Fabrication and manipulation of ionotropic hydrogels cross-linked by paramagnetic ions

This article describes the fabrication and manipulation of millimeter-scale spheres fabricated from ionotropic hydrogels that are cross-linked with paramagnetic metal ions (e.g., Ho3+). These ionotropic hydrogels experience a force in a magnetic field gradient that correlates with the concentration of the paramagnetic cations cross-linking the polymer. In an externally applied magnetic field, the paramagnetic hydrogel spheres assemble into ordered arrays or confined geometrical structures in the regions of highest magnetic field. These spheres can be separated from heterogeneous mixtures of diamagnetic materials using a simple bar magnet. Two applications using these recoverable hydrogel spheres were demonstrated: (i) When prepared with embedded indicator dyes bound to paper, the spheres were used as colorimetric sensors for pH. (ii) When prepared with embedded activated carbon powder, they were used to remove organic materials from aqueous solutions.