Dear Mahboobeh:

Nanoparticles functionalized with ionizable ligands can exist in either dispersed or aggregated states at the same value of pH. This bistability and the related hystreresis accompanying pH changes derive from a subtle interplay between electrostatic and van der Waals forces. Upset pH change. If it does not, change the pH and using centrifugation.

DOI: 10.1021/jz100406w |J. Phys. Chem. Lett. 2010, 1, 1459–1462

Best regards.

In this publication you can see a method which allows tuning of the size of Au NPs between 2.5 and 5.5 nm 

http://gradworks.umi.com/34/48/3448426.html

If your sample is a simple colloid of nude Au NPs in water, NaCl -induced precipitation should work... If that is the case you perhaps should use a more concentrated NaCl solution. If your nanoparticles are functionalized with something, NaCl may not be so effective if it has not the power to destroy functionalization first..

Another possibility is using a nitric acid solution at 1% concentration or more, to be mixed to a similar volume of your colloid. The HNO3 action on the ultrathin oxide layer that originally may surround your Au NPs should favor aggregation and hence precipitation in few minutes

Hope all that may help you