Ethiopian Journal of Science and Sustainable Development

e-ISSN 2663-3205 Volume 8 (1), 2021

Journal Home Page: www.ejssd.astu.edu.et ASTU

Research Paper

Medicinal Plant Syzygium Guineense (Willd.) DC Leaf Extract Mediated Green

Synthesis of Ag Nanoparticles: Investigation of their Antibacterial Activity

Tegene Desalegn1,, H C Ananda Murthy1,*, Yeshaneh Adimasu2

1Department of Applied Chemistry, School of Applied Natural Science, Adama Science and Technology University, P O Box

1888, Adama, Ethiopia

2Department of Applied Biology, School of Applied Natural Sciences, Adama Science and Technology University, P O Box: 1888,

Adama, Ethiopia

Article Info Abstract

Article History:

Received 27 July 2020

Received in revised form

29 September 2020

Accepted 10 October 2020

The medicinal plant, Syzygium guineense (Willd.) DC mediated green silver nanoparticles (SyG-

Ag NPs) were successfully synthesized for the first time in Ethiopia. The synergistic influence

of biomolecules of the plant leaf extract such as alkaloids, phenolic compounds, tannins,

saponins and glycosides with Ag NPs towards the antibacterial activity has been investigated.

The synthesized nanoparticles were characterized by UV-Vis, UV-DRS, FT-IR, XRD, SEM,

EDXA, TEM, HRTEM and SAED techniques. The presence of absorbance maxima, λmax at 452

nm confirms the formation of SyG-Ag NPs. The energy gap, Eg of NPs, was found to be 2.1 eV.

FTIR spectra confirmed the presence of biomolecules in the extract and NPs. The presence of 4

sharp peaks in the XRD pattern of NPs confirmed highly crystalline nature of NPs. The purity

of the NPs was confirmed by SEM-EDAX analysis. The average particle size of NPs was found

to be 27.62 nm. TEM-HRTEM-SAED analysis resulted in d-spacing values of 0.2396 nm which

corresponds to Ag (111) lattice fringes of SyG-Ag NPs. The d-spacing value of the derived

diffraction planes from 4 major spots; d111Ag = 0.2428 nm, d200Ag = 0.2056 nm, d220Ag = 0.1483

nm and d311Ag = 0.1263 nm, are in agreement with the XRD data. The antibacterial test results

showed synergistic effect against both Gram positive bacteria, S. aureus (11±0.04mm), and

Gram negative bacteria E. coli (12±0.012 mm), P. aeruginosa (10±0.1 mm), and E. aerogenes

(13±0.032 mm), respectively, proving potentiality of SyG-Ag NPs as a remedy for infectious

diseases caused by tested pathogens.

Keywords:

Medicinal plants

Green synthesis

Syzygium guineense (Willd.)

SyG-Ag NPs

Antibacterial activity

1. Introduction

Ethiopia is one of the six centers of biodiversity in

the world with 6,500 species of higher plants where

traditional medicine plays a significant role and vast

majority of population lives in rural areas with little

access to health services (Gebrehiwet Tesfahuneygn and

Gebremichael Gebreegziabher, 2019). In recent years

numerous Ethiopian medicinal plants have been

validated in a scientific empirical framework through



Corresponding author, e-mail: tegened@yahoo.com and anandkps350@gmail.com

https://doi.org/10.20372/ejssdastu:v8.i1.2021.265

phytochemical analysis and subsequent bioassays

(Judžentienė, 2016). The medicinal plants species are used

to treat many diseases in Ethiopia (Balcha Abera, 2014).

Syzygium guineense (Willd.) DC is a medium-sized or tall

evergreen tree, 15 - 30 m high. Its Amharic name is

‘dokma’, whereas, in English it is called by several names

such as water pear, water boom and water berry. The ripe

fruits are edible by humans, birds, and some wild animals.

http://www.ejssd.astu.edu/
mailto:tegened@yahoo.com
https://doi.org/10.20372/ejssdastu:v8.i1.2021................

