Shruti Khanna

Turbidity indicates the presence of larger sub- and post-micron material. Is this present from the start or does this occur with time?

Are you diluting your system for analysis? If so, with what liquid? Are you filtering your materials/liquid continuous phases before analysis (probably to better than 0.2 microns). You should have a purple transparent solution for a 40 nm Au colloid.

Alan F Rawle Dear Alan, Thank you for your kind reply.

We are not diluting our system with any solvent, nor are we filtering our solution. Previously, I got very beautiful red colored solution but now slightly violetish coloration is observed.

Shruti Khanna

OK, the dark violet color plus turbidity indicates larger material present and perhaps different shapes of agglomerates/aggregates. See for example this previous discussion in RG:

https://www.researchgate.net/post/Does-anyone-know-how-to-prepare-stable-different-sizes-of-gold-nano-particles

My first thought would be to filter your Au sol through a 0.2 micron filter to remove this extraneous large material and see if you get stability here.

However, I suspect you have some form of contamination in your system. You won't need many ppm (20 say) of polyvalent ions to destabilize such a colloidal system. The usual suspects are Fe3+ (from rust), Cr3+ (from stainless steel), Al3+ (if Al foil or Al boats are used to weigh material), Ni2+ (from Ni spatulas - this is an extremely common situation - you may see chloroauric acid reduce when weighing it out when a Ni spatula is used), Ca2+ (from water if DI/distilled isn't used) and so on.

I'd look to clean up all your glassware with aqua regia and thoroughly washing with 18 MOhm.cm DI water before making new preparations. Ideally background/continuous phases should be filtered through 0.2 or 0.02 μm filter systems to prevent particulate contamination.

Did you check for the purity of the samples using XRD when you synthesized them first?