RES E A R C H A R T I C L E Open Access
A novel green synthesis and
characterization of silver nanoparticles
using gum tragacanth and evaluation of
their potential catalytic reduction activities
with methylene blue and Congo red dyes
Murali Krishna Indana, Bhagavanth Reddy Gangapuram, Ramakrishna Dadigala, Rajkumar Bandi and
Veerabhadram Guttena
*
Abstract
Background: A facile and eco-friendly method for green synthesis of silver nanoparticles (AgNPs) has been
developed using gum tragacanth (GT) (Astragalus gummifer), an abundantly available natural phyto-exudate in
India, employing a novel method of ultrasonication process.
Methods: Silver nanoparticles were prepared by the reduction of silver nitrate solution by the aqueous extract of
gum tragacanth by ultrasonication method at 45 °C for about 45 min. The aqueous extract of the gum acts as a
reducing as well as stabilizing agent.
Results: The resultant AgNPs were characterized by ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform
infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. The
influence of gum concentration and silver precursor concentration on the synthesis of AgNPs was studied. The role
and effectiveness of nanoparticles in the catalytic reduction of a cationic dye methylene blue (MB), and an anionic
azo dye Congo red (CR), were also studied. FTIR analysis revealed that –OH groups present in the gum matrix
might be responsible for the reduction of Ag+ into AgNPs. The X-ray diffraction studies indicated that the resulting
AgNPs were highly crystalline with face-centered cubic geometry. TEM studies showed that the average particle
size of the synthesized AgNPs was 18 ± 2 nm.
Conclusions: The study highlights the green synthesis of GT-capped AgNPs and the rapid reduction of carcinogenic
and toxic contaminants such as MB and CR with the help of GT-capped AgNPs in an eco-friendly manner.
Keywords: Gum tragacanth, Silver nanoparticles, Ultra-sonication, Catalytic reduction, Methylene blue, Congo red
Background
Metal nanomaterials, in recent times, play a crucial role
in a multitude of applications as they were proved to ex-
hibit rema rkable improvements in their electronic, phys-
ical, chemical, optical, and biological properties when
compared to the ir pure metal counterparts (Thakkar et
al. 2010). These colloidal metal nanoparticles possess
unique physical properties owing to their extremely
small size and very high surface/volume ratio (Pandey et al.
2016). The extremely small size of these metal nanoparti-
cles offers a very high reactive surface area that contributes
to their significant improvement in the properties in com-
parison to their metal compatriots (Guo and Wang 2007).
The extremely small s ize of nanoparticles, especially in
the case of silver nanoparticles (AgNPs), finds their util-
ity in chemical applications such as catalysis, reduction,
stabilization, and colorimetric sensors (Murugadoss and
Chattopadhyay 2008; R a stogi et al. 20 14). The biological
applications involving the integration of nanoparticles
* Correspondence: gvbhadram@gmail.com
Department of Chemistry, University College of Science, Osmania University,
Hyderabad, Telangana State 500007, India
Journal of Analytical Science
and Technolog
y
© 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.
Indana et al. Journal of Analytical Science and Technology (2016) 7:19
DOI 10.1186/s40543-016-0098-1
with biological molecules led to the development of
therapeutic medicines, diagnostic tools, antimicrobial
agents , drug delivery systems , bioimaging tools, labeling
agents , therapies in cancer, etc. (Roy et al. 2015, Song
and Kim 2009, Kokila et al. 2015, Daniel and A s truc
2004).
The AgNPs are generally synthesized using various
methods such as chemical, photo chemical, electrochem-
ical, laser ablation, γ-irradiation, and biological conver-
sion (Solomon et al. 2007; Callegari et al. 2003; Pyatenko
et al. 2004; Li et al. 2007; Raju et al. 2014). The most
popular method by which AgNPs are synthesized is by
reducing agents such as sodium borohydride and sodium
citrate. Most of these synthetic methods are hazardous
since they employ toxic chemicals leading to environmen-
tal degradation (Mehndiratta et al. 2013) and biological
hazards. Much effort is being focussed, in recent times, by
researchers to mitigate the impact of these harmful chemi-
cals’ utilization through the integration of “green chemistry
principles” for the synthesis of metal nanomaterials using
plant extracts (Parveen et al. 2016), biosurfactants, etc.