Tegene Desalegn et al. Ethiop.J.Sci.Sustain.Dev., Vol. 8 (1), 2021

In southern Ethiopia S. guineense is a much-

appreciated shade tree for both the homestead and the

home garden.

The research on the fabrication of plant mediated Ag

NPs for biomedical, environmental and agricultural

applications has been in the forefront for the last few

years. The green Ag NPs have been used for

photocatalytic, electrocatalytic, organic dye degradation,

biomedical, pharmaceutical, cosmetic, energy and

catalytic applications since many decades (Ananda

Murthy and Prakash, 2020). These Ag nanostructures such

as nanoparticles, nanocrystals, nanorods, nanotubes,

nanosheets exhibit versatile properties and hence found

to exhibit inhibitory activity against many microorganisms

and bacterial strains (Hemmati et al., 2020). A very few

medicinal plants such as Azadirachta indica (Ahmed et

al. 2016), Dioscorea bulbifera (Ghosh et al., 2015),

Barleria prionitis (Sougata et al. 2016), Caesalpinia

bonducella (Saranya Sukumar and Agneeswaran

Rudrasenan, 2020), Hagenia abyssinica (Murthy et al.,

2020) have been applied to synthesize silver, gold,

copper and their oxides in the recent past for

multifunctional applications. But no significant work

has been done especially with the application of

biomolecules of medicinal plant extracts as reducing

and capping agents for the synthesis of Ag NPs in

Ethiopia for biomedical, photocatalytic, electrochemical

sensor and antibacterial applications.

In this work, a simple and eco-friendly green

synthesis of Ag nanoparticles using medicinal plant

Syzygium guineense (SyG) leaf extract at low

temperature was proposed to investigate synergistic

influence of phytochemicals and Ag NPs on bacterial

strains. The synthesised SyG-Ag NPs have been

characterised by UV-Vis, UV-DRS, FT-IR, XRD, SEM,

EDXA, TEM, HRTEM and SAED techniques and

investigated for their in vitro antibacterial activity.

2. Materials and Methods

2.1. Materials and reagent

All the chemicals, AgNO3, ethanol, Dimethyl

sulfoxide (DMSO) used in the experiments were of

analytical grade (purchased from Merck chemical

Industrial Company) and used without any further

purification.

2.2. Collection and Authentication of Plant Materials

Syzygium guineense (Willd.) DC plant leaves were

collected from the Arsi zone, Ethiopia. The identification

of the plant Syzygium guineense (Voucher code EB006)

was performed at the National Herbarium, Department of

Biology, Addis Ababa University, Addis Ababa, Ethiopia.

2.3. Preparation of Plant Leaf Extract

The leaves of Syzygium guineense were surface

cleaned and washed repeatedly with tap water followed

by distilled water to remove dust particles and then

allowed to dry under shadow for 15 days to remove

moisture contents from the leaves. This procedure has

been adapted from the previously published work

(Murthy et al., 2020). The dried leaves were ground

using grinding machine followed by packing in a brown

bottle. The extraction was carried out by taking 20 g of

powdered leaves of Syzygium guineense in a 500 ml of

conical flask containing 400 ml of deionized water. The

flask was later covered with aluminum foil, to prevent

the effect of light. After that the mixture was shaken

using mechanical shaker for 90 min. and allowed to

warm at 50°C for 1 hr. on magnetic stirrer, then it was

allowed to cool down to room temperature overnight.

The prepared solution was filtered through whatman

No.1 filter paper to get clear solution. The filtrate was

stored at 4°C for future experiments.

2.4. Green Synthesis of SyG-Ag NPs

A 0.2 M aqueous AgNO3 solution was prepared and

stored in brown bottles. 100 ml of leaf extract was mixed

with 400 ml of 0.2 M AgNO3 solution (1:4) slowly

dropwise with constant stirring (Kumar et al., 2019).

The mixture has been incubated at room temperature

for 24 hrs. The change in color from blue to light

brownish visually indicates the formation of Ag NPs and

then the solution was centrifuged for 15 min at 10000

rpm. The obtained SyG-Ag NPs were washed thrice by

deionized water and ethanol to remove any impurities.

