Design, Self-Assembly, and Switchable Wettability in Hydrophobic, Hydrophilic, and Janus Dendritic Ligand–Gold Nanoparticle Hybrid Materials

Controlling nanoparticles’ (NPs) surface polarity, colloidal stability, and self-assembly into well-defined complex architectures is of paramount importance for emergent nano- and biotechnologies, and each depends strongly on the ligand shell composition and chemical nature. In this study, a series of dendritic ligands with hydrophobic, hydrophilic, and Janus surface groups was synthesized, grafted onto Au NPs, and their effects on the self-assembly behavior and surface polarity of the corresponding hybrid materials were investigated. A generalized, flexible strategy was utilized for ligand synthesis that independently introduces dendritic end groups, responsible for the surface polarity and colloidal properties, and specific surface NPs binding groups, reducing the number of synthetic steps. The dendritic ligands obtained were grafted onto NP surfaces through solution phase ligand-exchange, and the resulting NP–dendron hybrids were studied using a variety of techniques such as transmission electron microscopy, UV–vis, and small-angle X-ray scattering. When the solvent evaporation rate during self-assembly is controlled, these dendronized Au hybrids self-organize into highly ordered thin films comprised of close-packed arrays of NPs where the interparticle separation can be varied as a function of the dendritic generation and end group chemistry. Moreover, contact angle and colloidal observations revealed the strong dependence of the dendron end-group and generation on the NP surface polarity. Uniquely, the hybrid material of Au NPs and the Janus dendron exhibits controlled surface wetting, where the surface polarity is dependent on solvent exposure, revealing a surface polarity memory effect, making this material a model system for surfaces that demonstrate switchable wettability.