Water‐Dispersive Hydrophobic Suprastructures: Specific Properties

Abstract

As shown in other chapters, 3D superlattices of nanocrystals, called either supracrystals or colloidal crystals, provide a new class of materials with collective properties. Mostly, these 3D superlattices are obtained from hydrophobic nanocrystals and therefore cannot be manipulated in aqueous systems limiting their range of applications. To produce various suprastructures of hydrophobic nanocrystals dispersed in aqueous solution, we use two concepts based on either instabilities or intertwine between vesicles between lipids and alkyl chains used to coat nanocrystals. The interactions between the alkyl chain used as coating agent and lipid vesicles favor the dispersion of the hydrophobic shaped 3D superlattices in aqueous solution. With Co nanoparticles as building blocks, such novel composites retain the crystalline structure and magnetic properties of the starting material. Furthermore, they move with an applied magnetic field. By extending such know-how to hydrophobic Au nanocrystals, the suprastructure optical properties reveal two peaks, assigned to photonic modes of the assembly and localized surface plasmon resonance of the nanocrystals. Furthermore, the fingerprint of Au nanocrystal is preserved even for large crystalline 3D superlattices demonstrating that the nanocrystal could be used as a probe for investigating the optical properties of such assemblies. Other types of suprastructures dispersed in aqueous solution are based on instabilities. Hollow suprastructures of hydrophobic Fe 3 O 4 nanocrystals self-assemble in shells and are dispersed in aqueous solution. Simultaneously, spherical solid face-centered cubic (fcc) assemblies called supraballs are produced through superlattice-matched epitaxial overgrowth along the existing colloidosomes. The nanomechanical properties of these two suprastructures are shown to be load-dependent in aqueous medium. Colloidosomes demonstrated higher flexibility and deformability than the supraballs. The differences in nanomechanical properties between these types of suprastructures are mainly due to their structures (hollow core–shell or fcc 3D superlattices). The photothermal properties of these various water-soluble suprastructures display a universal mechanism presiding over the light–heat conversion in this novel kind of nanomaterials. These suprastructures operate as efficient nanoheaters by exploiting the high absorption from the individual nanocrystals, enabled by the dilution of the inorganic phase, which is followed by a relatively fast heating of the embedding organic matrix, occurring on the 100-ps time scale. Note that such “universal” nanoheaters exist for the various suprastructures described here and cannot be observed in dried systems.