustainability with reducing antioxidant properties, suggesting that biogenic AgNPs can serve as effective medicinal agents.


Introduction
Resistance to antibiotics and a wide variety of microorganisms in the public health system has become a major obstacle, and almost every single variant of microorganisms has

Synthesis of Silver Nanoparticles
A crude extract (5 mL) of B. tomentosa Linn flower powder was transferred into 45 mL of a 1 mM aqueous AgNO 3 solution in an Erlenmeyer flask. The flask was incubated in the dark at room temperature for 5 h to minimize photoactivation of silver nitrate. The AgNP solution was purified by repeated centrifugation at 10,000 rpm for 15 min (REMI-C-30BL, Centrifuge, REMI Electrotecnik Limited, Chennai, Tamil Nadu, India) followed by washing of the pellets with deionized water and finally drying to collect the AgNPs [56][57][58].

Characterization of Silver Nanoparticles
The confirmation of biosynthesized B. tomentosa Linn flower powder-extract-derived AgNPs was accomplished using ultraviolet-visible light (UV-vis) spectrophotometry (Lambda 265, Perkin Elmer Health Sciences Pvt. Ltd., Chennai, Tamil Nadu, India; range: 300-800 nm) [58]. Characterization of AgNPs through Fourier-transform infrared spectroscopy (FTIR, Perkin Elmer FTIR-Spectrometer 1725 X, Perkin Elmer Health Sciences Pvt. Ltd., Chennai, Tamil Nadu, India) was used to detect the characteristic peaks of the functional groups attached to the surface of AgNPs in a spectral range of 400 to 4000 cm −1 [59,60]. Scanning electron microscopy (SEM) was used to study morphological information on the sample at the submicron scale and elemental information at the micron scale [61,62]. The dried samples were coated with gold (Polaron Emitech SC7640 sputter coater, Quorum Technologies Ltd., Newhaven, East Sussex, UK), and microscopic images were taken at 250× and a voltage of 10 kV by a Jeol JSM-6480LV SEM machine (JEOL Ltd., Tokyo, Japan) to characterize the particle size and morphology of the AgNPs. Energy-dispersive X-ray (EDX) analysis helped determine the elemental composition of Antioxidants 2021, 10, 1959 4 of 18 the AgNPs [63]. X-ray powder diffraction (XRD) was applied for phase identification of the Cu Kα radiation (1.5405 Å) of the AgNPs (Philips PANanalytical X'Pert XRD System (model # 3040), Amsterdam, The Netherlands) [57,64].
2.7. Antimicrobial Activities of Biosynthesized AgNPs 2.7.1. Antibacterial Activity The antibacterial activity of biosynthesized B. tomentosa Linn flower-powder extractderived AgNPs was investigated against Gram-negative (E. coli) and Gram-positive (S. aureus) bacterial pathogens using agar disk diffusion [28,56,61,65,66]. Briefly, a nutrient agar medium was prepared in a Petri dish and the bacterial cultures were swabbed on test media with a sterile cotton swab. The discs were dipped with the following four components (30 µL): (i) biosynthesized AgNPs, (ii) B. tomentosa Linn flower powder extract, (iii) AgNO 3 solution, and (iv) standard antibiotic solutions (chloramphenicol, 30 µg/mL). The dried discs were pressed gently over the surface of the culture-swabbed medium at equal distances to avoid overlapping of the inhibition zones. The plates were then incubated at 37 • C for 24 h. After incubation, the antibacterial activity of the biosynthesized AgNPs was evaluated according to the diameters of the clear inhibition zones [67].

Antifungal Activity
Antifungal activity of biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs was analyzed against A. flavus and C. albicans by disk diffusion. The following four different components (30 µL) were applied on separate Whatman No. 1 filter paper discs 6 mm in diameter: (i) biosynthesized AgNPs, (ii) B. tomentosa Linn. flower powder extract, (iii) AgNO 3 solution, and (iv) standard antifungal solution (fluconazole, 30 µg/mL). Each was allowed to dry before being placed on a PDA medium carrying the fungal strains and then incubated for 48 h. The diameter of the zones was measured in centimeters with the help of a scale, and the results were tabulated [28,58,68,69].

Molecular Docking of Silver Nanoparticles
The structures of target proteins and small molecules (AgNPs, chloramphenicol, and fluconazole) were retrieved from the Protein Data Bank (PDB) and the PubChem database, respectively (

Statistical Analysis
Experiments were performed in at least three biological replicates (antibacterial, antifungal, and antioxidant assays) and data are presented as mean ± standard deviation. A Student's t test was applied using SPSS software (IBM SPSS Statistics; Armonk, NY, USA) [75].

