|Year : 2022 | Volume
| Issue : 77 | Page : 22-28
Phyto-mediated green synthesis of silver nanoparticles using Acmella oleracea leaf extract: Antioxidant and catalytic activity
Sweta Bhardwaj, Suman Lata, Rajni Garg
Department of Chemistry, University School of Sciences, Rayat-Bahra University, Mohali, Punjab, India
|Date of Submission||25-Dec-2020|
|Date of Decision||04-Aug-2021|
|Date of Acceptance||28-Jan-2022|
|Date of Web Publication||28-Mar-2022|
University School of Sciences, Rayat-Bahra University, Mohali - 140 104, Punjab
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Background: Green synthesis has attracted the attention of all researchers due to its nontoxic, energy-saving, cost-effective, and eco-sound features. In the last decades, the usage of medicinal plants or herbs in the form of extract has been pursued to synthesize nanosized materials with added potential benefits that are manifested in various sectors. Aim: In the current study, an easy, economical and environmentally friendly one-pot synthesis technique has been proposed to obtain silver nanoparticles (AgNPs) using Acmella oleracea leaf extract. Materials and Methods: Color transition and ultraviolet-visible spectroscopy were availed to assess the formation of bio-mediated AgNPs while Fourier transform infrared (FTIR) was used to detect the functionalities in the leaf extract and the obtained 4-nitrophenol (NPs). Field emission scanning electron microscope (FESEM) analysis was used to analyze the morphological features of the NPs. The AgNPs were analyzed for the imminent antioxidant impact using 2,2-diphenyl-1-picrylhydrazyl test. In addition, the catalytic potential of AgNPs in reducing NP to 4-aminophenol (AP) was explored. Results: The absorption maximum for AgNPs was obtained at 435 nm and the FTIR analysis confirmed the capping of NPs by secondary metabolites of leaf extract with shift in peaks. FESEM analysis revealed the spherical shape of NPs with size ranging from 45 to 60 nm. AgNPs were found to exhibit significant antioxidant potential with IC50 value as 298.69 μg/mL and efficiently reduced NP to AP. Conclusion: The study signified that A. oleracea extract can be employed for the phyto-mediated synthesis of AgNPs with the latent antioxidant and catalytic applications that could be applied in the bio-medical field and environmental remediation in addition to other potential applications.
Keywords: Acmella oleracea, antioxidant activity, catalytic activity, phyto-mediated synthesis, silver nanoparticles
|How to cite this article:|
Bhardwaj S, Lata S, Garg R. Phyto-mediated green synthesis of silver nanoparticles using Acmella oleracea leaf extract: Antioxidant and catalytic activity. Phcog Mag 2022;18:22-8
|How to cite this URL:|
Bhardwaj S, Lata S, Garg R. Phyto-mediated green synthesis of silver nanoparticles using Acmella oleracea leaf extract: Antioxidant and catalytic activity. Phcog Mag [serial online] 2022 [cited 2022 Aug 15];18:22-8. Available from: http://www.phcog.com/text.asp?2022/18/77/22/341082
- Nanoparticles exhibit their significant role in biotechnology, optics, water purification, cosmetics, drug delivery. Green synthesis of nanoparticles by using the extract from various medicinal plants as reducing medium and capping agent has been found to be efficient, eco-friendly, and economical. The present work reports the easy and green synthetic technique to synthesize silver nanoparticles (AgNPs) with potential antioxidant and catalytic properties. Acmella oleracea, a medically important plant was used to prepare leaf extract that was further used as a reductant to obtain AgNPs using AgNO3 as a precursor. Characterization of 4-nitrophenol (NPs) revealed the spherical shape and size lying in between 45 and 60 nm. The NPs were found to exhibit significant antioxidant properties. These NPs were further used to catalyze the reduction of NP, a significant organic pollutant to 4-aminophenol. Thus, the results provided affirmation for use of A. oleracea leaf extract as a reductant for the synthesis of AgNPs. All the authors have equally contributed to the work including conceptualization, visualization; investigation, data curation, software analysis, writing, and editing of the paper.