Thereafter, the NPs were allowed to dry and ground so as

to be used for further analysis (Murthy et al., 2018).

2.5. Characterization Techniques

The UV-Vis absorbance and reflectance spectra of the

samples were recorded in the range of 200–800 nm using

Shimadzu’s UV-2600, UV-vis spectrophotometer. The

Ag nanoparticle solution was prepared by mixing the

freshly prepared plant extract solution with Ag solution

Tegene Desalegn et al. Ethiop.J.Sci.Sustain.Dev., Vol. 8 (1), 2021

after appropriate dilution. Fourier transform-infrared

spectroscopy (FT-IR) Spectrum (65 FT-IR PerkinElmer)

was recorded using KBr pellets in the range of 400–4000

cm−1. X-ray diffraction (XRD-Shimadzu x-ray

diffractometer (PXRD-7000) analytical technique was

used to reveal the crystalline nature of g-Cu NPs.

The scanning electron microscopy with energy-

dispersive X-ray spectroscopy (SEM-EDX-EVO 18

model with low vacuum facility and ALTO 1000 Cryo

attachment) and transmission electron microscope with

high-resolution (JEOL JEM 2100 HRTEM) were used

for understanding morphological and structural features

of SyG-Ag NPs (Kumar et al., 2020). Gatan Digital

Micrograph Software was used to evaluate the d-spacing

values of lattice fringes. Particle size was computed by

using imageJ application.

2.6. Method of Antibacterial Evaluation

All the antibacterial tests were conducted at Oromia

Public Health Research, Capacity Building & Quality

Assurance Laboratory, Adama, Ethiopia. The in-vitro

antibacterial activity of SyG-Ag NPs was evaluated

using Agar disc-diffusion method against selected one

Gram positive bacterial strain (Staphylococcus aureus)

and three Gram negative pathogenic bacterial strains

(Escherichia coli, Pseudomonas aeruginosa and

Enterobacter aerogenes). Prior to antibacterial activity

test, the bacterial strains were cultured in nutrient broth

for 24 hrs to obtain logarithmic growth phase of the test

bacteria. A standardized inoculum of the bacteria is

swabbed onto the surface of Mueller-Hinton Agar

(MHA) plate. The actively growing bacterial cultures of

1.3×108 CFU/mL concentration were inoculated/spread

onto the MHA plate (turbidity was adjusted with TSB to

match 0.5 McFarland standard). The nanoparticles

extract was prepared with four different concentrations

in Dimethyl Sulfoxide. Four concentrations (6.25, 12.5,

25 and 50 µg/µl) of the synthesized nanoparticles were

added to the respectively labeled wells.

The antibiotic discs of 6 mm diameter were applied

to agar surface using forceps with gentle pressure and

then impregnated with the dissolved extract.

Chloramphenicol disc was used as a positive control

while DMSO was taken as negative control. The plates

were incubated at 35 ±2°C in an ambient air incubator

for 18-24 hrs. The antibacterial activity was evaluated

in terms of zone of inhibition, measured to the nearest

millimeters (mm) using a ruler and recorded.

3. Results and Discussions

3.1. Synthesis of SyG-Ag NPs

The SyG-Ag NPs were synthesized by using silver

nitrate as a precursor and Syzygium guineense (Willd.)

DC plant leaf extract as a reducing and capping agent.

The presence of alkaloids, phenolic compounds, tannins,

saponins, anthraquinone glycosides and cardiac

glycosides was confirmed during the phytochemical

screening of Syzygium guineense leaf extract. The

details of the phytochemicals present in the extract are

as given in Table 1. Three stages of formation of NPs

includes: reduction of metal ions, formation of cluster

and growth of nanoparticles. It is learnt that the

tautomeric transformation of polyphenols from enol

form to keto form would release reactive hydrogen atom

that reduces silver ions to silver nanoparticles. In

addition, the enzymes of leaf extract also assist silver

ions to form an enzyme substrate complex resulting into

formation of protein capped silver NPs (Roy et al.,

2019). It is also understood that the phenolic compounds

act as ligand and bind to metal ions and reduce them and

cap them to form nanoparticles. These ligands also act

as particle size controllers as reported by the earlier

researcher (Azarbani & Shiravand, 2020).