Biosynthesis of AgNPs
Biosynthesis of B. tomentosa Linn flower-powder extract-derived AgNPs was monitored via the redox reaction (reduction of silver ions to metal and the formation of AgNPs) as recorded by UV-vis spectrophotometry (Figure 1). Over a period of 4 h the absorption peak shifted from approximately 400 nm to 420 nm due to the reduction of Ag + to Ag 0 (color shift from brown to yellowish), indicating that AgNPs were obtained.

Statistical Analysis
Experiments were performed in at least three biological replicates (antibacterial, antifungal, and antioxidant assays) and data are presented as mean ± standard deviation. A Student's t test was applied using SPSS software (IBM SPSS Statistics; Armonk, NY, USA) [75].

Biosynthesis of AgNPs
Biosynthesis of B. tomentosa Linn flower-powder extract-derived AgNPs was monitored via the redox reaction (reduction of silver ions to metal and the formation of AgNPs) as recorded by UV-vis spectrophotometry (Figure 1). Over a period of 4 h the absorption peak shifted from approximately 400 nm to 420 nm due to the reduction of Ag + to Ag 0 (color shift from brown to yellowish), indicating that AgNPs were obtained. Over a period of 4 h the absorption peak shifted from approximately 400 nm to 420 nm due to the reduction of Ag + to Ag 0 , indicating that AgNPs were obtained.

Fourier-Transform Infrared Analysis of Biosynthesized AgNPs
FTIR spectroscopy (in a range from 400 to 4000 cm −1 ) was used to detect functional groups in biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs. Characteristic absorption peaks corresponding to the functional groups of secondary metabolites, such as aliphatic primary amine (N-H bonds, peak at 3227.92 cm −1 ), terminal alkyne (C=C bonds, peak at 2099.24 cm −1 ), imine/oxime (C=N bonds, peak at 1263.68 cm −1 ), Antioxidants 2021, 10, 1959 6 of 18 ether (C-O bond, peak at 1187.09 cm −1 ) and aliphatic bromo components (C-Br bond, peak at 1081.58 cm −1 ), were evident. Formation of reduced silver atoms (Ag 0 , peaks at 706.63 cm −1 to 408.76 cm −1 ) and capping of the synthesized AgNPs by the phytochemicals present in the extract were also observed ( Figure 2).

Fourier-Transform Infrared Analysis of Biosynthesized AgNPs
FTIR spectroscopy (in a range from 400 to 4000 cm −1 ) was used to detect functional groups in biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs. Characteristic absorption peaks corresponding to the functional groups of secondary metabolites, such as aliphatic primary amine (N-H bonds, peak at 3227.92 cm −1 ), terminal alkyne (C=C bonds, peak at 2099.24 cm −1 ), imine/oxime (C=N bonds, peak at 1263.68 cm −1 ), ether (C-O bond, peak at 1187.09 cm −1 ) and aliphatic bromo components (C-Br bond, peak at 1081.58 cm −1 ), were evident. Formation of reduced silver atoms (Ag 0 , peaks at 706.63 cm −1 to 408.76 cm −1 ) and capping of the synthesized AgNPs by the phytochemicals present in the extract were also observed ( Figure 2).

Energy-Dispersive Spectroscopy Analysis of Biosynthesized AgNPs
An EDX analysis of biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs revealed signal energy peaks for silver atoms in a range of 2-4 keV, with weaker signals for chloride; pure silver (59.08%) was the major element compared to chloride (41.92%) ( Figure 3 and Table 2).
Strong signals of silver (59.08%) are clearly visible in the spectrum. The other signals can be attributed to the organic capping layer. The significant intensity of the peaks indicates the presence of a sufficient coating layer on the biosynthesized AgNPs [27,61,76].

Energy-Dispersive Spectroscopy Analysis of Biosynthesized AgNPs
An EDX analysis of biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs revealed signal energy peaks for silver atoms in a range of 2-4 keV, with weaker signals for chloride; pure silver (59.08%) was the major element compared to chloride (41.92%) ( Figure 3 and Table 2).   The data indicate the successful biosynthesis of AgNPs with some amount of chlorine impurities [77].  Strong signals of silver (59.08%) are clearly visible in the spectrum. The other signals can be attributed to the organic capping layer. The significant intensity of the peaks indicates the presence of a sufficient coating layer on the biosynthesized AgNPs [27,61,76].

