Introduction

Science has continued to advance at a fast pace, an aspect valid by the observation of nanotechnology, which is embarking on a phenomenal growth in the development of new products with multifunctional applications inherent in them and, in equal measure, applied in the field of medicine and environmental conservation1,2,3,4. Metal nanoparticles, with their intriguing physicochemical properties, have played a significant role in the technological advancement of nanotechnology. Nonetheless, finding eco-friendly and cost-effective materials and synthesis procedures is a significant problem, which has driven increased interest in green technologies5,6. Among all these materials, the silver nanoparticles (AgNPs) have such great interest owing to their enhanced bactericidal, antifungal, and catalytic properties as well as other physicochemical properties7,8,9,10. Earlier, AgNPs synthesis has been carried out by chemical and physical methods, which are expensive, time-consuming, energy consuming and involve toxic reducing agents, raising certain environmental and safety concerns. To these challenges, the green synthesis approach using bio-sources, including plants, fungi, and bacteria, has therefore emerged as the best and safest way of preparing nanoparticles11,12. Considering the biogenic sources for the synthesis of nano silver, the Manilkara zapota (M. zapota) could be more effective for green synthesis of AgNPs13,14,15. This tree-borne fruit is native to Mexico and Central America and is widely grown globally for the edible nuts as well as medicinal values due to the existence of phytochemicals like tannins, flavonoids, saponins, and other bioactive compounds16. Such phytochemicals have antioxidant, antimicrobial, and anti-inflammatory properties; therefore, M. zapota can be considered a renewable bio-template for synthesizing functional nanomaterials17,18. M. zapota is considered bioactive, so it enhances this property to synthesize AgNPs and follow the green chemistry strategy to minimize the negative impact of nanoparticles on the environment.

The biosynthesis of AgNPs using the plant extract of M. zapota belongs to the biogenic category, whereby there is a reduction of Ag+ to AgO. The process is generally carried out under slightly acidic pH, and the process does not involve the use of toxic solvents or other such chemicals. This results in the synthesis of AgNPs, which are entrapped by capping agents; these are other phytochemicals that are present in the plant extract that assist in preventing the coagulation of the nanoparticles19,20. This is a fast method when it comes to nanoparticle synthesis since it is cheaper and yields nanoparticles with properties pegged to the phytochemicals present in the plant extract. The bio-assisted combustion with calcination approach of synthesizing nanomaterials has a significant advantage over standard, environmentally friendly synthesis in that it allows for the rapid, energy-efficient synthesis of highly crystalline nanoparticles with regulated shape. Optimized calcination at 750 °C eliminates residual organics and promotes crystallinity without particle aggregation, enhancing the functional qualities of the nanomaterial21,22,23.

The biomedical applicability of AgNPs is widely recognized and documented in various studies, which emphasize their effectiveness against many pathogens such as bacteria, fungi, and viruses. The activity of AgNPs as antimicrobials has been ascribed to the ability to generate reactive oxygen species (ROS), destroy microbial cell membranes, and interfere with essential metabolic processes in these cells. Nevertheless, depending on the size, shape, surface charge, and capping agents on nanoparticles that depend on the synthesis method and the reducing agent being used, their action mechanisms differ24,25,26.

There has been an increasing interest in the recent past on using plant-mediated AgNPs for environmental applications, particularly in degrading organic pollutants. Among the most stubborn and harmful chemicals that are in use in the textile, paper, and leather industries are organic dyes that persist in wastewater27,28,29. In developing effective and sustainable wastewater treatment tools, a lot of research has been directed towards the catalytic degradation of these dyes with AgNPs. The capacity of AgNPs to initiate the disintegration of organic dyes is largely reliant upon their surface area, electronic configuration, and presence of active sites, which can be tailored by controlling the synthesis method and capping agents used30,31. Herein lies a double advantage of using M. zapota L. extract for the biosynthesis of AgNPs. First, new antibacterial agents could be made by exploiting the antimicrobial properties of M. zapota-mediated AgNPs, especially at such times when there happens to be a rise in antibiotic resistance levels. Second, these nanoparticles can work as catalysts for breaking down dangerous organic dyes, hence providing an eco-friendly technique for removing environmental pollutants.

The present work novels on evaluating the multifunctional potency of synthesized AgNPs through a green approach. The prepared AgNPs were characterized using various physicochemical techniques for structure, optical, and morphological properties. Further, the bio-combusted sample is employed in degrading the cationic dye, Methylene Blue (MB). The results were utilized in studying the kinetics and mechanism involved in degrading the same. Furthermore, the same has been utilized in anti-bacterial evaluation against Gram-positive and Gram-negative bacterial strains.

