From Precision Colloidal Hybrid Materials to Advanced Functional Assemblies

ConspectusThe concept of colloids encompasses a wide range of isotropic and anisotropic particles with diverse sizes, shapes, and functions from synthetic nanoparticles, nanorods, and nanosheets to functional biological units. They are addressed in materials science for various functions, while they are ubiquitous in the biological world for multiple functions. A large variety of synthetic colloids have been researched due to their scientific and technological importance; still they characteristically suffer from finite size distributions, imperfect shapes and interactions, and not fully engineered functions. This contrasts with biological colloids that offer precision in their size, shape, and functionality. Materials science has searched for inspiration from the biological world to allow structural control by self-assembly and hierarchy and to identify novel routes for combinations of functions in bio-inspiration.Herein, we first discuss different approaches for highly defined structural control of technically relevant synthetic colloids based on guided assemblies of biological motifs. First, we describe how polydisperse nanoparticles can be assembled within hollow protein cages to allow well-defined assemblies and hierarchical packings. Another approach relies on DNA nanotechnology-based assemblies, where engineered DNA structures allow programmed assembly. Then we will discuss synthetic colloids that have either particularly narrow size dispersity or even atomically precise structures for new assemblies and potential functions. Such colloids can have well-defined packings for membranes allowing high modulus. They can be switchable using light-responsive moieties, and they can initiate packing of larger assemblies of different geometrical shapes. The emphasis is on atomically defined nanoclusters that allow well-defined assemblies by supramolecular interactions, such as directional hydrogen bonding. Finally, we will discuss stimulus-responsive colloids for new functions, even toward complex responsive functions inspired by life. Therein, stimulus-responsive materials inspired by biological learning could allow the next generation of such materials. Classical conditioning is among the simplest biological learning concepts, requiring two stimuli and triggerable memory. Therein we use thermoresponsive hydrogels with plasmonic gold nanoparticles and a spiropyran photoacid as a model. Heating is the unconditioned stimulus leading to melting of the thermoresponsive gel, whereas light (at a specified wavelength) originally leads to reduced pH without plasmonic or structural changes because of steric gel stabilization. Under heat-induced gel melting, light results in pH-decrease and chain-like aggregation of the gold nanoparticles, allowing a new plasmonic response. Thus, simultaneous heating and light irradiation allow conditioning for a newly derived stimulus, where the logic diagram is analogous to Pavlovian conditioning. The shown assemblies demonstrate the different functionalities achievable using colloids when the sizes and the dispersity are controlled.