Hello Tarmo,

To avoid preparation of new solutions,

You can simply decrease the volume of the citrate solution while maintaining the gold precursor (concentration and volume), the temperature and stirring rate you used earlier.

Another way, you can increase the volume of the gold precursor you have prepared and used earlier, and maintain the volume of the citrate solution, temperature and stirring rate.

As you had earlier started it is also possible to use those particles as seeds to grow larger particles. Nevertheless, you may need to prepare different growth solutions and also optimize the concentration of the seed you may need in the seed-mediated growth.

Hope these suggestions works for you.

Good luck

Hei Tarmo Mitä kuuluu? The first thing to do is a Stokes' Law calculation for Au spheres of various sizes to see what can and will remain in suspension. Such a table is found in E2490-09 (15). You'll find that gold particles of 100 nm will settle reasonably rapidly and this is the reason for the difficulty in making such large systems.

Hyvä Suomi!

Hei! Kiitos teille vastauksista!

I decreased the amount of citrate (to 0.7:1 ratio for citrate:Au) and increased the temperature, which resulted in blue color (larger particles). However, those are not so stable. I added PVP, which make them stable. Nevertheless, I would like to use citrate only. I tried to add more citrate after the synthesis, but it didn't work.

I think that seeding may help in increasing the size of the particles, but not the stability of the particles.

Ystävällisin terveisin,

Tarmo

Hei Tarmo The issue here is to try to prevent probable sedimentation with the high specific gravity of Au (19.3 g/cm3). Have you done the Stokes' Law calculation? Citrate is a small molecule (and is, of course, the reducing agent in the Turkevich method) and an excess is as bad as too little with an ionic/electrostatic stabilizer. Using a steric stabilizer (such as 50 kDa PEG or PDI) may help somewhat as it reduces the density of the particles and also prevents aggregation of the Au core as the Au can't interact directly with another Au. Larger sizes are favored by higher concentrations of precursor (chloroauric acid usually). Obviously thickening the liquid/continuous phase will also reduce the sedimentation rate and may allow larger particles to stay in suspension. Some general information is present in this webinar:

Metal colloids - their preparation, application and characterization

https://www.brainshark.com/malvern/vu?pi=529368070&b=1&tx=120486&c1=22

Terveisin,

Alan

"Have you done the Stokes' Law calculation? "

Nope. Seems to be easy to calculate but how I can use the value?

Anyway, I used PVP and got stable particles around 100 nm. Mostly spherical but there was triangles too.

Stokes' Law will give the minimum time for settling based on spheres. Everything else conspires to keep the particles in suspension (irregularity, lower density due to polymer layers, convection currents, Brownian motion). Gold has a high density and if you calculate that the particles will fall 1 cm in 1 hour (terminal velocity) then you can be sure that they'll settle in time. That boundary is somewhat above 100 nm for Au. Incorporation of PVP lowers the effective density of the particles and keeps them in suspension longer. The point of doing the calculation is to set an upper boundary for the size that can be kept in suspension. For example, magnetite with a density of ~ 5.5 g/cm3 will settle 1 cm in 4.2 days (100 nm) but at 1 micron (1000 nm) will settle 1 cm in 1 hour approximately.

Thank you for your demonstrative answer! I learned something new again. :)

For higher size particles you can reduce the concentration of precursor, Chloroauric acid or try seed mediated method using sodium borohydrate seed particles and trisodium citrate for higher size

Poorani Gananathan Stokes' Law still applies no matter what the precursors are or what state they are in. In some instances one can increase the viscosity of the continuous phase. I've never been a fan of borohydride reduction as B is incorporated into the metal and acts as a catalyst poison. Typically I would prefer hydrazine hydrate (5 or 10%) or hydrogen depending on the application.

Tarmo Nuutinen

Hei Tarmo!

It's been awhile since your posting but the EU has done some research on some gold sols. Here is one sentence from the reference (attached):

'The 250 nm as well as 500 nm particles showed very strong sedimentation and therefore it was decided to take into account only 10 nm, 30 nm and 100 nm particles.'

As I'd said, the onset of sedimentation is somewhere in the 100 - 250 nm region.

Ystävällisin terveisin

Alan

Thank you! Kiitos!

Tarmo Nuutinen Ole hyvä

Hi you can do seed mediated synthesis in the presence of hydroquinone as reducing agent and trisodium citrate as capping.. check my profile for Nanotechology paper