There are many natural biopolymers used for metal nano-
particle synthesis such as gum kondagogu (Rastogi et al.
2014), salmalia malabarica gum (Murali krishna et al. 2015)
guar gum (Pandey et al. 2012, Pandey and Mishra 2011,
Pandey and Mishra 2013, Pandey et al. 2013, Pandey and
Mishra 2014, Pandey and Mishra 2016), chitosan (Wang
et al. 2015), gum ghatti (Kora et al. 2012) and cyclodextrin
(Maciollek and Ritter 2014). These biopolymers act as sta-
bilizing as well as reducing agents. The characteristics of
the metal nanoparticles were further improved when these
metal nanoparticles were reacted with natural gums such
as gum tragacanth (GT) (Raveendran et al. 2003) due to
the increase in the number of active sites, i.e., reactive func-
tional groups on nanoparticles through capping process.
Synthetic dyes are extensively used in a variety of in-
dustries such as textiles, paper, polymers, adhesives, cer-
amics, construction, cosmetics, food, glass, paints, ink,
soap, and pharmaceuticals (Zalikha et al. 2012). Dye resi-
dues form a group of glaring and grave contaminants
since they impart color to the wastewater e ven at a very
low concentration level. The catalytic reduction studies
of these dyes assume greater significance in the present
context as most of these dyes are known to be toxic
when inhaled or ingested orally and pose health hazards
such as skin and eye irritation in humans. The adverse
effects of these synthetic dyes to the environment in-
clude the ability to deplete oxygen in the surface waters
and streams thereby affecting the very sustenance of
aquatic flora and fauna and causing an inhibitory effect
on the photosynthetic activity of plants. The anaerobic
degradation products of azo dyes are amines which are
very toxic, carcinogenic, and mutagenic in nature. The
effluents from industries containing these hazardous
dyes need to be removed bef ore they are let out for on-
land disposal purposes. The treatment technologies such
as adsorption, ion exchange (Karcher et al. 2002), filtra-
tion, electrode deposition, and chemical precipitation are
being widely used to remove the dye contaminants from
water. An attempt has bee n made to alleviate the detri-
mental effects of these chemicals on the surrounding en-
vironment by trying to reduce the dye concentration in
the treated effluents such as methylene blue (MB) and
Congo red (CR) by employing GT-capped AgNPs as a
nanocatalyst. The previous studies on the reduction of
MB and CR by using gold and silver nanoparticles
(Wanyonyi et al. 2014; Ganapuram et al. 2015) as nano-
catalysts were also reported.
GT is a naturally occurring complex, an acidic polysac-
charide obtained from the sap of the genus Astragalus, in-
cluding A. adscendens, A. gummifer, A. brachycalyx,andA.
tragacanth us, a native genus of Middle Eastern countries
and West and South Asia. GT is a highly viscous, odorless,
tasteless, water-soluble mixture of highly branched and het-
erogeneous hydrophilic polysaccharides obtained from the
exudate of roots and bark of the plant which can be dried
andpowdered(KoraandArunachalam 2012). The molecu-
lar weight of a typical gum is reported to be about 8.4 ×
10
5
g/mol. The biopolymer is a mixture o f two poly-
saccharide fractions viz. water-soluble tragacanthin
and water-swellable tragacanthic acid or bassorin. The
tragacanthin comprises about 30–40 % in the gum
and is a neutral, highly branched, type II arabinoga-
lactan comprising (1→6)- and (1→3)-linked core chains
containing galactose and arabinose units and side groups
of (1→2)-, (1→3)-, and (1→5)-linked arabinose units oc-
curring as a monosaccharide or oligosaccharides. Bassorin,
a pectin component, has a chain of (1→4)-linked α-D-
galacturonic acid units, some of which are substituted at
O-3 with β-D-xylopyranosyl units and some of these being
terminated with D-galactose or L-fucose (Tischer et al.
