Adsorption of carboxylate-modified gold nanoparticles on an octadecylamine monolayer at the air/liquid interface

Gold nanoparticles (Au NPs) were prepared and surface-modified by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid groups (MSA-Au). Octadecylamine (ODA) was used as a template monolayer to adsorb the Au NPs dispersed in the subphase. The effect of MSA concentration on the incorporation of Au NPs on the ODA monolayer and the relevant behavior of the mixed monolayer were studied using the pressure-area (pi-A) isotherm and transmission electron microscopy (TEM) observations. The experimental results showed that the adsorbed density of Au NPs is low without the surface modification by MSA. When MSA was added into the Au NP-containing subphase, the incorporation amount of Au NPs increased with increasing MSA concentration up to approximately 1 x 10-5 M for the particle density of 1.3 x 1011 particles/mL. With a further increase in the MSA concentration, the adsorbed particle density decreases due to competitive adsorption between the free MSA molecules and the MSA-Au NPs. It is inferred that free MSA molecules adsorb more easily than the MSA-Au NPs on the ODA monolayer. Therefore, an excess amount of MSA present in the subphase is detrimental to the incorporation of gold particles. The study on the monolayer behavior also shows that the pi-A isotherm of the ODA monolayer shifts right when small amounts of Au NPs or free MSA molecules are incorporated. However, when larger amounts of particles are adsorbed at the air/liquid interface, a left shift of the pi-A isotherm appears, probably due to the adsorption of ODA molecules onto the particle surface and the transferring of the particles from beneath the ODA monolayer to the air/water interface. According to the present method, it is possible to prepare uniform particulate films of controlled densities by controlling the particle concentration in the subphase, the MSA concentration, and the surface pressure of a mixed monolayer.