Hindawi Publishing Corporation
Journal of Nanotechnology
Volume 2014, Article ID 954206, 11 pages
http://dx.doi.org/10.1155/2014/954206
Review Article
Reviewing the Tannic Acid Mediated Synthesis of
Metal Nanoparticles
Tufail Ahmad
Applied Science and Humanities Section, University Polytechnic, Faculty of Engineering & Technology,
Aligarh Muslim University, Aligarh 202002, India
Correspondence should be addressed to Tufail Ahmad; [email protected]
Received 10 July 2013; Revised 22 November 2013; Accepted 27 February 2014; Published 11 March 2014
Academic Editor: Thomas Thundat
Copyright © 2014 Tufail Ahmad. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Metal nanoparticles harbour numerous exceptional physiochemical properties absolutely different from those of bulk metal as a
function of their extremely small size and large superficial area to volume. Naked metal nanoparticles are synthesized by various
physical and chemical methods. Chemical methods involving metal salt reduction in solution enjoy an extra edge over other
protocols owing to their relative facileness and capability of controlling particle size along with the attribute of surface tailoring.
Although chemical methods are the easiest, they are marred by the use of hazardous chemicals such as borohydrides. This has led to
inclination of scientific community towards eco-friendly agents for the reduction of metal salts to form nanoparticles. Tannic acid,
a plant derived polyphenolic compound, is one such agent which embodies characteristics of being harmless and environmentally
friendly combined with being a good reducing and stabilizing agent. In this review, first various methods used to prepare metal
nanoparticles are highlighted and further tannic acid mediated synthesis of metal nanoparticles is emphasized. This review brings
forth the most recent findings on this issue.
1. Introduction
Owing to the nanoscale dimension (in the range of 1–
1000 nm), nanoparticles enjoy a leading edge in the fields
of nanoscience and nanotechnology. Recent years have
witnessed increased interests of scientific community in
nanomaterials particularly metal nanoparticles in various
areas ranging from material science to nanotechnology [1–
3]. Although nanomaterials have begun to be sought after
only recently, the notion dates back to the early 20th century
[3]. Humans have developed and used nanomaterials since a
very long time as evidenced by the ruby red colour of some
glass which is due to the entrapment of gold nanoparticles
in the glass matrix. Appearance of ruby red colour of the
colloidal gold solution results as a formation of small gold
nanoparticles [4]. Medieval potteries embellished with glaze
or luster display different characteristic colours that result
from special optical properties of the glaze which themselves
arise due to the random dispersion of metallic spherical
nanoparticles within the glaze [3]. The properties of the glaze
have been well enumerated in 1857 by Michael Faraday in
his revolutionary study “Experimental relations of gold (and
other metals) to light” [3, 5].
The nanoscale dimension and high surface area to volume
ratio of nanoparticles makes their physicochemical properties quite different from those of the bulk materials [3, 5]. This
makes nanomaterials capable of being potentially applied in
diverse fields including photonics and electronics, sensing,
imaging, information storage, environmental remediation,
drug delivery, and biolabelling [3]. It has been well documented that the optical, electronic, and catalytic properties
of metal nanoparticles are functions of nanoparticle size,
shape, and crystal structure. For instance, differently shaped
nanostructures of silver and gold embody unique optical
scattering properties [3, 6]. While a single scattering peak is
shown by highly symmetric spherical particles; multiple scattering peaks in the UV-vis range are exhibited by anisotropic
nanoparticles like rods, triangular prisms, and cubes exhibit
as a result of highly localized charge polarizations at corners
and edges [3]. This developed increased interest of scientific
community in the synthesis of metal nanoparticles of defined
morphology. Various procedures have been developed for the
2
Journal of Nanotechnology
HO
OH
OH O
HO
OH
OH
O
HO
OH
O
O
OH
O
O
O
O
HO
OH
O
O
O
O
O
O
HO
O
O
OH
O
HO
O
HO
OH
O HO
HO
OH
OH
HO
O
OH
O
HO
HO
OH
Figure 1: Molecular structure of tannic acid (C76 H52 O46 ) carrying numerous phenolic groups.
