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.

Chemical Methods: Turkevich method, Brust method, seeded growth method, etc. e.g. reduction of HAuCl4

Physical Methods: γ- irradiation method, UV-induced photochemical method, ultrasoundassisted method, laser ablation method, etc.

Synthetic Routes of

Biological Methods: Microbial mediated method, extracellular method, intracellular method, plant mediated method, etc.

Chemical Methods

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.

• Turkevich method

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.

• 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:

Produce modestly monodisperse spherical AuNPs (10-20 nm) in water;

When producing larger particles, the monodispersity will be lost.

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.

• 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.

Physical Methods

• 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.

• 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 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.

Key Features:

Proved to be the best method for the synthesis of AuNPs with controllable size and high purity.

Key Features:

Used for the formation of single crystallite-based AuNPs;

Particle size and shape controllable.

• 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.

• 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.

Biological Methods

Key Features:

Eco-friendly and rapid synthesis of AuNPs;

Ideal for various biotechnological applications.

Key Features:

Accurate and reproducible results;

A full-fledged physical approach to produce AuNPs with tunable features.

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.

Email: info@cd-bioparticles.com

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.

Extracellular method

Penicillium crustosum

Key Features: Eco-friendly and cost-effective; No hazardous chemicals and toxic derivatives.

Mechanism of extracellular and intracellular synthesis of AuNPs.

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. Email: info@cd-bioparticles.com Tel: 1-631-624-4882

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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

Sesbania drummondii

Key Features: Reliable; Eco-friendly reducing and capping agents.

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.

Biological Source Nanoparticles Morphology Size (nm) Biosynthesis Location

Bacteria

Bacillus subtillus Octahedral 5-30Intracellular Pseudomonasaeruginosa Spherical 5–30Extracellular Escherichia coli Triangular 25–33Intracellular 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 Klebsiellapneumoniae Spherical 35–65 Extracellular Marinobacterpelagius Spherical >20Extracellular Geobacillussp.strainID17 quasi-hexagonal 5–50Intracellular Fungi

Fusariumoxysporum Spherical and Triangular 8–40 Intracellular Rhizopusoryzae

Different shapes (rod, triangle, hexagon) 9–10 Intracellular Algae

Shewanellaalgae Different shapes (triangular, hexagonal, nanoplates) ~10 Extracellular Sargassum wightii Greville Spherical 8–12 Extracellular Chlorellavulgari

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

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Plant extract solution

Alkaloids Terpenoids Phenolics Proteins Vitamins Sugars Co-enzymes Naphthoquinones Anthraquinones Nitrate reductase

Gold salt solution AuNPs

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.

Jatropa curcas

Tridax procumbens Solanum melongena

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),

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.

Calotropisgigantea Carica papaya Datura metel Citrus reticulate, Citrus limon, Citrus sinensis Email: info@cd-bioparticles.com

Creative Diagnostics provides a comprehensive list of gold nanoparticles including spherical gold nanoparticles, gold nanorods and special shape gold particles, which meets various research and development needs. Please visit our website to see more.

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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.

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