The disclosure relates to a switchable aptamer having a high affinity for a selected target such as a virus, cell or antibody when in the presence of a binding ion and a low affinity for said target in the absence of said binding ion. The switchable aptamer may be isolated from a pool comprising a mixture of aptamers by incubating the pool with the target ligand and a binding ion to form target-aptamer complexes; separating unbound aptamer molecules from the target-aptamer complexes; contacting the target-aptamer complexes with a chelating agent having affinity for the binding ion wherein a switchable aptamer specific to said target is released from the target-aptamer complexes; and isolating the switchable aptamer released in the preceding step.