synthesis of metal nanoparticles and nanomaterials including physical, chemical, and biological methods. Numerous
reducing agents have been used to reduce metal salts to form
metal nanoparticles; for example, tri-sodium citrate [7–9]
and sodium borohydride [10] are being used for the reduction
of gold chloride and silver nitrate solutions to form gold and
silver nanoparticles, respectively, for decades. Tannic acid, a
plant derived polyphenolic compound [11] (Figure 1), has also
been exploited as a reductant of metal salt solutions. Ostwald
in as early as 1917 reported that chloroauric acid solutions
can be reduced to gold nanoparticles employing tannin even
when tap water is used to prepare aqueous solutions [12].
Ostwald’s protocol was replicated by Turkevich et al. [13] in
1951 who reported generation of gold particles with size to
be 12.0 ± 3.6 nm. Mülpfordt in 1982 [14] prepared colloidal
gold nanoparticles employing tannic acid as an additional
reductant. Moreover, tannic acid has also been an essential
component of the extensively used Slot and Geuze protocol
proposed in 1985 [15] for synthesizing gold nanoparticles in
the size range of 3–17 nm. This gives the historical significance
of tannic acid being used as a reductant for metal nanoparticle
synthesis. Despite that the first exploitation of tannic acid
mediated nanoparticle synthesis possibly dates back to early
20th century [11], its use remained subdued for a finite
period of time after that owing to the extra edge enjoyed
by citrate as a reducing agent for metal salts. However,
scientific community has recently revisited interest in this
compound owing to its properties of being a reducing as well
as stabilizing agent [16, 17].
Tannic acid has been well studied for its antioxidant,
antimutagenic, and anticancarcinogenic properties [18, 19].
Tannic acid has been reported to harbour inhibitory
action against skin, lung, and forestomach tumors caused
by polycyclic aromatic hydrocarbon carcinogens and
N-methyl-N-nitrosourea in mice [18, 20, 21]. Glucose
occupies the central core position in the tannic acid structure
whose hydroxyl groups are attached to one or more galloyl
residues [16]. At its natural acidic pH, tannic acid behaves as
a weak reducing agent which can induce growth of only seeds
to nanoparticles at room temperature [16, 22]. Tannic acid
owing to the pKa value between seven and eight, as a function
of the degree of dissociation, partially gets hydrolysed into
glucose and gallic acid moieties under mild acidic/basic
conditions [16, 23, 24]. At alkaline pH, gallic acid induces
formation of silver nanoparticles from silver nitrate rapidly
at room temperature [16, 25], but the poor stabilization
potential of gallic acid leads to aggregation of particles in
solution. However, glucose harbours the property of being
a good stabilizing agent at alkaline pH but concomitantly
is a weak reducing agent at room temperature [26]. These
facts enumerate the attribute of tannic acid of being an ideal
reducing and stabilizing agent under alkaline conditions
at room temperature. The aggressive reducing properties
of tannic acid owe to the numerous phenolic groups in
its structure. These phenols take part in redox reactions
by forming quinones and donating electrons. The donated
electrons reduce the oxidised metal ions in metal salts to form
corresponding metal nanoparticles. The reaction mechanism
of phenol (present in tannic acid) based reduction of metal
ions is outlined in Figure 2. In this review first we will
discuss the various methods used for the synthesis of metal
nanoparticles in general. Further, we will specifically review
the synthesis procedures for silver, gold, and palladium
nanoparticles with special emphasis on tannic acid mediated
synthesis of these particles since tannic acid has begun to be
used as a universal reductant of gold, silver, and palladium
salts for the production of respective nanoparticles. We will
discuss the most recent findings on this issue.
Journal of Nanotechnology
3
Phenolic form
HO
O
OH
OH O
HO
OH
Quinone form
+
−
O + 2H + 2e
OH
O
HO
OH
O
OH
O
O
O
M+
(from metal salt)
O
HO
OH
O
O
O
O
O
O
HO
OH
O
O
O
OH
O
HO
M0
O
HO
OH
O HO
HO
OH
OH
HO
O
OH
O
HO
HO
OH
Tannic acid
Figure 2: Reaction mechanism of tannic acid based reduction of metal salts. The phenolic groups in the tannic acid get oxidised to quinones
with subsequent release of electrons which reduce the metal ions.
