Surface Modification of Gold Nanoparticles Using Template Controlled Ligand Exchange Reactions

                                         Dr. Laura Gilberg


Monolayer protected gold clusters (MPC) have attracted interest in many fields of research because
of easily tunable properties. Another attractive feature of MPC is that surfacebound alkane thiols
can be exchanged with functionalized thiols allowing the preparation of nanoparticles with
characteristic surface functionalities, so-called mixed-monolayer protected gold clusters (MMPC).
Decorating MPC with suitable binding sites gives rise to synthetic receptors, for instance, whose
properties are characteristically influenced by the gold core.

Exchange of thiols on the surface of a gold cluster is a reversible reaction, which proceeds under
thermodynamic control. Equilibrium is thus controlled by parameters such as concentration of free
thiols in solution, temperature, as well as steric and electronic effects of the substituents in
the functionalized thiols stabilizing a certain MMPC surface distribution. So the distribution of
functional groups on the surface of an MMPC might also be controlled by a suitable template
molecule present during the ligand exchange reaction, which binds to the surface functionalities.
This concept relates to the molding strategy in dynamic combinatorial chemistry used for the
identification of synthetic receptors. If successful, this approach could extent the repertoire of
reversible reactions in dynamic combinatorial chemistry and would introduce a new route for the
synthesis or optimization of nano-sized MMPC-derived receptors.

This concept was evaluated on the basis of the flavine binding MMPC described by Rotello as a model
system. An alkylthiol passivated gold nanoparticle was equilibrated with pyrene and diaminopyridine
functionalized thiols. Ligand exchange reactions were carried out in the absence and presence
template, and the resulting nanoparticles compared concerning their surface composition and binding
affinities.