Introduction

Nanomaterials are becoming more significant as a means of addressing material science issues. Preparation of nanosized silver nanoparticles (AgNPs) is one of the most promising fields of nanotechnology. Green synthesis of silver-based nanoparticles has been accomplished using a variety of physical, chemical and biological approaches (Lombardo et al. 2016; Treshchalov et al. 2017). Many research fields focus on green chemistry to improve and/or safeguard our global environment. Silver nanoparticles' applications are extensive, from food processing, cosmetics, home cleaning, catalytic and garment production to medicinal applications (Bansod et al. 2015; Benn et al. 2010; Zhu et al. 2022). Medicinally AgNPs have been used for the treatment of diseases such as cancer, HIV, diabetes, malaria and tuberculosis (Casañas Pimentel et al. 2016; Kalmantaeva et al. 2020; Kasithevar et al. 2017; Muthukumaran et al. 2015). Green chemistry-based strategies have been addressed in the synthesis of nanoparticles in recent years, among the various chemical and physical ways of nanoparticle synthesis (Noah and Ndangili 2022). When plant extracts are used to reduce and stabilize silver nanoparticles, they do not contain any synthetic chemical compounds on their surface. They are not toxic to human and the environment (Tamuly et al. 2013). The phytochemicals sticking to the surface of nanoparticles are responsible for the scavenging effect of AgNPs against free radicals (Ansar et al. 2018).

Eupatorium adenophorum Spreng. (E. adenophorum) [also known as Ageratina adenophora (Spreng.)] of Asteraceae family, have been used in traditional medicine for the treatment of wounds, diabetes, inflammation, fever, jaundice and dysentery (Awah et al. 2012; Giri et al. 2022; Sharma et al. 1998; Tiwary et al. 2015; Uprety et al. 2011). It is well known that consummation or even exposure of the plant has serious adverse effects on humans as well as animals, due to presence of toxins (sesquiterpenes) (Cui et al. 2021; Ren et al. 2021). However recent investigations have shown that some extracts and/or isolated secondary metabolites are safe for use and possess considerable antioxidant, antifungal and/or medicinal properties. Sesquiterpenoids, triterpenes, flavonoids, phenolics, coumarins, steroids and phenylpropanols have been isolated and identified from E. adenophorum (Liu et al. 2021). Various pharmacological studies revealed that E. adenophorum extract had antimicrobial (Manandhar et al. 2019), antioxidant (Lallianrawna et al. 2013), anticancer (Mani et al. 2019), wound healing (Garg and Paliwal 2011), analgesic (Mandal et al. 2005), antipyretic (Ringmichon and Gopalkrishnan 2017), anti-inflammatory (Shi et al. 2019) activities and antiviral activity against COVID-19 main protease (Neupane et al. 2021).

The present work aimed to explore E. adenophorum's potential for the first time in the synthesis of AgNPs. X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible absorption (UV–Vis) spectroscopy, dynamic light scattering (DLS) and scanning electron microscopy (SEM) were used to analyze the biosynthesized AgNPs. Biosynthesized nanoparticles were also studied for antioxidant, antibacterial and photocatalytic activities.

Materials and methods

Materials

Silver nitrate (AgNO3) and Rhodamine B were procured from Sisco Research Laboratory, Mumbai, India. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and Mueller Hinton Broth (MHB) were supplied by HiMedia (Mumbai, India). All other chemicals and reagents used were of highly pure analytical grade.

Leaves of E. adenophorum were collected from Jaigaon, Alipurduar, West Bengal, in March 2022. The plant was authenticated (Reference no. CNH/Tech.II/2021/11) by the Botanical Survey of India, Shibpur, Howrah (West Bengal). This plant specimen has been preserved in the laboratory for further reference.

Preparation of plant extract

The collected E. adenophorum leaves were washed thoroughly with distilled water several times to remove dust and dried under shade. The dried leaves were ground to get fine powder, and its aqueous extract was prepared using the Soxhlet apparatus. The extracts were filtered to remove particulate matter and lyophilized using lyophilizer (CoolSafe, Labogene, Allerod, Denmark) to yield the powdered (amorphous) crude extracts.

