Use of attractive forces to create a self-assembled film of charged nano-particles with a controlled packing

Abstract

A positively charged monolayer was formed at an air/water interface with a Langmuir trough by spreading a solution of dioctadecyldimethylanmonium bromide (DODAB) in chloroform with a syringe at an air/water interface. Compression of the monolayer to high surface pressures gave a densely packed monolayer with a high charge density. Negatively charged bare gold nano-particles could adsorb to the DODAB monolayer, if they were injected under the monolayer. The number of nano-particles adsorbing to the monolayer and their ordering could be controlled via the nano-particle size and the injected volume. Particles with a diameter of 10nm gave a reasonably well-packed monolayer of gold particles, while particles with a diameter of 50nm gave only a sparse monolayer of gold particles. The adsorption is thought to be controlled by the attractive electrostatic and van der Waals interactions acting between the gold particles and the DODAB molecules, the flow of the water in the subphase, and the gravitational force arising from the weight of the gold particles.