Materials Sciences and Applications, 2010, 1, 272-278
doi:10.4236/msa.2010.15040 Published Online November 2010 (http://www.SciRP.org/journal/msa)
Bifunctional Role of Thiosalicylic Acid in the
Synthesis of Silver Nanoparticles
Ramasamy Indumathy, Kalarical Janardhanan Sreeram, Muralidharan Sriranjani, Cheravathoor
Poulose Aby, Balachandran Unni Nair
Chemical Laboratory, Central Leather Research Institute, Council of Scientific and Industrial Research, Chennai, India.
Received July 30th, 2010; revised October 19th, 2010; accepted October 30th, 2010.
ABSTRACT
Conventional synthesis of silver nanoparticles employs a reducing agent and a capping agent. Surfactants are effective
capping agents as they prevent the aggregation of nanoparticles during storage and use. However, the biocompatibility
of several of the surfactants is questionable. In this report, the use of thiosalicylic acid as both reducing and capping
agent is reported. Compared to conventional synthesis, this methodology requires higher temperature for synthesis,
which then is expected to result in aggregates of larger size. The ability of three different synthesis methodologies –
direct heating, photochemical and microwave dielectric treatment were evaluated and assessed on the basis of the size,
size distribution and stability of the particles. Microwave irradiation was found to be most suitable for achieving parti-
cles with a hydrodynamic diameter of 10 nm. Our studies indicate that -COO- group is involved in the reduction of Ag+
and –SH group of TSA is involved in the capping of the nanoparticles.
Keywords: Silver Nanoparticles, Microwave Dielectric Heating, Reducing cum Capping Agent, Thiosalicylic Acid,
Photon Correlation Spectroscopy, Zeta Potential
1. Introduction
Metal nanostructures owing to their unique physical,
chemical, electrical and optical properties have ac-
quired immense attention from the researcher’s point of
view. The potential application of the nanostructures
can be tailored by controlling their size, shape, compo-
sition, and crystalline character [1-3]. Among the noble
metals, silver nanoparticles has gained more impor-
tance due to its application in catalysis, photonics, real
time optical sensors, printed electronics, surface en-
hanced Raman scattering and as anti-microbial agents
[4-9]. They also find applications in the field of biol-
ogy as antibacterials, in DNA sequencing etc., because
they are biocompatible and possess less cytotoxicity.
The synthesis of monodispersed and nanometer sized
particles remains as great task due to which the re-
search in the field of synthetic methodology of nano-
materials is being held as an endless endeavor. Recent
reports on nanomaterial synthesis reveal that the size,
shape and stability of the nanoparticles depend strongly
on the specificity on the method and experimental con-
ditions implemented for their synthesis. Thus, new
synthetic strategies are frequently reported in the lit-
erature. A variety of approaches have been reported in
literature for synthesizing silver nanoparticles. This
includes chemical reduction in aqueous media with or
without stabilizers [10,11], formation in micro emul-
sion, thermal decomposition, use of Langmuir-Blodgett
films, biosynthesis using microorganisms, fungus and
the use of liquid crystalline phases made of surfactant
aggregates, etc [12-21].
The solution chemistry relating to the synthesis of sil-
ver nanoparticles has predominantly been those involv-
ing reduction of silver colloidal systems with reductants.
Sodium citrate and sodium borohydride are often used as
the most effective reducing agents [22]. To circumvent
the problem of aggregation, researchers have been em-
ploying surface active agents or capping agents in addi-
tion to reducing agents [23,24]. Use of organic com-
pounds, alkanethiols and bisulfides as well as polymers
has been described, the most important ones being sur-
factants and carboxylic acids [25]. Amongst the several
Funding support from CSIR through the NWP0035 – Nanomaterials and
nanodevices for applications in health and diseases as well as the
OLP-57 EMPOWER project on collagen stabilization using functional-
ized nanoparticles.

Bifunctional Role of Thiosalicylic Acid in the Synthesis of Silver Nanoparticles 273
approaches developed for the nanoparticle synthesis,
photo activation [26,27] and microwave [28-30] assisted
techniques serves as a simple and straight forward meth-
ods for the rapid and size controlled synthesis of metal
nanoparticles.
In this work, a combination approach based on classi-
cal colloidal chemistry (for reduction) and modern na-
notechnology (for stabilization) has been employed to
synthesize stable silver nanoparticles. We present here a
thiosalicylic acid derivatized system, wherein the TSA
under appropriate mole ratios, serves a dual role of both
as a reducing and capping agent for the generated silver
nanoparticles. The rate of silver nanoparticles formation
and their stability is evaluated by adopting three different
synthetic methodologies namely, thermal, photochemical,
and microwave dielectric heating.
