Structural characterization, antibacterial and catalytic effect of iron oxide nanoparticles synthesised using the leaf extract of Cynometra ramiflora

Abstract

In the present investigation, the leaf extract of Cynometra ramiflora was used to synthesize iron oxide nanoparticles. Within minutes of adding iron sulphate to the leaf extract, iron oxide nanoparticles were formed and thus, the method is very simple and fast. UV-VIS spectra showed the strong absorption band in the visible region. SEM images showed discrete spherical shaped particles and EDS spectra confirmed the iron and oxygen presence. The XRD results depicted the crystalline structure of iron oxide nanoparticles. FT-IR spectra portrayed the existence of functional groups of phytochemicals which are probably involved in the formation and stabilization of nanoparticles. The iron oxide nanoparticles exhibited effective inhibition against E. coli and S. epidermidis which may find its applications in the antibacterial drug development. Furthermore, the catalytic activity of the nanoparticles as Fenton-like catalyst was successfully investigated for the degradation of Rhodamine-B dye. This outcome could play a prominent role in the wastewater treatment.

Full text

Structural characterization, antibacterial and catalytic effect of iron
oxide nanoparticles synthesised using the leaf extract of Cynometra
ramiflora
Silvia Groiss, Raja Selvaraj
*
, Thivaharan Varadavenkatesan, Ramesh Vinayagam
Department of Biotechnology, Manipal Institute of Technology, Manipal, Karnataka, 576104, India
article info
Article history:
Received 25 June 2016
Received in revised form
16 August 2016
Accepted 11 September 2016
Available online 12 September 2016
Keywords:
Iron oxide nanoparticles
Cynometra ramiflora
Green synthesis
Rhodamine B degradation
Antibacterial activity
abstract
In the present investigation, the leaf extract of Cynometra ramiflora was used to synthesize iron oxide
nanoparticles. Within minutes of adding iron sulphate to the leaf extract, iron oxide nanoparticles were
formed and thus, the method is very simple and fast. UV-VIS spectra showed the strong absorption band
in the visible region. SEM images showed discrete spherical shaped particles and EDS spectra confirmed
the iron and oxygen presence. The XRD results depicted the crystalline structure of iron oxide nano-
particles. FT-IR spectra portrayed the existence of functional groups of phytochemicals which are
probably involved in the formation and stabilization of nanoparticles. The iron oxide nanoparticles
exhibited effective inhibition against E. coli and S. epidermidis which may find its applications in the
antibacterial drug development. Furthermore, the catalytic activity of the nanoparticles as Fenton-like
catalyst was successfully investigated for the degradation of Rhodamine-B dye. This outcome could
play a prominent role in the wastewater treatment.
©2016 Elsevier B.V. All rights reserved.
1. Introduction
Nowadays, the exploration of nanoparticles has earned a lot of
momentum as a consequence of their unique characteristics [1].
Environmentally benign, sustainable and green processes are
captivating the nanotechnology researchers by virtue of the prob-
lems concomitant with the conventional chemical methods. As
reported in many studies, the conventional chemical methods of
nanoparticle synthesis employ many organic solvents, toxic
chemicals and non-biodegradable stabilizing agents [2]. Therefore,
synthesis of nanoparticles using naturally available waste materials
like extracts of various parts of plants, various microorganisms,
their metabolites and few natural humic substances [3,4] as
reducing and capping agents are becoming popular in the field of
green nanotechnology. Many metallic nanoparticles such as silver,
gold, iron, copper and zinc have been synthesised by green syn-
thesis procedures.
Iron nanoparticles are of considerable interest due to their ca-
pacity to remove various pollutants from wastewater. Few of them
include the removal of heavy metals such as lead [5,6], chromium
[7,8], arsenic [9], copper, zinc and manganese [10], removal dyes
such as direct yellow 12 [11], methylene blue, methyl orange [12]
and malachite green [13], degradation of chlorinated organics
[14], removal of steroidal estrogens [15] and the removal of total
nitrogen and phosphorus from wastewater [16]. Moreover the iron
nanoparticles have the innate antibacterial and antifungal proper-
ties [17], thereby finding applications in water disinfection [18].
In this study, we propose a green synthesis method using a
novel source ethe leaf extract of a medicinal plant Cynometra
ramiflora for the production of nanoparticles. This indigenous tree
is a member of the Caesalpiniaceae family and is distributed across
tropical areas in Africa, Australia as well as south Asian countries
like India and Sri Lanka. The leaves exhibit a leathery and a smooth
surface area and consist of two to six leaflets. Traditionally the
leaves of this plant have been used for the treatment of hyperten-
sion, diabetes and hypercholesterol. The anti-cancer activity of the
methanolic extract has been demonstrated by Kharisov et al. [19].
Paguigan et al. [20] have shown the anti-ulcer activity of the leaves
and analyzed the phytochemical contents of the leaf extract. Their
study revealed the existence of flavonoids, tannins, alkaloids, car-
diac glycoside, phenols, saponin and steroids in the leaf extract
which were responsible for the anti-ulcer activity. These phyto-
chemicals are water-soluble, non-toxic and bio-degradable which
*Corresponding author
E-mail address: rajaselvaraj@gmail.com (R. Selvaraj).
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
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Journal of Molecular Structure 1128 (2017) 572e578