Experimental

Chemicals and analytical techniques

The pristine analytical grade silver nitrate (AgNO3) (≥ 99.0%), purchased from Sigma-Aldrich (USA), was used as a precursor. The Bruker D8 Advance Powder X-ray diffractometer (Microstar Proteum 8) was used as an experimental method of examining the structure of the as-synthesized sample (). Moreover, the FESEM JEOL JSM-7100 F device was utilized to determine the surface morphology and elemental composition of the samples. The precisely resolved JOEL JEM-2100 plus TEM Japanese Transmission Electron Microscope was employed to give a more accurate average particle size assessment utilizing ImageJ, a free software available at https://imagej.net/ij/index.html. The spectrophotometer employed in probing the band gap and conducting photocatalysis was Lab India UV-3200. FTIR analysis was performed using SHIMADZU IR, XRoss. Horiba nanoparticle analyzer SZ-100 was employed for Dynamic Light Scattering (DLS) analysis.

Preparation of M. zapota L. extract

The preparation of M. zapota L. extract follows the method reported in our previous work40. The M. zapota leaves were identified with the help of a taxonomist (Dr. Murali, University of Mysore), and fresh leaves were plucked from the plant in the botanical garden located at Manasagangotri campus, University of Mysore (deposited in the Herbarium lab at University of Mysore having voucher number 12/1941). The leaves were properly cleansed with double-distilled water and placed in a beaker measuring 250 milliliters along with 100 mL of water. The mixture was heated for an extended period until 100 mL was reduced to 25 mL of the leaf extract (L. extract). The combination was then filtered, and the filtrate was stored at 4 °C for further characterization.

Synthesis of M. zapota L. extract mediated silver nanoparticles (AgNPs)

A bio-assisted combustion synthesis was used in synthesizing the sample of AgNPs40. First, 1.69 g of AgNO3 was calculated for the stoichiometric amount and put into a combustion crucible. Then, 5 mL of freshly extracted M. zapota L. extract was added as a fuel along with the double-distilled water, and the combination was allowed to agitate for 30 min to get a highly neutral solution. The same was introduced into a preheated muffle furnace maintained at a temperature of 500 °C, resulting in a typical combustion product that was taken to obtain the black amorphous product. Additionally, the obtained sample was calcined for 2 h at 750 °C to obtain AgNPs of the desired phase. A brief synopsis of the synthesis process is depicted in Fig. 1. The same was conducted in triplicate to achieve the appropriate yield 1.086 g, 64.3%) sufficient for sample characterization.

Fig. 1
figure 1

Schematic representation of bio-combustion of AgNPs and its reaction mechanism involved.

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Catalyst activity against MB dye

The bio-combusted AgNPs were utilized as a photocatalyst in removing 10 ppm (10 mg/L) MB dye solution in acidic medium, conducted in a 3 mL cuvette cell at an ambient temperature of 27 ± 5 °C. A dosage of 60 mg of the previously mentioned photocatalyst was used in these solutions. These solutions were further stirred in the dark for 60 min inside a UV chamber equipped with an amiciVision UV flashlight, attaining adsorption-desorption equilibrium. Furthermore, every 3 mL solution at a time interval (10 min) starting from the blank solution was picked and analyzed for absorption spectrum between 525 and 725 nm wavelength using Lab India’s UV-3200 spectrophotometer. The degradation rate was evaluated using Eq. (1)32,

$$\:\%D=\frac{{A}_{0}-{A}_{t}}{{A}_{0}}\times\:100$$
(1)

Where A0 and At resemble absorption at the 0th minute and particular time illumination, respectively.

Results and discussion

Absorption studies

Estimation of the band gap is important in understanding the electronic characteristics of a material, which in turn defines the appropriateness of the material for semiconductor, photocatalyst, and optical applications. Hence, a reflectance spectrum (Fig. 2a) between 200 and 600 nm was recorded for the synthesized AgNPs, which was further involved in calculating the energy band-gap using the Kubelka-Munk (K-M) relation (2)33,

$$\:\left[F\left(R\right)\right]=\raisebox{1ex}{$K$}\!\left/\:\!\raisebox{-1ex}{$S$}\right.$$
(2)
$$\:K={(1-R)}^{2}\:\:\:\:\:\:S=2R$$

Where R indicates the reflectance coefficient. The evaluated band-gap energy was found to be 3.31 eV as depicted in Fig. 2b.

Fig. 2
figure 2

(a) Recorded reflectance spectrum and (b) K-M plot of synthesized AgNPs.