Preparation of E. adenophorum silver nanoparticles

Green synthesis method is used to synthesize AgNPs with slight modification (Kumar et al. 2014). E. adenophorum powdered extract (500 mg) was dissolved in 100 mL of distilled water (Fig. 1A). Then, 10 mL of the above extract was added to 90 mL of 0.1 M AgNO3 solution in a conical flask and heated at the temperature range between 60 and 80 °C with continuous stirring in a magnetic stirrer at a speed of 400 rpm. The solution color turned brown, indicating the formation of AgNPs (Fig. 1B). The contents were centrifuged for 20 min at 10,000 rpm, and the precipitate was collected after discarding the supernatant. The collected AgNPs were dried in a freeze dryer without using a cryoprotector before being utilized for characterizations and antibacterial and antioxidant activities.

Fig. 1
figure 1

E. adenophorum leaf extract (A) and formation of AgNPs by adding AgNO3 in E. adenophorum leaf extract (B)

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Characterizations of E. adenophorum silver nanoparticles

UV–visible spectrum for E. adenophorum silver nanoparticles

UV–visible spectral analysis was done by using Shimadzu UV–visible spectrophotometer (UV-1900i, UV visible spectrophotometer, Shimadzu, Japan). The analysis was carried out in a scan range of 800–200 nm with a resolution of 1 nm. Both the E. adenophorum leaf extract and the AgNPs synthesized from the leaf extract were scanned by UV–visible spectroscopy to confirm the formation of AgNPs. After completion of the reaction, one milliliter of the AgNPs sample was pipetted out and scanned at room temperature without dilution.

FTIR analysis for E. adenophorum silver nanoparticles

The dried sample extract and biosynthesized nanoparticles were individually mixed with the KBr powder and pressed into a pellet. The FTIR spectrum was recorded in the IR region of 4000–400 cm−1 on a compact FTIR Spectrometer (Bruker, alpha II spectrometer, USA).

X-ray diffraction (XRD) analysis for E. adenophorum silver nanoparticles

E. adenophorum silver nanoparticles were characterized to distinguish their crystal structure using X-ray diffraction technique. The XRD pattern was recorded using computer-controlled XRD system (Rigaku, Japan, SmartLab 9kW).

Particle size and zeta potential analysis of E. adenophorum silver nanoparticles

The particle size and zeta potential of E. adenophorum silver nanoparticles were determined using particle size analyzer (Litesizer 500, Anton Paar GmbH, Austria). In the present study, a small, weighed quantity of the experimental sample was dispersed in deionized water by vortex mixing followed by sonication for 1 h and placed in a disposable cuvette for determining average particle size. The Anton Paar's Omega cuvettes were used for zeta potential measurement by the use of the same prepared sample solution.

Scanning electron microscopy and energy-dispersive X-ray spectroscopy

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed by using a scanning electron microscope (JEOL, Tokyo, Japan) with an accelerating voltage of 15 kV. The elemental composition of AgNPs was assessed using SEM fitted with an EDX (OXFORD) detector.

Antioxidant activity

The ability of AgNPs to scavenge DPPH radical was determined according to the method of Mensor et al. with a slight modification (Mensor et al. 2001). In brief, 2 mL of AgNPs solution in water was mixed with 1 mL of DPPH solution (0.3 mM) at various concentrations (5, 10, 25, 50, 75 and 100 µg/mL). To make the control, 1 mL of DPPH was mixed with 2 mL of water. The reaction's absorbance was measured at 517 nm after 30 min. Distilled water served as the blank. The percentage of DPPH radical scavenging was estimated using the following formula.

$$\% \,{\text{DPPH}}\,{\text{radical}}\,{\text{scavenging}}\,{\text{activity}} = \left[ {\left( {{\text{Ac}} - {\text{As}}} \right)/{\text{Ac}}} \right] \times {1}00$$

where Ac and As are the absorbance of the control and the sample, respectively.