may be accountable for reduction and capping of nanoparticles
[21].
There are no existing studies concerning the synthesis of iron
oxide nanoparticles (IONPs) using the leaf extract of Cynometra
ramiflora. The aim of this investigation is to synthesize IONPs using
Cynometra ramiflora leaf extract as a reducing agent and to char-
acterize the molecular structure of IONPs using various techniques.
Furthermore, the antimicrobial activity was tested against two
pathogens namely Escherichia coli and Staphylococcus epidermidis
and the ability of the nanoparticles to degrade a carcinogenic
pollutant dye, Rhodamine B (Rh B) in the presence of hydrogen
peroxide was also investigated.
2. Experimental
2.1. Chemicals
FeSO
4
.7H
2
O and hydrogen peroxide were purchased from
Merck. Analytical grade Rhodamine B (HiMedia) was used without
any further purification. Millipore-Milli-Q water was used in the
synthesis procedure.
2.2. Preparation of leaf extract and IONPs
Fresh leaves of Cynometra ramiflora were collected in July month
in the Manipal University premises. The leaves were first cleaned
with raw water for 10 min and washed with Millipore-Milli-Q
water. The leaves were dried on a blotting paper for 1 h at room
temperature. 10 g of cleaned and dried Cynometra ramiflora leaves
were cut into fine parts and boiled in 100 mL of water for 20 min.
The resulting extract was settled for 1 h and filtered. The collected
leaf extract was maintained at 4

C for storage purposes. For the
synthesis of IONPs, a solution of 0.10 mol/L FeSO
4
$7H
2
O was pre-
pared and mixed with the leaf extract (1:2). The instantaneous
black color appearance indicated the formation of IONPs.
2.3. Characterization of IONPs
The absorbance spectrum of nanoparticles was monitored using
Shimadzu UVeVIS spectrophotometer. The morphological struc-
ture was characterized through Scanning electron microscope (EVO
MA18) and the elemental composition was characterized by Energy
dispersive X-ray analysis (Oxford). A thin film of as-synthesised
IONPs was made on a glass slide and the images were recorded at
various magnifications. X-ray diffraction (XRD) patterns of the
IONPs were recorded using a RigakuMiniflex 600 X-Ray diffrac-
tometer using a high-power Cu K
a
source at 40 kV/15 mA to
identify the crystallite structure. The scanning was done within the
2
q
range of 20

e80

. FTIR analysis (SHIMADZU-8400S spectro-
photometer) was performed to identify the chemical groups within
the C. ramiflora leaf extract which are accountable for the reduction
and capping process of iron nanoparticle synthesis.
2.4. Antimicrobial activity
Green-synthesised IONPs were investigated for their antimi-
crobial activity against E. coli and S. epidermidis by standard Kirby-
Bauer diffusion assay. 20 mL of agar was loaded to sterile petri
plates. After solidification 100
m
L of overnight bacterial culture was
spread to get bacterial lawn. Two circular wells were made, one
serving as a negative reference with sterile water and the other one
for IONPs (70
m
L). After incubation of the plates for 24 h at 37