2002; Hassan et al. 2013; Chenlo et al. 2010). GT is also
recognized as a food-grade additive by the Indian Bureau
of Standards under Indian Standard IS 7238: 1997.
Thepresentstudyhasbeentakenuptoexplorethe
innumerable potential possibilities that have been fore-
seen with the green synthesis of AgNPs with GT a s
a reducing, capping , and stabilizing agent. Also, the at-
tributes of GT which prompted us to get into the study
are (i) wi despread natural availability, (ii) nontoxic and
noncarcinogenic nature, (iii) excellent reducing and sta-
bilizing capability, (iv) compli ance with eco-friendly
green synthesis principles, and (v) efficient catalytic re-
duction of MB and CR. The synthesized AgNPs were
characterized for ultraviolet-visible (UV-Vis), F ourier
transform infrared (FT IR) spectroscopy, X-ray diffrac-
tion (XRD), and transmission electron microscopy
(TEM) parameters.
Indana et al. Journal of Analytical Science and Technology (2016) 7:19 Page 2 of 9
Methods
Materials
Silver nitrate (AgNO
3
), NaBH
4
, CR, and MB, all of ana-
lytical grade, were purchased from E. Merck Limited,
Mumbai, India, and GT (99 % purity) was purchased
from Loba Chemie, Mumbai, India.
Synthesis of AgNPs using GT
A 0.5 % (weight/volume (w/v)) homogeneous gum stock
solution was prepared by dissolving 0.5 g of GT in
100 mL of deionized water and stirring overnight at
room temperature. The solution was then centrifuged at
6000 rpm for 10 min to separate out any undissolved
matter, and the supernatant is used for the experiments
to be carrie d out in the study. A 0.5 % (w/v) solution of
AgNO
3
is prepared by dissolving 0.5 g of analytical grade
AgNO
3
in 100 mL of deionized water. Then, 5 mL of
gum extract was mixed with 5 mL of AgNO
3
solution in
a boiling tube, and this mixture was subjected to ultra
sonication at 45 °C temperature for a 45-min reaction
time (Firdhouse et al. 2012). The conversion of the col-
orless reaction mixture to the characteristic clear yellow
color indicates the formation of AgNPs. A series of sam-
ples were prepared by varying the concentration of
AgNO
3
(0.1 to 0.5 %) while keeping the concentration of
GT (0.5 %) constant. Likewise, the second set of samples
were prepared by varying the concentration of GT (0.1
to 0.5 %), keeping the concentration of AgNO
3
constant
(0.5 %) at the same sonication conditions as mentioned
earlier (45 °C for 45 min).
Characterization of silver nanoparticles
The UV-Vis spectral analysis of GT-AgNP solution was
carried out on a dua l-beam UV-Vis spectrophotometer
(Shimadzu-3600, Kyoto, Japan). The FTIR spectra of GT
alone and GTcapped AgNPs were recorded using FTIR
spectrophotometer (model: IRAffinity-1, Shimadzu Cor-
poration, Japan) equipped with attenuated total reflect-
ance (ATR) accessory in the scanning range of 650–
4000 cm
−1
. XRD studies of GT-AgNPs were carried out
on X’pert Pro MRD X-ray diffractometer (PANalytical
BV, The Netherlands) operating at 40 kV and a current
of 30 mA at a scan rate of 0.388 min
−1
. TE M analysis
was conducted for the morphology and size distribution
measurements of the GT-AgNPs using a transmission
electron microscope (model: 1200 EX, JEOL Ltd., Japan)
operated at an accelerating voltage of 200 kV, casting
nanoparticle dispersion on carbon-coated copper grids
and allowing for drying at room temperature. The par-
ticle size distribution was measured by using ImageJ
software analysis of TEM micrographs. Zeta potential
was determined using dynamic light scattering (Malvern
instrument Ltd., Malvern, UK).
Catalytic activity
MB and CR were subjected to reduction using sodium
borohydride in the presence of AgNPs in order to assess
the efficacy of the catalytic activity of the synthesized
AgNPs (Suvith and Philip 2014; Junejo et al. 2014).