2. Materials and Methods
2.1. Materials
Silver nitrate (AgNO3) (99.9% purity) and thiosalicylic
acid (TSA) (98.0% purity) were sourced from M/s. SD
Fine Chemicals India Pvt Ltd. All chemicals were used
as received and de-ionized water was used throughout
the work.
2.2. Particle Synthesis
Three different synthetic methodologies were adopted in
order to compare the rate of formation and stability of the
nanoparticles. The photochemical and microwave treat-
ment were done only for the samples of definite mole
ratio which showed very slow rate of nanoparticle forma-
tion at thermal conditions. Typically, 0.01 M aqueous
solutions of TSA and AgNO3 were prepared and stored
under dark conditions. To a known volume of TSA,
AgNO3 solution was added drop wise under constant
stirring, such that after the completion of the addition, the
resultant solution had a TSA: AgNO3 concentration of
1:0.25 to 1:2. The solution was kept stirring for 10 min,
during which time the silver particles were formed. All
reactions were carried out at room temperature. For the
photochemical approach, appropriate amounts of TSA
and AgNO3 were taken in a quartz cuvette of 3 ml capac-
ity. The cuvette was placed at distance of 5 cm from the
light source and was irradiated for about 20 min. For
microwave heating, the reactions were carried out in a
domestic microwave oven with an operating frequency of
2450 MHz, where TSA and AgNO3 of different mole
ratios were mixed at ambient temperature and subjected
to microwave heating for about 30 sec. For obtaining the
silver nanoparticles in solid state, the solution was cen-
trifuged at 10,000 rpm for 10 min.
2.3. Experimental Techniques
UV-vis absorption spectrum of the silver containing so-
lution was measured using a Lambda 35 UV-visible
spectrophotometer from M/s. Perkin Elmer Ltd., after
appropriate dilution. The size of the nanoparticles was
determined using transmission electron microscopy
(TEM) on a JEOL 3010 electron microscope under an
acceleration voltage of 300 kV. For this, a drop of silver
colloid was placed on a holey carbon film supported by a
300 mesh copper grid and the solvent allowed to evapo-
rate. The oxidation state of the silver in the dried sample
was determined by using X-ray Photoelectron Spectros-
copy (XPS). The infrared (IR) spectrum was recorded on
a Perkin Elmer RX-1 model FT-IR spectrophotometer on
the sample pelletized with KBr powder. Thermo Gra-
vimetric Analysis (TGA) was performed using a univer-
sal V 3.9 A equipment from M/s. TA instrument. These
studies used ramp setting of 10oC/min from 32o to 1000oC.
Light scattering measurements were carried out at 90o on
a photon correlation spectrometer (PCS) from Malvern
Instruments, Zetasizer 3000 HSA equipped with a digital
autocorrelator. The solution was used as such for the
measurement of particle size and zeta potentials. The
electrophoretic mobility measurements were performed
using the same setting equipped with a platinum elec-
trode. The electrode was cleaned for 10 min in an ultra-
sonic bath prior to each measurement and pre-equilibra-
ted for 2 min in an aliquot of the sample. The particle
size (hydrodynamic diameter) and the particle size dis-
tribution (through CONTIN analysis) were obtained di-
rectly from the instrument. The size and size distribution
of the particles were measured for the AgNPs obtained
after centrifugation and drying and then re-dispersion of
the silver particles in water through sonication.
3. Results and Discussion
The thiosalicylic acid mediated synthesis of silver nano-
particles is a slow reaction (in the order of minutes), and
hence the reaction has been monitored through UV-visi-
ble spectroscopy. Nanoparticle formation is favored only
by deliberate addition of AgNO3 to the aqueous solution
of TSA and also for certain mole ratios, namely, 1:0.25
to 1:2 (TSA: AgNO3), beyond which it results in either
aggregation or precipitation. The formation of metal col-
loids through the reduction of their ions in solution in-
volves two stages: nucleation and growth, whose relative
rates determine the size distributions of colloids. These
two processes can be affected by many factors, amongst
which the concentration of the two reactants plays an
important role. Originally described by Ritchie [31], sur-
face plasmon resonance is an important optical property
of nanoparticles and is described as coherent fluctuations

Bifunctional Role of Thiosalicylic Acid in the Synthesis of Silver Nanoparticles 274
in electron density occurring at a “free electron” metal/
dielectric interface. “Free electron” metals are those met-
als, which have lone electron in valence shell such as Au,
Ag, Al and Cu. The surface plasmon absorptions are re-
sponsible for “red” and “yellow” colors of the gold and
silver colloids, respectively. In general bare silver
nanoparticles exhibit surface plasmon resonance in the
region 380–400 nm [32]. The UV absorption spectrum
for silver nanoparticle formation for different mole ratios
of AgNO3 and TSA is presented in Figure 1. In the pre-
sent systems, we observed bands in the range 420–
432 nm, which matches with earlier observations [33]. A
shift in the Surface Plasmon Resonance to higher wave-
length has been attributed to an increase in the size of the
nanoparticles [34].
