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In the functionalization of gold nanoparticles (GNPs) through the sulfur-metal bond, this article (among others) states that $\ce{NaCl}$ is slowly added in the GNPs solution after addition of thiol compound.

What is the role of $\ce{NaCl}$ in the functionalization step?

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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. NaCl is easily broken into Na+ and Cl- so that the 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 Mg2+ impart even greater pacification.

Here are some good references:

Preparation of aptamer-linked gold nanoparticle purple aggregates for colorimetric sensing of analytes J Liu, Y Lu - Nature protocols, 2006 - nature.com Homogeneous immunoassay based on aggregation of antibody-functionalized gold nanoparticles coupled with light scattering detection B Du, Z Li, Y Cheng - Talanta, 2008 - Elsevier … in 10 mM phosphate-buffered solution (pH 7.0) containing 1.0 mmol l −1 NaCl at room … 7.

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