Freshly prepared 1.0 mL of 10 mM sodium borohydride
solution was mixed with 1.5 mL of 1 mM MB, and the
mixture was made up to 10 mL using double-distilled
water and then stirred for 5 min. Similarly, freshly pre-
pared 1.0 mL of 10 mM NaBH
4
solution was mixed with
1.5 mL of 1 mM CR, and the solution mixture was made
up to 10 mL using double-distilled water and then
stirred for 5 min as well. Sufficient quantities of synthe-
sized AgNPs were added to both these solutions and
mixed for 30 min with good agitation, and the UV-Vis
spectrum of the react ion mixture of MB was recorded at
1-min intervals of time for a period of 12 min and a
wavelength range of 400–750 nm at 25 °C whereas, for
CR, the absorption spectrum was re corded at a wave-
length range between 300 and 650 nm with 1-min inter-
vals of time for a period of 15 min at 25 °C (Ganapuram
et al. 2015). The test was conducted in a standard quartz
cuvette of about 3.0-mL volume. The rate constant of
the redox reaction was dependent on the variation in ab-
sorption band at 664 nm as a function of time.
Results and discussion
UV-Vis spectral analysis
UV-Vis spectroscopy emerged as one of the most power-
ful analytical tools for characterization of metal nanopa r-
ticles. The synthesized AgNPs were characterized by
UV-Vis spectroscopy and observed very well absorption
patterns in the UV-Vis spectral analysis. The absorption
behavior of AgNPs was attributed to the surface plas-
mon resonance (SPR), which results from the coherent
oscillation of electrons in the conduction band induced
by the ele ctromagnetic field, and a characteristic surface
plasmon absorption band can be seen in the spectral
range of 418–428 nm (Rai et al. 2009). The role of
AgNO
3
concentration on the production of AgNPs was
studied by UV-Vis spectra of the synthesized AgNPs
with different concentrations of AgNO
3
(0.1 to 0.5 %)
with 0.5 % GT for 45 min of ultra sonication at 45 °C
(Fig. 1). It was observed from Fig. 1 that the absorbance
intensity of the reaction mixture increased with the
increase in AgNO
3
concentration which indicates the in-
crease in the efficiency of nanoparticle synthesis. Further,
the effect of gum concentration on the synthesis of
AgNPs wa s studied by UV-Vis absorption spectra pro-
duced by the ultra sonication of different concentrations
of GT (0.1 to 0.5 %) with 0.5 % of AgNO
3
solution for
45 min at 45 °C (Fig. 2). It was noticed that the absorb-
ance intensity of the react ion mixture increased with the
Indana et al. Journal of Analytical Science and Technology (2016) 7:19 Page 3 of 9
increase in the concentration of GT indicating the
higher production rate of nanoparticles.
FTIR analysis
The stability of synthesized AgNPs was studied in rela-
tion to the interaction of various functional groups of
GT with AgNO
3
solution, and the FTIR spectra were re-
corded for the aqueous extract of GT and GT-stabilized
AgNPs (Fig. 3). The major absorbance peaks for gum-
stabilized AgNPs were observed at 3442, 2870, 1645,
1380, and 1068 cm
−1
. The broad band observed at
3442 cm
−1
corresponds to the stretching vibration of
O–H groups in GT. The band at 2870 cm
−1
could be
attributed to symmetric stretching vibrations of aliphatic
–CH
3
groups present in the gum. The bands present at
1645 and 1380 cm
−1
represent, respectively, the charac-
teristic asymmetrical and symmetrical stretching vibra-
tions of the –COO
−
group. The absorption band at
1068 cm
−1
could be attributed to stretching vibrations of
the C–O bond in either group. It was clearly observed
from the FTIR spe ctra of both the GT aqueous extract
and GT-stabilized AgNPs that the shifting of peaks from
3470 to 3442 cm
−1
, 2876 to 2870 cm
−1
, 1662 to 1645 cm
−1
,
and 1073 to 1068 cm
−1
with decreased peak intensity sug-
gesting that hydroxyl, methyl, carboxylate, and carbonyl
functional groups, respectively, are involved in the forma-
tion and stability of AgNPs.
