Hi Sachin,

You could first coat the gold nanoparticle (AuNP) with 11-Mercaptoundecanoic acid (11-MUA) and activate the acid group with EDC/NHS. Then to use these particles to react with antibodies in the aqueous solution with constant stirring or sonication bath to facilitate dispersion of hydrophobic AuNP. At this step, ethanol or other non-toxic organic solvent can be added in small quantities (say, 5 - 30%). If the alcohol is low concentration, it may not interfere with the stability of the antibody.

Alternatively, gold nanoparticles can be coated with appropriate ratio of PEG-SH (or any other hydrophilic thiolated molecule, e.g. cysteine) and 11-MUA to have a aqueous soluble AuNP. Then proceed with EDC coupling reaction.

Best Wishes.

Thanks Shubaash.

Thanks John. This is really helpful

You can use modified PEG such as carboxy PEG thiol. Then by using EDC/NHS chemistry, the antibody can be conjugated with the activated -COOH group of PEG by forming an amide bond.