2. Synthesis of Metal Nanoparticles
Numerous physical and chemical methods have been used
to carry out the synthesis of metal nanoparticles that include
laser ablation, ion sputtering, solvothermal synthesis, chemical reduction, and sol-gel method [3, 27–31]. Moreover,
biological method is also being used to fabricate metal
nanoparticles [3, 32, 33]. Nanoparticle synthesis approaches
can be basically divided into two: the top-down approach
and the bottom-up approach [3]. The concept of top-down
approaches is to create nanoscale objects by using bulky,
externally controlled microscopic devices for directing their
assembly, while bottom-up approaches adopt molecular components that are built up into more complex assemblies
[3]. Microfabrication techniques are often used for topdown approaches, wherein externally controlled equipment
are exploited for shaping materials into the required shape
and size by cutting or milling. Photolithography and inkjet
printing which fall under micropatterning techniques are
well-known examples of top-down approach. On the other
contrary, bottom-up approaches use the self-assembled properties of single molecules into some useful conformation [3].
Various physical and chemical methods for metal nanoparticle synthesis are described in the following section.
2.1. Laser Ablation. Laser ablation can be used to obtain
colloidal nanoparticles solutions in a number of solvents
[3, 28]. Plasma plume generated in response to laser ablation
of bulk metal plate dipped in liquid solution on condensation
forms nanoparticles. This method being conceived as “green
technique” is used in place of the chemical reduction method
for synthesizing noble metal nanoparticles. Nevertheless, this
method suffers a setback owing to high energy required per
unit of metal nanoparticles generated and the feeble control
over the growth rate of the metal nanoparticles.
2.2. Inert Gas Condensation. The most widely applied
method at laboratory scale for metal nanoparticle synthesis
is inert gas condensation (IGC) which was introduced by
Gleiter in 1984 for iron nanoparticle synthesis [3]. In IGC,
the small particles synthesized as a result of condensation of
metals evaporated in ultra high vacuum chamber grow by
Brownian coagulation and coalescence and ultimately form
nanoparticles. This technique has recently been applied for
size-controlled synthesis of gold or palladium nanoparticles
and varied-sized gold nanoclusters for nonvolatile memory
cell applications [34, 35].
2.3. Sol-Gel Method. The sol-gel process is a recently developed wet-chemical technique for the synthesis of nanomaterials. Sol-gel process is used for the generation of inorganic
nanostructures by first forming a colloidal suspension (sol)
and then gelation of the sol to integrated network in continuous liquid phase (gel). Size and stability of metal and
metal oxide nanoparticles have been controlled by inverted
micelles, polymer blends, block copolymers, porous glasses,
and ex situ particle-capping techniques [3, 36–39]. Despite
these features, the aqueous sol-gel chemistry is marred by
the complexity of process and amorphous nature of the assynthesized particles.
4
2.4. Hydrothermal and Solvothermal Synthesis. Synthesis of
inorganic materials by hydrothermal and solvothermal technique is amongst the important methodologies for nanomaterial synthesis [3]. For hydrothermal synthesis, the synthetic process is required to occur in aqueous solution at
temperature higher than the boiling point of water, while
in solvothermal method the reaction takes place in organic
solvents above their boiling points (at temperatures 200–
300∘ C). TiO2 photocatalysts have been reported to be produced by hydrothermal process [40]. Low cost and lower
energy consumption make hydrothermal process suitable for
industrial production. Solvothermal process on the other
hand increases the synthetic diversity as it enables to choose
among numerous solvents or mixture thereof. For instance,
highly reactive TiO2 nanocrystals were produced in hydrogen
fluoride and 2-propanol solvent mixture [41].
