Assembled Monolayers of Hydrophilic Particles on Water Surfaces

A facile and quick approach to prepare self-assembled monolayers of water-dispersible particles on the water surface is presented. Particle suspensions in alcohols were dropped on a water reservoir to form long-range ordered monolayers of various particles, including spherical solid particles, soft hydrogel particles, metal nanoparticles, quantum dots, nanowires, single-wall carbon nanotubes (SWCNTs), nanoplates, and nanosheets. A systematic study was conducted on the variables affecting the monolayer assembly: the solubility parameter of spreading solvents, particle concentration, zeta potential of the particles in the suspension, surface tension of the water phase, hardness of the particles, and addition of a salt in the suspension. This method requires no hydrophobic surface treatment of the particles, which is useful to exploit these monolayer films without changing the native properties of the particles. The study highlights a quick 2D colloidal assembly without cracks in the wafer scale as well as transparent conductive thin films made of SWCNTs and graphenes.