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We describe the remarkable capabilities of polymer–nanoparticle hybrids for cation binding and separation. The materials investigated here are hydrogels of N-isopropylacrylamide (NIPA) prepared using nanoparticles of the synthetic clay, Laponite® RD, as the crosslinkers. When immersed into an aqueous solution containing a cationic amine-based dye (methylene blue), the gel rapidly soaks up the dye from solution. Additionally, the gel is able to selectively extract the above cationic dye from a mixture of cationic and anionic dyes. These ion-exchange properties are driven by the strong binding affinity of certain cations for the anionic surfaces of the clay nanoparticles within the gel matrix. A comparison of the nanostructured gel with a typical cation-exchange resin (polystyrene-sulfonate) shows that the former is quicker and more efficient at extracting cationic species from solution. We also show that the solute adsorbed within the gel can be further concentrated by exploiting the shrinking property of NIPA gels when heated. Additionally, we demonstrate the disassembly of Laponite-crosslinked gels by exposure to an organic solvent or a hydrophilic oligomer, which allows the adsorbed solute to be released and thereby recovered.