Plant-mediated synthesis of silver nanoparticles with potential antioxidant and catalytic applications
Abbreviations used: AgNPs: Silver nanoparticles; NPs: Nanoparticles; UV-Vis: UV-Visible; FTIR: Fourier transform infrared; FESEM: Field Emission Scanning Electron Microscope; DPPH: 2,2-diphenyl-1-picrylhydrazyl; IC50: Half Maximal Inhibitory Concentration; NP: 4-nitrophenol and AP: 4-aminophenol.
| Introduction|| |
Green nanotechnology is the development of clean and green technology that proactively affects the nanomaterials and their products through their design at the nanoscale. The green approach triggers the clean production of the nanomaterials from natural sources. Although 4-nitrophenol (NPs) are mainly synthesized by employing conventional physical, chemical, and biological approaches, yet, nowadays biological methods have been explored a lot due to low-cost process rate, easy to handle, bulk production, use of waste materials, relatively high energy saving, as well as eco-friendly features as it reduces the use of toxic substances. Plant-mediated green synthesis is more beneficial over the microbe-mediated synthesis as their reduction rate is fast and stability of NPs can be established using a single precursor. Moreover, the usage of medicinal plant or herbal extracts for the synthesis of NPs incorporates the medicinal properties in the synthesized NPs. In the green/biogenic/biosynthesis of the NPs, the phytochemicals extant in the extract function as reductants, capping besides stabilizing mediators. Karnan and Selvakumar prepared zinc oxide NPs by the use of Rambutan (Nephelium lappaceum) peel extract while Filip et al. have prepared gold and silver NPs using an aqueous fruit extract of Cornus mas. Devi et al. have synthesized the silver NPs using fruit extract of Aegle marmelos. Moodley et al. highlighted that major phytochemicals in the plant extract that help in synthesizing as well as stabilizing the NPs are phenols, dihydric phenols, alkaloids, terpenoids, flavonoids, and steroids.
Midst the most commonly used nanoparticles, the metal NPs, specifically silver nanoparticles (AgNPs) have been extensively used. Nanosized metallic silver particles display unique characteristics due to fine size such that the physicochemical and biological properties differ from its bulk particles. Bio-mediated AgNPs have profound claims in various fields such as antioxidant, dye degradation, catalytic reduction, biosensors for food and agriculture diagnostics, antimicrobial activity, anticancer, and wastewater treatment to name a few. Nowadays, human beings are facing many degenerative diseases that distress our normal body functions due to the presence of free radicals. Such free radicals popularly known as reactive oxygen species can induce the risk of DNA damage, inflammation, asthma, diabetes, cancer, etc. Antioxidants are compounds that inhibit oxidation reactions which produce free radicals and protect humans by terminating the chain reactions. AgNPs can act as efficient free radical scavengers thus showing excellent antioxidant properties.
Degradation and removal of hazardous compounds from the environment is another problem that continues to pose a significant threat to humanity, and any significant contribution in this direction will be enormously beneficial internationally. Alamelu and Jaffar Ali signified that many nitro-aromatic compounds such as NPs are released as byproducts from many pharmaceutical and dye manufactured industries. These compounds act as important intermediates that are formed during the synthesis of dyes, pesticides, fungicides, pharmaceuticals, etc. These are highly toxic compounds for the environment and difficult to remove naturally by degradation. Many metal NPs including AgNPs show effective catalytic potential for reducing such highly environment polluting organic compounds as reported by Zaheer and Qu et al.
Acmella oleracea is a medically important plant with many therapeutic applications due to the presence of biologically active secondary metabolites/phytochemicals in its various parts as reported by Abeysiri et al. The present work addresses the phyto-mediated biosynthesis of AgNPs employing leaf extract of A. oleracea. As per literature survey, A. oleracea has not been explored much for synthesizing AgNPs. Various techniques namely ultraviolet-visible (UV-Vis), Fourier transform infrared (FTIR), and field emission scanning electron microscope (FESEM) analysis was used for the characterization of AgNPs and their antioxidant and catalytic activities were also analyzed.
| Materials and Methods|| |
Plant and chemicals
A. oleracea was acquired from village Bagana in Distt. Kangra, Himachal Pradesh, India at altitude of 613 m. Silver nitrate (AgNO3, 99.9%) was procured from S. D. Fine Chem. Limited, Mumbai. All analytical grade reagents including those for phytochemical screening were utilized without any additional processing. All formulations were prepared using double distilled water.