Table 1: The details of phytoconstituents screening of Syzygium guineense plant leaves extract.

Sl. No. Phytoconstituents Test / Reagent Result

1 Alkaloids Wagner’s reagent +

2 Tannins KOH +

3 Flavonoids Shinoda Test -

4 Terpenoids Salkowski Test -

5 Anthraquinone glycosides Borntrager's Test +
6 Cardiac glycosides Keller-Kiliani Test +

7 Saponins Frothing Test +

8 Phenols FeCl3 +

‘+’ indicates the presence and ‘-’ indicates the absence

Tegene Desalegn et al. Ethiop.J.Sci.Sustain.Dev., Vol. 8 (1), 2021

Primary components of Syzygium guineense extract

are alkaloids, phenolic compounds, tannins, saponins,

anthraquinone glycosides and cardiac glycosides. The

antioxidant properties of phenolic compounds are

primarily due to their high inclination towards chelating

the metals. Phenolic compounds contain hydroxyl and

carboxylic groups which have very high tendency to

bind metal ions. Metal ions in solution interact with

phenolic compounds which help in the nucleation and

formation of Ag NPs.

3.2. Characterization of SyG-Ag NPs

The SyG-Ag NPs were characterized using UV-Vis,

UV-DRS, FT-IR, XRD, SEM, EDXA, TEM, HRTEM and

SAED techniques.

3.2.1. UV-Vis spectral analysis

The UV-Vis absorbance spectrum recorded for

instantaneously synthesized SyG-Ag NPs exhibited λmax of

452 as shown in Figure 1a, just after 10 min of mixing plant

extract with copper nitrate solution in 1:4 ratios.

This absorption band is basically due to surface

plasmon resonance of SyG-Ag NPs. The absorbance

spectrum recorded after 30 min of forming NPs, exhibited

another identical absorbance band at λmax of 452 nm

(Figure 1b). The enhanced absorbance in the 2nd band,

clearly confirm the increased concentration of NPs. As the

time proceeds on, the reduction of copper ions is followed

by nucleation of small cluster of copper atoms to form

nanoparticles in the presence of biomolecules of plant

extract which are possibly believed to have acted as

reducing agent and stabilizing agent. Poly phenolic

compounds usually help in the reduction of silver ions into

silver atoms which form cluster and nanoparticles at the

later stage.

Figure 1: (a) and (b) UV-Vis absorbance spectrum of SyG-Ag NPs at different time intervals (c) UV-Vis diffused

reflectance spectrum of SyG-Ag NPs. (d) Tauc plot of SyG-Ag NPs showing Eg Value.

Tegene Desalegn et al. Ethiop.J.Sci.Sustain.Dev., Vol. 8 (1), 2021

Figure 2: The XRD pattern of SyG-Ag NPs.

A similar result was reported after the analysis of

synthesized Ag NPs by using the Lustrum lucidum leaf

extract (Huang et al., 2020) and Persea americana seed

extract (Girón-Vázquez et al., 2019). The surface

plasmon absorbance presents a set of different λmax

values for NPs synthesised using different plant extracts

which is possibly due to morphological features of the

NPs. Similarly, the UV-Vis diffused reflectance

spectrum was recorded (Figure 1c). The band gap

energy of SyG-Ag NPs was evaluated using Tauc plot as

shown in figure 1d by using the data obtained in

reflectance spectra utilizing Kubelka-Munk function.

The band gap energy, Eg of SyG-Ag NPs was found to

be 2.1 eV.

3.2.2. XRD analysis

The XRD diffraction pattern of SyG-Ag NPs is

presented in figure 2. The peaks at 2θ values = 38.14o,

44.72o, and 64.50o, and 77.42o corresponds to (111),

(200), (220) and (311) lattice planes of face centred

cubic structure of Ag NPs and the diffraction results

were in good agreement with the data of ICPD file no.

00-004-7383 (Fm3m) (Sadeghi & Gholamhoseinpoor,

2015).