X-ray Diffraction Analysis of Biosynthesized AgNPs
The data indicate the successful biosynthesis of AgNPs with some amount of chlorine impurities [77].

X-ray Diffraction Analysis of Biosynthesized AgNPs
The XRD method was used to determine the crystalline phase of the biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs. The XRD pattern includes diffraction peaks at 2θ = 37 • , 49 • , 63 • , and 76 • , corresponding to the planes of silver (111, 200, 220, 311), respectively ( Figure 4). The XRD data and pattern confirmed the crystalline structure of the biosynthesized AgNPs. No significant peaks corresponding to other crystalline phase impurities were detected. All peaks in the XRD pattern can be assumed to correspond with the structure of silver.

Scanning Electron Microscopic Analysis Biosynthesized AgNPs
An SEM analysis revealed uniformly distributed AgNPs on the surfaces of the nanoparticles. An SEM image of silver nanoparticles synthesized using B. tomentosa Linn flower extract shows spherical and relatively uniform shapes with a diameter near 32 nm ( Figure 5).

Scanning Electron Microscopic Analysis Biosynthesized AgNPs
An SEM analysis revealed uniformly distributed AgNPs on the surfaces of the nanoparticles. An SEM image of silver nanoparticles synthesized using B. tomentosa Linn flower extract shows spherical and relatively uniform shapes with a diameter near 32 nm ( Figure 5).

Antibacterial Activity of Biosynthesized AgNPs
The antibacterial activity of the biosynthesized AgNPs was determined using diffusion. The antibacterial activity of the biosynthesized AgNPs tested against G negative (E. coli) and Gram-positive (S. aureus) bacterial pathogens showed a larger of formation against S. aureus (9.25 mm ± 0.956 mm) compared with that of E. coli mm ± 0.957 mm) (Figures 6 and 7).

Antibacterial Activity of Biosynthesized AgNPs
The antibacterial activity of the biosynthesized AgNPs was determined using disk diffusion. The antibacterial activity of the biosynthesized AgNPs tested against Gram-negative (E. coli) and Gram-positive (S. aureus) bacterial pathogens showed a larger zone of formation against S. aureus (9.25 mm ± 0.956 mm) compared with that of E. coli (6.75 mm ± 0.957 mm) (Figures 6 and 7).

Antibacterial Activity of Biosynthesized AgNPs
The antibacterial activity of the biosynthesized AgNPs was determined using disk diffusion. The antibacterial activity of the biosynthesized AgNPs tested against Gramnegative (E. coli) and Gram-positive (S. aureus) bacterial pathogens showed a larger zone of formation against S. aureus (9.25 mm ± 0.956 mm) compared with that of E. coli (6.75 mm ± 0.957 mm) (Figures 6 and 7).    Figure  6). Data are presented as mean ± standard deviation of four independent experiments (* p < 0.01 [E. coli compared with S. aureus]. AgNP has the same efficacy as chloramphenicol.

Antifungal Activity of Biosynthesized AgNPs
The antifungal activity of biosynthesized B. tomentosa Linn flower-powder extractderived AgNPs was determined by disk diffusion against the fungal strains A. flavus and C. albicans. Fluconazole was used as a standard antifungal agent. The AgNPs achieved superior inhibition against A. flavus (zone of inhibition: 7 ± 0.812 mm) compared with C. albicans (zone of inhibition 5.75 ± 0.447 mm) (Figures 8 and 9).   Figure 6). Data are presented as mean ± standard deviation of four independent experiments (* p < 0.01 [E. coli compared with S. aureus]). AgNP has the same efficacy as chloramphenicol.

Antifungal Activity of Biosynthesized AgNPs
The antifungal activity of biosynthesized B. tomentosa Linn flower-powder extractderived AgNPs was determined by disk diffusion against the fungal strains A. flavus and C. albicans. Fluconazole was used as a standard antifungal agent. The AgNPs achieved superior inhibition against A. flavus (zone of inhibition: 7 ± 0.812 mm) compared with C. albicans (zone of inhibition 5.75 ± 0.447 mm) (Figures 8 and 9).  Figure  6). Data are presented as mean ± standard deviation of four independent experiments (* p < 0.01 [E. coli compared with S. aureus]. AgNP has the same efficacy as chloramphenicol.

