Gold Nanoparticles Article Summaries.
Original Article Summary: Turkevich, Stevenson, & Hillier, 1951.
In their 1951 article, Turkevich, Stevenson, and Hillier explore the nucleation processes and growth of colloidal gold nanoparticles during synthesis under different methods. The paper explores the standard preparation methods of colloidal gold which all use a nucleating agent which usually comprises reducing agents that later form a mixed polymer with a chloroauric ion before reduction to the nucleus occurs. The authors of this paper explore gold colloids synthesis methods such as Bredig’s electrical arc synthesis, the Faraday method, and reduction methods using acetone, tannin gold solution, oxalic acid, Hydroxylamine hydrochloride, citric acid, acetylene and carbon monoxide. However, the synthesis method highlighted due to its efficacy is the reduction by citrate method since the citrate first acts as the reducing agent and later as the stabilizing agent for gold nanoparticles by controlling the electrostatic interactions of the lone electron pair on the metal surface and oxygen. This ensures that the particles do not grow freely and exponentially, and later agglomerate to form non-uniform gold nanoparticles which are not desired.
In the citrate reduction synthesis method, Gold Chloride is dissolved in water while heating and stirring the mixture and then Trisodium citrate dihydrate is dissolved in some water and added to the heated mixture. The mixture is set to reflux for a few minutes as the citrate reduces gold to form a uniform wine-red solution. The colloid formed using this technique have uniform diameters of about 20 nm when observed under an electron microscope. However, the average size of the gold colloid particles, the characteristics of particles sizes as seen on the distribution curves, and the deviation of particle sizes from the average sizes were found to be determined by the nucleation process involved, the amount of gold, and the law of growth which was observed to be exponential. For example, in the citrate reduction synthesis method, the size of gold nanoparticles relies heavily on the citrate concentration. Apart from these general ideas, the paper goes on to explore the intricate chemistry behind the roles of Sodium citrate, the rate of auric ions disappearance in the citrate reduction method, nuclei isolation, the nucleation curves characteristics, as well as nucleation and particle growth mechanisms.
Turkevich Method Revisited Article Summary: Kimling et al., 2006.
In this paper, the authors Kimling et al. (2006) revisit the Turkevich method of Gold nanoparticle synthesis over half a century later. In the article, the citrate reduction synthesis technique proposed by Turkevich et al. in 1951, and further refined by G Frens in 1973 is explored in more detail and compared to other synthesis methods such as ascorbate reduction and Ultraviolet initiated reduction. The refined process by G Frens helped Kimling et al. (2006) find a general relation between the gold-to-reducing agent ratio, and the final gold nanoparticle sizes. It was found that the relation is generally autonomous of the absolute concentrations. However, there are limitations for producing stable particles for long-term use which is below 2 millimolar (mM) and even between 1-2 mM gold concentration, it is found that the particle sizes are highly sensitive to the concentration. The best Gold nanoparticle sizes are achieved at gold concentrations below 0.8 mM, and for particles smaller than 40 nm a 13-16% size dispersion can be achieved. The investigation reveals that larger particles are elongated, and the size definition worsens. In contrast, in the UV initiated reduction method, the particles produced are more spherical even for larger particles although platelets and triangle shapes are also abundant. Finally, the ascorbate reduction technique is found to produce particles with the best spherical definition compared to the other methods.
The paper also explores particle formation kinetics and a multistep process of cluster formation, appearance of primary particles, collapsing into larger particles, and the growth of these particles by the reduced gold residue in the solution is observed. This last stage in the process is quite critical in helping understand why the particles attain a spherical definition upon the capping/passivation effect of the citrate.
Conclusively, the results of the research by Kimling et al. (2006) indicate that controlling the reduction conditions in the conventional Turkevich synthesis method can help define the particle sizes and shape. The authors also point out that the synthesis procedures explored in their research can be applied in a wide range of precursors such as the formation of palladium, silver or platinum particles by controlling the strength of the reductants, and also the reaction conditions such as UV irradiation and temperature. Following the successful results of their research, Kimling et al. (2006) finally conclude that their proposed technique is viable for the production of various particle materials (apart from Gold) by combining the right measures of reducing agents e.g. ascorbate and citrate with the appropriate reaction conditions such as UV irradiation and temperature to achieve more uniform and stable nanoparticles.
References:
Kimling, J., Maier, M., Okenve, B., Kotaidis, V., Ballot, H., & Plech, A. (2006). Turkevich Method for Gold Nanoparticle Synthesis Revisited. The Journal of Physical Chemistry B, 110(32), 15700-15707. http://dx.doi.org/10.1021/jp061667w
Turkevich, J., Stevenson, P., & Hillier, J. (1951). A study of the nucleation and growth processes in the synthesis of colloidal gold. Discuss. Faraday Soc., 11, 55-75. http://dx.doi.org/10.1039/df9511100055