Photo irradiation of the mixture, namely, AgNO3 and
TSA, resulted in a yellowish brown color solution. The-
formation of the nanoparticle was corroborated by
UV-visible spectrometry. Figure 2 shows the UV ab-
sorption spectrum for the AgNPs obtained by photo irra-
(a)
(b)
(c)
Figure 1. Absorption spectra of the TSA-Ag solution with
different mole ratios (TSA: AgNO3) prepared at room tem-
perature where (a) 1:0.25; (b) 1:0.5 and (c) 1:2.0.
Figure 2. UV-Vis spectra of the TSA-Ag solution prepared
by photochemical approach at different time intervals.
diation at different time intervals.
Almost monodispersed AgNPs were obtained. By this
method in addition to the excellent reproducibility, pho-
tochemical method is more advantageous as it results in
the formation of nanoparticles at a faster rate when com-
pared to that of the reaction carried out at ambient tem-
perature. No significant difference between the absorp-
tion spectra shapes existed after a particular duration of
time, i.e., after 20 min, indicating no difference in the
morphology and size of the formed silver particles.
Microwave dielectric heating ended up with a brown
color solution. The reaction was found to be highly ex-
peditious, in order of about 10 seconds. Figure 3 shows
the UV absorption spectrum for the AgNPs obtained by
microwave irradiation at different time intervals.
This observation can be attributed to the fact that the
high penetration depth of microwaves leads to fast and
uniform heating which results in minimization of thermal
gradients. The process of particle formation was also
Figure 3. UV-Vis spectra of the TSA-Ag solution prepared
by microwave heating at different time intervals for the
sample with 1:0.5 mole ratios of TSA and AgNO3.

Bifunctional Role of Thiosalicylic Acid in the Synthesis of Silver Nanoparticles 275
monitored by UV-visible spectra for the nanoparicles
synthesized by microwave irradiation. The absence of a
band at 275 nm indicates the absence of Ag+/Ag2+ ions.
The presence of well characterized absorption peak at
420 nm is an indicative of the effectiveness of thiosali-
cylic acid as a reductant, similar to our earlier observa-
tions. The narrow peaks observed give indirect evidence
to the ability of the reductant to function as a stabilizer of
the nanoparticles against aggregation.
Transmission electron microscopic (TEM) analysis of
the silver nanoparticles obtained by thermal, photo-
chemical and microwave irradiation is presented in Fig-
ure 4. TEM images clearly signifies that the particles
obtained by all the methodologies were more or less uni-
form in size and have a spherical morphology, with an
average diameter of 6 nm. The formation of uniform and
well-shaped particles can be accounted as the conse-
quence of balance between stabilization and crystal-
growth. The effective separation of the nanoparticles in
the TEM image clearly indicates that the nanoparticles
with well passivated surface by the TSA molecules. The
same has been further confirmed from the lattice image
HRTEM and SAED pattern presented in Figure 5. It is
clear from the SAED pattern that the silver nanoparticles
are single crystalline and have been indexed on the basis
of fcc structure of silver. The diffraction spots marked by
circles correspond to the (111) reflections and (200) re-
flections. The presence of uniformly sized nanoparticles
of silver, with no prominent aggregation is indicative of
the ability of thiosalicylic acid to function as a stabilizer
of the nanoparticles.
The particle size for the silver nanoparticles in solution
synthesized by microwave irradiation, has been measured
using Photon Correlation Spectroscopy and by employ-
ing Mie theory. The number average diameter was found
to be 2 nm (using CONTIN method of analysis). It was
observed that the particle size distribution was narrow,
lying between 1 nm and 8 nm. The polydispersity index
was 0.53, indicating a predominant monodisperse char-
acter for the generated silver nanoparticles. In order to
compare the particle size characteristics of the AgNPs,
the particle size distribution for the regenerated AgNPs
were also studied. This was done by centrifugation of the
reaction mixture followed by washing of the particles in
water and subsequent drying. The dried silver nanoparti-
cles were then redispersed in water and the particle size
and particle size distribution pattern was analyzed. The
number average diameter of the particles was 7 nm and
the polydispersity index was 0.59. It is obvious from the
particle size measurements, that there is no or minimal
variations in the particle size and its distribution between
original and regenerated AgNPs prepared by microwave
irradiation method.
