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.