2.5. Colloidal Methods. Colloidal methods have been used
for long to control the nucleation and growth of metal
nanoparticles [3, 12, 42–44]. In colloidal method, metal salts
are reduced by chemical reducing agents such as borohydride,
citrate and hydrazine to produce metal nanoparticles which
are further stabilized by capping agents to avoid coalescence
of the particles. Varying the concentration of these chemical
reductants influences the size, shape, and dispersity of produced nanocrystals. Moreover, concentration of metal salts
also affects the size and dispersity of generated nanoparticles
when such chemical reducing agents are used to reduce metal
salts. Recently, reduction of metal salts by tannic acid has
attracted attention of scientific community since it fulfils the
requirement of a chemical reductant, but itself is a plant
derived compound. Hence, it renders the characteristics of
green synthesis. In the succeeding section, we will be discussing synthesis of silver, gold, and palladium nanoparticles
employing tannic acid.
3. Silver Nanoparticles Synthesized Using
Tannic Acid
The unique size and shape dependent optical, electrical, and
chemical properties of silver nanoparticles open avenues for
their application in diverse fields. They possess antimicrobial
properties and can be incorporated in biosensor materials,
composite fibres, cryogenic superconducting materials, cosmetic products, and electronic components [10]. Numerous physical and chemical methods have been exploited
for generating and stabilizing silver nanoparticles [45, 46].
Silver nanoparticles have been synthesized using chemical
reduction method, electrochemical procedure, physiochemical reduction, or radiolysis. For chemical reduction, various
organic and inorganic reducing agents are in use [10]. These
chemicals are usually hazardous and sometimes require
energy inputs. Therefore, there is increasing interest in producing nanoparticles using environment friendly methods,
that is, green methods. In this context, use of environmentally safe “green” reducing agents is entailed. Several recent
reports have made significant progress towards this goal by
using amino acids, vitamins, polysaccharides, and extracts
Journal of Nanotechnology
of bioorganisms. However, very recently, tannic acid being a
polyphenolic plant extract has been given attention since it is
an aggressive reducing agent and behaves as stabilizer as well.
The study conducted by Cataldo et al. [47] reveals that silver nanoparticles synthesized employing tannin make more
stable colloidal solutions than those prepared by reduction
of silver salt by NaBH4 . Sivaraman et al. have demonstrated
that silver nanoparticle size can be controlled by molar ratio
variation of tannic acid to silver nitrate [16]. They found
that tannic acid can be used as a reducing and stabilizing
agent for silver nanoparticle synthesis within a few minutes.
An increase in particle size with increasing molar ratio of
tannic acid/silver nitrate indicated that tannic acid acts as an
organizer for facilitating nucleation. Moreover, the synthesis
was found to occur at room temperature itself. The silver
nanoparticles were synthesized over a wide range of values
of the initial molar ratio of tannic acid to silver nitrate. Stable
colloidal dispersions were formed in all instances. Therefore,
they infer that each tannic acid molecule acts as a five-armed
chelator and that atomic reorganization occurs within such
complexes facilitating nucleation.
Although tannic acid has been previously utilized as a
reducing agent in the presence of additional stabilizers [48]
or as both the reducing and stabilizing agent [49, 50] but
the reaction times reported lie between half to two and a
half hours at 80∘ C, whereas the particle sizes reported are
>15 nm. The crucial difference between the study conducted
by Sivaraman et al. [16] and the earlier reports is that the pH of
the tannic acid solution was adjusted prior to the addition of
metal salt. The alkaline pH environment enhanced the reducing and stabilizing capability of tannic acid allowing room
temperature synthesis of silver nanoparticles in seconds, and
also enabling variation of the mean size from 3 to 22 nm.
The increase in particle size with increasing molar ratio of
tannic acid to silver nitrate indicates a third role for tannic
acid as an organizer for facilitating nucleation. This concept
of using tannic acid at alkaline pH as a reducing, organizing,
and stabilizing agent is easily extendable to other elements
that are known to chelate with tannic acid [16].
Moreover, Călinescu et al. [51] have demonstrated the
synthesis of silver nanoparticles in the presence of tannic
acid along with poly-vinyl alcohol. Their study enumerates
that by varying the molar ration between silver nitrate/polyvinyl alcohol and silver nitrate/tannic acid at different NaOH
concentration reaction time, temperature and microwave
power level renders to develop a fine control over the
nanoparticle size and distribution. Gupta et al. [52] have
found that silver nanoparticles synthesized employing tannic
acid bear antibacterial activity against multidrug resistant
human pathogens. Silver nanoparticles synthesized using
tannic acid have also been evaluated for release of silver
in natural waters [53]. It has been found that tannic acid
functionalized silver nanoparticles are more prone to silver
release in comparison to citrate coated silver particles in water
reservoirs.
