Thanks Meral. I will run the gel longer and check again. The problem with doing a 280nm protein quant is that I could not separate the conjugated gold nanoparticles from the excess antibody. Therefore, I can not be sure if the reading I am getting is because of the excess antibody or conjugated NPs. I posted a new question just now asking how to remove the excess antibody. 

Dear Vaideesh

You can check the antibody tagging with gold nanoparticles by gel retardation assay using agarose gel electrophoresis.

As suggested by Meral Yuce, the other methods include UV-Vis spectrophotometry to check the 280 nm protein shoulder peak on the conjugated samples.  In addition, The gold nanoparticles exhibit a characteristic surface plasmon resonance at 520 nm and with addition of antibody it shows a red shift. The red shift indicates a successful tagging of antibody to the gold nanoparticles.

Thank you Prabir. Can you see the answer I posted above regarding separating the excess antibody and let me know if this problem is well-known or if it's just me who's experiencing it.

hello,

you can use 1% Agarose gel electrophoresis to visualize aunp-antibody conjugates. if the surfaces of aunps were well conjugated with antibodies or well blocked with linkers, they would run though gel whereas naked aunps would precipitate. based on my experience, in order to understand the surface of aunps after conjugation agarose gel electrophoresis is nice method to see with naked eye :). Of course, later you are suppose to confirm the conjugatioon via several optical techniques such as UV/Vis spectroscopy. But, agarose gel electrophoresis is my favorite to see the linkage between aunps and biomolecules.