preparation of gold nanoparticlec

    There are basically two different procedures to form gold nanoparticles, depending on whether they are going to be dispersed in a liquid or supported on a solid.

     In spite of the fact that the formation of gold nanoparticles was known since ancient times (Astruc 2004), many researchers consider that the first systematic study of the formation of colloidal gold nanoparticles started with the
pioneering work of Turkevich (Turkevicph, Stevensoan et al. 1951), who studied the size and shape of gold
nanoparticles obtained by treating aqueous solutions of AuCl₄⁻
with various reducing agents. Sodium citrate was a convenient reducing agent,
giving narrow particle size distribution around 20 nm. Another common
experimental procedure to form colloidal gold solutions is the two - phase method
(Fink, Kiely et al. 1998) (Kang and Kim 1998) (Ghosh, Nath et al. 2004) (Praharaj, Ghosh et al. 2005), in which, starting from a AuCl₄⁻
salt in aqueous solution, the reduction with hydrazine, a metal hydride, etc.
is performed in the presence of an immiscible organic solvent (toluene), a
phase transfer catalyst (a quaternary ammonium salt) and a ligand able to
coordinate with the gold nanoparticles (phosphine). A low molar AuCl₄⁻
/ligand ratio has a positive consequence on the stability of the resulting
colloid against agglomeration, but can play a negative role on the catalytic
activity. Upon formation of colloidal gold in the aqueous phase, the
nanoparticles will coordinate with the ligand, rendering the colloids
hydrophobic and soluble in the organic solvent. This coordination will produce
the phase transfer from water to the organic solvent. The colloidal suspension
containing gold has to be used for further applications, since solvent removal
is normally not advisable due to massive particle agglomeration (Fig 1).

Fig 1: Summarizes the two - phase methodology.

For the preparation of gold nanoparticles supported on insoluble solids, the most widely used procedure is the precipitation – deposition method (Ivanova, Petit et al. 2004) (Yan, Mahurin et al. 2005) (Zhu, Liang et al. 2006) . Starting from an aqueous solution of HAuCl₄,
addition of a base leads to precipitation of a mixture of Au(OH)₃
and related oxy/hydroxides that adsorbs into the solid and is then reduced to
metallic gold by boiling the adsorbed species in methanol or any other alcohol.
In this procedure, it has been established that the pH of the precipitation and
the other experimental conditions (nature of the alcohol, temperature and time
of the reduction, calcinations procedure, etc.) can provide a certain control
of the particle size of the resulting nanoparticles  (Haruta, Yamada et al. 1989)(Fig 2).

Fig 2: Illustrates the steps required in the formation of supported gold nanoparticles

Astruc, M.-C. D. a. D. ( 2004). "Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis,
and Nanotechnology." Chem. Rev. 
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Turkevicph, J., P. c. Stevensoan, et al. (1951). "A STUDY OF THE NUCLEATION AND GROWTH PROCESSES IN THE SYNTHESIS OF COLLOIDAL GOLD." Discuss. Faraday Soc.  11.

Ghosh, S. K., S. Nath, et al. (2004). "Solvent and Ligand Effects on the Localized Surface Plasmon Resonance (LSPR) of Gold Colloids." J. Phys. Chem. B  108:
13963-13971.

Fink, J., C. J. Kiely, et al. (1998). "Self-Organization of Nanosized Gold Particles." Chem. Mater. 10: 922-926.

Kang, S. Y. and K. Kim (1998). "Comparative Study of Dodecanethiol-Derivatized Silver Nanoparticles Prepared in One-Phase and Two-Phase Systems." Langmuir  14:
226-230.

Zhu, H., C. Liang, et al. (2006). "Preparation of Highly Active Silica-Supported Au Catalysts for CO Oxidation by a Solution-Based Technique." J. Phys. Chem. B 110: 10842-10848.

Yan, W., S. M. Mahurin, et al. (2005). "Effect of Supporting Surface Layers on Catalytic Activities of Gold Nanoparticles in CO Oxidation." J. Phys. Chem. B 109: 15489-15496.

Praharaj, S., S. K. Ghosh, et al. (2005). "Size-Selective Synthesis and Stabilization of Gold Organosol in CnTAC: Enhanced Molecular Fluorescence from Gold-Bound Fluorophores." J.
Phys. Chem. B 109: 13166-13174.

Haruta, M., T. Kobayashi, et al. (1987). "Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature far Below 0 °C " Chemistry Letters   16: 405-408

Ivanova, S., C. Petit, et al. (2004). "A new preparation method for the formation of gold nanoparticles on an oxide support." Applied Catalysis A: General   267:
191–201.