Owing to a higher surface energy, the nanoparticles
tend to aggregate with time. The stability of nanoparti-
cles against aggregation is a major requirement for sev-
eral applications, most importantly in medicine. Zeta
potential measurements are indicative of the stability of
the particles to remain discrete in a given medium. The
zeta potential values for the silver nanoparticles in the
reaction medium after the completion of reaction under
thermal, photochemical and microwave heating were
found to be −24.8 ± 1.6 mV, −26.7 ± 1.6 mV, and −41.5 ±
1.6 mV respectively. A large negative zeta potential is a
measure of the stability of the nanoparticles against ag-
gregation. From the zeta potential values, it is evident
that the nanoparticles prepared by the three methods
were found to stable. The enhanced stability offered by
the microwave irradiation may be due to the complete
reduction of metal salt along with effective capping of-
fered by TSA under microwave heating. The stability of
the nanoparticles against aggregation needs to be corre-
lated to the possible capping that the medium provide.
In order to understand the role of the medium (thio-
Figure 4. Transmission electron micrograph of silver nanoparticles synthesized through (a) thermal treatment, (b) photo
irradiation and (c) microwave dielectric heating.

Bifunctional Role of Thiosalicylic Acid in the Synthesis of Silver Nanoparticles 276
Figure 5. (a) High resolution transmission electron micro-
graph and (b) SAED pattern of the silver nanoparticles
prepared through microwave dielectric heating.
salicylic acid) in providing stability to silver nanoparti-
cles, a FT-IR spectroscopic measurement of the dry sil-
ver nanoparticles was carried out. FT-IR spectrum re-
corded for TSA derivatized AgNPs and free TSA is
shown in Figure 6. In the FT-IR spectrum of AgNPs, the
appearance of the strong band at 1633 cm−1 and com-
paratively a weak band at 1461 cm−1 can be assigned to
the symmetric and antisymmetric stretching vibration of
COO- from the thiosalicylate. Weak band at 1576 cm−1 is
ascribed to the C=C stretching. It is evident from the
spectrum of TSA-AgNP, that the -COO- group is in-
volved in the reduction of Ag+ and –SH group of TSA is
involved in the capping of AgNPs( presence of weak
band around 2400 cm-1). Thus, the above observation
provides direct evidence to the presence of an organic
capping around silver nanoparticles.
X-ray photoelectron spectroscopy was used to study
the change in the oxidation state of silver before and after
the experimental treatments. The study was carried out
with dry sample obtained from centrifuging colloidal
silver solution. The spectral profile for the TSA derivat-
ized AgNPs (Figure 7) shows two peaks at 368.54 and
374.64 eV due to Ag 3d5/2 and Ag 3d3/2 orbital, respect-
Figure 6. FTIR spectrum of free TSA and TSA capped
AgNPs.
Figure 7. XPS spectrum of silver nanoparticles.
tively, which indicates that the silver present in the clus-
ter is in the Ag (0) state.
The capping ability of TSA on the AgNPs was further
substantiated from thermogravimetric analysis (TGA).
Dry sample was utilized for this study. Figure 8 shows
the thermogram for the synthesized TSA capped nano-
particles at thermal condition, which depicts continuous
weight losses in the temperature region 100–800oC. The
dotted line in the figure shows the thermogram of TSA
alone. A total weight loss of about 9.3% is indicative of
presence of organic molecules. Hence, weight losses
from the thermogram of AgNPs can be ascribed to the
ready decomposition of TSA molecules from the surface
of AgNPs. Therefore, the above observation apparently
deduces the weakly bonding nature of TSA molecules on
to the metal surface.
In conclusion, thiosalicylic acid under appropriate
mole ratio to Ag ions serves as both reductant and stabi-
lizer in the synthesis of silver nanoparticles in aqueous
media. The rate of formation of the nanoparticles can be
further enhanced by adopting different methodologies
like photochemical and microwave dielectric heating.
Among the three methods employed, microwave heating
has been found to be advantageous in terms of energy
saving, shorter processing time, uniformity of products,
reduction of particle size and the nanoparticle stability.
Figure 8. Thermogram of TSA and TSA capped AgNPs.
Inset shows the thermogram in the 100-80% weight loss
region under magnification.

Bifunctional Role of Thiosalicylic Acid in the Synthesis of Silver Nanoparticles 277
UV-vis spectroscopy confirms the presence of surface
plasmon at 420 nm, characteristic of Ag nanoparticles.
Particle size measurement, zeta potential analysis and
TEM results indicate that the silver nanoparticles synthe-
sized by microwave irradiation have a size of less than 7
nm and are spherical. Stable silver sol, wherein attach-
ment of thiosalicylic acid to Ag nanoparticle has been
obtained as confirmed from the TGA and IR measure-
ments. Unaltered particle morphology with long stability
even after drying has also been obtained. All the three
synthetic methodologies employed are simple, repro-
ducible and could be adopted for direct one step synthe-
sis and for achieving stable, monodisperse metal sols in
mass
4. Acknowledgements
Dr B Sreedhar of Indian Institute of Chemical Technol-
ogy, Hyderabad, India is thanked for the XPS measure-
ments.
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