Preparation of Uniform Magnetic Microspheres through Hydrothermal Reduction of Iron Hydroxide Nanoparticles Embedded in a Polymeric Matrix

A novel method is described for the preparation of nearly monodispersed and highly magnetic responsive microspheres with magnetite nanocrystals formed in a polymeric matrix by hydrothermal reduction. The method is based on the formation of iron hydroxide/polymer composite microspheres by acid-catalyzed condensation polymerization of urea and formaldehyde in the presence of colloidal iron hydroxide. The iron hydroxide colloids entrapped in the polymer matrix are then in situ converted to magnetite nanocrystals by reaction with sodium borohydride under hydrothermal conditions. Characterization of the resulting microspheres with electron microscopy and vibrating sample magnetometry confirmed that these particles possessed a uniform spherical morphology, narrow particle size distribution, and high magnetic susceptibility. More interestingly, the magnetic nanoparticles embedded in the polymer matrix are of cubic shape and highly crystalline structure. While the growth of uniform composite microspheres is accounted for by the well-known LaMer model, the formation of the cubic magnetite nanocrystals appears to involve a dissolution−recrystallization process. After being coated with silica by the sol−gel approach, the magnetic particles were used as adsorbents for isolation of genomic DNA from biological samples, with results comparable to those obtained by magnetic silica microspheres. Incorporation of iron hydroxide colloids into polymer microspheres coupled with chemically induced phase transformation represents a new cost-effective approach to the preparation of uniform magnetic microspheres that is more controllable with respect to particle properties and more amenable to large-scale production

Inverse Mixed-Mode Chromatography for the Evaluation of Multivalency and Cooperativity of Host–Guest Complexation in Porous Materials

A new separation-based analytical method was developed to evaluate the multivalency and cooperativity of supramolecular host–guest complexation in porous materials. The method is based on inverse mixed-mode chromatography in which a porous material with a multivalent functional group is packed into a column and bound with a complementary guest molecule to form a multivalent complex. The bound guest molecules are eluted in the mobile phase and detected by appropriate methods such as UV absorption. The retention factor of the guest molecule is determined and broken down into the contributions of noncovalent interactions between binding sites (e.g., hydrophobic and ionic components), thereby calculating the effective molarity and cooperativity factor of the complexation. Two model systems denoted as RP/SCX and RP/SAX were analyzed by the established method. On average, the RP/SCX system has an effective molarity (EM) of 0.14 M and a cooperativity factor (β) of 0.86, while the RP/SAX system has an EM value of 0.18 M and a β value of 2.3. Interestingly, experiments have shown that these values do not change with changes in the intrinsic binding strength of the constituent sites. In summary, the developed method allows for quantitative assessment of multivalency and cooperativity effects in porous materials, providing a valuable complement to the analytical toolbox for supramolecular chemists and materials scientists