Magnetic behavior of cyclodextrin-based nanosponges bearing nitroxyl persistent radicals

Paramagnetic organic materials are attracting an increasing interest due to their potential use in nanomedicine. In particular, it has been reported that polymeric matrices containing TEMPO radical moieties are capable to combine drug delivery properties with intracellular scavenging action of reactive oxygen species. Paramagnetic cyclodextrin nanosponges (pmCDNS) are a new class of nanodevices with potential application in nanomedicine, due to the following properties: a) adsorption and release capability of bioactive molecules; b) detectability by EPR and MRI spectroscopies during in vitro and in vivo experiments thanks to the presence of spin labels. Moreover, they may open the way to the synthesis of metal-free organic magnets. pmCDNS were prepared by reaction in anhydrous DMSO of a mono-functionalized β-cyclodextrin (βCD), bearing a TEMPO radical moiety, with suitable cross-linkers (CL) like EDTA or pyromellitic dianhydride (PMA), with variable βCD/CL molar ratio. An alternative approach consisted into the one-pot reaction between pristine βCD, CL and a phthalic anhydride derivative bearing a TEMPO radical unit. The paramagnetic behavior of the obtained materials was investigated by EPR spectroscopy and SQUID magnetometry. Magnetic measurements, corrected for the diamagnetic contribution of the nanosponge matrix, displayed a Curie-Weiss behaviour with a negative paramagnetic Curie temperature (p = -4.3 K), suggesting that antiferromagnetic interactions were predominant in this system. The isothermal magnetisation at 2K was well fitted by the Brillouin function with a single spin moment (J=½), as expected by the TEMPO radical. A quantitative evaluation of the magnetic data suggested that around 50 mg of TEMPO radicals were present per gram of material.