A hybrid monolayer of An nanoparticles and dioctadecyl-dimethylammonium chloride (DODAC) was formed at the air-water interface. The An nanoparticles were found to form loop structures. The loops were 0.1-1 mum in size with a width of around 5 nm, and consisted of single particle strings. The loop formation depended on the density of the DODAC monolayers, but not on the size distribution of the particles.