C, a
clear zone of growth inhibition was examined.
2.5. Catalytic ability
The catalytic ability of IONPs was checked based on the pro-
cedure given by Nijagi et al. [21] with a few modifications. Briefly, a
known concentration of RhB dye was taken in a cleaned quartz
cuvette to which 2% H
2
O
2
and various concentration of IONPs were
loaded to visualize the degradation process. In order to check the
degradation capacity of H
2
O
2
alone, one set of control experiments
was performed without the addition of IONPs. For all these ex-
periments, blanks were prepared with water without the addition
of dye. As soon as IONPs were added, the cuvette contents were
mixed thoroughly using a pipette and the absorbance readings
were monitored with respect to time at the
l
max
of 555 nm.
3. Results and discussions
3.1. UV-VIS spectra analysis
After the addition of FeSO
4
, the yellow leaf extract changed to
black instantaneously which confirmed the formation of IONPs
(Fig. 1A). This reduction process is feasible and spontaneous
because the standard reduction potential of the polyphenols found
in the leaf extract is between 0.534 and 0.540 V [22] and the
standard reduction potential of iron is 0.44 V [23]. The spectrum
analysis of FeSO
4
, leaf extract and IONPs in the range 200e800 nm
are shown in Fig. 1a. The aqueous solution of FeSO
4
does not have
any peak and shows a continuous absorption band in the entire
spectrum. The leaf extract shows a specific peak at 275 nm which
corresponds to polyphenolic compounds present in the extract
[23]. The absence of peak at 275 nm in the IONPs spectrum in-
dicates that polyphenolic compounds react with FeSO
4
to form
IONPs. This phenomenon also ascertains that the phenolic com-
pounds act as reducing agent. A continuous absorption in the
visible range, one of the characteristic features of IONPs [24,25] has
been observed. The polyphenols of the leaf extract are accountable
for the reduction of FeSO
4
to form IONPs as observed by Machado
et al. [26] for various plant leaf extracts.
3.2. XRD
The crystallite structure of freshly synthesised IONPs are shown
in Fig 1B. The characteristic peak (2
q
¼44.9

) for zero-valent iron
nanoparticle is not observed in the XRD pattern which confirms the
capping and stabilizing effect of the leaf extract on the surface of
the nanoparticles [13]. The dominant peak at 2
q
¼19

is due to the
polyphenols of the leaf extract [27]. The double peaks around
28

e29

belong to maghemite (
g
-Fe
2
O
3
). Magnetite (Fe
3
O
4
) shows
a typical peak at 36.5

[13]. Additionally, several peaks at around
23

e25

indicate the presence of iron oxohydroxide (FeOOH) due
to partial oxidation during the synthesis [28].
3.3. FTIR
An FTIR spectrum of IONPs was investigated to identify the
functional groups within the plant extract as well as on the parti-
cles themselves. The spectra in Fig. 1C presents a very strong band
at 1624 cm
1
which corresponds to stretching of eC]Cedouble
bonding. The presence of weak eC]Ceabsorption is indicated by
the band at 1443 cm
1
which suggests aromatic skeletal com-
pounds. Typical eC]Cestretching bands indicating the presence
of alkenes would appear at around 1612e1618 cm
1
. The shift in the
absorption band can be seen as an indication for the oxidation
process of the functional groups during nanoparticle synthesis ac-
cording to Huang et al. [29]. The features around
3570 cm
1
e3600 cm
1
correspond to strong eOH vibrations, the
S. Groiss et al. / Journal of Molecular Structure 1128 (2017) 572e578 573

