Hydrophilic. To render the surface hydrophobic you need to have something hydrophobic on the surface. Nevertheless, the carboxylic groups can be protonated at low pH and this will decrease the hydrophilicity of the surface.

Citrate-capped gold nanoparticles are systems that, depending on the environment to which they are exposed, can be classified as hydrophobic or hydrophilic, so it is normal that you have seen papers in which these particles are classified differently ...

@Omat Gonzalez-Ortega Francisco Manuel Marquez thank you for your reply.

Hello Ibrahim,

as a synthetic inorganic chemist I must admit that I'm not a specialist in this field enough to provide a qualified answer to your very interesting question. From reading some relevant papers I got the impression that Francisco Manuel Marquez is right in that both situations (hydrophobic and hydrophilic) are possible depending on the environment. In this context I recommend that you read the following very interesting review article:

Hydrophobic and Hydrophilic Au and Ag Nanoparticles. Breakthroughs and Perspectives

Article Hydrophobic and Hydrophilic Au and Ag Nanoparticles. Breakth...

Luckily this very useful paper has been posted by the authors as public full text on RG. Thus you can freely download it as pdf file. I hope it helps claryfying your question.

Good luck with your research and best wishes, Frank Edelmann