A rapid, facile and green synthesis of Ag@AgCl nanoparticles for the effective reduction of 2,4-dinitrophenyl hydrazine

Article abstract of DOI:10.1039/c5nj02367j

In this study, we have developed a green and single-step process for the fabrication of Ag@AgCl nanoparticles (NPs) using the leaf extract of Momordica charantia. This method does not require the utilization of any surfactant/halide source or external ene

Full text of DOI:10.1039/c5nj02367j

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A rapid, facile and green synthesis of Ag@AgCl nanoparticles for the effective reduction of
, 4ꢀdinitrophenyl hydrazine
2
Thengujam Babita Devi, Md. Ahmaruzzaman* Shamima Begum
National Institute of Technology Silchar, Pinꢀ788010, Assam, India
Abstract
In this study, we have developed a green and single step process for the fabrication of Ag@AgCl
nanoparticles (NPs) using leaf extract of Momordica charantia. This method does not require the utilization
of any surfactant/halide source or external energy. The uniqueness of the developed method lies in its fast
synthesis rates (1 min for Ag@AgCl NPs).The phytochemicals present in the leaves extract act as a reducing
as well as stabilizing agent and is responsible for the formation of Ag@AgCl NPs. The synthesized
Ag@AgCl NPs exhibited a catalytic activity for the reduction of 2, 4ꢀdinitrophenyl hydrazine (DNPH). It
followed the pseudoꢀfirst order reaction and the rate constant (k) for the reduction of 2, 4 DNPH was found to
ꢀ2
ꢀ1
be 5.2×10 min . For the first time, the reduction of 2,4ꢀdinitrophenyl hydrazine was reported using
Ag@AgCl nanoparticles as a catalyst. The photo catalytic activity of the asꢀprepared Ag@AgCl nanoparticles
remains unchanged after 4ꢀ5 cycles of operation
Keywords: Ag@AgCl, nanoparticles, green, reduction, 2,4ꢀdinitrophenyl hydrazine.
*
corresponding author: M. Ahmaruzzaman (mda2002@gmail.com) phone No: +91ꢀ3842ꢀ242915
1
. Introduction
Noble metal nanoparticles (NPs) have shown remarkable prospective for applications in catalysis, electronic,
quantum confinement effect, chemical, biological, biomedical issues (anticancer, anti diabetic, antimicrobial,
anti fungal, antibacterial effects) and wastewater treatment [1]. Wetꢀchemical process has been extensively
utilized for the synthesis of metal NPs. However, the utilization of hazardous and other harmful organic
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reagents in the chemical synthesis increased the toxicity levels, pollutes the environment and raising concerns
regarding the impact of chemically grown metal NPs on living organisms [2]. Therefore, there is a need for
the development of environmentally benign methods for the preparation of metal NPs. The utilization of green
and clean technology is expected to minimize the pollution in the environment. Therefore, plant and waste
derived materials have been exploited for the green fabrication of various NPs [3ꢀ11]. They are easily
obtained, inexpensive and environment friendly and renewable in nature. One of the major advantages in
biological synthesis of metal NPs is the presence of phytoꢀcompounds in plants. These phytoꢀcompounds act
as reducing and stabilizing agents in the synthesis of metal NPs and added advantages over routine chemical
and physical methods. However, a major drawback in biosynthetic procedures compared to the chemical
methods is the longer reaction time required for the synthesis of metal nanoparticles.
Recently, much attention has been paid to the plasmonic photo catalysts (Ag/AgX, Ag/TiO , Au/ZrO and Au/
2
2
SiO ) which combine the Plasmon resonance of noble metal nanoparticles (such as silver and gold) and its
2
immobilization to improve the quality of metal nanoparticles [12ꢀ21]. It is reported that AgX alone does not
have the ability to act as a catalyst in visible region of the light. Therefore, Ag metal is doped on the surface
of AgCl to increase the catalytic activity and may be an excellent catalyst for the removal of toxic organic
compounds from wastewater.