Preparation of aqueous leaf extract of Acmella oleracea
A. oleracea leaves were used to synthesize AgNPs as these are readily available, cost-effective and have medicinal properties. Fresh leaves were subjected to washing using tap water subsequent by double distilled water to ensure the removal of dirt and other contaminants. 20 g of the air-dried and powdered leaves were boiled for 30 min in 200 mL double distilled water with constant stirring [Figure 1]. The extract was kept at 4°C for further usage in experimental studies after cooling and filtration with Whatman filter paper.
|Figure 1: Preparation of plant extract from Acmella oleracea leaves (a-d)|
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Preparation of silver nanoparticles
The reduction of Ag+ (+1 oxidation state) to Ag0 NPs (zero oxidation state) was carried out using prepared leaf extract and AgNO3 solution (50 mM) in the ratio of 2:8, respectively [Figure 2]. The solution mixture was heated at 60°C ± 2°C temperature for 30 min. The color change was observed with an increase in time and after 30 min, the color transformed from yellowish-green to blackish leading to confirmation of Ag+ ions reduction to AgNPs. The stable color of the solution specified the accomplishment of the reaction. The formation of NPs was also established using the ultraviolet-visible spectroscopic technique at varying time intervals. The synthesized AgNPs were carried to centrifugation at 12,000 rpm (rounds per minute) for 10 min. Supernatant was poured out and the AgNPs pellets were re-suspended in deionized water. The procedure was repeatedthree times to purify the AgNPs and then dried in oven. The resultant AgNPs were further analyzed through spectrochemical characterization techniques.
|Figure 2: Diagrammatic representation of extract-mediated synthesis of silver nanoparticles|
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Phytochemical screening of leaf extract
Various qualitative tests were carried out to analyze phytochemicals in leaf extract of A. oleracea. Several secondary metabolites or phytochemicals extant in the extract helps in stabilizing the NPs. Such phytochemical constituents were screened through standard protocol. The plant extract formulated in acetonitrile (50 mg/3 mL) was analyzed using GC-MS Thermo Scientific (TRACETM 1300 ISQTM LT). The chromatogram was generated using Thermo Scientific™ Xcalibur™ tools and the compounds were classified from the standards database.
Spectrochemical characterization of synthesized AgNPs was carried out using UV-Vis, FTIR, and FESEM. The absorption spectra of AgNPs were achieved by Agilent Cary 60 UV-Vis Spectrophotometer in the range of 300–700 nm of wavelength. The possible functional groups in leaf extract as well as in synthesized NPs were identified through Fourier transform inferred spectrometer (Thermo Scientific NICOLET 1S50) at the registered amplitude waves ranging from 450 to 4000 cm−1. Morphology of nanoparticles was examined by FESEM (JEOL JSM-6100 [Jeol] machine).
Spectrochemical characterization of synthesized AgNPs was performed to ascertain the chemical and morphological characteristics. The absorption spectra (300–700 nm) of AgNPs were attained by UV-Vis Spectrophotometer (Agilent Cary 60). The probable functionalities in leaf extract as well as in synthesized NPs were identified through Fourier transform inferred spectrometer (Thermo Scientific NICOLET 1S50) at the registered amplitude waves ranging from 450 to 4000 cm− 1. Morphological features of NPs were scrutinized by FESEM (JEOL JSM-6100 [Jeol] machine).
The quantitative investigation of the antioxidant potential of AgNPs was carried out with the aid of in-vitro 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The radical scavenging activity was estimated as per the scheme given by Jokar et al. with slight modifications. Briefly, the sample solution of AgNPs was prepared as 100, 200, 300, 400, 500 μg/mL equal volume (1.5 mL) of various concentrations of the sample was mixed freshly prepared DPPH reagent in methanol (with the final concentration of 0.1 mM). The mixture was blended thoroughly and was subjected to unilluminated incubation for 40 min. The reduction in the absorption peak of the DPPH solution, at 517 nm, due to the incorporation of the antioxidant was assessed. The assay was triply executed while using ascorbic acid as a reference. The percentage scavenging effect of DPPH free radical was computed as per equation (1):
where Absorbancecontrol and Absorbancesample is the absorbance of DPPH + methanol and DPPH + sample, respectively. The concentration of the sample solution entailed to scavenge 50% of DDPH free radical known as IC50 can also be estimated from the graph plotted between the percentage of inhibition (%) versus concentrations of the sample (μg/mL) using linear regression statistical data analysis.
