What is the role of NaCl in the functionalization of gold nanoparticles?
$\ce{NaCl}$ acts as a charge suppressor so that the negatively charged gold nanoparticle and a negatively charged ligand (with thiol) are not longer charge repelled and can experience a chemical bond. $\ce{NaCl}$ is easily broken into $\ce{Na+}$ and $\ce{Cl-}$ so that the $\ce{Na+}$ can pacify the negative charge on the surface of the gold nanoparticle to become less negative and approach zero charge. Other higher charged salts such as $\ce{Mg^2+}$ impart even greater pacification.
Here are some good references:

J Liu, Y Lu. Preparation of aptamer-linked gold nanoparticle purple aggregates for colorimetric sensing of analytes. Nature protocols, 2006. nature.com

B Du, Z Li, Y Cheng. Homogeneous immunoassay based on aggregation of antibody-functionalized gold nanoparticles coupled with light scattering detection. Talanta, 2008. Elsevier. From this last ref:



in $\pu{10 mM}$ phosphate-buffered solution (pH 7.0) containing $\pu{1.0 mmol L−1} ~\ce{NaCl}$ at room …