From the literature, it is also revealed that bioꢀsynthesis of Ag/AgX NPs possessed several limitations, such as
multistep synthesis, using toxic solvent, external halide source and a longer reaction time [22ꢀ24]. In order to
overcome the above shortcomings, in the present work, we design a novel and totally green method for the
synthesis of stable colloidal Ag@AgCl NPs using Momordica Charantia leaves extract. In the present work,
Ag@AgCl NPs was synthesized within 1 min in a single step, without using any reducing and external halide
+
0
source. The phytoꢀchemical present in the leaves extract helps in the reduction of Ag ion to Ag species. The
presence of chloride ion in the Momordica Charantia leaves was confirmed by the Mohr’s method. The
2

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prepared NPs were well characterized by XRD, TEM, HRꢀTEM, SAED, FTꢀIR, SEMꢀEDX and UV−visible
spectroscopy. The synthesized Ag@AgCl NPs were spherical in nature with an average diameter of 15 nm.
The catalytic activity of the asꢀprepared Ag@AgCl NPs was explored for the reduction of 2, 4ꢀdinitrophenyl
hydrazine into 2,4ꢀdiaminophenyl hydrazine. The probable mechanisms for the formation of Ag@AgCl NPs
and their catalytic properties were also investigated.
2
. EXPERIMENTAL METHODS
.1. Materials
2
All the reagent, such as silver nitrate, sodium borohydride and 2,4ꢀdinitro phenyl hydrazine were
obtained from Merck India Limited and used without further purification. All the chemicals utilized
were of analytical grade. All solutions were prepared in doubleꢀdistilled water. All apparatus were
rinsed with aqua regia (3:1 solution of HCl: HNO ) and then washed with doubleꢀdistilled water
3
before use.
2
.2. Preparation of Momordica Charantia leaves Extract
A total of 5 g of fresh leaves of Momordica charantia were taken and washed thoroughly with distilled
water to remove dust particles. These leaves were then cut into small pieces and boiled for 20 min with
3
4
2
00 mL of distilled water in a roundꢀbottom flask. The leaves extract was filtered using whatman No.
0
1 filter paper to obtain the pure leaves extract. The extract was stored at 4 C for further experiments.
.3. Biosynthesis of Ag@AgCl nanoparticles
A total of 15 ml of 1.0 mM of aqueous solution of silver nitrate was taken in a container and 15 ml of
Momordica charantia leaves extract was added. The green color of leaves extract was changed into a
brown color [Figure 1(a) (b)] within 1min which indicates the formation of colloidal Ag@AgCl NPs
(Scheme 1). The colloidal solution of Ag@AgCl NPs was then allowed to stabilize for 1 day. After
1
day, the solutions with brown color sediments were formed at the bottom of the container and the
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NPs were separated by centrifugation, washed with double deionized water several times to remove
any unbound polymers, unwanted materials and organic matter from the Ag@AgCl nanoparticles.
The prepared NPs were then dried in a desiccator for 3 days.
Scheme1. Mechanism for the formation of Ag @AgCl NPs
Figure 1: Momordica charantia leaves extract (A).Colloidal form of synthesized Ag@AgCl
NPs (B).
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2.4. Detection of chloride ion by precipitation titration with silver nitrate (Mohr’s method)
A total of 5 ml of Momordica charantia leaves extract was taken in a test tube. The homogenate
is centrifuged for 15 min and the supernatant filtered through qualitative filter paper. An aliquot
−
of filtrate is used for the determination of Cl by precipitation titration with silver nitrate by
Mohr’s method. When the AgNO solution is slowly added to the leaves extract, a precipitation
3
of silver chloride is formed. The end point of the titration occurs when all the chloride ions are
precipitated. Then additional silver ions react with the chromate ions of the indicator, potassium
chromate, to form a red brown precipitation of the silver chromate. The percentage of chloride
ion in the Momordica charantia leaves extract was found to be 30+5 [25]. The presence of
chloride ion in the Momordica charantia leaves extract was further confirmed by EDX and XRD
analyses.
