Eco-friendly approach in synthesis of silver nanoparticles and evaluation of optical, surface morphological and antimicrobial properties
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Silver nanoparticles (Ag NPs) were synthesized using Alternanthera sessilis leaf and Oregano root extract in an eco-friendly fashion and their significant physicochemical and optochemical properties were ascertained for nano-defined characteristics. The UV–visible spectrum showed a single and distinct absorbance peak at 433 nm (Alternanthera sessilis) and 425 nm (Oregano), typical SPR (surface plasmon resonance) for silver. Structural studies revealed nano-crystal with face centre cubic (FCC) symmetry with monodispersed nature. SEM studies showed spherical-shaped particles and the purity determined from EDX spectrum. The synthesized Ag NPs showed that antibacterial activity was studied. There was a significant inhibitory effect toward clinically important pathogens viz. B. subtilis, S. aureus, P. aeruginosa, and E. coli exposed to Ag NPs at different concentrations.
KeywordsGreen synthesis Silver nanoparticles Alternanthera sessilis Oregano Antimicrobial activity
Materials and methods
Alternanthera sessilis leaves were procured from a local market in periyakulam, and silver nitrate (AgNO3 ≥ 99.8%; AR grade) was purchased from Sigma-Aldrich and used without further purification. All the glass wares used were cleaned in chromic acid and autoclaved.
Preparation of the extracts of Alternanthera sessilis and Oregano root
50 g of fresh Alternanthera sessilis leaves and Oregano root were thoroughly washed in running water followed by distilled water to remove any dust particles. They were initially dried on an absorbent paper and chopped into small pieces using a pair of scissors. Prior to surface cleansing, Alternanthera sessilis leaves and Oregano roots were blended separately in a mixer grinder for less than a minute with 10 mL of distilled water. The blending was checked for paste-like consistency and collected in two separate Erlenmeyer flasks. To the pastes, 100 mL of double distilled water was added and kept in a shaking incubator at 80 °C for 10 min. Both the mixtures were brought down to room temperature and subjected to filtration using syringe filter (pore size 0.2 µm) and the filtrate maintained at 4 °C.
Synthesis of Ag NPs by Alternanthera sessilis leaf and Oregano root extracts
To 100 mL of 1 mM silver nitrate taken in two separate flasks, 5–10 mL of Alternanthera sessilis leaf and Oregano extracts were added under the stirring condition for 20 min at 60 °C. Reaction pertaining to nanoparticle synthesis with respect to time was observed. Furthermore, separation and purification of the colloids were performed using repeated washing and centrifugation. Finally, the particles were dried and stored in airtight containers for further experiment.
UV–Vis Schimadzu 1800 spectrophotometer was used to record the absorbance spectrum (200–700 nm) of the synthesized silver nanoparticles operated at a resolution of 1 nm. The phase purity of synthesized silver nanoparticles was determined using a Philips X’pert Pro diffractometer (Schimadzu) aided with CuKβ radiation. FT-IR spectrum was recorded on JASCO 4400 in the spectral range of 4000–400 cm−1 with sample pelleted using potassium bromide (1:100). Electron microscopic studies (Carl Zeiss MA15) were performed on the sample sputtered with gold to analyze the surface properties and assembly characteristics. The composition of the synthesized silver nanoparticles was determined using X-ray Energy Dispersion Spectroscopy (Inca, Oxford Instruments, Buckinghamshire, UK).
The antimicrobial activity of phytochemically synthesized silver colloids was determined using agar diffusion method using clinically important pathogens containing Gram-positive and Gram-negative test strains. The pure cultures of the strains at 1 × 108 CFU/mL were swabbed uniformly onto the Mueller–Hinton agar (MHA) medium using sterile swabs. Four hollow blocks of medium (dia 6 mm) were cut from the MHA plates using 100 µL sterile pipette tips. Ag NPs synthesized from two different extracts at a concentration of 25, 50, and 75 µL was added to the wells using a sterile micropipette. Amoxicillin antibiotic (25 µL) was used as a positive control. The culture-inoculated plates treated with Ag NPs and antibiotic were incubated for 18–24 h at 37 °C for the observation of any inhibition zone.
Results and discussion
SEM and EDX analysis
Fourier transform infrared spectroscopy (FT-IR)
Antibacterial mechanism exhibited by metal nanoparticles depends on the degree of susceptibility of microbes. The nanoparticles when encountered with the microbe adhere to the bacterial surface via electrostatic interaction. Their significantly smaller size helps gain entry into the bacterial cell via the transmembrane proteins and by the influence of proton motive force. It has a greater affinity towards sulfur groups present in proteins to form thiols  and on phosphates forming complexes resulting in DNA damage. It was demonstrated that Ag NPs’ interaction with cysteine residues results in the generation of ROS by inhibiting electrons at terminal oxidase, thereby inducing bacterial cell death. The difference in susceptibility pattern toward Ag NPs exposed to Gram-positive and Gram-negative strains relies on their cell wall make up. Gram-positive strains possess a thick cell wall made of peptidoglycan that helps to prevent intrusion of foreign body selectively, whereas the Gram-negative strain lacks such component that falls easy prey to an antimicrobial agent, in this case, Ag NPs, which sustains severe damage leading to cell death [30, 31, 32, 33].
The present work highlights the most simple and economical approach in the synthesis of Ag NPs using plant extracts of Alternanthera sessilis (leaf) and Oregano (root) as reducing agents. Spectroscopic and Microscopic analyses revealed typical nano-characteristics of silver. FT-IR results confirmed the contribution of phytochemicals, viz., terpenes, flavonoids, and proteins for effective synthesis. There was a significant bactericidal activity against Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli as evidenced from agar well-diffusion method. This biosynthesized Ag NPs represented a promising antimicrobial with potential biomedical applications. Therefore, this green chemistry approach towards the synthesis of Ag NPs has been the most sought-after method in terms of economic viability.
One of the authors (Suresh Sagadevan) acknowledges the honor, namely the “Senior Research Fellow” at Nanotechnology & Catalysis Research Centre (NANOCAT), University of Malaya 50603 Kuala Lumpur, Malaysia. The author wishes to place on record his heartfelt thanks that are due to the authorities concerned.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
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