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Structural phase analysis

Powder X-ray diffraction (PXRD) analysis is of great importance in defining the phases and structural details of nanomaterials, which act as a key factor in a wide range of applied fields. The prepared AgNPs were characterized by a powder X-ray diffraction (PXRD) technique to provide structural information. Figure 3 depicts the diffracted peaks of AgNPs in accordance with JCPDS card #96-901-3046, thus confirming that the observed phase is cubic (Fm-3 m). The obtained parameters were (α = β = γ = 90° and a = b = c = 4.0860 Å). Planes (111), (200), (220), (311), and (222) were allocated to 2θ = 38.14°, 44.32°, 64.46°, 77.43°, and 81.56°, respectively. For the same peak values, Scherrer’s relation (3) was applied in evaluating the average crystallite size34.

$$\:{D}_{hkl}=\frac{0.9*\lambda\:}{{\beta}_{hkl}\text{cos }\theta\:}$$
(3)

Where, Dhkl is referred to as crystallite size, βhkl as Half Maximum of Full Width (FWHM) of the observed peaks. Also, θ resembles the diffracting angle, and λ is the wavelength of the X-rays used. The average crystallite size was evaluated to be 22.38 nm. The same was affirmed by applying the Williamson-Hall (W-H) relation (4) for the obtained results, as shown in Fig. 435. The obtained value was well aligned with the crystallite size from the PXRD analysis (24.75 nm) with a strain component (ε) value of 3.2 × 10− 3 and R2 value of 0.987.

$$\:{\beta}_{hkl}\text{cos }\theta\:=4\epsilon\:\text{sin }\theta\:+\frac{0.9\lambda}{{D}_{hkl}}$$
(4)

The same was verified by the Stress-Strain Plot (SSP) (Text S1), which was found to be Dhkl = 25.02 nm and ε = 5.4 × 10− 3 (Figure S1).

Fig. 3
figure 3

PXRD profile of as-prepared AgNPs is in good agreement with the card #96-901-3046.

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Fig. 4
figure 4

W-H plot of AgNPs using the respective diffraction peaks.

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IR studies

The vital part of molecular analysis using vibrational transitions is in determining molecular structures and the chemical composition of a particular object. Figure 5 demonstrates the IR spectra recorded between 500 and 4000 cm− 1 of both M. zapota L. extract and AgNPs. Prominent bands at 849 cm− 1, 882 cm− 1, 1105 cm− 1, 1444 cm− 1, 1554 cm− 1, 1656 cm− 1, and 3335 cm− 1 were observed. Upon referring to the available literature, the band at 3335 cm− 1 is assigned to O-H stretching. Bands at 1105, 1444/1554, and 1656 cm− 1 can be attributed to C-N stretching, N-H stretching, and C = C/C = O moieties, respectively. The band ranging between 2458 –1897 cm− 1 is attributed to C-H stretching from methylene groups. At last, the bands at 849 cm− 1 and 882 cm− 1 were observed due to the presence of metal peroxides (Ag-O) (< 1000 cm− 1)36,37,38,39.

Fig. 5
figure 5

Recorded FT-IR spectrum of as-synthesized AgNPs.

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Morphology

Morphology evaluation is one of the key aspects in predicting the functional properties of nanostructures since it is associated with material shape and size distribution. Hence, the as-synthesized AgNPs were subjected to study the shape and size distribution using SEM and TEM microscopy. Figure 6 depicts the SEM images at 3 μm (Fig. 6A), 2 μm (Fig. 6B), 1 μm (Fig. 6C), and a magnified version of Fig. 6B (Fig. 6D), showing an agglomerated assembly morphology for synthesized AgNPs. This agglomeration may be due to the usage of green fuel and calcinating temperature after combustion synthesis40. Figure 7 presents the EDX spectrum, which confirms the elemental composition of the synthesized AgNPs. The peaks indicating the major components are silver (Ag), exhibiting a notably high weight% of approximately 76.5%. The presence of oxygen (~ 17.5%) suggests the potential existence of surface oxide or phytochemical capping derived from the extract of M. zapota L. A minor carbon signal (~ 5.9%) is apparent, likely resulting from residual plant metabolites or adsorbed atmospheric CO₂ during the sample preparation process. The results validate the effective green synthesis of AgNPs with high purity, and the elemental composition aligns with the PXRD and FTIR analyses, further substantiating both the structure and composition of the nanomaterial.

Fig. 6
figure 6

SEM images at different magnifications (A) 3 μm, (B) 2 μm, (C) 1 μm, and (D) magnified region of (B) 2 μm.

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Fig. 7
figure 7

EDAX spectrum of AgNPs affirming the precursor elements.