2.5. Spectroscopic Characterization (UV, FTꢀIR, SEMꢀEDX, XRD, TEM, SAED, HRꢀTEM,
LCꢀMS)
The UV–Vis absorption spectra of the synthesized nanoparticles were recorded on Cary 100 Bio
spectrophotometer (k max in nm) equipped with 1ꢀcm quartz cell. The FTIR spectra were
measured using Bruker Hyperion 3000 FTIR spectrometer, using thin transparent KBr pellets to
characterize the functional groups of synthesized NPs. The elemental analysis of the synthesized
nanoparticles was performed using EDX on the SEM (FEGꢀSEM, Model: JSMꢀ7600F,
Magnification: x 25 to 1,000,000). The Powder Xꢀray diffraction (XRD) patterns were carried
out on a Xꢀray Diffractoꢀmeter (Model: Philips X’ pert MPD system) with X’pert software using
30 mA, 40 kV, Cu K radiation (k=1.546 Å). The size, morphology and diffracted ring pattern of
Ag@AgCl nanoparticles were determined by JEOLꢀJEM 2100, operated at an accelerating
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voltage of 200 kV. The Liquid chromatography mass spectrometry (LCꢀMS) was carried out
using HRꢀLCMS, Model: 1290 infinity UHPLC machine.
2
.6. Catalytic activity of synthesized Ag@AgCl nanoparticles
The conversion of 2,4ꢀdinitrophenyl hydrazine (DNPH) to 2,4ꢀdiamino phenyl hydrazine
DAPH) was carried out using sodium borohydride (NaBH ) in aqueous medium at room
(
4
temperature in the presence of Ag@AgCl nanoparticles. The catalytic reduction of DNPH was
carried out in a standard quartz cuvette with 1 cm path length. A total of 2 ml of distilled water
ꢀ3
was taken in the quartz cuvette and 60µl of (6.0 ×10 M) DNPH was added and the absorbance
was recorded using the UV spectrometer. After that a solution of 350 µl of (0.1 M) NaBH was
4
added to the DNPH solution present in the cuvette and again run the UV spectrometer and
absorbance was recorded. Now, 150 µl of (0.008 g) Ag@AgCl NPs solution was added to the
mixture and record the absorbance till the peak due to the presence of nitro group disappeared
completely. The catalytic performance of Ag@AgCl nanoparticles showed that the catalytic
property was not changed after 4ꢀ5 cycle of reduction of DNPH.
3. Results and discussion
3.1. UV visible spectroscopy
Figure 2(a) showed the UVꢀvisible spectrum of the asꢀprepared Ag@AgCl nanoparticles. The
complete reduction of silver ions occurs within 1 min, as evidenced by the most intense SPR
peak occurring at that time. We have checked the stability of these synthesized nanoparticles
after one month, and found that the optical properties of Ag@AgCl NPs showed the same
plasmonic absorption in visible region at 450 nm. This absorption may be due to the presence of
silver ions on the AgCl surface [9ꢀ10].
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3.2. XꢀRay Diffraction (XRD) Studies
Figure 2(b) shows the Xꢀray diffraction (XRD) pattern of the synthesized Ag@AgCl
nanoparticles. These pattern is compatible with the cubic phase of Ag with diffractions points at
2θ values of 38.2°, 44°, 64.6°and 77.5° and can be indexed to the (111), (200), (220), and (311)
planes of the face centered cubic (FCC) structure, respectively (JCPDS file: 65ꢀ2871). The XRD
pattern also showed the presence of the cubic phase of AgCl at 2θ values of 27.9°, 32.3°, 46.3°,
55.0° and 57.6° corresponds to the (111), (200), (220), (311) and (222) planes, respectively
(JCPDS file: 31ꢀ1238).
The average crystallite size of the synthesized NPs was determined using the Debye−Scherrer
equation
Dv = Kλ/β cos θ……………………………………………. (1)
where Dv is the average crystalline size, K is a dimensionless shape factor, with a value close to
unity (0.9); λ is the wavelength of Cu Kα, β is the full width at halfꢀmaximum of the diffraction
peaks, and θ is the Bragg’s angle. No other characteristic peaks were found in the XRD spectra
which indicated the high purity of the asꢀprepared Ag@AgCl nanoparticles. The intense and
narrow diffraction peaks revealed the crystalline nature of the synthesized nanoparticles [2]. To
calculate the average crystalline particle size of the synthesized Ag@AgCl nanoparticles, we
preferred the most intense peaks of Ag and AgCl. We have selected (111) and (200) lattice
planes of Ag and AgCl to calculate the average crystalline particle size of Ag@AgCl NPs. The
calculated average crystalline particle size of Ag@AgCl NPs was found to be 20.5 nm and 16
nm for Ag and AgCl, respectively.
