Assembly of Linked Nanocrystal Colloids by Reversible Covalent Bonds

The use of dynamically bonded molecules designed to reversibly link solvent-dispersed nanocrystals (NCs) is a promising strategy to form colloidal assemblies with controlled structures and macroscopic properties. In this work, tin-doped indium oxide NCs are functionalized with ligands that form reversible covalent bonds with linking molecules to drive assembly of NC gels. We monitor the gelation by using small angle X-ray scattering and characterize how changes in the gel structure affect the infrared optical properties arising from the localized surface plasmon resonance of the NCs. The assembly is reversible because of the designed linking chemistry, and we disassemble the gels using two strategies: addition of excess NCs to change the ratio of linking molecules to NCs and addition of a capping molecule that displaces the linking molecules. The assembly behavior is rationalized using a thermodynamic perturbation theory to compute the phase diagram of the NC-linking molecule mixture. Coarse-grained molecular dynamics simulations reveal the competition between the loop and bridge linking motifs essential for understanding NC gelation. This combined experimental, computational, and theoretical work provides a platform for controlling and designing the properties of reversible colloidal assemblies that incorporate NC and solvent compositions beyond those compatible with other contemporary (e.g., DNA-based) linking strategies.