Nanoplates as well as nanoshells of silver have been
synthesized using tannic acid. Tannic acid has been reported
to induce formation of silver nanoplates at room temperature.
The synthesis has been found to be a seedless process where
Journal of Nanotechnology
tannic acid plays the role of a reducing as well as a capping
agent [54]. Very recently, tannic acid (along with sodium
citrate) mediated controlled synthesis of low polydispersity
Ag@SiO2 core-shell nanoparticles has been reported. These
nanoparticles have been found to have applications in plasmonics [55].
4. Gold Nanoparticles Synthesized Using
Tannic Acid
The unique optical, electronic, and molecular-recognition
properties of gold nanoparticles make them the subject
of substantial research embodying applications in a wide
variety of areas, including electron microscopy, electronics,
nanotechnology, and materials science [18]. Specifically gold
nanoparticles in the 1–10 nm size range have size-tunable
physicochemical properties that are useful in a wide variety of applications such as cancer diagnostics, catalysis,
Raman and fluorescence spectroscopy, selective ionisation of
biomolecule, water purification, drug delivery, photothermal
therapy, and optoelectronics [18, 56–59]. Colloidal synthetic
procedure being versatile and relatively easy is better than
other chemical methods in vogue for the synthesis of gold
nanoparticles [60]. This method allows control over seed size
and seed size distribution since the experimental parameters
like reactant concentration, addition of stabilizers, mixing
rate, and so forth, can be tuned and manipulated as per the
requirement. A variety of chemical reagents being able to
reduce gold ions have been exploited for gold nanoparticle
synthesis. Preparation of thiol-functionalized gold nanoparticles by chemical reduction of soluble Au(1)-thiolates has
been reported byCorbierre and Lennox [61]. Long et al.
[62] prepared gold nanoparticles exploiting trisodium citrate
as reductant and further studied their optical properties.
Gold nanoparticles have also been generated by ethanol
assisted reduction of tetrachloroaurate ion in the presence of
sodium linoleate [63]. Moon et al. [64] have demonstrated
the cetyltrimethylammonium bromide (CTAB) mediated
synthesis of gold nanoparticles in sodium dodecyl sulfonate
(SDS) aqueous solution.
Synthesis of gold nanoparticles using tannic acid has
begun to be investigated recently owing to its attribute of
being a “green technique.” Influence of tannic acid alone and
in combination with citrate has been observed on the size
of synthesized gold nanoparticles [65]. It has been reported
that when the mixture of tannic acid and citrate is used
to reduce gold chloride solution, the gold nanoparticles
formed are significantly smaller than those observed when
only tannic acid is used as a reductant [65]. Mahl et al.
[66] have reported synthesis of gold, silver, and gold-silver
bimetallic nanoparticles using a modified citrate reduction
method where tannic acid has been used. The use of mixture
of citrate and tannic acid led to reduction in size of formed
nanoparticles by a factor of 3. Sivaraman et al. [11] studied
how pH affects the size distribution of gold nanoparticles
produced in response to reduction of gold chloride solution
by tannic acid. They report that the pH of precursor solutions,
mode of contacting, and the dynamics of stabiliser adsorption
5
vis-à-vis Brownian collision frequency play critical roles
in tuning nanoparticle formation, growth, and coalescence.