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(A)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
400
500
600
700
800
Wavelength(nm)
Figure 2: UVꢀspectra of synthesized Ag@AgCl NPs (A). XRD spectra of the synthesized
Ag@AgCl NPs (B).
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3.3. Transmission electron microscopy, HRꢀTEM and SAED pattern
Figure 3(a) and (b) represents the TEM and HRTEM images of Ag@AgCl NPs, and indicates
the morphology and size distribution of the synthesized NPs. Figure 3(a) clearly indicates that
the synthesized NPs are spherical in shape and uniform in size with an average particle sizes of
15 nm. The image also exhibits that the Ag@AgCl NPs are well dispersed and not aggregated.
The lattice spacing of the synthesized NPs were calculated from the HRTEM image [Figure3 (b)]
and found to be 0.33nm, and 0.28nm, and corresponds to the (111), (200) lattice of face centered
cubic (FCC) structure of AgCl. The HRTEM image also showed the lattice spacing of 0.12nm,
and 0.11nm, and corresponds to the (311) and (222) lattice structure (FCC) of Ag [23]. Figure
3(c) presents the SAED pattern of the synthesized Ag@AgCl NPs. The concentric diffraction
rings of SAED pattern showed that the synthesized NPs are polycrystalline in nature and also
supports the unit cell structure of Ag and AgCl [16].
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Figure3 TEM image of the synthesized Ag@AgCl NPs (A) HRTEM image of the
synthesized Ag@AgCl NPs (B) SAED pattern of the synthesized Ag@AgCl NPs (C).
3.4. FTIR spectroscopy
Figure 4(a, b) represents the FTIR spectra of Momordica charantia leaf extract and synthesized
Ag@AgCl NPs. The FTIR analysis was carried out to identify the stabilizing agent and
functional groups present on the nanoparticles synthesized by leaf extract of Momordica
charantia. The FTIR spectra showed that the phytoꢀchemicals present in the leaf extract were
+
0
responsible for the reduction of silver ions (Ag ) into silver nanoparticles (Ag ). The FTIR
ꢀ1
spectrum of leaf extract showed absorption bands at 3465, 1636 and 669 cm . The appearance of
−
1
an intense absorption band around 3465 cm corresponds to hydrogen bonded OꢀH stretching
vibrations of alcohols and phenols and also due to the presence of amines NꢀH group in leaf
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−1
extract [26ꢀ29]. The peaks at 1636 and 669 cm were attributed to >C=O stretching and N–H
wagging or out of plane O–H bending vibrations, respectively.
The FTIR spectrum of synthesized Ag@AgCl NPs showed prominent peaks at 3446, 1634 and
ꢀ1
1
380 cm due to the presence of OꢀH, C=O, CꢀN stretching. In particular, the shift of peak from
−
1
3
465 to 3446 cm may also indicate the involvement of OꢀH functional group in the reduction
+
of Ag ions and formation of Ag@AgCl NPs [8, 26ꢀ29]. The FTIR spectra also indicated that
+
0
the phenolic group in methyl gallate reduced the Ag ion to Ag and itself get oxidized to form
quinone [29]. The presence of methyl gallate in the Momordica charantia leaves and the
formation of quinone were further confirmed by LCꢀMS spectroscopy. The absorption bands
appeared in the FTIR spectrum of the leaf extract revealed that phyto constituents also protected
the asꢀprepared Ag@AgCl NPs from aggregation.
(a)
leaf extract
60
50
669
40
30
1636
20
10
3465
0
4000
3500
3000
2500
2000
1500
1000
500
-1
wave numbers(cm )
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(b)
Ag/AgCl nanoparticles
100
1380
90
1634
80
3446
70
3500
3000
2500
2000
1500
1000
500
-1
Wave numbers(cm )
Figure 4: FTIR spectra of the leaves extract (a) and synthesized Ag@AgCl NPs (b).