3.2.3. FT-IR spectral analysis

The FTIR spectra of SyG plant extract and SyG-Ag

NPs is presented in figure 3. FTIR spectra confirmed the

presence of biomolecules in the extract and NPs. The

broad peaks appeared in the region between 3391 and

2926 cm-1 corresponds to –OH stretching vibration and

sp
3
C-H stretching vibrations, respectively. The peak at

1623 cm-1 corresponds to C=O stretching of carbonyl

groups. The sharp peak around 1450 cm−1 shows the

presence of −COO group of carboxylic acid. It is also

believed that the amine and carboxylate group present

in the SyG plant leaf extract responsible for the binding

of proteins with the surface of Ag and thereby leading

to the stabilization of the biosynthesized nanoparticles.

The peaks represented by 1350 cm-1 shows C-N

stretching of amide. The medium peak at 1160 cm-1

corresponds to C-O stretching of phenolic compounds.

The C-O-C stretching appears at 1038 cm-1. The

bending vibrations of Ag–O–H bonds resulted in a small

peak at 800 cm−1 which can be attributed to the presence

of the Ag–O bond. The last peak at 665 cm-1

corresponds to bending modes of vibrations of C–H

bond. The small shift in the IR bands indicates the

possible reaction of silver ions and synthesis of

nanoparticles in the extract. FTIR analysis results

confirmed the presence of various phytochemicals of

Syzygium guineense leaf extract such as alkaloids,

phenolic compounds, tannins, saponins, anthraquinone

glycosides and cardiac glycosides involved in the

synthesis of SyG-Ag NPs.

Figure 3: The FTIR spectra of SyG plant extract and

SyG-Ag NPs.

3.2.4. Morphological and compositional analysis

by SEM-EDAX

The morphological features of synthesized SyG-Ag

NPs as depicted by SEM micrographs are shown in

figure 4a and 4b. The SEM images also depicted

Tegene Desalegn et al. Ethiop.J.Sci.Sustain.Dev., Vol. 8 (1), 2021

varieties of nanosized particles with diversified size

ranges as well as shapes. All the possible nearly

spherical and diverse shapes such as hexagonal,

cylindrical, triangular and prismatic shapes of Ag NPs

with varying particle sizes were found in the

micrographs. The average grain size of Ag NPs was

found to be in the range between 10 and 40 nm

(Velgosova et al., 2019).

The presence of less agglomerated Ag NPs possibly

due to moderate surface area which yielded small sized

particles. The Ag NPs were found to be as small as 10.1

nm in their dimension which clearly confirms the

efficient role of biomolecules as stabilizing agents

preventing the growth of clusters of silver atoms to

bigger NPs. The chemical composition of the NPs was

studied by EDAX analysis. Figure 4c shows the EDAX

spectrum obtained for the SyG-Ag NPs. The peaks

corresponding to elemental Ag, C and Cl were clearly

identified and additional peak for Au appeared due to

the usage of standard during the analysis.

It is also possibly believed that the presence of C is

basically from the capped bioactive compounds. The

reduction of silver ions to Ag NPs is facilitated by the

biomolecules of plant extract containing surface

hydroxyl groups.

Figure 4: (a) and (b) SEM micrographs of VeA-Ag NPs (c) EDAX spectrum of SyG-Ag NPs.

Tegene Desalegn et al. Ethiop.J.Sci.Sustain.Dev., Vol. 8 (1), 2021

3.2.5. TEM, HRTEM and SAED analysis

In order to explore deep insights into the

morphological and structural features of the SyG-Ag

NPs, TEM, HRTEM and SAED technical micrographs

and patterns were utilized. The HRTEM images of as-

synthesized SyG-Ag NPs (Figure 5) shows that the

synthesized NPs are nearly spherical but exhibited

varieties of shapes.

Figure 5: The TEM micrographs showing SyG-Ag NPs

with a) cylindrical, b) prismatic c) hexagonal,

and d) triangular shapes with nearly spherical

shapes.

The smaller NPs as small as 10.1 nm confirm the

efficient role of bioactive components of Syzygium

guineense plant extract as capping and stabilizing

agents. Otherwise NPs would have been agglomerated

to yield larger NPs in an elongated form.