Antibacterial activity of E. adenophorum silver nanoparticles

The antibacterial assays were done on human pathogenic Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). By using the broth dilution method, the minimum inhibitory concentration (MIC) was ascertained (Wiegand et al. 2008). Mueller Hinton Broth (MHB) was used for bacterial strains. Fresh cultures at 106 CFU/mL were added to each tube of sterile MHB. Then, AgNPs was loaded at different concentrations (10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 and 120 µg/mL). In the experiment, broth and inoculum served as the positive control, whereas broth and AgNPs and broth alone served as the negative control. For 16–24 h, the bacteria-inoculated tubes were incubated at 37 °C. To obtain the MIC, the absorbance was measured at 600 nm. The MIC was the lowest concentration with above 90% inhibition.

Photocatalytic activity

For measurement of catalytic activity of biosynthesized AgNPs we carried out the degradation and reduction of Rhodamine B (Baruah et al. 2019). AgNPs' photocatalytic activity was calculated in visible light using Rhodamine B dye. The degradation of Rhodamine B was also performed in absence of AgNPs with leaf extract and without leaf extract. To reach equilibrium between absorption and desorption, the reaction mixture was agitated for 20 min in the dark. Five milligrams of the nanoparticles was added to 10 mg/L of Rhodamine B solution, while the mixture was still being stirred in the presence of sunlight. At intervals of 15, 30, 45, 60, 75 and 90 min after starting the reaction under sunlight, the reaction mixtures' maximum absorbance was measured by using a spectrophotometer to study its degradation. The rate of dye degradation (%) was calculated using the formula below.

$${\text{Dye}}\,{\text{degradation}}\,{\text{rate}}\,{(\%)} = \left[ {\left( {C_{0} - C_{t} } \right)/C_{0} } \right] \times {1}00$$

where C0 represents the starting concentration of rhodamine solution and Ct represents the concentration of dye solution following "t" hours of exposure to sunlight.

Results and discussion

Green synthesis of E. adenophorum silver nanoparticles

In this study, we have synthesized E. adenophorum silver nanoparticles using green approach. In this work, reduction of AgNO3 was achieved by constituents of Eupatorium adenophorum leaf aqueous extract with application of temperature in the range of 60–80 °C with continuous stirring. The formation of AgNPs was indicated by the color change from straw yellow to brown. Similar observations were reported by other investigators who used plant extracts as a reducing agent (Jayapriya et al. 2019). Based on the study by Rajput and team (Rajput et al. 2020), the temperature of the solution was maintained at a temperature more than 60 °C to get optimum particle size (E. adenophorum). Many studies showed that synthesis of smaller size AgNPs using other plant leaf extracts occurs when the reaction is performed at higher temperature (Alharbi et al. 2022; Kumar et al. 2018; Verma and Mehata 2016). AgNPs of particle size 51 nm were also synthesized in the presence of sunlight by green approach using AgNO3 and Rivina humilis leaf extract (Raghava et al. 2021). However, recently AgNPs were synthesized by the green method using E. adenophorum without applying heat during the reaction, and AgNPs formed were dried using hot air oven (Gautam et al. 2021). They report that the nanoparticles obtained were face-centered cubic geometry based on XRD data having particle size mostly of 24 nm estimated using the Debye-Scherrer equation (Gautam et al. 2021). In the present work, the particle morphology was characterized using SEM image, and average sizes were obtained using Litesizer.

UV–visible spectroscopy

The formation of AgNPs of silver nitrate containing E. adenophorum extract was confirmed by observing a visible color change from light brown to dark brown. The synthesized AgNPs shown in Fig. 2 was confirmed by the final solution's constant λmax at 400 nm.

Fig. 2
figure 2

UV–Vis spectra of E. adenophorum leaf extract and AgNPs synthesized using E. adenophorum leaf extract

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FTIR analysis for E. adenophorum silver nanoparticles

FTIR spectrometry of the aqueous extract of E. adenophorum leaves and AgNPs produced with E. adenophorum leaf extract is shown in Fig. 3. The FTIR spectra obtained from extract showed different characteristic peaks at 3165 cm−1 for –OH stretching vibration, at 1722 cm−1 for –C–H stretching vibration, at 1615 cm−1 for –C=O stretching of –COOH group of polyphenols, at 1400 cm−1 and 1039 cm−1 for –C–O stretching vibration. These characteristic peaks substantiate the presence of polyphenolic compounds in the extract, which might act as bioreductant in the formation of AgNPs through reduction of silver nitrate. The FTIR spectra obtained from AgNPs showed characteristic peaks at 3165 cm−1, 1615 cm−1 and 1400 cm−1 for –OH stretching, –C=O and –O–C– stretching vibration, which indicates the presence of polyphenolic compounds at the surfaces of nanoparticles. Another peak at 1112 cm−1 was observed for –C–O stretching. The peaks at 617 cm−1 and 482 cm−1 were assigned to –O–H bond between oxygen atom of Ag2O and H-atom of phenolic compound at the surface of nanoparticles, which indicates the formation of AgNPs.