Antifungal Activity of Biosynthesized AgNPs
The antifungal activity of biosynthesized B. tomentosa Linn flower-powder extractderived AgNPs was determined by disk diffusion against the fungal strains A. flavus and C. albicans. Fluconazole was used as a standard antifungal agent. The AgNPs achieved superior inhibition against A. flavus (zone of inhibition: 7 ± 0.812 mm) compared with C. albicans (zone of inhibition 5.75 ± 0.447 mm) (Figures 8 and 9).

Antioxidant Activity of Biosynthesized AgNPs
The radical scavenging activity of biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs was quantified spectrophotometrically by changing the DPPH color from brown to yellow. Inhibition of DPPH radical scavenging activity was found to be dose-dependent with half-maximal inhibitory concentration (IC50) values of 56.77 μg/mL and 43.03 μg/mL for AgNPs and ascorbic acid (control), respectively ( Figure 10).

Molecular Docking of Biosynthesized AgNPs
The antimicrobial mechanisms of AgNPs against bacterial or fungal pathogens remain unclear. AgNPs can directly attack and disrupt or penetrate cell walls to induce intracellular redox reactions mediating cytotoxicity. Moreover, AgNPs can interact with pivotal microbial proteins to inhibit their activities and cause cell death [78][79][80][81]. Accordingly, we selected representative proteins for each species to study the possible three-dimensional (3D) interaction of AgNPs with bacterial DNA gyrase [82,83], fungal CYP51

Antioxidant Activity of Biosynthesized AgNPs
The radical scavenging activity of biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs was quantified spectrophotometrically by changing the DPPH color from brown to yellow. Inhibition of DPPH radical scavenging activity was found to be dose-dependent with half-maximal inhibitory concentration (IC 50 ) values of 56.77 µg/mL and 43.03 µg/mL for AgNPs and ascorbic acid (control), respectively ( Figure 10).

Antioxidant Activity of Biosynthesized AgNPs
The radical scavenging activity of biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs was quantified spectrophotometrically by changing the DPPH color from brown to yellow. Inhibition of DPPH radical scavenging activity was found to be dose-dependent with half-maximal inhibitory concentration (IC50) values of 56.77 μg/mL and 43.03 μg/mL for AgNPs and ascorbic acid (control), respectively ( Figure 10).

Molecular Docking of Biosynthesized AgNPs
The antimicrobial mechanisms of AgNPs against bacterial or fungal pathogens remain unclear. AgNPs can directly attack and disrupt or penetrate cell walls to induce intracellular redox reactions mediating cytotoxicity. Moreover, AgNPs can interact with pivotal microbial proteins to inhibit their activities and cause cell death [78][79][80][81]. Accordingly, we selected representative proteins for each species to study the possible three-dimensional (3D) interaction of AgNPs with bacterial DNA gyrase [82,83], fungal CYP51

