Clément Sanchez tells Journal of Materials Chemistry about his hot paper.

1. Can you briefly describe what you achieved in this article? 
Functional metal nanoparticles in solution can be viewed as a nanometric piece of metal covered by a ligand shell. In this paper, we described syntheses of gold nanoparticles stabilized by a mixture of two different ligands that can self segregate at the surface because they feature little chemical affinity for one another. Among several pieces of evidence for the self segregation of the ligands, we report herein a striking blue shift of the surface plasmon band of the mixed-shell nanoparticles that strongly suggests not only a phase segregation of the ligands but also a polarization of the nanoparticles.

2. Could you explain the significance of your article to the non-specialist? 
We showed in this paper that if two ligands can self-segregate at the surface of nanoparticles and form two phases at the two poles of the particle, then two things happen: 
1. the functionalities at the end of the ligands (at the interface with the surrounding medium) can be tuned to achieve amphiphilic properties, or multi functionality (two different receptors or active sites on the same particle) 
2. the functionalities anchored at the surface can be tuned to alter the electronic properties of the particle itself: polarization of the particle can be achieved.

3. What has motivated you to conduct this work? 
In the past, we worked with the phosphinine (the phosphorus equivalent of the pyridine) as a ligand for gold nanoparticles. This molecule is not only a good ligand for nanoparticles, it also features very different electronic behavior toward the metal surface than that of thiols.  Mixed ligand shells, with thiols and phosphinines, seemed a good strategy to us to gain insight into those differences. We chose these two ligands because they have different electronic donor and electronic acceptor properties towards gold. At the scale of the particle, it creates a push-pull effect responsible for the polarization. On a broader perspective, Janus type nanoparticles are increasingly important in materials science, and the synthesis of Janus type metal nanoparticles was still a major challenge when we started the project. 

4. Where do you see this work developing in the future? 
This work should lead to applications in the field of material science, where amphiphilic nano objects are highly sought. We have demonstrated a method for achieving an amphiphilic state without the presence of an interface.  For example, with this strategy, we can make giant surfactants in an isotropic environment. On the other hand, the polarization of the nanoparticles described in the article may find applications in the field of catalysis with metal nanoparticles, as the electronic state of the surface is known to have a major impact. Such a property, namely the presence of two distinct functionalities on either side of a particle, could procure interesting applications in catalysis or sensing. 

5. Are there any particular challenges facing future research in this area? 
The major challenges in metal nanoparticle functionalisation are to completely rationalize the nature of the bonding between a nanoparticle and its ligands, the dynamics of nanoparticle synthesis and the dynamics of ligand exchange reactions. More generally, we report here on a new strategy for the synthesis of Janus type nanoparticles which consists in choosing surface functionalities that are incompatible enough to make the Janus state a free enthalpy minimum. We anticipate that we can apply this to nanoparticles of many substrates, like silica or transition metal oxide. In those cases, the surface functionalities are covalently or iono-covalently bonded, so grafting usually occurs under kinetic control. However, one can choose conditions under which the grafts acquire some mobility (temperature, presence of catalysts.), in order to let the system evolve towards its thermodynamic sink, therefore opening a route to Janus type nanoparticles of silica or transition metal oxide. For all these reasons, we think that this result is an important breakthrough.