preparation of gold nanoparticlec
There are basically two different procedures to form gold nanoparticles, depending on whether they are going tobe 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 systematicstudy of the formation of colloidal gold nanoparticles started with thepioneering work of Turkevich (Turkevicph, Stevensoan et al. 1951), who studied the size and shape of goldnanoparticles 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 commonexperimental 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), aphase transfer catalyst (a quaternary ammonium salt) and a ligand able tocoordinate with the gold nanoparticles (phosphine). A low molar AuCl4−/ligand ratio has a positive consequence on the stability of the resultingcolloid against agglomeration, but can play a negative role on the catalyticactivity. Upon formation of colloidal gold in the aqueous phase, thenanoparticles will coordinate with the ligand, rendering the colloidshydrophobic and soluble in the organic solvent. This coordination will producethe phase transfer from water to the organic solvent. The colloidal suspensioncontaining gold has to be used for further applications, since solvent removalis 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 usedprocedure 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)3and related oxy/hydroxides that adsorbs into the solid and is then reduced tometallic 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 andthe other experimental conditions (nature of the alcohol, temperature and timeof the reduction, calcinations procedure, etc.) can provide a certain controlof 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
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