X-ray diffraction
The crystalline structure of AgNPs stabilized with GT
was evidenced using the analytical technique of X-ray
diffraction. The recorded XRD spectrum showed four
distinct and well-characterize d intense diffraction peaks
at scattering angles (2θ) of 38.5°, 44.6°, 64.64°, and 77.45°
corresponding, respectively, to (111), (200), (220), and
(311) sets of lattice planes of face-centered cubic (fcc)
structure of metallic silver (Fig. 4). The intensity of the
diffraction peak corresponding to the (111) plane was
more significant than that of the rest of the diffraction
peaks in the recorded spectrum, and it can be construed
from the above that the (111) plane is the preferred
plane of orientation for the silver nanopa rticle crystal
lattice. The TEM images recorded on the samples fur-
ther corroborate the crystalline nature of the synthesized
AgNPs. The Scherrer formula was used to calculate the
average size of the synthesized AgNPs employing the
width of the peak, as a representative one, corresponding
to the (111) diffraction plane with an applied geometric
Fig. 1 The UV-Vis absorption spectra of AgNPs synthesize d with
different concentration of silver nitrate solutions containing 0.5 % of
gum tragacanth by ultra sonication for 45 min at 45 °C
Fig. 2 The UV-Vis absorption spectra of AgNPs synthesize d with
different concentration of gum tragacanth extracts containing
0.5 % of silver nitrate by ultra sonication for 45 min at 45 °C
Fig. 3 FTIR spectra of GT aqueous extract (red) and GT-capped
AgNPs (black)
Indana et al. Journal of Analytical Science and Technology (2016) 7:19 Page 4 of 9
factor of 0.97 (Kora and Arunachalam 2012). The result-
ant average particle size was found to be about 17.5 nm,
and the same was found to be in agreement with the size
obtained from the corresponding TEM image of AgNPs.
TEM analysis
The typical TE M images of GT-capped AgNPs synthe-
sized with a composition of 0.5 % (w/v) aqueous AgNO
3
solution and 0.5 % (w/v) GT sonicated at 45 °C for
45 min were depicted in Fig. 5. It was e vident from the
TEM images that the AgNPs so formed are spherical in
shape and were well distributed in the polymer matrix of
the gum. The histograms plotted on the obtained data
(Fig. 6) to study the particle size distribution reveals that
the size of the nanoparticles ranged from 10 to 29 nm
and the average particle size was about 18 ± 2.0 nm. The
selected area electron diffraction (SAED) pattern of the
synthesized AgNPs (Fig. 7) shows concentric rings dot-
ted with bright spots. This indicates the highly crystal-
line nature of these nanoparticles, and these concentric
rings can be attributed to the diffraction from the (111),
(200), (220), and (311) planes of the fcc structure of the
metallic silver. Further, the stability of the aqueous solu-
tion of AgN Ps was assessed by zeta potential analysis. A
zeta potential value of −11.21 mV (Fig. 8) indicates that
the dispersed AgNPs were capped by negatively charged
groups proving that they are stable.
Catalytic activity of AgNPs
The catalytic ability of AgNPs was studied a s it is a very
well-known fact that some of the reactions are, other-
wise, difficult to take place. Here, the catalytic reduction
of MB and CR was investigated by employing NaBH
4
as
the reducing agent and GT-capped AgNPs as the cata-
lyst. It is evident from the previous studies that the effi-
cacy of the metal nanopa rticles as electron transfer
Fig. 4 X-ray diffraction pattern of GT-capped AgNPs in an aqueous
system using gum tragacanth as reducing and stabilizing agent,
showing face-centered cubic (fcc) crystal structure
Fig. 5 Typical TEM image of GT-capped AgNPs in an aqueous system using gum tragacanth as reducing and stabilizing agent a at 0.2-μm scale
and (b) at 100-nm scale
Fig. 6 Histogram showing particle size distribution of
GT-capped AgNPs
Indana et al. Journal of Analytical Science and Technology (2016) 7:19 Page 5 of 9