Their study also demonstrates that gold nanoparticles with
reduced polydispersity are produced when chloroauric acid
solution is added into tannic acid slowly as compared to
those produced by faster addition protocols. They reported
that dropwise addition protocol leads to synthesis of sizecontrolled gold nanoparticles (in the size range of 2–10 nm)
in a few minutes at room temperature. They stated that
“the optimal process is shown to be similar to a oneshot nucleation-seeded growth technique and the growth
mechanism is identified to be surface-reaction controlled
under these conditions. These insights on independently
manipulating reactivity and stabilisation can be extended to
other redox reactions for rapidly synthesising size-controlled
metal nanoparticles, as most systems exhibit pH dependent
reactivity and stabilization.” Aswathy Aromal and Philip [18]
have reported facile one-pot-synthesis of varied sized gold
nanoparticles at 373 K employing tannic acid as reducing and
stabilizing agent. They have investigated tannic acid mediated
synthesis of stable and spherical gold nanoparticles. They
come up with a simple, economically viable, and efficient
protocol for preparing gold nanoparticles. Nanoparticle size
was found to be a function of the amount of tannic acid and
pH. They state that phenyl hydroxyl and carboxylate groups
present in the tannic acid might play a putative role in the
formation of gold nanoparticles. Their study has enumerated
that adjusting one or more experimental parameters is
required to harness gold nanoparticles in a particular size
range and with particular morphology. Aswathy Aromal and
Philip [18] also report that gold nanoparticles formed using
tannic acid exhibit good catalytic activity in the reduction of
4-nitrophenol to 4-aminophenol by excess NaBH4 . Ahmad
and Khan [67] very recently investigated the effect of higher
chloroauric acid concentrations on the size evolution of gold
nanoparticles synthesized using tannic acid. They observed
two different patterns of size evolution of gold particles in
response to higher chloroauric acid concentrations. The sizes
obtained below 1 mM chloroauric acid solution exhibited a
general decrease with increase in molarity of chloroauric
acid solution. In contrast, the sizes of particles formed at
chloroauric acid concentrations greater than or equal to 1 mM
were found to increase with increase in chloroauric acid
concentration. Very recently, tannic acid functionalized gold
nanoparticles have also been found to assist in SNP detection
in the NanoBioArray chip at room temperature [68].
In addition to forming gold nanoparticles (nanospheres),
tannic acid has also been reported in synthesizing particles of other morphologies like nanoplates and nanorods.
Nanostructures of various morphologies are obtained by
controlling aspect ratios of particles in seeded growth. Tannic
acid is popularly known to form metal seeds which under
the influence of variation in crucial growth factors like pH,
temperature and molar ratio result in nanoparticles of diverse
shapes. Zhang et al. [69] have demonstrated tannic acid
assisted synthesis of gold nanoplates. They have reported
that mixing aqueous tannic acid and HAuCl4 solutions at
room temperature may be an effective route towards green
preparation of gold nanoplates. Catalytic property of gold
6
nanoplates towards the oxidation and reduction of H2 O2 has
been enumerated in the study. Moreover, they successfully
demonstrate that such gold nanoplates can be used as
building blocks for creating biosensor capable of detecting
glucose in buffer as well as human serum. Moreover, Untener
et al. [70] have demonstrated a distinctive uptake of tannic
acid coated gold nanorods by endosomes and their unique
distribution within the cell. Their study reveals that tannic
acid functionalized gold nanorods maintain their integrity
after being endocytosed by the keratinocyte endosomes
which indicates their potential candidature as nanodelivery
agents, nanobioimaging tools, and nanotherapeutics.
Journal of Nanotechnology
than those formed at latter condition. Hence, they conclude
that stable palladium nanoparticles can be prepared at room
temperature by sheer use of tannic acid as reducing agent for
palladium salt and capping agent of formed nanoparticles.
FTIR analysis performed in the study demonstrates the role
of poly-phenolic groups in reducing Pd2+ ions. They state the
development of that simple method for the synthesis of metal
nanoparticles is desirable over other methods because of its
facile, environment friendly, quick, and one-step approach.
Moreover,Devarajan et al. [90] have reported that simultaneous addition of trisodium citrate and tannin to the gold
and palladium salt aqueous solutions leads to formation of
4–7 nm sized bimetallic gold-palladium nanoparticles.
