Hierarchical Self-Assembly of pH-Responsive Nanocomposites with Molecular-Scale and Mesoscale Periodicities

We have demonstrated a novel synthesis of mesoscopically ordered stimulus-responsive silica-based hybrid materials through a hierarchical self-assembly process. Two cationic surfactants with a special functional chromophoric group at one end of a carbon chain and a trimethylammonium headgroup at the other end were synthesized and used as both structure-directing agents and functional nano building blocks. Using these two cationic ammonium surfactants with different chromophoric groups at their hydrophobic tails, the micrometer-sized particles with a two-dimensional hexagonal mesostructure and spherical particles with a wormlike mesostructure were produced, respectively. Both of the mesostructured composite particles exhibit strongly fluorescent and pH-responsive behaviors with a rapid and recyclable signal response. Significantly, the symmetry of the self-assembled architecture demonstrated here not only guarantees a high filling and total homogeneous distribution of organic functionalities but also affords precise molecular-scale control over the spatial distribution of the organic component in the mesoscopically ordered inorganic network, which endows the hierarchical materials with mechanical robustness, improved thermal stability, and faster responses to chemical stimuli.