Template synthesis of nanostructured materials via layer-by-layer assembly

The layer-by-layer (LbL) templating technique has attracted significant interest as a simple, highly versatile approach that has been widely used to prepare nanostructured materials with tailored properties. The process involves the sequential deposition of species, such as polymers, nanoparticles, lipids, proteins, and dye molecules, onto various templates, which are subsequently removed to yield free-standing structures. Although fine control of the material properties (e.g., size, composition, thickness, permeability, function) is afforded by the type of species LbL-assembled, the morphology and composition of the templates also play a crucial role in determining the properties, and hence potential applications, of the materials generated. In this review, we focus on the two main classes of templates that have been employed to prepare nanostructured materials: planar and colloidal. The use of porous planar and colloidal substrates in LbL templating synthesis is also presented, as this offers opportunities to fabricate novel materials with advanced structuring. Particular emphasis will be placed on using colloidal templates of different composition (e.g., polymer, inorganic, liquid droplets, gas bubbles) and geometry (e.g., spheres, rods, fibers). The assembly routes employed to prepare a range of nanostructured materials, including films, capsules, nanotubes, nanoporous particles, and macroporous and biomimetic structures, are described, together with the potential applications of the materials in fields such as adsorption/immobilization, catalysis, drug delivery, sensing, separations, and synthesis. Future research directions of the LbL templating technique are also discussed.