5. Palladium Nanoparticles Synthesized Using
Tannic Acid
Although in the last few decades, the coinage metals like gold
and silver had been the focus of the nanoscientific community
leaving the transition metals at the backdrop of the research
field. However, recently transition metals have begun to prove
their mettle when their properties as metal nanoparticles are
being harnessed [71, 72]. Transition metal nanoparticles are
majorly being exploited in the area of catalysis as the high surface area to volume ratio of nanomaterials makes them highly
efficient as potential catalysts. Palladium nanoparticles are
one of the transition metal nanoparticles being widely applied
in homogenous and heterogenous catalysis [72–75] although
they find applications in sensing, chemooptical transducers,
and plasmonic wave guiding as well [72, 76–78]. Palladium
nanoparticles being capable of adsorbing hydrogen are extensively applied in hydrogen storage [72, 79, 80]. Palladium
nanoparticles have been prepared by various methodologies
including chemical and electrochemical reduction [81, 82],
ion exchange [83], vapor deposition, thermal decomposition
[84, 85], and polyol method [86]. As discussed earlier,
biological and eco-friendly green technologies are at the
leading edge of nanoscience and nanotechnology in the
current era [87, 88]. Moreover, wide and unique applications
of nanoparticles demand economically viable approaches for
their synthesis. Metal nanoparticle synthesis entails the use of
a reducing agent and a stabilizer for the reduction of a metal
salt and further capping of metal nanoparticles, respectively,
to avoid nanoparticle collapse and coalescence [89]. Tannic
has been used as a reducing and protective agent in gold
nanoparticle synthesis since time immemorial and recently
it has begun to be widely applied for synthesis of other metal
nanoparticles which include silver and palladium.
Very recently, Kumari et al. [72] demonstrated an economically viable and efficient procedure for the preparation of palladium nanoparticles exploiting tannic acid.
They enumerate a green method for the reduction of Pd2+
ions to nanometer size using tannic acid. The procedure
demonstrated by them neither necessitates the use of any
superfluous stabilizing or capping agent nor involves any
extreme operating conditions such as high pressure. They
synthesized palladium nanoparticles both at room temperature and boiling conditions and found that the nanoparticles
formed at the former environmental cue were more stable
6. Other Metal Nanoparticles Synthesized
Using Tannic Acid
There are also other metals which have been reduced to
nanoparticles employing tannic acid wholly or as a coreductant. Tannins isolated from the biomass of Medicago sativa
(alfalfa) have been found to be capable of reducing gold
chloride to gold nanorods and zinc salts to zinc nanoparticles
[91]. Moreover, they were also found capable of forming
bimetallic nanoparticles, lanthanide clusters, iron oxide, and
magnetite nanoclusters [91]. Herrera-Becerra et al. [92]
have also demonstrated tannin mediated biosynthesis of iron
oxide nanoparticles. In a recent study, magnetic carbon-iron
oxide nanoparticles have been synthesized using tannin in
combination with a microwave-based thermolytic process
[93]. The iron oxide nanoparticles generated employing this
process have been found to be embedded within a carbon
matrix in small nanoclusters (lees than or equal to 100 nm).
Recently, Chang et al. [94] have reported preparation of 1,5diaminoanthraquinone nanofibers (DAAQNFs) decorated
with small platinum nanoparticles (PtNPs) synthesized using
tannic acid as a reducing agent. They found the resultant
PtNP/DAAQNF composites to exhibit a good catalytic activity toward reduction of 4-nitrophenol to 4-aminophenol by
NaBH4. In an earlier study, Huang et al. [95] exploited
bayberry tannin as a stabilizer to prepare supported platinum nanocatalysts. Preparation of gold-platinum bimetallic
nanoparticles has also been reported employing tannic acid
[96]. To a mixture of gold chloride and platinum chloride,
addition of tannic acid and citrate led to the formation of
bimetallic gold-platinum nanoparticles [96].
Ni/graphene nanocomposites have also been prepared
using tannic acid. Very recently, Lu et al. [97] prepared novel
Ni(II)-based metal-organic coordination polymer nanoparticle/reduced graphene oxide (NiCPNP/rGO) nanocomposites employing hydrothermal treatment of the mixture of
tannic acid functioned graphene oxide and NiCl(2) aqueous solution in N,N-dimethylformamide. They found that
the NiCPNP/rGO nanocomposite-modified electrode could
exhibit high electrocatalytic activity for glucose oxidation in
alkaline medium and hence could be used as a glucose sensor
in human blood serum.