Fig. 3
figure 3

FTIR spectroscopy analysis of E. adenophorum leaf extract and AgNPs

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X-ray diffraction (XRD) analysis for E. adenophorum silver nanoparticles

On examining XRD pattern (Fig. 4) of AgNPs, the prominent peaks at 2θ = 38.030°, 44.214°, 64.354°, 77.267° and 81.299° represent the (111), (200), (220), (311) and (222) Bragg's reflections of the face-centered cubic structure of silver, respectively. It confirmed the crystalline structure of AgNPs synthesized using E. adenophorum leaf extract. The preferred direction of the growth of the silver nanoparticles was indicated by the most intense peak, which falls within the (111) plane. The phase purity of the material as synthesized was simply verified by the absence of any other peaks. The highly crystalline structure of the silver nanoparticles as synthesized was clearly indicated by the sharp and strong peaks.

Fig. 4
figure 4

XRD pattern of AgNPs synthesized using E. adenophorum leaf extract

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Particle size and zeta potential analysis of E. adenophorum silver nanoparticles

From the particle size graph, the average particle size of the AgNPs obtained was 117.75 nm (Fig. 5A). The particles obtained were as small as 30 nm and as large as 400 nm. The zeta potential is a property of nanoparticles that quantifies charge. It measures the electrical charge that a nanoparticle has on its surface. The zeta potential provides information about the stability of the particles. Figure 5B displays the zeta potential distribution graph. The zeta potential of E. adenophorum AgNPs was found to be − 33.4 mV (Fig. 5B). The zeta potential is negative, which denotes repulsion and increased stability in colloidal state.

Fig. 5
figure 5

Zetasizer measurement of the average particle size of AgNPs synthesized using E. adenophorum leaf extract (A) and zetapotential of AgNPs (B)

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Scanning electron microscopy (SEM)

The SEM image of the biosynthesized AgNPs (Fig. 6) indicates the presence of extremely tiny, spherical nanoparticles. Particle aggregation is also depicted in the illustration. The evaporation of solvent during sample preparation may cause AgNPs to aggregate. Additionally, the formation of aggregation of AgNPs was due to freezing without cryoprotectants which in turn induce stress from the formation of ice crystals. This might have caused the particle size variance.

Fig. 6
figure 6

SEM image of AgNPs synthesized using E. adenophorum leaf extract

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The EDX profile confirmed that the particles were AgNPs by displaying a high silver signal and remarkably stronger peaks. The presence of carbon in the EDX image (Fig. 7) substantiates the presence of carbon compound on the surface of the particles, which might be phytoconstituents present in the plant extract. Thus, EDX data confirm the formation of AgNPs using E. adenophorum extract.

Fig. 7
figure 7

EDX spectrum of AgNPs with a higher percentage of the silver signal

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Antioxidant activity

The DPPH radical is a free radical with one electron and a relatively stable structure (Priyadarsini et al. 2003). The color of the solution changes when it comes into contact with a scavenger. The scavenging efficacy of AgNPs on DPPH radical was significantly linked with concentration, as shown in Fig. 8. The DPPH radical scavenging experiment showed that AgNPs had significant radical scavenger characteristics, with an IC50 value of 48.96 ± 0.84 µg/mL (Fig. 8) when compared to ascorbic acid's IC50 value of 14.35 ± 0.16 µg/mL. Previous studies reported the IC50 value of 92.791 µg/mL of methanolic extract of E. adenophorum (Khazeo et al. 2018). The DPPH activity of the synthesized AgNPs was found to increase in a dose-dependent manner.

