[confusedstud]

Recently in an lab experiment to synthesize gold nanoparticles we used trisodium citrate to reduce the gold ions into gold. We added the sodium citrate while stirring and heating it and analyzed the solution in intervals of 5, 15 and 30 minutes. I have a few questions regarding this.


1. Why are there 2 peaks in the Dynamic Light Scattering report?
As of now, the possible reasons i came up with by researching on the topic is that there could be gold nanorods forming instead of nanospheres causing 2 peaks of different sizes to form. Another reason could be that the stirring was not uniform and so a multidiverse gold nanoparticles are formed causing 2 peaks to form. Are these reasons viable? And are there other reasons?

2. How would the size of the nanoparticles change over time (regarding the 5, 15 and 30 minutes interval)?
Because my data has 2 peaks I'm not sure which data to use. However I feel that the size should increase over time. This is because the concentration of the citrate ions decreases over time as more of the gold ions are being reduced. Since the citrate acts as a capping agent as well, the decrease concentration would mean that it is less able to stabilize the newly formed gold nanoparticles and so there is an increase in size.

3. How would the strength of the reducing agent affect the size of the nanoparticles?
I found out online that the stronger the reducing agent, the smaller the size. However I could not find any reasons for this.

[Enthalpy]

3. I expect that more nucleating sites create smaller particles. Stronger reactants could indirectly multiply the nucleating sites, but that's probably not the strongest factor.

2. Can the nanoparticles do anythng else than grow? Metallic gold is seriously difficult to dissolve.

[confusedstud]

Hmm since the nanoparticles increases in size then why would the colour change from violet to red as shown in this video? https://www.youtube.com/watch?v=-FRHoYuLeqo

I thought that the colour change should have been from a red to violet instead. This is because initially when the particles are small, they would absorb shorter wavelengths and reflect out the longer wavelengths (so red would be reflected out). And when the particles become larger they would absorb longer wavelengths and reflect out the shorter wavelengths (so violet would be reflected). Why was the opposite trend observed?

[zetasizer]

Concerning the extra peak, this may be due to rotational diffusion occasionally observed for colloidal gold nanoparticles:

http://www.materials-talks.com/blog/2014/11/17/tips-tricks-for-nanoparticle-characterization/#gold
http://www.materials-talks.com/wp-content/uploads/2014/11/Rotational-Diffusion-and-Results-From-DLS-Measurements.pdf

[jeffmoonchop]

how clean are your solvents? could be impurities in the solvent. Also light is scattered in emulsion colloids as well as sol colloids. Ive done experiments where I haven't let my sample fully relax before testing size and compared to relaxed samples. Turns out the unrelaxed samples appeared larger than the relaxed sample because the colloidal suspension had not finished crystal growth in tiny pockets of high concentration which are undergoing crystal growth. These bubbles refracted the light at an increased diameter because once crystal growth had finished the overall size had decreased and showed a sharper peak.

Could be unreacted citrate giving you the other peak was your ratio exactly 1:1? Or impurities in solvents did you run blank samples on the DLS to make sure they were clean?

[jeffmoonchop]

and yes the stronger the reactants the higher the supersaturation of the solution and the higher likelihood of many nucleation sites. The particles will be smaller because the more nucleation sites the less the particles can grow further because the gold has been used up in other sites.