Synthesis of a Free-Standing Monolayer of Covalently Bonded Gold Nanoparticles

We present synthesis of a free-standing monolayer film of gold nanoparticles (AuNPs) which are linked by covalent bonds. In the method developed, the free-standing film is obtained by chemical cross-linking of the AuNPs of the core diameter of 5.6 nm that form a dense monolayer at the oil liquid interface. As the cross-linking agent, naphthalene dianhydride derivative, which forms amide bonds with the ligand molecules, is used. The AuNPs are coated with aminothiolate ligands that can change their character from a hydrophilic to a hydrophobic one via the reversible protonation/deprotonation mechanism. When adsorbed at the oil water interface, such functionalized AuNPs display amphiphilic (Janus-like) structure and self assemble into a highly stable monolayer. To bring the AuNPs at the oil water interface, an excitation of the system that leads to the formation of the oil-in-water emulsion is required. After the excitation, the AuNPs are transported onto the oil water interface on the surface of the oil droplets that carry them as their cargo. A thermodynamic mechanism explaining this cargo effect is put forward. The as-synthesized free-standing film can be easily transferred from the oil water interface onto solid support, as well as porous grids, and it is found to be stable in air.