Current Methods for Synthesis of Gold Nanoparticles

1. Current Methods for Synthesis
of Gold Nanoparticles
Metal nanoparticles possess quantum size effect and thus have specific electronic structures, which
makes them exhibit unique physical and chemical properties different from those of the bulk materials
or atoms. Among them, gold nanoparticles (AuNPs) may be the most remarkable members of the
metal nanoparticle groups. They have attracted plenty of researchers' interests and driven a diversity
of potential applications in catalysis, biology, drug delivery, and optics. Here we are specifically
focusing on the principles and most recent improvements disclosed in the literature on various types of
AuNPs synthesis.
CD
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Synthetic Routes
of AuNPs
Chemical Methods:
Turkevich method, Brust method,
seeded growth method, etc.
e.g. reduction of HAuCl4
Physical Methods:
γ- irradiation method, UV-induced
photochemical method, ultrasound-
assisted method, laser ablation
method, etc.
Biological Methods:
Microbial mediated method,
extracellular method, intracellular
method, plant mediated method,
etc.

2. Chemical Methods
• Turkevich method
In chemical methods, AuNPs are usually produced by reduction of (hydro)chloroauric acid (HAuCl4
),
using some sort of stabilizing agents. The first step is to dissolve HAuCl4
and then stir the solution
quickly and add a reducing agent at the same time to reduce the Au3+
ions to neutral gold ions.
This method involves the reaction of small amounts of hot HAuCl4
in the presence of reducing agents such as citrate, amino acids,
ascorbic acid or UV light. The AuNPs will form due to the presence
of citrate ions as both a reducing agent and a capping agent.
When producing larger particles, the amount of sodium citrate
should be reduced to 0.05% and thus there would not be enough
citrate ions to reduce all the gold. Since the citrate ions are
responsible for stabilizing the particles, less sodium citrate will
cause the small particles to aggregate into larger ones until the
total surface area of all particles is small enough to be covered
by the existing citrate ions in solution. Finally, the larger particles
are produced.
Key Features:
Produce modestly
monodisperse spherical
AuNPs (10-20 nm) in water;
When producing larger
particles, the monodispersity
will be lost.
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• Brust method
It can be used to produce AuNPs in organic liquids that are
normally not miscible with water (e.g. toluene). It involves the
reduction of HAuCl4
solution with tetraoctylammonium bromide
(TOAB, an anti-coagulant) solution in toluene and sodium
borohydride (NaBH4
, a reducing agent). The diameter of AuNPs
here will be 2 to 6 nm and TOAB is both the phase transfer
catalyst and the stabilizing agent. The gold ions are then reduced
using NaBH4
in presence of an alkanethiol. The alkanethiols
stabilize the AuNPs, resulting in a color change of the reaction
from orange to brown.
Key Features:
A method of two-phase
synthesis and stabilization with
thiol;
Particle diameter and
grain-size distribution
controllable;
Functionalization of the particle
surface with alkanethiols.

3. Physical Methods
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• Seeded growth method
Seed mediated growth is the most widely used method to
produce AuNPs in other shapes. Firstly, the seed particles will be
produced by reducing gold salts with a strong reducing agent
such as NaBH4
. Then, the seed particles will be added to a
solution of metal salt in presence of a weak reducing agent (e.g.
ascorbic acid) and a structure-directing agent to prevent further
nucleation and accelerate the anisotropic growth of AuNPs.
Key Features:
Used for producing other
shaped gold particles (e.g.
rods, cubes, tubes);
By using reducing agents,
structure-directing agents
and varying the
concentration of seeds, the
geometry of AuNPs can be
altered.
• γ- irradiation method
The γ- irradiation method is adopted to synthesize AuNPs with 2
to 40 nm in diameter. In this method, the natural polysaccharide
alginate solution is used as a stabilizer. Akhavan A. et. al. gave a
single step γ-irradiation method to synthesize AuNPs of size 2 to 7
nm by using bovine serum albumin protein as a stabilizer.
Key Features:
Proved to be the best
method for the synthesis of
AuNPs with controllable
size and high purity.
• UV-induced photochemical method
Using photochemistry, AuNPs with controllable size were
successfully synthesized. The presence of UV radiation with
different wavelengths will encourage chemical reactions in
aqueous Au solution. For example, with γ-rays irradiation, the
aqueous solution of chloroauric acid can form 80 nm AuNPs.
Moreover, the presence of surfactant/polymer reagent will
impact the particle dimensions, namely the particle size will
decrease by increasing the polymerization degree.
Macromolecular polymers, dendrimers, and surfactants can
provide the required steric hindrance effect and thus prevent the
aggregate formation, which acts as soft templates during AuNPs
fabrication.
Key Features:
Used for the formation of
single crystallite-based
AuNPs;
Particle size and shape
controllable.

