Magnetic Nanobeads as Support for Zinc(II)–Cyclen Complexes: Selective and Reversible Extraction of Riboflavin

Riboflavin (vitamin B2) is essential for the health of humans and animals. Its structure is part of flavin adenine dinucleotide (FAD) and riboflavin-5’-phosphate (flavin mononucleotide, FMN), redox cofactors found among others in dehydrogenases, oxidases and photolyases. A lack of vitamin B2 can cause growth failure, seborrheic dermatitis or even cataracts and normocytic anemia. Therefore, extraction and quantification of riboflavin in body fluids, like urine or blood plasma, or in nutrition supplements, for example vitamin tablets, is of analytical interest. It has been shown that artificial receptors containing zinc(II)–cyclen complexes can reversibly coordinate imide moieties, like thymine, uracil, and flavins. Recent reviews outline the molecular interactions of zinc(II)–cyclen and its derivatives. We previously reported on a zinc(II)–cyclen-functionalized polymer that reversibly and selectively binds flavins at physiological pH and enables the extraction of riboflavin from aqueous solutions. A magnetic support would further improve this concept by allowing rapid agitation and separation by applying an external magnetic field. Various surface-modified magnetic nanoparticles have been successfully employed for the extraction of histidine-tagged proteins, DNA/RNA, biotin-labeled antibodies, or heavy metal ions from aqueous media. We report here the immobilization of zinc(II)– cyclen complexes as binding motifs for the extraction and release of riboflavin from aqueous media on carbon-coated magnetic metal nanoparticles, featuring both high thermal and chemical stability, as well as high magnetization levels (158 emug ). This concept may be applied to the extraction of other target molecules by an exchange of the receptor molecules on the surface of the magnetic nanobeads. Previously, these nanobeads have been applied as supports for catalysts or as magnetic scavengers. The synthesis of cyclen-functionalized nanobeads started from threefold tert-butoxycarbonyl (Boc)-protected cyclen 1. Propargylation applying standard conditions afforded 2 in good yields (Scheme 1). Polymer-coated magnetic support 3