Molecular Docking of Biosynthesized AgNPs
The antimicrobial mechanisms of AgNPs against bacterial or fungal pathogens remain unclear. AgNPs can directly attack and disrupt or penetrate cell walls to induce intracellular redox reactions mediating cytotoxicity. Moreover, AgNPs can interact with pivotal microbial proteins to inhibit their activities and cause cell death [78][79][80][81]. Accordingly, we selected representative proteins for each species to study the possible three-dimensional (3D) interaction of AgNPs with bacterial DNA gyrase [82,83], fungal CYP51 (cytochrome P450 monooxygenase (CYP) superfamily) [51,84] and fungal dihydrofolate reductase [85]. To predict the biological interactions of the biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs with these possible microbial target proteins, we performed molecular docking analysis using a Patch dock server for the 3D structures of PDB proteins 3G7B (DNA gyrase, S. aureus), 4WUB (DNA gyrase, E. coli), 5TZI (cytochrome P450, C. albicans), and 6DRS (dihydrofolate reductase, A. flavus). Silver nanoparticle bound microbe structures (DNA gyrase, cytochrome P450, and dihydrofolate reductase) were visualized for interaction by PyMOL (Version 2.3.0, PyMol Molecular Graphics system, Schrödinger, LLC, New York, NY, USA). By the molecular rendering approach, interaction of AgNPs with amino acid (AAs) in the target protein structures was identified. The AA residues interacted with silver through hydrophobic contact ( Figure 11). (cytochrome P450 monooxygenase (CYP) superfamily) [51,84] and fungal dihydrofolate reductase [85]. To predict the biological interactions of the biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs with these possible microbial target proteins, we performed molecular docking analysis using a Patch dock server for the 3D structures of PDB proteins 3G7B (DNA gyrase, S. aureus), 4WUB (DNA gyrase, E. coli), 5TZI (cytochrome P450, C. albicans), and 6DRS (dihydrofolate reductase, A. flavus). Silver nanoparticle bound microbe structures (DNA gyrase, cytochrome P450, and dihydrofolate reductase) were visualized for interaction by PyMOL (Version 2.3.0, PyMol Molecular Graphics system, Schrödinger, LLC, New York, NY, USA). By the molecular rendering approach, interaction of AgNPs with amino acid (AAs) in the target protein structures was identified. The AA residues interacted with silver through hydrophobic contact ( Figure 11).
We applied multiple biophysical and biochemical methods to characterize our biosynthesized AgNPs. A UV-vis spectroscopic analysis showed a characteristic absorbance peak shift from 400 nm to 420 nm during the formation of biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs (Figure 1), which can be attributed to the formation of larger particles [57,89,90]. An EDX analysis helped demonstrate the elemental composition of the biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs (Figure 3, Table 2). The dense peak corresponding with silver strongly confirmed the reduction of AgNO3 and the formation of AgNPs. An EDX analysis also proved that
We applied multiple biophysical and biochemical methods to characterize our biosynthesized AgNPs. A UV-vis spectroscopic analysis showed a characteristic absorbance peak shift from 400 nm to 420 nm during the formation of biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs (Figure 1), which can be attributed to the formation of larger particles [57,89,90]. An EDX analysis helped demonstrate the elemental composition of the biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs ( Figure 3, Table 2). The dense peak corresponding with silver strongly confirmed the reduction of AgNO 3 and the formation of AgNPs. An EDX analysis also proved that the required phase of silver was present in the biosynthesized AgNPs [27,61,63,76,91]. The crystalline nature of the biosynthesized AgNPs was confirmed in the form of XRD diffraction peaks at 2θ = 37 • , 49 • , 63 • , and 76 • (corresponding to the planes of silver 111, 200, 220, 311), respectively (Figure 4), which are typical XRD values of biosynthesized AgNPs [65,76,[92][93][94]. Additionally, FTIR spectroscopy confirmed the various functional (amine, alkyl, ether, and aliphatic) groups and chemical bonding of biosynthesized AgNPs, while SEM analysis revealed the surface morphology and size of the AgNPs, which assumed spherical, uniform shapes ( Figure 5) [11,61,62].
To determine possible biomedical applications of the biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs we examined their potential antimicrobial activity. The biosynthesized AgNPs exhibited efficient anti-Gram-negative and anti-Grampositive bacterial activity, with higher efficiency against Gram-positive bacterial pathogens (Figures 6 and 7). Moreover, the biosynthesized AgNPs exhibited significant antifungal activity, as determined by the disk diffusion method, against A. flavus and C. albicans, respectively (Figures 8 and 9). Recent data point to the possible redox-potential of B. tomentosa Linn-derived AgNPs and their possible uses as antimicrobial agents [67]. The antimicrobial activity of our biosynthesized AgNPs may be mediated by a redox reaction, which was confirmed by the reduction and radical scavenging potential of silver in green biosynthesized AgNPs (against DPPH). The lowest concentration of the biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs was 20 µg/mL, with an effectivity of 15.30 ± 0.40% and an IC 50 of 56.77 (Figure 10), which was superior and in the range of previously described AgNPs using other green sources [63,95,96]. Therefore, our results presented here indicate that our biogenic AgNPs are superior to other biosynthesized AgNPs in terms of higher in vitro antioxidant [34,47,95] and higher in vitro antimicrobial efficacy (Table 3) [34,45,47,[56][57][58][61][62][63]65,69,70,96]. Finally, we used molecular modeling and docking analyses to investigate the antibacterial and antifungal mode of action of the biosynthesized AgNPs. We observed AgNP-mediated cytotoxicity and identified the AA residues SER-303, ASN-294 (DNA gyrase from Escherichia coli), ILE-67, THR-212, GLN-210 (DNA gyrase from S. aureus), ALA-107, PHE-105 (cytochrome P450 from C. albicans), and VAL-214, ALA-216 (dihydrofolate reductase from A. flavus) as possible participants in hydrophobic interactions with validated silver in the biosynthesized AgNPs, which are potentially responsible for the antibacterial and antifungal redox reactions mediating microbial cytotoxicity. We inferred from molecular modeling and docking studies that the biosynthesized AgNPs can effectively bind to microbes and act as antimicrobial agents ( Figure 11) [66,70,99].
Thus, the use of B. tomentosa Linn extracts for the synthesis of biomedically important Ag-NPs therefore has several advantages, since the environmentally friendly synthesis provides stable and highly effective AgNPs with a highly effective redox potential for highly effective antimicrobial activity and possible biomedical applications ( Figure 12) [7,34,41,[43][44][45][46][47]. reductase from A. flavus) as possible participants in hydrophobic interactions with validated silver in the biosynthesized AgNPs, which are potentially responsible for the antibacterial and antifungal redox reactions mediating microbial cytotoxicity. We inferred from molecular modeling and docking studies that the biosynthesized AgNPs can effectively bind to microbes and act as antimicrobial agents ( Figure 11) [66,70,99]. Thus, the use of B. tomentosa Linn extracts for the synthesis of biomedically important AgNPs therefore has several advantages, since the environmentally friendly synthesis provides stable and highly effective AgNPs with a highly effective redox potential for highly effective antimicrobial activity and possible biomedical applications ( Figure 12) [7,34,41,[43][44][45][46][47].