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Figure 8 (a–c) shows the TEM images of as-synthesized AgNPs at 500, 100, and 50 nm magnification, confirming the agglomeration of AgNPs in the clusters. The same has been utilized in evaluating the average particle size of ~ 24 nm using the line tool in ImageJ software (ImageJ 1.53t, https://imagej.net/ij/). This result is aligned with the calculated crystallite size in PXRD analysis. Also, Fig. 8d indicates the two prominent peaks (111) and (200) and confirms the crystallinity of the sample. At last, Fig. 8e and f show the HRTEM resolution of the sample, which is used further to evaluate the d-spacing value, which is to be found at 0.351 nm. Further, the same TEM images have been utilized to obtain the particle size distribution histogram as shown in Fig. 8g, which is well aligned with the evaluated average particle size of ~ 24 nm, which was further authenticated by particle size analysis of the recorded DLS data as depicted in Figure S2 of the supporting information.

Fig. 8
figure 8

(a–c) TEM images of synthesized AgNPs at 500, 100, and 50 nm, respectively. (d) SAED image affirming two prominent peaks (111) and (200), (e, f) HRTEM images with d-spacing evaluation, and (g) obtained particle size distribution histogram.

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Catalytic activity of as-synthesized AgNPs against MB dye

The removal of cationic MB dye was observed through absorption studies conducted using a UV-visible spectrophotometer between 525 and 725 nm wavelengths, as seen in Fig. 9(a). Initially, a UV chamber equipped with an AmiciVision UV flashlight was utilized to conduct the absorbance studies. 60 mg of AgNPs catalyst was added to 100 mL MB dye solution with a 10 ppm concentration and stirred in the dark for 60 min to attain the adsorption-desorption equilibrium. Later, once the illumination was made, 3 mL solution was taken out every 10 min for recording the absorption spectrum. The dye observed a noticeable 83.33% degradation after 110 min of UV illumination inside a UV chamber (Fig. 9b). Further, the same absorbance results were utilized in studying the degradation using the zeroth-order kinetic relation (5)41,

$$\:{A}_{t}={A}_{0}-kt$$
(5)

This shows that the degradation results were best aligned with R2 = 0.98 and k0 = 0.007 as depicted in Fig. 9c.

Fig. 9
figure 9

(a) Time-sensitive absorption spectrum depicting the degradation of cationic MB dye, (b) time-sensitive degradation rate curve, (c) zeroth-order kinetics plot, and (d) pH effect on degradation rate of MB dye.

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Investigating the pH influence on dye degradation is vital as the pH affects both the efficacy and mechanism of degradation, thus affecting the general effectiveness of existing treatment procedures in removing dyes from water. Hence, the solution containing 10 ppm of MB dye and 60 mg of prepared catalyst, AgNPs, was maintained at pH levels 2, 4, 6, and 8 using a suitable buffer. The same was kept under UV illumination to examine the degradation of MB dye, and found that the maximum dye removal occurred in acidic medium (pH 2) (Fig. 9d).

Furthermore, the degradation rates were also studied in the presence of different scavengers. The selected scavengers: (i) Ascorbic Acid (AA), (ii) Ethylenediamine tetraacetic acid (EDTA), and (iii) 2 Propanol (IPA) serve as the trappers of superoxide radicals (·O2−), holes (h+), and hydroxyl radicals (·OH), which are responsible for dye removal. Results depicted in Fig. 10 reveal that the MB degradation was minimally affected by the selected scavengers. AA affects the degradation rate from 83.3% to 79.5%, EDTA, to 79.8%, and IPA, to 80.8%.

Fig. 10
figure 10

Scavenger effect of as-synthesized AgNPs on the degradation rate of MB dye.

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Degradation mechanism

When UV-Vis light is exposed to a photocatalyst, an electron (e) is excited from the valence band into the conduction band, generating a positive hole (h+). The cleavage of water molecules by h+ or e− reduces surface absorbed oxygen species to produce superoxide ions (02−)42,43,44. Nevertheless, in either case, highly reactive hydroxyl radicals attack certain organic compounds, thus permitting their decomposition into smaller intermediates and finally carbon dioxide (CO2) and water (H2O) without any danger45,46,47,48,49. All the observed results and the scavenger effect suggest a possible degradation mechanism proposed in Fig. 11.

Fig. 11
figure 11

Suggested photo-degradation of MB dye using AgNPs.

