Carbonate is regularly used and i do not know why. You just need a buffer that does not impair the adsorption process. Moreover you should use a buffer that favors the adsorption (through hydrophobic interactions) of your protein

Fotgot to answer the question. I would use 100 mM.

Hi Robert,

In my experiment, I used 100 mM sodium bicarbinate buffer (pH=8.3 or 9) for Ag or Ab coating.

Best,

Carbonate-bicarbonate buffer has no hydrophobic ions that would interfere with protein/peptide binding with polystyrene wells and has somewhat reasonable pH buffering range not far from physiological pH, most proteins and peptides survive it. Phosphate ions may bind to certain proteins and change their conformation, so phosphate buffers are not suitable as a standard system for Ag or Ab coating. The use of 100 mM carbonate-bicarbonate buffer may favour hydrophobic interactions between proteins and polystyrene support better than 50 mM buffer. Also you should choose the buffer pH (usually in the pH range 8.4-9.6) depending on pI of your protein/peptide - proteins and peptides with the sum zero charge will be more readily adsorbed onto a plate.  

Thanks everyone