Conclusions
Silver nanoparticles from different natural sources are useful industrial and medicinal tools. B. tomentosa Linn flower-powder extract-derived AgNPs were characterized through UV-vis spectrophotometry, FTIR, XRD, and EDX. We observed the reduction of Ag + to Ag 0 with an accompanied UV-vis spectral peak shift from 400 nm to 420 nm over 4 h. The FTIR analysis revealed the functional (amine, alkyl, ether, and aliphatic) groups of AgNPs, while XRD analysis showed that the biosynthesized AgNPs had a crystalline structure. Results of SEM analysis revealed the AgNPs were spheres approximately 32 nm in diameter. The results of EDX examination confirmed the presence of Ag 0 in biosynthesized AgNPs with reducing antioxidant properties validated by DPPH assays. Biologically synthesized AgNPs exhibited antibacterial activity against E. coli (Gram-negative) and S. aureus (Gram-positive) as well as antifungal activity against C. albicans and A. flavus. Figure 12. Overview of the study of B. tomentosa Linn flower extract-derived biogenic AgNPs. Biosynthesized AgNPs (change in the color (brown to yellow) of the solution over time when aqueous plant extract was added to a AgNO 3 solution). Biophysical characterization of biosynthesized AgNPs by UV-vis, FTIR, XRD, and SEM confirmed the nature of AgNPs. Biochemical and cellular analyses confirmed the antioxidant (dose-dependent DPPH radical scavenging activity) and antimicrobial (antibacterial (Gram-positive (G + ) and Gram-negative (G − )) and antifungal) properties of the biogenic AgNPs. Molecular modelling and docking studies indicated the possible antimicrobial activity mechanism of the biogenic AgNPs: inhibition of key enzymes such as DNA gyrase, cytochrome P450, and dihydrofolate reductase.

Conclusions
Silver nanoparticles from different natural sources are useful industrial and medicinal tools. B. tomentosa Linn flower-powder extract-derived AgNPs were characterized through UV-vis spectrophotometry, FTIR, XRD, and EDX. We observed the reduction of Ag + to Ag 0 with an accompanied UV-vis spectral peak shift from 400 nm to 420 nm over 4 h. The FTIR analysis revealed the functional (amine, alkyl, ether, and aliphatic) groups of AgNPs, while XRD analysis showed that the biosynthesized AgNPs had a crystalline structure. Results of SEM analysis revealed the AgNPs were spheres approximately 32 nm in diameter.
The results of EDX examination confirmed the presence of Ag 0 in biosynthesized AgNPs with reducing antioxidant properties validated by DPPH assays. Biologically synthesized AgNPs exhibited antibacterial activity against E. coli (Gram-negative) and S. aureus (Grampositive) as well as antifungal activity against C. albicans and A. flavus. A possible mode of reducing antibacterial and antifungal activities was studied by molecular docking analysis, which indicated that the biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs may be able to inhibit key enzymes, such as bacterial DNA gyrase and fungal cytochrome P450 (C. albicans) and dihydrofolate reductase (A. flavus). This study may pave the way for the development of new and potentially antimicrobial compounds based on biosynthesized B. tomentosa Linn flower-powder extract-derived AgNPs ( Figure 12).