In the present work it was established that benzoic acid on the surface is located vertically . The area of the carboxyl group is 0.21 nm2. Therefore, using the TEM method, determine the average nanoparticle size. Then determine the concentration of nanoparticles total area and calculate the number of molecules of benzoic acid.

Spectrochim Acta A Mol Biomol Spectrosc. 2013 Mar;104:41-7. doi: 10.1016/j.saa.2012.11.103. Epub 2012 Dec 7.

Adsorption of benzoic acid, phthalic acid on gold substrates studied by surface-enhanced Raman scattering spectroscopy and density functional theory calculations.

Gao J1, Hu Y, Li S, Zhang Y, Chen X.

Best regards.

You can perform Boehm titration which is an acid-base titration method used to determine the amount of surface oxygen groups. Although the method is reported for quantification of oxygen functionalities on carbon surfaces like graphene oxide, I think it can work for gold nanoparticles as well.  Please go through the following link:

http://www.sciencedirect.com/science/article/pii/S0008622310003271

In one of our works, we had used NaHCO3 following this method to calculate the acidic functionalities on carbon dot surface:

http://pubs.acs.org/doi/abs/10.1021/acs.joc.6b02914

The surface concentration of carboxylic group on the particles could be estimated by combining acid-base titration with the determination of the surface area.