Carboxylic Acid Functionalized Gold Nanoparticles, unlike other gold nanoparticles, are pre-ready for covalent conjugation. The carboxlic acid groups on the surface of the gold nanoparticles are easily reacted with primary amines via a condensation reaction which yields strong amide bonds. Usually, this is achieved by using a water-soluble carbodiimide linker such as EDC.1,2
The process to attach a ligand using EDC chemistry is most efficient in acidic conditions and should be conducted in buffers lacking extraneous carboxyls and amines. The process is explained briefly as:
(Note the O-acylisourea intermediate is unstable in aqueous solutions. If unreacted it will hydrolyze and regenerate the carboxyl group.)
Another popular, and more efficient, route for conjugation of carboxylic acid functionalized nanoparticles uses N-hydroxysuccinimide (NHS) or the water-soluable analogue (Sulfo-NHS). Including NHS in EDC coupling creates a stable amine-reactive intermediate and improves the overall conjugation efficiency.
Briefly the stages of this protocol are:
 Jazayeri, Mir Hadi, et al. "Various methods of gold nanoparticles (GNPs) conjugation to antibodies." Sensing and Bio-Sensing Research 9 (2016): 17-22.
 R.-A. Sperling, W. Parak “Surface modification, functionalization and bioconjugation of colloidal inorganic nanoparticles” Philos. Trans. R. Soc. Lond. A Math. Phys. Eng. Sci., 368 (1915) (2010), pp. 1333–1383.