.5. Energy dispersive Xꢀray spectroscopy
3
Figure 4(c) presents the SEM/EDX spectrum of the synthesized NPs. The elemental analysis of
the Ag@AgCl nanoparticles was performed using EDX on the SEM. The freezeꢀdried
nanoparticles were mounted on specimen stubs with doubleꢀsided taps, coated with gold in a
sputter coater and examined under FEGꢀSEM, and then the EDX spectrum of spherical NPs was
recorded. The spectrum showed the presence of two peaks at around 3 keV and 2.7 keV and
corresponds to the binding energies of Ag and Cl. The above findings confirmed that Ag and Cl
are the major elements found in the composition of Ag@AgCl nanoparticles [30]. The energy
dispersive Xꢀray analysis (EDX) reveals the strong signal for the silver and chlorine region and
confirmed the formation of Ag@AgCl nanoparticles.
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Figure 4(c) EDX spectrum of synthesized Ag@AgCl NPs.
3.6. Liquid Chromatography and mass spectroscopy (LCꢀMS spectroscopy)
Figure 5(a) and (b) represents the LCꢀMS spectra of Momordica charantia leaf extract and
residual (aqueous) extract obtained after the formation of Ag@AgCl nanoparticles. Triterpenes,
proteins, steroids, alkaloids and other phenolic compounds are the major phytoꢀchemicals
presents in the Momordica charantia leaves. The LCꢀMS spectra of leaves extract showed
fragmentation of the phytochemicals present in the Momordica charantia leaf (Figure 5a). The
presence of phytochemicals and their fragmentation are summarized in Table 1. The main
intense peaks that were identified in the LCꢀMS spectra of Momordica charantia leaves were at
89, 102, 176, 184, 219, 291, 349, 376, 381, and 416 (m/z value). The appearance of these peaks
indicated the presence of alanine (C H NO ), butyric acid gammaꢀamino (C H NO ), ascorbic
3
7
2
4
9
2
acid (C H O ), methyl gallate (C H O ), pantothenic acid (C H NO ), catechin (C H O ),
6
8
6
8
8
5
9
17
5
15 14
6
Zeatin ribose (C H N O ), Riboflavin (C H N O ), somalenone (C H O ), and sligmastanol
1
5
21
5
5
17 20
4
6
26 40
2
(
C H O), respectively.
29 48
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Table 1: LCꢀMS spectra study of Momordica charantia leaves and aqueous residual part after
formation of Ag@AgCl NPs
Momordica charantia leaves extract
Compound name
Aqueous residual part after formation of Ag@AgCl NPs
Molecular Formula Fragmentation
1
2
3
4
5
6
7
8
9
1
.Alanine
C H NO
Peak remain same
Peak remain same
Peaks at 61 and 115
Peak at 182
ꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀ
Peaks at 31, 88, 131 and 241
ꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀ
3
7
2
.Butyric acid gammaꢀ C H NO
4
9
2
. Ascorbic acid
. Methyl gallate
. Pantothenic acid
. Catechin
. Zeatin ribose
. Riboflavin
C H O
6
8
6
C H O
8
8
5
C H NO
5
9
17
C H O
6
15
14
C H N O
15
21
5
5
C H N O
17
20
4
6
. Somalenone
0. Sligmastanol
C H O
2
6
40
2
C H O
29
48
Figure 5b showed the LCꢀMS spectra of residual (aqueous) part obtained after the synthesis of
NPs. The peaks at 89 (alanine) and 102 (butyric acid gammaꢀamino) remain unchanged and
+
0
indicated that these phytoꢀchemicals is not taking part in the reduction of Ag to Ag . The peaks
at 31, 88, 131, 135, and 241 are fragmentation of Riboflavin, and peaks at 61 and 115 are
fragmentation of ascorbic acid. The most intense peak at ~182 (m/z values) corresponds to the
quinone compound (C H O ). Here, methyl gallate was one of the major constituent present in
8
6
5
Momordica charantia leaf [29]. In the FTꢀIR spectra, the appearance of an intense peak at 3465
ꢀ1
cm corresponds to the OH stretching vibrations of phenolic compounds, such as methyl gallate.
