We have developed an efficient, one-step method to create magnetic nanowires consisting of permanently linked chains of magnetic beads of varying flexibility tethered to a patterned glass surface using simple amidation chemistry. The flexibility of the nanowire was governed by the molecular weight of the molecule used to covalently link the beads and its length by the height of the microchannel in which it was synthesized. The nanowire diameter was determined both by the bead size and by the number of beads adhering to each dot in the microstamped, patterned array. Longer nanowires can form loops attached at two points on the glass surface. Both single flexible chains and flexible loops can adopt different configurations (straight, hairpin, S-shaped, etc.) when subjected to magnetic fields, the configurations depending on the directions of these fields. Shorter, less flexible nanowires align with the field always and do not exhibit the more exotic configurations seen for long, flexible chains and loops. These magnetic nanowires can have potential use in microfluidic pumping and mixing processes and in microparticle manipulation.
An inexpensive and versatile approach is reported for the synthesis of monodisperse magnetoresponsive rods of desired diameter, length, and magnetic susceptibility based on the confined alignment of magnetic beads in microchannels of selected channel height, followed by localized hydrolysis of sol-gel precursors within polyelectrolyte shells adsorbed on the beads. The layer-by-layer technique was used to coat the polystyrene beads with polyelectrolytes of alternating charge and with charged magnetic nanoparticles, and the polystyrene cores could be removed either by solvent dissolution or by calcination to form hollow-shelled chains. The reorientation dynamics of single and clustered chains following the application of an external magnetic field was evaluated theoretically, with favorable comparisons with the experimental data.
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