Fig. 8
figure 8

DPPH radical scavenging activity of ascorbic acid and AgNPs synthesized using E. adenophorum leaf extract

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Antibacterial activity

The silver nanoparticles' minimum inhibitory concentrations (MIC) against the gram-negative bacteria E. coli and the gram-positive bacteria S. aureus were determined to be 82.5 µg/mL and 64.5 µg/mL, respectively. The antibacterial activity of AgNPs is connected due to any of the four mechanisms. Firstly, AgNPs interact directly with the bacterial cell membrane, leading to subsequent membrane damage and the formation of complexation with the cellular components. Secondly, AgNPs adhere to the surface of cell membrane and wall and cause damage there. Thirdly, AgNPs interact with thiol (−SH) groups and induce the formation of reactive oxygen species (ROS) in the cells that cause oxidative stress. Lastly, AgNPs modulate the signal transduction pathways (Kung et al. 2018; Rai et al. 2014). The spherical AGNPs showed promising and powerful antibacterial activity although might be less that cuboid- or triangular-shaped AgNPs (Baghbani-Arani et al. 2017; Dakal et al. 2016; Qing et al. 2018). AgNPs with smaller size (10–50 nm) are more effective in terms of their biocompatibility, stability and enhanced antimicrobial activity (Dakal et al. 2016). The nanoparticles were of different sizes starting from 27 nm. Hence, prepared nanoparticles could able to prevent bacterial infection effectively.

Photocatalytic activity

To determine catalytic activity, Rhodamine B was employed as an experimental probe. This well-known water-soluble color is utilized in various industrial fields, including textile, leather, printing, paper and pharmaceuticals. Under sunlight, the photocatalytic activity of AgNPs was calculated. To minimize the impact of light, Rhodamine B was used as a control in the absence of a catalyst because it displayed a clear absorption spectrum at 554 nm. A gradual reduction in the absorption peak at 554 nm represents the reduction and degradation of Rhodamine B, as shown in Fig. 9A. This decline in the absorption peak clearly illustrates the catalytic activity of nanoparticles and the conversion of Rhodamine B to a colorless solution. Figure 9B, C shows the experimental result of the degradation of the Rhodamine B without AgNPs and without the AgNPs but containing the leaf extract. The result depicts no significant degradation of Rhodamine B in the absence of AgNPs or in the presence of only leaf extract without AgNPs. It was also noticed when Rhodamine B was treated with AgNPs, the rate of photodegradation of the dye increased with time (Fig. 9D). Rhodamine B dye showed time-dependent photodegradation, which was followed by a significant photodegradation (78.69%) with decolorization after 90 min. Previous studies showed 93% degradation achieved in 216 h (Awad et al. 2021). When compared photocatalytic activity at 90 min, 81.12% (Alshehri and Malik 2020) and 86.51% (Shaikh et al. 2020) degradation of Rhodamine B was reported under UV radiation in their studies. The Ln (Ct/C0) versus time plot from Rhodamine B is shown in Fig. 9E. Rhodamine B's calculated degradation rate constant is 1.56 × 10–2/min. The reaction process followed pseudo-first-order kinetics, as evidenced by the fact that Ln (Ct/C0) value decreased over time.

Fig. 9
figure 9

Photocatalytic activity showing degradation of Rhodamine B by AgNPs (A), without AgNPs (B), with the leaf extract but without the AgNPs (C). Degradation rate of Rhodamine B by AgNPs (D), and kinetic plot of degradation of Rhodamine B by AgNPs (E)

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Conclusion

Aqueous leaf extract from E. adenophorum was utilized successfully to synthesized silver nanoparticles where the extract plays a dual role as bioreductant and capping agent. In addition to showing bacterial inhibitory effects against human infections that were comparable to those of amoxicillin, the AgNPs also showed improved free radical scavenging activity. According to the results of the photocatalytic investigation, E. adenophorum AgNPs were effective at degrading rhodamine dye when exposed to sunlight. As a result, the textile and water purification sectors can greatly benefit from this. Our results clearly support the idea that plant-mediated nanoparticles can be employed in the near future for the treatment of diseases caused by free radicals as well as the purification of waste water be removing hazardous dye from waste water, in addition to being used as effective therapeutic agents against human pathogens.