The peak at 184 (m/z) due to the presence of methyl gallate (Figure 5a) was disappeared and
appearance of a new peak at ~182 (m/z) (Figure 5b) revealed that phenolic ꢀOH groups present
+
0
in methyl gallate undergo oxidation and form quinone with consequent reduction of Ag to Ag .
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Therefore, the above discussion clearly showed that the methyl gallate is responsible for the
formation of Ag@AgCl NPs [29].
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Figure 5: LCꢀMS spectra of Momordica charantia leaf extract (a) LCꢀMS spectra of
aqueous residual part after formation of Ag@AgCl NPs (b).
3.7. Mechanistic aspects on biogenic synthesis of Ag@AgCl nanoparticles.
The aqueous solution of AgNO is not stable and can be easily decomposed either by heating or
3
by UV light illumination. Under such conditions, Ag (0) forms as a sole product by releasing
oxygen and nitrogen dioxide in the absence of stabilizing agent [31]. Thus, the presence of
stabilizing agent was very much essential for the synthesis of Ag@AgCl NPs. Here, the leaf
extract of Momordica charantia acts as reducing and stabilizing agent in the synthesis of
Ag@AgCl NPs.
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The major phytoꢀchemicals present in Momordica charantia leaf were triterpenes, proteins,
steroids, alkaloids and other phenolic compounds [11, 32ꢀ34]. In this work, the possible
mechanism for the synthesis of Ag@AgCl NPs was explained by the chloride ion and phenolic
groups (OH), such as methyl gallate present in the Momordica charantia leaf extract [35ꢀ36].
This phytoꢀchemical was a strong antioxidant along with high reducing capacity and facilitated
the reduction of silver ion to silver NPs. The above reduction takes place due to the electron
donating ability of these phenolic compounds.
+
Scheme 2 represents the possible mechanism for the formation of AgCl and reduction of Ag to
0
Ag . In the first step, AgCl is formed by the combination of the silver ion from the metal
precursor and the chloride ion from the Momordica charantia leaf extract. Once the solution
containing AgCl was formed at room temperature, these silver ions are reduced to metallic silver
+
by the reducing agent, methyl gallate present in the leaves extract. The Ag ions then formed an
intermediate complex with OH groups present in the methyl gallate. This then undergoes
+
0
oxidation and formed quinone, and subsequently reduced Ag to Ag NPs [29]. In this scheme,
the quinone compound was produced because of the oxidation of the phenolic OH group in
methyl gallate and adsorbed on the surface of nanoparticles, accounting for their suspension
0
stabilization. Hence, the Ag /AgCl NPs were formed and illustrated in scheme 2 as shown below
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Step 1
Step 2
AgNO3
Ag +
NO3
AgCl
Ag
+
cl
OCH3
Step 3
OCH3
OAg
OH
O
2
Ag
+
O
ꢀ
2H+
OAg
OH
OH
OH
Methyl Gallate(Phenolic form)
Step4
O
OAg
O
OAg
OH
+
2Ag0
OCH3
O
OCH3
O
OH
(
Quinone Form)
Scheme2. Schematic illustrated the mechanism for the formation of Ag/AgCl NPs.
.8. Catalytic study for the reduction of 2,4ꢀdinitro phenyl hydrazine to 2,4ꢀdiamino phenyl
3
hydrazine using Ag@AgCl nanoparticles
The catalytic efficiency of Ag@AgCl nanoparticles was investigated by carrying out the
reduction of 2,4ꢀdinitro phenyl hydrazine (DNPH) in aqueous medium using NaBH . The UV–
4
visible spectra (Figure 6a) of 2,4ꢀdinitrophenyl hydrazine in aqueous medium showed a
maximum absorbance at 350 nm [37]. No change in absorption spectrum was observed on
addition of freshly prepared aqueous solution of sodium borohydride (0.1M) in 2,4ꢀDNPH
solution. The absorption spectra did not change even keeping the solution for one day. This
indicates that the reduction of 2, 4ꢀDNPH by sodium borohydride is kinetically not favorable,
although the reduction reaction being thermodynamically favorable. When the Ag@AgCl
solution was added, the reaction proceeds very fast. A new peak at 300 nm due to the formation
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of reduction product (amino group) developed immediately [25, 38] as soon as Ag@AgCl NPs
solution was added and the peak due to the 2, 4ꢀDNPH at 350 nm was disappeared. The
complete elimination of absorbance peak at 350 nm and subsequent emergence of peak at 300
nm within a few min indicated the catalytic activity of the Ag@AgCl nanoparticles. The
absorbance band at 350 nm was completely disappeared within 60 min which shows a good
catalytic property of the synthesized Ag@AgCl NPs.
