I don't think that diluting the gold is necessary, as I have worked in the past at a higher gold concentration. In my experience, the 'student lab' gold synthesis procedure (http://education.mrsec.wisc.edu/277.htm) is very robust. In general you bring 1mM concentration of gold salt to boil and add 10% of that volume with 1% trisodium citrate. (In the lab they start with 20mL but I have done it also with 100mL gold solution). This produces good quality 12nm gold nanoparticles.

http://education.mrsec.wisc.edu/277.htm

In a lot of paper they "forget" to mention the use of filtration to make the product more homogenous. Try filters (be carefully to choose one that won't interact with your sample) or use the extruder equipment with nitrogen.

I agree Roni's suggestion.  Previously, I have got better results after filtration

try continuous stirring during synthesis, this will help you get monodispersivity in your nanoparticles synthesized. all the best. heating by microwave irradiation would also serve your purpose. focus on calculating Full width half maxima (FWHM) of the nanoparticles thru SPR reading that ull get and compare it with the nanoparticle tracking analysis *NTA) analysis if you have access. ALL D BEST 

can you write the exact steps you used and the polydispersity you got? I have done this synthesis many times, and got good particles with PDI of 0.15-0.20 for synthesis of 12nm diameter gold nanoparticles.It is known that for larger particles ( 30nm+) the Frens method does not produce monodisperse particles and seeded growth methods are better.

Thank you Ron Gill,

I have prepared 0.01% w/w HAuCl4 aqueous solution (0.254 mM) and 1% w/w trisodium citrate dihydrate aqueous solution (34 mM). I then boiled 12.5 mL of the HAuCl4 solution followed by quick addition of 0.125 mL (or 0.25 mL) of the citrate solution, with continuous stirring. I have chosen heating temperatures ranging from 80 to 120 degree. Ruby red colour was observed and stable at between 40 min and 20 min depending on the temperature used. However when DLS was used to analyse the samples after cooling, PDI between 0.520 and 0.628 as well as double distribution curves with st. deviation ranging from 12 to 28 was achieved. 

However, when I tried to reduce the HAuCl4 conc by twice, thrice and 4X dilution, light red (maybe brilliant red) colour was observed after 2 hours, 1 hour and 3 hours respectively. Surprisingly these solutions show high monodispersity with PDI of 0.114, 0.090 and 0.160 and Zaverage of 23.55, 23.33 and 30.76 respectively but the long synthesis time used is of concern to me as this is not reported in literature.

I don't think that diluting the gold is necessary, as I have worked in the past at a higher gold concentration. In my experience, the 'student lab' gold synthesis procedure (http://education.mrsec.wisc.edu/277.htm) is very robust. In general you bring 1mM concentration of gold salt to boil and add 10% of that volume with 1% trisodium citrate. (In the lab they start with 20mL but I have done it also with 100mL gold solution). This produces good quality 12nm gold nanoparticles.

http://education.mrsec.wisc.edu/277.htm

Hi George,

if you like you can try this method:

http://pubs.acs.org/doi/abs/10.1021/la503209b

It works extremely well for me.  Larger AuNP can be obtained by seeded growth, e.g. with this strategy:

http://pubs.acs.org/doi/abs/10.1021/la201938u

Regarding the DLS measurements you should consider that organic byproducts of the synthesis can also contribute to the signal. To have a meaningful analysis you can try syringe filtration of the sample (0.2 µm, PTFE, for AuNP with d ~ 12 nm) before DLS measurements; complementary TEM analysis would be ideal.