Effect of reversible cross-linker, N,N′-bis(acryloyl)cystamine, on calcium ion adsorption by imprinted gels

Imprinted gels incorporating two different breakable cross-linkers, a PbMAA₂ complex and a disulfide (S-S) bond, were prepared by radical polymerization. After the lead ions were removed by washing, these gels showed a high affinity for calcium ions. Breakage and subsequent reconnection of the S-S bonds in the absence of Ca²⁺ decreases the Ca²⁺ binding affinity of the gel. This indicates that random reconnection of the S-S bonds produces a frustration in the adsorption of Ca²⁺ by the carboxyl groups. However, if the S-S bonds were reconnected in the presence of Ca²⁺ and the Ca²⁺ was subsequently removed (the post-imprinting technique), the resulting gels showed a higher binding affinity for Ca²⁺. This indicates that the post-imprinting technique creates a more favorable conformation for Ca²⁺ binding in the polymer network. We interpret our data to mean that "memory" of target-binding sites was encoded effectively into the polymer network by the initial imprinting technique and then enhanced by the post-imprinting technique.