In addition, the variation in size of SyG-Ag NPs is

probably due to the presence of polyphenolic

compounds which have strong attractive forces and

holds the particles together.

The TEM micrographs which exhibited very finely

grained SyG-Ag NPs with cylindrical, prismatic,

hexagonal, triangular shapes as well as nearly spherical

shapes are presented in Figure 5a to 5d.

The nearly spherical particles with varying sizes

from 10.1 nm to 54.7 nm with an average particle size

of 27.62 nm as determined by image J application are as

shown in Figure 6a and 6b.

Figure 6: TEM images of as-synthesized SyG-Ag NPs

at (a) lower magnification (100 nm) and (b)

higher magnification (50 nm) (c) SAED

pattern with 1 to 6 spots and (d) HRTEM

image showing lattice fringes of SyG-Ag NPs

with d-spacing of 0.2396 nm.

The SAED pattern of SyG-Ag NPs (Figure 6c)

contained six spots each corresponding to specific

crystal planes. The most prominent 4 spots were

represented in colored concentric circles which

represents (111), (200), (220) and (311) planes. The d-

spacing of 0.2396 nm for Ag (111) plane is as shown in

Figure 6d. HRTEM morphology of SyG-Ag NPs with

magnified lattice fringes, IFFT patterns and profile of

IFFT with d-spacing value for Ag (111) plane (Figure

6d) are presented in Figure 7a, 7b, 7c and 7d

respectively. The image analysis to arrive at d-spacing

value has been carried out by using Gatan Digital

Micrograph Software application which resulted in dhkl

value of 0.2396 nm a set of crystal planes at the surface

of Ag NPs.

The d-spacing values for the most prominent four

spots depicted in the SAED pattern of SyG-Ag NPs

(Figure 6c) are presented in Table 2. Each spot on the

SAED pattern corresponds to specific set of lattice

planes. The XRD pattern of NPs presented earlier in

Figure 3, revealed 4 major peaks corresponding to (111),

(200), (220) and (311) planes of fcc structure of pure Ag

(ICPD file no. 00-004-7383 (Fm3m)). The d-spacing

Tegene Desalegn et al. Ethiop.J.Sci.Sustain.Dev., Vol. 8 (1), 2021

Table 2: The d-spacing values for SyG-Ag NPs from SAED pattern.

Spot No. d- spacing (nm) Rec. Pos. (1/nm) Degrees to Spot 1 Degrees to x- axis Amplitude

1 0.2428 4.145 0.00 80.37 2839.10

2 0.2126 4.678 2.04 78.33 4168.58

3 0.2056 4.895 52.10 28.27 4484.95

4 0.1483 6.756 6.11 74.26 1295.65

5 0.1263 7.954 5.87 86.24 5564.48

6 0.09527 10.40 19.92 60.45 4062.21

values of the derived diffraction planes corresponding to

spots 1, 3, 4 and 5; d111Ag = 0.2428 nm, d200Ag = 0.2056

nm, d220Ag = 0.1483 nm and d311Ag = 0.1263 nm, are in

agreement with the standard d-spacing values of Ag

(ICPD file no. 00-004-7383 (Fm3m)) structure.

Figure 7: HRTEM morphology of SyG-Ag NPs (a)

Magnified lattice fringes (b) IFFT patterns (c)

Profile of IFFT with d-spacing distance.

3.3. Antibacterial Activity

The SyG-Cu/Cu2O/CuO NPs exhibited broad range

of antibacterial activities against all tested pathogens; S.

aureus, E. coli, P. aeruginosa, and E. aerogenes. The

present work evaluated synergistic influence of

biomolecules with NPs against 4 pathogens. The zone

of inhibitions for Chloramphenicol, DMSO and NPs

with four concentrations (6.25, 12.5, 25 and 50 µg/µl).

Pure Ag NPs were proved to exhibit excellent

antibacterial activity. Accordingly, SyG-Ag NPs were

found to show better antibacterial activity against Gram

positive bacteria than Gram negative bacteria which is

believed to be due to the structural differences in the cell

walls of bacteria (Abebe et al., 2020).

