Do I understand correctly that You would like to obtain NP's with a mixed ligand (containing both -COOH and 4,4 biphenol dithiol) on the surface?

Hello

You have not mentioned which types of nanoparticles?

Anyway in general following possibilities can be there:

1. As wrote by you, if the grafting density of your already present 4,4 biphenol dithiol on the particle surface is low and you still have bare surface of the particles for additional grafting, you can graft mercapto succinic acid or 11-mercaptoundecanoic acid etc. to obtain particles with both thiol and carboxylic acid functionalities

2. If you are sure that the terminal thiol groups are free (normally they get oxidized to disulfides easily) You can carry out thiol-ene click reaction by using acrylic acid so you will convert some of the thiols into carboxylic acid terminated groups.

3. A smart approach will be, first to prepare thiol terminated self-assembled monolayers (SAMs) on glass surface then mobilize particles on them it with desired pattern. It will form the particle-SAM bonds only on the surface of particles facing towards the lower 2-D SAMs and then graft carboxy terminated compounds/ligands on the bare surface of the particles.

You will get lot of references for all approaches mentioned here in the literature.

Good luck

Thank you @ Sushilkumar A. Jadhav we agreed to use 11-MUA.

@ Sławomir Lach yes my goal is have both functional groups, free thiols to make SAM on glass surface and -COOH for EDC/NHS activation.