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Antibacterial activity

The as-synthesized AgNPs were evaluated for antimicrobial activity by agar well diffusion method, as described by previous reports50, with slight modifications. The prepared AgNPs were labeled as AgS for convenience purposes and tested against the bacterial strains, gram-positive Staphylococcus aureus MTCC-7443 and gram-negative Escherichia coli MTCC-7410. The inoculum was adjusted to approximately 5 × 105 CFU/mL with sterile saline solution. The sample was dissolved at 10 mg/mL in DMSO as a stock solution and loaded into different concentration ranges, 200 µg to 600 µg, for different wells. The medium used was Muller Hinton agar (Hi-media) and incubation at 37 °C for 12 h After incubation, the diameter of the inhibition zone (mm) was measured. The results tabulated in Table 1 show that the prepared sample was active for both gram-positive and gram-negative beyond 400 µg/mL, which is shown in Fig. 12 in comparison with standard drugs, Penicillin G for gram-positive and Ciprofloxacin for gram-negative bacterial species at 60 µg/mL.

Table 1 Antibacterial test results against two bacterial strains.
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Fig. 12
figure 12

Images depicting the antibacterial activity of as-synthesized AgNPs against (A) S. aureus and (B) E. coli.

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The synthesized AgNPs have shown strong antibacterial action against both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli pathogens, with greater inhibition at doses over 400 µg/mL. Greater inhibition zone widths with higher nanoparticle concentrations indicate a dose-dependent impact of antibacterial activity. The somewhat increased sensitivity shown in E. coli compared to S. aureus may be explained by structural changes in the cell wall. Gram-negative bacteria have an outer membrane containing negatively charged lipopolysaccharides, which may interact electrostatically with positively charged AgNPs surfaces, increasing nanoparticle uptake. Gram-positive bacteria, on the other hand, have a thicker peptidoglycan coating that may inhibit nanoparticle penetration and hence provide a reduced sensitivity.

The mechanism for AgNPs’ antibacterial effect is considered to be complex. First, AgNPs attach to the bacterial cell’s surface, disrupting membrane permeability and integrity. This causes the leaking of critical intracellular components, which leads to cell lysis. AgNPs produce reactive oxygen species (ROS) such as hydroxyl radicals (•OH), superoxide anions (O2−), and hydrogen peroxide (H2O2), causing oxidative stress and damage to bacterial proteins, lipids, and nucleic acids. AgNPs produce Ag+ ions, which attach to thiol (-SH) groups in bacterial enzymes and proteins. This inhibits respiratory enzymes and ATP synthesis. Furthermore, Ag+ ions may bind to bacterial DNA and hinder replication and transcription activities. These combinatorial actions lead to the effective destruction of bacterial cells. Thus, AgNPs’ antibacterial effect is the result of a combination of physical membrane rupture, chemical interaction via ion release, and oxidative stress induction. The variety of these pathways not only increases the efficacy of AgNPs but also reduces the likelihood of bacterial resistance development, making them ideal candidates for use in antimicrobial coatings, wound care products, and disinfectant applications51,52. Further, the comparative analysis of reported AgNPs and its photocatalytic and antibacterial activities are tabulated in Table 2.

Table 2 Comparison analysis of various AgNPs as potential catalytic and antibacterial agents.
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Conclusion

To sum up, AgNPs were successfully synthesized using M. zapota leaf extract as a reducing agent through a simple bio-combustion method. The synthesized AgNPs were characterized for optical, phase, and morphology using various analytical techniques such as UV-Visible spectroscopy, PXRD, SEM, and TEM. The optical studies reveal that the band-gap energy of the as-synthesized AgNPs is 3.31 eV. The structural evaluation of as-prepared AgNPs aligns the observed peaks with JCPDS #96-901-3046 with a cubic phase and Fm-3 m space group. The calculated crystallite size was found to be 22.38 nm, and the same was affirmed by the W-H plot (24.75 nm). Further, the IR spectral analysis reveals the characteristic O-H stretching peak at 3335 cm− 1 and metal-peroxide peak at 849 cm− 1 (< 1000 cm− 1). The clustered agglomeration was characterized with SEM and TEM images. Initial precursors were also affirmed through EDAX analysis. Furthermore, the prepared sample was utilized in degrading the cationic dye, MB, through absorption studies. The results show a noticeable 83.33% degradation, which aligns with 0th order kinetics (R2 = 0.98, k0 = 0.007). pH effect reveals that the maximum degradation rate was observed in the acidic medium (pH 2). Also, a minute decrement was observed in the degradation rate after the incorporation of selected scavengers into the degradation solutions. The as-prepared AgNPs were used to evaluate the antibacterial potency against Staphylococcus aureus (gram-positive) and Escherichia coli (gram-negative) bacterial strains. The results affirm a responsive activeness from 400 µg/mL against both the bacterial strains. All the observed results suggest that the as-prepared AgNPs as a potent catalytic and antibacterial agent.