hydroxyl groups present in C. ramiflora leaf extract. The spectra
shows typical eCeH absorption at 2924 cm
1
which relates to
stretching vibrations of methyl groups. The absorption frequencies
at low wave numbers (<100 0 cm
1
) like 995 cm
1
and 810 cm
1
attribute to strong ]CeH bending. The band at 1273 cm
1
corre-
spond either to asymmetric stretching bonds or confirms the
presence of eC-N compounds on the nanoparticle surface. The
adsorption bands less than 800 cm
1
correspond to the vibration of
FeeO bonds of various IONPs [5,25].
The various bands endorse the presence of different poly-
phenols in the IONPs complex. Therefore it is hypothesised that the
eOH functional groups of the polyphenolic compounds could act
as metal chelators in the formation of nanoparticles. The proposed
structure of IONPs with a ferrous ion (from FeSO
4
) positioned in
nanoparticles chelated by the polyphenols residing in the leaf
extract is shown in Fig. 2. Similar kind of mechanism has been
postulated by Wang et al. [30] for the formation of iron nano-
particles using Salvia officinalis leaves with FeCl
3
.
3.4. SEM/EDS
The SEM image of the IONPs synthesised from the Cynometra
ramiflora leaf extract is shown in Fig. 3A. It can be visualized from
the image that the IONPs exist as singular nanoparticles and small
aggregates of spherical nanoparticles. These findings are concur-
rent with the results obtained by other investigators [13,28]. The
aggregate formation may be due to the existence of various phy-
tochemicals of the leaf extract [16].
EDS analysis was performed to study the elemental composition
of synthesised IONPs. Apart from Fe and O, the EDS spectrum
(Fig. 3B) symbolises intense peaks of Ca, Na and Si. The signals for
these impurities must have originated from the glass underlay that
holds the sample. The intense peaks between 6 and 7 keV is specific
for elemental iron. The presence of peak for oxygen suggests the
oxidation of the iron nanoparticles because of the exposure to air or
water [21].
3.5. Antimicrobial activity
It has been shown by many researchers [31e33] that metallic
nanoparticles exhibit antimicrobial property. To ascertain the
antibacterial activity of the synthesised IONPs, the well-diffusion
method was employed against E. coli (Fig. 4A) and S. epidermidis
(Fig. 4B). The well W was loaded with sterile water as a negative
control and another well was loaded with 70
m
Lof IONPs. Upon
incubation of the plates for 24 h at 37

C, a clear zone of inhibition
was observed in the well loaded with IONPs. This portrayed the
antibacterial potential of IONPs. This may be due to the
Fig. 1. (A) UVevis spectra of C. ramiflora leaf extract, FeSO4 and IONPs, (B) XRD pattern of IONPs, (C) FT-IR spectra of IONPs.
Fig. 2. Proposed structure of iron-oxide nanoparticles.
S. Groiss et al. / Journal of Molecular Structure 1128 (2017) 572e578574

accumulation of metallic nanoparticles inside the cell membrane
and the release of cellular compounds that eventually led to cell
death as described by Sondi &Salopek-Sondi [34] or the synergetic
effect of phytochemicals residing in the leaf extract and the reactive
oxygen species produced by IONPs [35].
3.6. Catalytic studies
Fig. 5A displays the degradation of RhB dye with time using 2%
H
2
O
2
and mixtures of 2% H
2
O
2
and various concentrations
(0.28 mMe1.11 mM) of green-synthesised IONPs. The residual
concentration of dye was measured from the absorbance at 555 nm.
The constant absorbance with respect to time in the presence of
just 2% H
2
O
2
can be visualized from the same figure. Nevertheless,
the dye started degrading in the presence of mixture of H
2
O
2
and
IONPs which indicated the degradation by free radical pathway
mechanism.
As the concentration of IONPs increases at a fixed concentration
of 2% H
2
O
2
, there was an increase in the degradation of dye. The
process was slowest in the presence of 2% H
2
O
2
and 0.28 mM IONPs
with only 46% being degraded after 15 min. In the presence of 2%
H
2
O
2
and 0.56 mM IONPs, about 87% dye was degraded within
15 min. The process was fastest when 2% H
2
O
2
and 1.11 mM IONPs
was used, which led to a 100% degradation of dye within 15 min.
These results prove that high concentration of IONPs accelerate the
degradation of dye in the presence of H
2
O
2
.
The degradation kinetics of dye was fitted into a first order
model [Eqn. (1)] with rate constant of k
1
and the values with their
Fig. 3. (A) SEM image of IONPs, (B) EDS spectrum of IONPs.
Fig. 4. Antibacterial activity of IONPs synthesised using the leaf extract of Cynometra ramiflora against E. coli and S. epidermidis.
S. Groiss et al. / Journal of Molecular Structure 1128 (2017) 572e578 575