The yellow color of 2,4ꢀDNPH solution was slowly faded after the addition of Ag@AgCl NPs
solution. This decolonization was monitored by UV–visible spectroscopy at a regular interval of
time. Figure 6b shows the UV–visible spectra for the reduction of 2,4ꢀDNPH using NaBH in
4
presence of Ag@AgCl nanoparticles as a catalyst. From Figure 6b, it was evident that the
intensity of the new peak at 300 nm gradually increases with increase in time because of the
reduction of 2,4ꢀDNPH to 2,4ꢀdiamino phenyl hydrazine (DAPH). Hence, it is found that 2,4ꢀ
DNPH can be completely reduced into 2,4ꢀDAPH using Ag@AgCl nanoparticles as a catalyst in
the presence of NaBH . About 98.5% of 2,4ꢀDNPH was catalytically reduced within 60 min
4
using Ag@AgCl nanoparticles.
According to Beer–Lambert’s law, the absorbance of 2, 4ꢀDNPH solution is directly proportional
to the concentration (c)
A =εbc………………………………….. (2)
where ε (the molar absorption coefficient) and b (the thickness of the absorption cell). Molar
absorption coefficient (ε) and thickness of the cell (b) are constant. The changes in the
concentration of 2,4 DNPH with time can be detected using UV–visible spectrometer. Figure 7
revealed that the reduction of 2,4ꢀDNPH followed the pseudoꢀfirst order reaction and its kinetics
may be expressed as follows [39]:
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ln (C /C) = kt ……………………………………(3)
0
where k is the rate constant, C and C are the absorbance or concentration before and after
0
reduction of 2,4ꢀDNPH, respectively. The rate constant can be calculated using Eq. (3). Figure 7
represents the plot of ln (C /C) versus time (t) and showed a linear relationship. The slope of the
0
linear line represented the rate constant (k) for the reduction of 2,4 DNPH and was found to be
ꢀ2
ꢀ1
5
.2×10 min .
(A)
1
0
0
.0
.5
.0
300
400
wave number (nm)
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(
B)
0 min.
1
2
0 min.
0 min.
1.5
1.0
0.5
0.0
3
0 min.
4
0 min.
2
,4ꢀDNPH
5
6
0 min.
0 min.
2,4ꢀDAPH
300
400
500
wave lenght(nm)
Figure 6 present the UVꢀvisible spectra of 2,4ꢀDNPH (A) UV–Vis absorption spectra
ꢀ3
recorded of 60 µl (6.07 × 10 M) 2,4ꢀDNPH + 350 µl (0.1 M) NaBH + 150 µl (.008g)
4
Ag@AgCl NPs in aqueous solution (B)
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ln C /Ct
0
linear fit of sheet lnC /Ct
0
0.55
0.50
0.45
0.40
0.35
0.30
0.25
10
20
30
40
50
60
Time(min)
Figure 7: Plot of ln (C /C ) against time for pseudoꢀfirstꢀorder reduction kinetics of 2,4ꢀ
t
0
DNPH in the presence of excess NaBH (0.1 M) in aqueous solutions
4
3.9. Mechanism of reduction of 2,4ꢀDNPH to 2,4ꢀDAPH by Ag@AgCl nanoparticles in
presence of NaBH4
This mechanism of reduction involves the surface adsorption of the reducing agent and the 2,4ꢀ
DNPH substrate on the catalyst. For the reduction of 2,4ꢀDNPH, electron is required to be
ꢀ
transferred. The borohydride anion (BH ) acts as a donor. The Ag@AgCl NPs are added to the
4
ꢀ
2
,4ꢀDNPH solution in presence of NaBH . The BH ions get adsorbed on the surface of NPs
4
4
and transfer hydrogen to the substrate for the reduction. As a result, 2,4ꢀDAPH is desorbed
from the surface of NPs and the catalyst, Ag@AgCl NP is regenerated, and can be used again.
