For UV/vis spec, you can make sure the absorbance comes less than the maximum absorbance unit (I guess 5 units) of the instrument. So that you can dilute 5 times or 10 times to get the proper absorbance values within limits.

For DLS/zeta sizer, you can actually do the analysis as such - without dilution. If you do for diluted samples also, it will give the desired results.

I agree with Saravanan Krishnan regarding UV-Vis. However, regarding DLS, I would like to add a point. If the gold nanoparticle suspension is a little turbid, you can dilute the sample, since otherwise it might lead to multiple scattering due to large number of particles. A 5-times dilution might suffice. However, the kcps of the diluted suspension must not be less than 200.

Nabodita Sinha

If there's turbidity in the Au sol, then it indicates that the preparation isn't 'nano' or there is substantial contamination. All solutions in the preparation should have been filtered to 0.02 microns or better. Your final material should be pristine transparent/clear (and colored based on the size - yellow, purple, blue are common).

Dilution should be avoided as it is likely to promote aggregation. If made, the dilution should be in the mother liquor which may be difficult to prepare for a nanosystem. A back-scatter (173 degrees) DLS set-up allows higher concentrations to be measured.

What is the color of the colloid that you synthesized? Is it stable as a colloid? Maybe you need to check its zeta potential also.

Just thought to share that some teaching labs use colloidal gold plus various added NaCl to observe a change in UV peak. This also shows up in DLS. As an example you might like http://www.unesco.org/new/fileadmin/MULTIMEDIA/FIELD/Cairo/images/Colloidal_Gold_Online_Lab_Handout.pdf