The catalytic activity is the other potential application of the extract-mediated synthesized AgNPs that was investigated in the current work. The catalytic property of AgNPs was examined for NP reduction to 4-aminophenol (AP) freshly prepared NaBH4 solution that acts as a reductant. 1.9 ml of NP (0.2 mM) was poured into 1 mL of NaBH4 solution (0.2 M) in a quartz cuvette. The UV-Visible spectra (200–700 nm) were noted after the addition of AgNPs (0.1 mL, 0.1%) solution for the completion of the reaction. The decrease in absorption peak at 400 nm of 4-nitrophenolate ion was monitored at various time intervals.
| Results and Discussion|| |
Phytochemical screening of Acmella oleracea leaf extract
From the phytochemical screening of aqueous leaf extract of A. oleracea, it was found to be a fine source of phytochemicals (secondary metabolites) such as glycosides, flavonoids, phenolic compounds, alkaloids, amino acids, tannins, terpenoids, saponins, and steroids as shown in [Table 1].
|Table 1: Qualitative phytochemical screening of Acmella oleracea leaf extract*|
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The gas chromatogram of the extract is illustrated in [Figure 3] and also shows the list of the identified phytochemical compounds. Such phytochemical constituents prevent the aggregation of the nanoparticles and are eventually accountable for the stability of the AgNPs. Mohanta et al. have highlighted that among these phytochemicals, the phenolic compounds and flavonoids are mainly accountable for the Ag+ ions reduction to AgNPs. The hydroxyl and carboxylic functionalities of phenolic compounds and flavonoids bind with the metal ion and thus act as good reducing agents. Besides flavonoids, tannins and glycosides may also help in the bio-reduction of Ag+ ions to AgNPs and can act as capping agents thus preventing the agglomeration. Saponins also assist the stabilization of the synthesized NPs. The secondary metabolites namely flavonoids and phenolic compounds can also serve as free radical scavengers as discussed in the application section of the present study.
|Figure 3: Chromatogram of Acmella oleracea leaf extract (retention time 50 min)|
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Ultraviolet-visible absorption studies
UV-Vis absorption spectroscopy is an imperative tool to observe the emergence as well as stability of the synthesized metal NPs. The absorption peak can be recorded and screened through the UV-Vis technique due to the surface plasmon phenomenon for various metal nanoparticles whose particle size varies within the nanometric range (<100 nm). In the present study, the absorption spectra of bio-synthesized AgNPs were recorded in aqueous solution at the wavelength range of 300–700 nm. [Figure 4] shows the UV-Vis absorption spectra of AgNPs synthesized using A. oleracea leaf extract that was observed to be around 435 nm. Besides the absorption peak, the change in color from greenish-yellow to blackish-brown was also an indication of the formation of AgNPs confirming the surface plasmon resonance [In set [Figure 4]]. The formation of the NPs was also monitored through absorption spectra as a function of time [Figure 5]. The intensity of the absorbance peak amplified with reaction time (from 5 to 30 min). The temperature was kept near about 60°C of temperature during the study. No further increase in absorption peak was observed after 30 min thus considering half an hour to be the optimum reaction time.
|Figure 4: Ultraviolet-visible spectra of silver nanoparticles (AgNPs) synthesized form Acmella oleracea leaf extract. In set, color changes from (a) greenish-yellow to (b) blackish-brown upon synthesis of AgNPs|
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|Figure 5: Ultraviolet-visible spectra of bio-synthesized silver nanoparticles at different time intervals (from 5 to 30 min)|
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Fourier transform infrared spectroscopy
FTIR spectroscopy was availed to analyze the functionalities of secondary metabolites that were accountable for capping as well as efficient stabilization of AgNPs. FTIR spectrum of synthesized AgNPs was observed with many frequency bands at the range in between 3300 and 1000 cm− 1 as shown in [Figure 6].
|Figure 6: Fourier transform infrared spectra of (a) leaf extract and (b) silver nanoparticles|
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The spectral band range between 3500 and 3100 cm− 1 consigned the stretching vibrations of O-H group or N-H group, therefore, the band at 3275 cm− 1 perhaps due to either stretching of O-H as alcohol intermolecular bond of the phenolic compound or N-H stretching of primary or secondary amine/amide [Figure 6]a. The band at 1605 cm− 1 was observed to associate with C = O stretching of amide bond of protein and that at 1312 cm− 1 indicate the presence of C-N stretching bond of amines. The absorption bands at 1007 cm− 1 can be ascribed to the vibration of the C = C bending of alkene. The shift in the frequency bands (for instance, the absorption peak shifts from 3275 to 3351 cm− 1 whereas 1605 to 1653 cm− 1) as shown in [Figure 6]b and reduction in the number of peaks is a clear manifestation of some sort of interaction taking place between the synthesized AgNPs and the biomolecules at the interface., The FTIR analysis in the present study have shown the active functional groups associated with various phytochemicals such as proteins, amino acids, flavonoids, phenolic acid, saponins, that are adequately existing in the leaf extract and assist the formation as well as the stability of AgNPs in consistence with literature reports.