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The probable mechanism for the reduction of 2,4ꢀDNPH to 2,4ꢀDAPH was illustrated in scheme
3) [40].
+
H ion adsorbed on the catalyst
(
Nitroarene capture H+ upon collision)
BH4
Catalyst
Catalyst
(Reduction of nitro arene to amino arene)
NH
2
NH2
NH
NH
NH2
NO
2
Ag@AgCl NPs
+
NaBH4
NH2
NO2
Scheme 3: Mechanism for the reduction of 2,4ꢀDNPH to2, 4ꢀDAPH using Ag@AgCl nanoparticles as a
catalyst in presence of NaBH4 in aqueous medium.
4. Conclusions
In summary, the current study revealed that Ag@AgCl nanoparticles can be synthesized at a low
temperature, and solvent free (reducing and stabilizing agent) condition using Momordica
charantia leaves extract. The developed method is a single step, eco friendly and cost effective.
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The advantages of this method were that Ag@AgCl nanoparticles were formed at a faster rate
than any biological green chemistry methods reported (~1min). The LCꢀMS and FIꢀTR analyses
revealed that the phytoꢀchemicals present in the leaf extract (methyl gallate) acts as reducing and
stabilizing agent in the synthesis of Ag@AgCl NPs. The TEM image showed that the
synthesized NPs were spherical in shape with an average diameter of 15 nm. The UVꢀvisible,
HRTEM, SAED pattern, XRD, and EDX spectroscopic analyses confirmed the formation of
Ag@AgCl NPs. For the first time, the catalytic activity of Ag@AgCl NPs was reported for the
reduction of 2,4ꢀDNPH into 2, 4ꢀDAPH. The complete reduction takes place within 60 min.
Acknowledgement
The authors are grateful to TEQIPꢀII of NIT Silchar for providing the financial assistance and
SAIF–NEHU Shillong, CSMCRI Gujarat and SAIFꢀIIT Bombay for providing the TEM, XRD,
LCꢀMS and EDX facilities, respectively.
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LIST OF FIGURES:
1
2
. Figure.1 (a) Momordica charantia leaves extract, (b) Colloidal formation of Ag@AgCl NPs
. Figure. 2(a) UV–Vis absorption spectra of synthesized Ag@AgCl NPs
(b) XRD spectra of the synthesize Ag@AgCL NPs
3. Figure. 3 (A) TEM images of Ag@AgCl NPs, (B) HRTEM images of Ag@AgCl NPs, (C)
SAED pattern of Ag@AgCl NPs
. Figure. 4 FTIR spectra of Momordica charantia leaves extract (a) Synthesized Ag@AgCl
NPs (b) EDX spectra of Ag@AgCl NPs(c).
. Figure. 5 (a) LCꢀMS spectra of Momordica charantia leaves (b) LCꢀMS spectra
of residual part after formation of Ag@AgCl NPs.
. Figure 6 (a) UV – visible spectra of 2,4ꢀDNPH (b) UV–Vis absorption spectra recorded of 60
4
5
6
ꢀ3
µl (6.07 × 10 M) 2,4ꢀDNPH + 350 µl (0.1 M) NaBH + 150 µl (.008g) Ag@AgCl NPs in
4
aqueous solution.
7
. Figure 7 Plot of ln (C /C ) against time for pseudoꢀfirstꢀorder reduction kinetics of 2,4ꢀDNPH
t 0
in the presence of excess NaBH (0.1 M) in aqueous solutions.
4
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O
At 70 c for 20min
AgNO3
Ag@AgCl Nps
Leaf extract
Momordica charantia leaf
CatalyticActivity
NH2
NH
2
NH
NH
NH2
NO
2
Ag@AgCl NPs
+
NaBH
4
NH2
NO2