  Nanocrystal gelation provides a powerful framework to translate nanoscale
properties into bulk materials and to engineer emergent properties through the
assembled microstructure. However, many established gelation strategies rely on
chemical reactions and specific interactions, e.g., stabilizing ligands or ions
on the surface of the nanocrystals, and are therefore not easily transferrable.
Here, we report a general gelation strategy via non-specific and purely
entropic depletion attractions applied to three types of metal oxide
nanocrystals. The gelation thresholds of two compositionally distinct spherical
nanocrystals agree quantitatively, demonstrating the adaptability of the
approach for different chemistries. Consistent with theoretical phase behavior
predictions, nanocrystal cubes form gels at a lower polymer concentration than
nanocrystal spheres, allowing shape to serve as a handle to control gelation.
These results suggest that the fundamental underpinnings of depletion-driven
assembly, traditionally associated with larger colloidal particles, are also
applicable at the nanoscale.