R
2
are shown in Table 1. The first order rate constants increase from
0.0408 min
1
at 0.28 mM IONPs to 0.3813 min
1
at 1.11 mM.
lnhC
o
.C
f
i¼k
1
t (1)
The linear relationship of k
1
with the increase in the concen-
tration of IONPs can be observed from the (Fig. 5B).
3.7. Mechanism of degradation of dye by IONPs
As discussed in the earlier section, the oxidation pathway by
H
2
O
2
has been accelerated by the presence of IONPs. This kind of
process is known as Fenton-like catalytic process in which IONPs
(iron oxides, iron oxohydroxide and zero valent iron) act as a source
for ferrous ions and serve the role of catalyst [36]. In contrast to this,
the dissolved ferrous ions act as catalysts in traditional Fenton
processes [37]. The major disadvantage of traditional Fenton pro-
cess is the production of large amount of sludge which require
further treatment. But in the Fenton-like process which employs
IONPs, this sludge formation can be avoided [38].
As observed in the XRD spectrum results, the nanoparticles
Fig. 5. (A) Degradation of Rhodamine B dye over time with H
2
O
2
catalysed by IONPs, (B) Linear plot of degradation of Rhodamine B dye over time with H
2
O
2
catalysed by IONPs.
Table 1
First order rate constant of degradation of Rhodamine B dye with H
2
O
2
catalysed by
IONPs.
Concentration of IONPs (mM) First order rate constant k1 (min
1
)R
2
0.28 0.0408 0.9968
0.56 0.1128 0.963
0.83 0.1899 0.9674
1.11 0.3813 0.9944
Fig. 6. (A) Degradation mechanism of Rhodamine B dye with H
2
O
2
catalysed by IONPs, (B) UV-VIS spectra of degradation of Rhodamine B dye over time with 2% H
2
O
2
and 1.1 mM
IONPs.
S. Groiss et al. / Journal of Molecular Structure 1128 (2017) 572e578576

contain various iron oxides. The interaction between these oxides
and the oxidant are outlined as follows [39]: A series of reactions
(Fig. 6A) are first initiated by the generation of hydroxyl radical
(

OH) from the reaction of Fe
2þ
(from IONPs) and H
2
O
2
with sub-
sequent formation of Fe
3þ
. The second reaction is the adsorption of
H
2
O
2
on the surface of the oxidised product Fe
3þ
to form the in-
termediate, Fe
3þ
H
2
O
2
. This is further reduced to Fe
2þ
and forms
perhydroxyl radical (

HO2) and ends the cycle. Finally the radicals
(

OH and

HO
2
) generated in reactions 1 and 3 react with the dye
molecules and degrades the dye (reaction 4). Similar kind of
mechanism has been explained by Xue et al. [40] for the Fenton-like
oxidation of RhB using iron oxides.
The visual and spectral features of RhB degradation in the
presence of 2% H
2
O
2
and 1.1 mM IONPs with respect to time is
shown in Fig. 6B. As shown in the figure, the absorption spectrum of
RhB dye is mainly characterized by a visible band at 555 nm. As the
reaction proceeds, this characteristic peak gradually decreases and
completely vanishes after the 10th minute, indicating the complete
degradation of dye.
4. Conclusions
In this research work, we have synthesised the IONPs by using
the leaf extract of Cynometra ramiflora for the first time. The UV-VIS
spectroscopy and FTIR studies revealed the role of polyphenols of
the leaf extract in the formation and stability of IONPs. SEM, EDS
and XRD techniques were used to further characterize the molec-
ular structure of IONPs. The synthesised IONPs have potential
antibacterial effect which may lead to the formulation of new
antibacterial drugs to treat various bacterial infections. Further-
more, the catalytic activity of IONPs to degrade a pollutant dye,
Rhodamine B was successfully studied. This outcome may find
applications in wastewater treatment to reduce environmental
pollution problems.
Acknowledgments
Silvia Groiss is thankful to IAESTE (International Association for
the Exchange of Students for Technical Experience) at Manipal
University for being selected as an intern to work in this project. All
the authors thank Department of Biotechnology, MIT, Manipal
University for providing the facilities to carry out the research
work.
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