SyG-Ag NPs were found to show better antibacterial

activity against Gram negative bacteria than Gram

positive bacteria which is believed to be due to the

structural differences in the cell walls of bacteria. The

antibacterial activity of NPs can be partially attributed

to the presence of bioactive compounds on the surface

of NPs as capping and stabilizing agents. In this regard,

the activity was pronounceable against S. aureus. The

highest zone of inhibition (mm) recorded with 50 µg/µl

of SyG-Ag NPs against E. aerogenes bacteria was

13±0.032 mm and the lowest zone of inhibition (mm)

recorded against P. aeruginosa bacteria was 10±0.1 mm

(Table 3).

The wide zone of inhibitions of SyG-Ag NPs against

pathogens confirm their great potentiality as a remedy

for infectious diseases caused by the tested bacterial

pathogens.

Additionally, the standard disc Chloramphenicol

showed comparable zone of inhibition with the SyG-Ag

NPs, which is small and it can be attributed to

development of resistance by bacteria against

Chloramphenicol. The zone of inhibition values for

SyG-Ag NPs was found to be moderately lower for S.

aureus, E. coli and E. aerogenes except P. aeruginosa

when compared with that of positive control

Chloramphenicol. It is mainly due to the differences in

the chemical structure of the bacterial cell walls and as

a consequence different types of interactions occur

between differently sized NPs and bacterial strains. The

interaction between Ag NPs and microorganisms starts

with adhesion of Ag NPs to the microbial cell wall and

membrane, which is based on electrostatic attraction

between the negatively charged microbial cell membrane

and positively or less negatively charged Ag NPs. Zeta

potential is another physico-chemical property influence

antimicrobial activity since the interaction between NPs

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Table 3: The variation of zone of inhibitions for different bacteria by SyG-Ag NPs.

Concentration of NPs

(µg/µL)

Bacterial strains and Zone of Inhibition in mm

S. aureus

ATCC25923

E. coli

ATCC25992

P. aeruginosa

ATCC27853

E. Aerogenes

ATCC13048

50 11±0.04 12±0.012 10±0.1 13±0.032

25 11±0.005 10±0.06 8±0.05 10±0.103

12.5 6±0.008 8±0.017 6±0.057 8±0.007

6.25 6±0.11 6±0.002 6±0.043 6±0.041

Chloramphenicol 24±0.14 20±0.015 6±0.001 30±0.016

DMSO 6±0.006 6±0.02 6±0.003 6±0.01

Table 4: Comparative data of antibacterial activities of Ag NPs synthesised by using various plant extracts.

Sl. No. Plant extract Zone of Inhibition (mm) Tested Pathogens Reference

1 Bergenia cili(Phull et al.,

2016) ata

8.5 S. aureus (Phull et al. 2016)

2 Balloon flower plants 12 E. coli (Anbu et al., 2019)

3 Aloe fleurentiniorum 12, 14.5 E. coli, S. aureus (Salmen & Alharbi, 2020)

4 Chenopodium murale 12.7 S. aureus (Abdel-Aziz et al., 2014)

5 Syzygium guineense 11,

12,

10,

S. aureus,

E. coli,

P. aeruginosa,

E. aerogenes.

Present work

6 Ocimum Sanctum (Tulsi) 14 E.coli (Jain & Mehata, 2017)

7 Citrus paradisi (grapefruit

red)

22 E.coli (Ayinde et al., 2018)

8 Coffea arabica 23, 21 E. coli, S. aureus (Dhand et al., 2016)

9 Banana peel extract 17,

20,

16,

E. coli,

P. aurognosa,

S. aureus,

B. subtilis.

(Ibrahim, 2015)

10 Allium cepa 24,

23,

E. coli,

S. aureus,

B. subtilis.

(Gomaa, 2017)

11 Rosmarinus officinali 17.2,

18.8,

31.2,

16.2

E. coli,

P. aurognosa,

S. aureus,

B. subtilis.

(Khafri, 2015)

12. Cassia roxburghii leaf 33 E. coli (Moteriya et al., 2017)

and the cell membrane is based on electrostatic adhesion

which is different for different bacterial strains.

