Janus Nanoparticles by Interfacial Engineering

The intense research interest in nanoscience and nanotechnology is largely fueled by the unique properties of nanoscale materials that may deviate vastly from those of their constituent atoms and bulk forms. In order to exploit these unprecedented materials properties for the fabrication of next-generation devices and circuitries, two key aspects that are intimately related to each other have to be addressed: design and synthesis of nanoscale building blocks, and controlled assemblies of these structural units into functional architectures. Thus, it is of critical importance to look for additional nanomaterials design parameters beyond size and shape. Among these, of particular interest is the creation of amphiphilic nanoparticles which exhibit hydrophobic characters on one side and hydrophilic on the other, akin to the dual-face Roman god, Janus. These particles represent a unique nanoscale analog to the conventional surfactant molecules and thus may be exploited in the formation of functional superstructures by virtue of self-assembly. Most previous studies of Janus particles are focused on polymer-based materials. Several effective routes have been reported towards the synthesis of these biphasic particles. For instance, microfluidic flow systems have been used to prepare amphiphilic particles by the polymerization of the Janus droplets formed within the microfluidic channels. Submicronsized Janus particles were prepared by biphasic electrified jetting of two polymer precursors. In a different report, Janus particles were prepared by sputtering gold onto the top face of a polymer bead array where further functionalization on the gold surface might be achieved. Paunov and co-workers reported the fabrication of Janus particles by the replication of particle monolayers at liquid surfaces using a gel trapping technique. It should be noted that in these earlier studies, the typical sizes range from a few hundred nanometers to a few micrometers; and reports on the synthesis of nanometer-sized Janus particles are actually rather scarce. In fact, the majority of nanometer-sized Janus particles refer to bifunctional heterodimers consisting of two different particle cores. In these snowman or dumbbell-like nanostructures, the focus is generally placed on the core materials rather than on the organic capping shells. So the surface wettability of these hybrid particles may actually be very similar to that of their respective monomeric particles. In the present investigation, we adopt the Langmuir method to create nanosized Janus particles that exhibit hydrophobic characters on one side and hydrophilic on the other, by taking alkanethiolate-protected nanoparticles as the illustrating example. These particles represent a unique class of nanomaterials. They consist of a nanosized metal core on which alkanethiolates form a densely packed self-assembled monolayer by virtue of the strong affinity of the thiol group to transition metal surfaces. It has been demonstrated previously that ligand place-exchange reactions (Scheme 1) can be employed to further functionalize the nanoparticle surface, which can then be used as a point of departure for more complicated surface functionalization, for instance, by surface coupling reactions. However, typically, in these earlier studies, the particles and the new ligands are mixed in the same phase. Thus, while the final composition (up to 100 % displacement) of the particle surface layers can be controlled by the initial concentrations of the particles and the new ligands, the incorporation of