Optical properties of aqueous colloidal dispersions of 2D electrolytes, if their aspect ratios are extra-large, can be determined by their orientation preferences. Recently, we reported that a colloidal dispersion of diamagnetic titanate­(IV) nanosheets (TiIVNSs), when placed in a magnetic field, is highly anisotropic because TiIVNS anomalously orients its 2D plane orthogonal to the magnetic flux lines due to its large anisotropic magnetic susceptibility. Herein, we report a serendipitous finding that TiIVNSs can be in situ photochemically reduced into a paramagnetic species (TiIV/IIINSs), so that their preference of magnetic orientation changes from orthogonal to parallel. This transition distinctly alters the structural anisotropy and therefore optical appearance of the colloidal dispersion in a magnetic field. We also found that TiIV/IIINSs is autoxidized back to TiIVNSs under non-deaerated conditions. By using an elaborate setup, the dispersion of TiIVNSs serves as an optical switch remotely operable by magnet and light.

Mechanical nonreciprocity, or the asymmetric transmission of mechanical quantities between two points in space, is crucial for developing systems that can guide, damp, and control mechanical energy. We report a uniform composite hydrogel that displays substantial mechanical nonreciprocity, owing to direction-dependent buckling of embedded nanofillers. This material exhibits an elastic modulus more than 60 times higher when sheared in one direction compared with the opposite direction. Consequently, it can transform symmetric vibrations into asymmetric ones that are applicable for mass transport and energy harvest. Furthermore, it exhibits an asymmetric deformation when subjected to local interactions, which can induce directional motion of various objects, including macroscopic objects and even small living creatures. This material could promote the development of nonreciprocal systems for practical applications such as energy conversion and biological manipulation.