Well, as far as I know

1,4-dithiothreitol, 2-mercaptoethanol or 2-mercaptoethylamine mediated chemical reduction of antibodies will generate antibodies fragment. These fragments can be assemble themselves on gold surface due to the exposed thiol group (of cysteine residue) in PBS or HEPES.

You can also read the paper shown in the link I have given below. They have mentioned other coupling methods in this paper as well.

http://www.ncbi.nlm.nih.gov/pubmed/10051846

 EDC/NHS chemistry is another way to attach Abs to gold NPs. 

You can utilize noncovalent electrostatic adsorption of the antibody molecules to the citrate-capped gold nanoparticles (http://www.ncbi.nlm.nih.gov/pubmed/12727808) or a more tedious process involving first, modify AuNP with cysteamine, then coupling the amino moiety to the specific bio-component. Be careful with the AuNP surfactant exchange with cysteamine step as AuNPs will tend to aggregate. You can refer to this paper for the second method (http://www.sciencedirect.com/science/article/pii/S0956566313004041)

 Use PEG which attract and give hendrence to