Stability of a HAuCl4 solution

[kephra]

Silver does not as long as the ppm is not too high.
40 ppm gold nanoparticles are about the same size as 20 ppm silver particles in the same size range, since gold is about twice as heavy as silver.  So at that concentration, gold is probably stable with nothing else.
I have never tried it, but the historical record says it works.  Perhaps you will give it a shot.  I don't if the exact metal makes a difference either, but it should be a metal which forms a chloride.

[LionHeart]

Silver does not need a capping agent?

Is the particle size affected by the ppm?

[kephra]

Silver does not need a capping agent?

Is the particle size affected by the ppm?

Below about 20 ppm, the electrostatic repulsion is enough to stabilize the particles.  When the ppm exceeds the amount that elecrostatic repulsion can handle, you need to add stabilizers.