Field emission scanning electron microscope
The FESEM micrograph [Figure 7] displays the presence of bio-synthesized AgNPs. The FESEM analysis verified the data about the nature, dispersity, shape, and size of particles. The micrograph depicts that the prepared AgNPs having a spherical shape, smooth surfaces, uniform, homogeneous, dispersed, and low levels of agglomeration with the average particle size of NPs ranging as 45–60 nm [Figure 7].
|Figure 7: Field emission scanning electron microscope analysis of Acmella oleracea leaf extract-mediated silver nanoparticles|
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Antioxidant property and catalytic activity of silver nanoparticles
Potential antioxidant property of A. oleracea synthesized AgNPs was examined through DPPH assay. DPPH is a stable free radical and its free radical scavenging is noticeable and can be assessed through visual color change from purple to greenish-yellow indicative of the reaction between methanolic DPPH solution and the antioxidant [Figure 8]. The decrease in absorption peak can be observed at 517 nm. However, it was detected that the decrease, as well as the color change, was solely depended on the concentration of the sample solution. Further, increased concentration has improved the free radical scavenging action of the AgNPs [Figure 8]b. The IC50 value calculated from the graph was found to be 298.69 μg/mL [Figure 8]c. The antioxidant efficacy of AgNPs could be attributed to phytochemicals such as flavonoids and phenolic compounds (from leaf extract) adhered to them that act as hydrogen or electron donor to DPPH. The DPPH radical acquires the electron/hydrogen to form a stable compound. The free radical generated in the first step can undergo several other chemical reactions to control the overall stoichiometry.
|Figure 8: 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay: (a) The reaction shows the transfer of the electron/hydrogen from antioxidant to DPPH free radical, (b) Concentration dependent antioxidant efficacy of silver nanoparticles (AgNPs) solution, (c) Increase in percentage of inhibition with increase in concentration AgNPs solution|
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As another potential application, the catalytic activity of AgNPs [Figure 9]a was analyzed through UV-Vis absorption spectra of NP in an aqueous NaBH4 solution. The reduction of NP to AP emerges as thermodynamically favorable, however, the kinetic barrier restricts the spontaneity of the reaction. Such a high energy barrier can be abridged in presence of AgNPs that act as good catalysts. The reaction shows absorption maximum at 400 nm in absence of AgNPs and no reduction is observed even after 24 h or more. However, in the presence of AgNPs color of the solution mixture gradually changes and peak intensity at 400 nm gradually decreases [Figure 9]b. The decrease in peak intensity perhaps is due to a decrease in the concentration of the intermediate (4-nitrophenolate ion). A new peak appears around 293 nm demonstrating the formation of AP [Figure 9]c. The increase in the speed of reaction can be attributed to the adhering hydrogen to AgNPs and providing it from the reducing agent to the reaction intermediate or perhaps facilitating the transfer of electrons.
|Figure 9: Catalytic activity of silver nanoparticles (AgNPs): (a) Reduction in the presence of AgNPs, (b) Ultraviolet-visible (UV-Vis) spectra of time-dependent reduction (c) UV-Vis peak for the formation of 4-aminophenol|
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| Conclusion|| |
In this research, the phyto-mediated synthesis of AgNPs using A. oleracea leaf extract has been developed. A. oleracea, as per knowledge, has not been reported so far for green synthesis of AgNPs, therefore, provides exclusivity to the present work. The confirmation of the bio-synthesized AgNPs was carried through color change as well through other characterization tools, namely UV-Vis, FTIR, and FESEM. The spherical morphology having an average size in between 45 and 60 nm of AgNPs was identified from the FESEM micrograph. The green synthesized AgNPs have shown potential free radical scavenging activity, as well as catalytic activity thus, may be introduced as promising applications in the biomedical field, and in environmental remediation.
The authors are thankful to Rayat Bahra University, Punjab, India, for providing the facility for the research work.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]