4. Biological Methods
Given the versatility of both physical and chemical synthesis strategies used for AuNPs fabrication,
various technologies have been successfully used during the latest research studies, including
aerosol-based synthesis, ultraviolet, and ultrasound radiation, lithography, laser ablation and
photochemical reduction of metallic gold. However, these physicochemical synthesis approaches often
require using hazardous chemicals, expensive equipment, and technologies, so the attention of the
research community recently turned into the bio-inspired methodologies for AuNPs synthesis.
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• Ultrasound-assisted method
Using an ultrasound wave generator in a water bath with
constant temperature, gold ions can be reduced with
ultrasonic-assisted in the presence of 2-propanol. For
reproducibility and tunability reasons, various stabilizers have
been used during the conventional ultrasound-assisted synthesis
method, such as citrate, poly (N-vinyl-2-pyrrolidone),
triphenylphosphine, disulphide, and several dendrimers.
Key Features:
Eco-friendly and rapid
synthesis of AuNPs;
Ideal for various
biotechnological applications.
• Laser ablation method
This method is based on the photo-induced effects of a 532 nm
wavelength laser beam which reduces the gold (III)
tetrachloroaurate metallic precursor to produce nanogold
particles with a size range lower than 5 nm. During this process,
aqueous solutions of sodium dodecyl sulfate (SDS) have been
used as a template agent and the researchers have studied the
influence of both SDS concentrations and laser influences on the
dimensions of the synthesized AuNPs. Gold nanospheres,
silica-gold nanoshells and gold nanorods synthesized by this
method have been widely used in biological, cell imaging and
photothermal therapeutic applications.
Key Features:
Accurate and reproducible
results;
A full-fledged physical
approach to produce AuNPs
with tunable features.

5. • Extracellular method
This approach refers to the reduction of chloroauric ions in the
presence of cells to produce AuNPs. The biosynthesis is
successfully accomplished due to the key role of the cell wall and
cell wall proteins. It has been shown that the
culture supernatants are enriched in nitroreductase enzyme
content that is subsequently involved in bacterial mediated
synthesis of colloidal gold.
Key Features:
Eco-friendly and
cost-effective;
Easy to synthesize;
Reduction and surface
accretion of metals may be
processed, by which bacteria
keep themselves from the toxic
effects of metallic ions.
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• Microbial mediated method
Both the eukaryotes and prokaryotes can synthesize gold colloids
from inorganic precursors due to the specific activity of their
secondary metabolites produced either intracellular or
extracellular. For example, the fungus
isolated from soil involved in success full synthesis of AuNPs
mediated by extracellular proteins. Enzymes such as ligninases,
laccases, reductases, and peptides are involved in growth and
nucleation of NPs, while free cysteine/amino and surface-bound
protein of microbes involve in the stabilization of these colloids.
Moreover, several factors including temperature, pH, substrate
concentration and static condition also affect the synthesis and
stability of AuNPs.
Key Features:
Eco-friendly and
cost-effective;
No hazardous chemicals and
toxic derivatives.
Mechanism of extracellular and intracellular synthesis of AuNPs.
Penicillium crustosum
Enterobacteriaceae

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• Intracellular method
Many reports have shown that plants have the tremendous ability
to in situ produce inorganic nanoparticles within the vegetal cells.
For example, growing seedlings in
chloroaurate solution resulted in the accumulation of stable
AuNPs within various plant tissues, as a consequence of
shoot-guided transport phenomena of the root-located
reduction processes. The obtained intracellular monodispersed
and immobilized gold nanoparticles may act as stable catalysts
Key Features:
Reliable;
Eco-friendly reducing and
capping agents.
Sesbania drummondii
Biological Source Nanoparticles Morphology Size (nm)
Biosynthesis
Location
Bacteria
Bacillus subtillus Octahedral 5-30 Intracellular
Pseudomonas aeruginosa Spherical 5–30 Extracellular
Escherichia coli Triangular 25–33 Intracellular
Rhodopseudomonas
capsulata
Spherical 10–20 Extracellular
Stenotrophomonas
maltophilia
Spherical 40 Extracellular
Brevibacterium casei Spherical 10–50 Extracellular
Bacillus licheniformis Cubic 10–100 Intracellular
Pseudomonas veronii Different shapes 5–25 Extracellular
Klebsiella pneumoniae Spherical 35–65 Extracellular
Marinobacter pelagius Spherical > 20 Extracellular
Geobacillus sp. strain ID17 quasi-hexagonal 5–50 Intracellular
Fungi
Fusarium oxysporum Spherical and Triangular 8–40 Intracellular
Rhizopus oryzae Different shapes (rod, triangle, hexagon) 9–10 Intracellular
Algae
Shewanella algae Different shapes (triangular, hexagonal, nanoplates) ~10 Extracellular
Sargassum wightii Greville Spherical 8–12 Extracellular
Chlorella vulgari
Different shapes (triangular, truncated triangular,
hexagonal)
800–2000 Extracellular
Plant
Aloe vera Triangular 2–8 Extracellular
Cassia auriculata Triangular, hexagonal 15–25 Extracellular
Hibiscus rosa-sinensis Spherical 16–30 Extracellular
Ananas comosus Spherical 10–11 Extracellular
for future applications. Moreover, the intracellular biosynthesized gold clusters capped with organic
ligands possess the ability to covalently attach to biological substances and structures and protein
molecules, indicating that they are promising tools with biological labeling potential applications.

