Hi Samrat, as you can neither use the normal phospate buffer, due to heavy metal= precipitation, I recommend you to use HEPES (pKA= 7.2.). On the other hand, I am curious why you can not use the normal Tris-HCl one, I have never observed silver precipitation in presence of this buffer. Regards. 

@Felipe : AgCl should be highly insoluble: 

https://en.wikipedia.org/wiki/Silver_chloride

This fact is even exploited in Silver Chloride electrodes:

https://en.wikipedia.org/wiki/Silver_chloride_electrode

There are even more hints in some resources online from electrode suppliers:

http://www.bio-rad.com/featured/en/tris-buffer.html

"When making a Tris buffer, it is important to use a Tris-compatible electrode when adjusting the pH. Single-junction Ag/AgCl electrodes can exhibit instability when used with Tris buffers because the silver gradually precipitates and clogs the electrode."

That being said, there is a lot of work already on the detection of metal ions and or the complexation of Ag+ by DNA:

http://pubs.rsc.org/en/content/articlepdf/2007/cc/c4cc02981j

Tom Vosh works on this as well:

http://synbio.ku.dk/researchers/tom_vosch/

You could check what these groups use as buffer systems.

Thanks Felipe and Kris for your suggestions.