Furthermore, Ag NPs could interact with protein

parts of the outer membrane of bacterial strain,

constitute complexes with oxygen, phosphorous,

nitrogen or sulphur atom containing electron donors,

and cause irreversibly changes in the cell wall structure.

The moderate zone of inhibitions exhibited by the SyG-

Ag NPs is possibly because of the application of very

small amounts of NPs compared to the amount of NPs

utilized by various researchers.

Many mechanisms of antibacterial activity have been

reported by the past researchers, accordingly, the action

of SyG-Ag NPs on the bacteria is yet to be explored fully.

It is assumed that the SyG-Ag NPs get adsorbed on to the

cell wall of bacteria and interacts with the electronegative

elements within the cell membrane.

These results in failed metabolism and thereby

leading to interference and disruption of transcription in

bacteria and hence causes antibacterial activity by SyG-

Ag NPs. It is also believed that the synergistic effect of

SyG-Ag NPs with bioactive compounds of extract

would have played significance influence to inhibit the

activity of pathogenic bacteria.

The antibacterial results obtained using SyG-Ag NPs

were found to be better when compared with an earlier

Tegene Desalegn et al. Ethiop.J.Sci.Sustain.Dev., Vol. 8 (1), 2021

work reported by many researchers (Table 4). The highest

zone of inhibition (mm) recorded with SyG-Ag NPs

against E. aerogenes bacteria was 13±0.032 mm which is

higher than the zone of inhibition exhibited by Ag NPs

synthesized by various plant extracts as mentioned in

Table 4. The last few plant extracts were proved to exhibit

a higher zone of inhibition which can be attributed to the

presence of different type of phytoconstituents.

Another possible mechanism of the inhibitory

phenomenon was that the cell walls of Gram-positive

bacteria bound larger quantities of metal (Ag) than

Gram-negative bacteria. Most studies have shown that

Gram-negative bacterial growth is more affected by Ag

NPs than Gram-positive bacteria which can be attributed

to thin cell walls that are easily penetrated, while Gram-

positive bacteria have thicker cell wall (Anbu et al.,

2019). It is possibly understood that the helical structure

of DNA molecules would have disrupted by the action

of SyG-Ag NPs. In addition, electrochemical potential

across the cell membrane decreases up on interaction

with the released Ag metal ion by SyG-Ag NPs affecting

integrity of the membrane.

4. Conclusion

The application of medicinal plant, Syzygium

guineense leaf extract towards the green synthesis of

silver (SyG-Ag) nanostructures was successful. The

UV-Vis spectra, XRD pattern and FTIR spectra

confirmed the formation of crystalline SyG-Ag NPs in

the presence of biomolecules such as ployphenolics,

tannins, glycosides and proteins of the plant extract.

SEM and TEM micrographs of SyG-Ag NPs revealed

cylindrical, prismatic, hexagonal, triangular and

spherical morphologies. HRTEM-SAED analysis

confirmed the presence of crystalline Ag by the

observation of Ag (111), (200), (220) and (311) lattice

fringes in SyG-Ag NPs with the calculated d-spacing

values matching exactly with the standard value for Ag.

The SyG-Ag NPs are identified as effective antibacterial

agents against multiple microbes of various strains. The

synergistic influence of bioactive compounds, SyG-Ag

NPs and antibiotic ampicillin proved to be highly

effective against pathogens, Gram-positive and Gram-

negative bacterial strains S. aureus, E. coli, P.

aeruginosa, and E. aerogenes. It is expected that this

accomplished work will initiate research towards the

green synthesis of fabricated metallic nanoparticles for

prospective biomedical applications.

Acknowledgments

This work has been funded by the project

(ANSD/04/0453/11-2018) approved by the Research

and Technology Transfer Office, sanctioned by Adama

Science and Technology University, Ethiopia. The

authors gratefully acknowledge Adama Science and

Technology University for the financial support and

laboratory facility to conduct this research work.

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