Nickel nanowires prepared by electrochemical growth in alumina templates have been removed from their templates and functionalized with luminescent porphyrins. The nanowires response to magnetic fields was quantified using video microscopy. In viscous solvents, magnetic fields can be used to orient the nanowires; in mobile solvents, the nanowires form chains in a head-to-tail configuration when a small magnetic field is applied. The dynamics for chain formation have been quantitatively modeled. The results demonstrate a new approach for assembling nanowires.

The use of magnetic nanowires is demonstrated as a new method for the application of force to mammalian cells. Magnetic separations were carried out on populations of NIH-3T3 mouse fibroblast cells using ferromagnetic Ni wires 350 nm in diameter and 35 microns long. Separation purities in excess of 90% and yields of 49% are obtained. The nanowires are shown to outperform magnetic beads of comparable volume.

Magnetic nanowires suspended in fluid solutions can be assembled and ordered by taking advantage of their large shape anisotropy. Magnetic manipulation and assembly techniques are demonstrated, using electrodeposited Ni nanowires, with diameter 350 nm and length 12 μm. Orienting suspended nanowires in a small magnetic field H≈10 G promotes self-assembly of continuous chains that can extend over several hundred μm. The dynamics of this process can be described quantitatively in terms of the interplay of magnetic forces and fluid drag at low Reynolds number. In addition, a new technique of magnetic trapping is described, by which a single magnetic nanowire can be captured between lithographically patterned magnetic microelectrodes. The use of three-segment Pt–Ni–Pt nanowires yields low resistance, Ohmic electrical contacts between the nanowires and the electrodes. This technique has potential for use in the fabrication and measurement of nanoscale magnetic devices.

Magnetic particles that can be bound to cells and biomolecules have become an important tool for the application of force in biology and biotechnology. Multifunctional magnetic nanowires fabricated by electrochemical deposition in nanoporous templates are a type of magnetic carrier that offers significant potential advantages over commercially available magnetic particles. Recent experimental work aimed at developing these wires for this purpose is reviewed. Results on chemical functionalization of Au and Au/Ni wires and magnetic manipulation of wires in suspension are described. Fluorescence microscopy was used to demonstrate the covalent binding of thiol-terminated porphyrins to Au nanowires, and to optimize functionalization of two-segment gold–nickel nanowires for selectivity and stability of the nanowire–molecule linkages. Magnetic trapping is a technique where single nanowires are captured from fluid suspension using lithographically patterned micromagnets. The influence of an external magnetic field on this process is described. The dynamics of magnetic trapping is shown to be well described by a model based on the interplay of dipolar forces and viscous drag.

Ferromagnetic nanowires are shown to perform both high yield and high purity single-step cell separations on cultures of NIH-3T3 mouse fibroblast cells. The nanowires are made by electrochemical deposition in nanoporous templates, permitting detailed control of their chemical and physical properties. When added to fibroblast cell cultures, the nanowires are internalized by the cells via the integrin-mediated adhesion pathway. The effectiveness of magnetic cell separations using Ni nanowires 350 nm in diameter and 5-35 micrometers long in field gradients of 40 T/m was compared to commercially available superparamagnetic beads. The percent yield of the separated populations is found to be optimized when the length of the nanowire is matched to the diameter of the cells in the culture. Magnetic cell separations performed under these conditions achieve 80% purity and 85% yield, a 4-fold increase over the beads. This effect is shown to be robust when the diameter of the cell is changed within the same cell line using mitomycin-C.