Modulating Layer-by-Layer Assembly of Oppositely Charged Nanoparticles Using a Short Amphiphilic Molecule

Layer-by-layer (LbL) assembly of silica (SiO 2 ) and titania (TiO 2 ) nanoparticles leads to the generation of multifunctional thin films with useful properties. However, favorable film growth for these all-nanoparticle LbL thin films occurs in a narrow pH range. In this work, we widen the processing window of TiO 2 /SiO 2 LbL films by introducing a small amphiphilic molecule, hexylamine (HA), into SiO 2 nanoparticle suspensions. Film thickness measurements show that significantly thicker LbL films are obtained after introducing HA into SiO 2 nanoparticle suspensions. The zeta potential of SiO 2 nanoparticles and that of LbL films are characterized to investigate the role of HA in LbL film assembly. Furthermore, quartz crystal microbalance (QCM) measurements are used to monitor the real-time assembly of LbL films. Our results indicate that HA in SiO 2 nanoparticle suspension widens the processing window of LbL assembly by suppressing the negative surface charge of SiO 2 nanoparticles and, at the same time, inducing complete charge inversion of LbL films, of which the outermost layer is TiO 2 nanoparticles. These two factors lead to the enhanced adsorption of SiO 2 nanoparticles and to the formation of TiO 2 /SiO 2 LbL films over a wide pH range. This new approach of using a short amphiphilic molecule is a convenient way of generating all-nanoparticle LbL films over a wide processing window.