When I started to do Gold nanoparticles, the gold and citrate solution was around 2 minutes boiling. The colour of the solution was blue-purple and size should be around 70nm because the peak of absorbation was around 700. So you can change this time to get it.

You can vary the ratio between the gold and the citrate to get the different sizes. Visit

the book published by Elsevier, "Nanomaterials for Medical Applications" at http://www.sciencedirect.com/science/book/9780123850898 and also check out

http://www.materialsviews.com/book-review-nanomaterials-for-medical-applications/

The book contains procedures for gold and other nanomaterial synthesis.

Thank you Hinojal Zazo.....I try that. But it is not stable particle size. With in 2-3 days the particles goes down or convert in to purple color. I want to make it stable more than 2 weeks. Need to try freezing mechanism....

gold NPs with diameters ranging from 15 to 20-nm

Hydrogen tetrachloroaurate (III) and trisodium citrate as starting materials. A 20-ml solution of 1 mM of HAuCl4 was prepared by adding distilled water, and the solution was boiled under continuous stirring. 1 % Na3C6H5O7 solution was prepared by dissolving 0.1 g of the salt in 10 ml of distilled water. 2 ml of this solution was added to the boiling solution of HAuCl4. The color of the solution mixture gradually changed from blue to deep red. The heating was stopped when the solution has turned deep red

This simple method not giving exact 10 nm slightly varring with citrate concentration. Red colour gold nano particle has wider range as you seen in the picture. Most of them are not stable, with the time it become 15-20 nm range and again with longer time all get aggregated. I also looking for method for stabilize those particle size.

We are making stable gold nanoparticles in the range you want. Read this paper

https://www.mdpi.com/1420-3049/26/19/5788