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• Plant mediated method
This method involves in revaluing the polyphenol-based
secondary metabolites from plants as efficient reducing agents
for metallic precursors. The hydroxyl groups within the
plant-derived polyphenols would be successfully taken part in
gold ions reducing process via encouraging the oxidation
reaction and the specific formation of quinine forms. Moreover,
when a hard ligand specifically binds soft metals, e.g. Au+
, no
complex compounds will be encouraged to form. However, the
concerned soft metal will undergo reduction processes and
finally form AuNPs. Many leaves and fruits have been successfully
used to produce AuNPs, such as L. (Barbados
nut), L. (Coat buttons),
Creative Diagnostics provides a comprehensive list of gold nanoparticles including spherical gold
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Key Features:
Spontaneous, eco-friendly and
cost-effective synthesis;
Suitable for large scale
production;
Avoidance of time-consuming
maintenance of cell cultures;
Particle size and shape
controllable.
Plant extract
solution
Alkaloids
Terpenoids
Phenolics
Proteins
Vitamins
Sugars
Co-enzymes
Naphthoquinones
Anthraquinones
Nitrate reductase
Gold salt
solution
AuNPs
Green synthesis
L. (Eggplant), L. (Calotropis), L. (Papaya), L. (Datura),
and banana peel powder. In addition to leaves and fruits, bark
or stem extract and seed extract are also applied to the synthesis of AuNPs.
Jatropa curcas
Tridax procumbens Solanum melongena
Calotropis gigantea Carica papaya Datura metel
Citrus reticulate, Citrus limon, Citrus sinensis
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8. References:
1. Turkevich, J., Stevenson, P. C., & Hillier, J. (1951). A study of the nucleation and growth processes in the
synthesis of colloidal gold. Discussions of the Faraday Society, 11, 55-75.
2. Gangwar, R. K., Dhumale, V. A., Kumari, D., Nakate, U. T., Gosavi, S. W., Sharma, R. B., ... & Datar, S.
(2012). Conjugation of curcumin with PVP capped gold nanoparticles for improving bioavailability.
Materials Science and Engineering: C, 32(8), 2659-2663.
3. Faraday, M. (1857). X. The Bakerian Lecture.—Experimental relations of gold (and other metals) to
light. Philosophical Transactions of the Royal Society of London, (147), 145-181.
4. Waters, C. A., Mills, A. J., Johnson, K. A., & Schiffrin, D. J. (2003). Purification of dodecanethiol
derivatised gold nanoparticles. Chemical Communications, (4), 540-541.
5. Sharma, N., Bhatt, G., & Kothiyal, P. (2015). Gold Nanoparticles synthesis, properties, and forthcoming
applications-A review. Indian Journal of Pharmaceutical and Biological Research, 3(2), 13.
6. Akhavan, A., Kalhor, H. R., Kassaee, M. Z., Sheikh, N., & Hassanlou, M. (2010). Radiation synthesis and
characterization of protein stabilized gold nanoparticles. Chemical Engineering Journal, 159(1-3), 230-235.
7. Mafuné, F., Kohno, J. Y., Takeda, Y., Kondow, T., & Sawabe, H. (2001). Formation of gold nanoparticles
by laser ablation in aqueous solution of surfactant. The Journal of Physical Chemistry B, 105(22), 5114-5120.
8. Barabadi, H., Honary, S., Ebrahimi, P., Mohammadi, M. A., Alizadeh, A., & Naghibi, F. (2014). Microbial
mediated preparation, characterization and optimization of gold nanoparticles. Brazilian Journal of
Microbiology, 45(4), 1493-1501.
9. Sengani, M., Grumezescu, A. M., & Rajeswari, V. D. (2017). Recent trends and methodologies in gold
nanoparticle synthesis–a prospective review on drug delivery aspect. OpenNano, 2, 37-46.
10. Ramezani, F., Ramezani, M., & Talebi, S. (2010). Mechanistic aspects of biosynthesis of nanoparticles
by several microbes. Nanocon, 10(12-14), 1-7.
11. Sharma, N. C., Sahi, S. V., Nath, S., Parsons, J. G., Gardea-Torresde, J. L., & Pal, T. (2007). Synthesis of
plant-mediated gold nanoparticles and catalytic role of biomatrix-embedded nanomaterials.
Environmental science & technology, 41(14), 5137-5142.
5
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