Journal of Nanotechnology
7
HO
OH
OH O
HO
OH
OH
O
HO
OH
O
+
M
O
OH
O
O
O
O
HO
OH
O
Reduction
O
O
O
M0
O
OH
O
HO
Electrosteric
stabilization
O
O
O
HO
O
HO
OH
O HO
HO
OH
OH
HO
O
OH
O
HO
HO
OH
Tannic acid (C76 H52 O46 )
: M0
: COO −
: polymer moiety
M : metal
Figure 3: Putative mechanism of metal nanoparticle synthesis and stabilization by tannic acid.
Nanospheres
M+
Tannic acid
Variation in pH, temperature,
e
molar ratio
Nanoplates
Reduced metal seeds
Nanorods
Figure 4: Schematic representation of various morphologies of
metal nanoparticles obtained upon tannic acid mediated reduction
of metal ions in response to variation in pH, temperature and molar
ratio.
7. Reduction Mechanism of Metal Salts by
Tannic Acid
Tannic acid mediated reduction of metal salts and synthesis
of metal nanoparticles is outlined in Figure 3. Gallic acid
molecules and glucose polymerise to form tannic acid. Tannic
acid being an antioxidant is rich in electrons and embodies
the capability of liberating freely reactive hydrogen atom
[18]. Presence of a hydrophobic “core,” a hydrophilic “shell,”
and above all the polyphenolic nature of tannic acid make
it an effective antioxidant [18] combined with an aggressive
reducing agent. Numerous polyphenols bear antioxidant
nature owing to the relative facileness of donating hydroxyl
group to a free radical and the potency of the aromatic ring
to carry an unpaired electron. Hence, the hydroxyls of the
phenolic groups present in the tannic acid may be responsible for reducing chloroauric acid. Carboxylic acid groups
(COOH) present in the tannic acid lose their hydrogen atom
to become carboxylate ion (COO− ) during the reduction
process. The COO− so formed attaches to the surface of
metal nanoparticles along with the remaining part of the
polymer to act as surfactant and stabilize metal nanoparticles
by electrosteric stabilization [18]. Various factors like pH,
temperature, and molar ratio of metal salts to tannic acid
have been found to play a decisive role in the generation of
nanoparticles of various shapes (Figure 4).
8. Conclusion and Future Perspective
Herein, we first reviewed various methods for metal nanoparticle synthesis and further shifted our focus to tannic acid
mediated synthesis of metal nanoparticles since tannic acid
being a good reductant and eco-friendly compound is currently in vogue for the reduction of metal salts to form metal
nanoparticles. From the findings enumerated in the review,
it becomes apparent that tannic acid can act as a universal
8
reducing agent for silver, gold, palladium, platinum, nickel,
zinc, iron, and other metallic salts to form their respective
metal nanoparticles. Exploitation of tannic acid provides an
opportunity for rapid, facile, economically viable, and ecofriendly synthesis of metal nanoparticles in general without
requiring specific chemical reductant for each metal salt. pH,
temperature, and molar ratio of the metal salt and tannic
acid have been found to influence the synthesis of metal
nanoparticles from tannic acid. However, a defined pattern
of nanoparticle synthesis for various metals in response to
variation in these parameters cannot be worked out from
state-of-the-art findings. To obtain a complete picture in
this regard, further studies on tannic acid mediated metal
nanoparticle synthesis have to be performed. Since the
nanoparticles formed employing tannic acid have smaller
diameters, this can open avenues in future for them to be
exploited in therapeutics, drug delivery, and bioimaging as
smaller particles cannot be uptaken by the phagocytic system
of the body, remain in circulation for a long time, and
therefore can have enhanced targeting potential.
Journal of Nanotechnology
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Conflict of Interests
The authors declare that there is no conflict of interests.
[18]
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Journal of
Journal of
Textiles
Ceramics
Hindawi Publishing Corporation
http://www.hindawi.com
International Journal of
Biomaterials
Volume 2014
Hindawi Publishing Corporation
http://www.hindawi.com
Volume 2014