Hi Diptiman, do you know the chemistry used to immobilize the cysteines to the gold nanoparticles you are working with? Is this an -S-Au bond?

There are ways to remove organic materials from gold that involve strong acids, if you want to go that way.

You can substitute it with another thiol like mercaptoethanol or DTT in sufficient concemtration - perhaps 0.1-0.2 M will work. However, you will get gold particles bound to that new thiol instead of Cys.

@ Dr. Stefansson. Thank you very much for your suggestions. It is an -S-Au chemistry. Can you please suggest me some literature for that. 

@ Dr. Dubey. Thanks for you suggestion as well. But I have a limitation of my system. I can't replace the "S" bound to gold to any other "S" molecule which has more affinity though. I need to make my gold particles free of sulphur. 

Hi Diptiman, here are some papers that I hope can help you

http://www.ncbi.nlm.nih.gov/pubmed/18241042

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0CDgQFjAD&url=http%3A%2F%2Fwww.bioee.ee.columbia.edu%2Fcourses%2Fupload%2FBibliography%2Fworley_ozone_sulfur_gold_1995.pdf&ei=OHdrVKSjL8f7sASC0IK4Dg&usg=AFQjCNGyyIB3tOJvDm6FvBxcE6LMeimDRw

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCsQFjAB&url=http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F7324059_Formation_and_removal_of_alkylthiolate_self-assembled_monolayers_on_gold_in_aqueous_solutions%2Flinks%2F00b7d524db4cfb619e000000&ei=OHdrVKSjL8f7sASC0IK4Dg&usg=AFQjCNH8Rkpb5IFqOVNY99d10nNr0cL-Bw