Monica Tanase

Magnetic trapping and self-assembly of multicomponent nanowires

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.

Cell manipulation using magnetic nanowires

The use of magnetic nanowires is demonstrated as a 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 μm long. Separation purities in excess of 90% and yields of 49% are obtained. The nanowires are shown to outperform magnetic beads of comparable volume.

Biological applications of multifunctional magnetic nanowires (invited)

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 fiel...

Optimization of yield in magnetic cell separations using nickel nanowires of different lengths.

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.

High-yield cell separations using magnetic nanowires

Ferromagnetic nanowires are demonstrated as a new tool in performing high-yield, single step cell separations on cultures of mammalian cells. The nanowires are made by electrochemical deposition in nanoporous templates, and when added to cultures of 3T3 mouse fibroblast cells, the nanowires can readily bind to the cells. The effectiveness in cell separations of Ni nanowires 350 nm in diameter and 5-35 /spl mu/m long in field gradients of 40 T/m were compared to commercially available superparamagnetic beads. The percentage 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 four-fold increase over the beads.

Magnetotransport properties of bismuth films on p-GaAs

Polycrystalline Bi films were deposited onto p-GaAs(100) by electrochemical deposition. Annealing resulted in the formation of large grains with a preferred [012] orientation. The p-GaAs/Bi junctions were rectifying and the barrier height and ideality factor decreased with increasing film thickness. For films greater than 0.5 μm in thickness, the barrier height was about 0.56 eV and the ideality factor was between 1.1–1.2 for both as-deposited and annealed films. The resistance of the as-deposited films exhibited a negative temperature coefficient whereas the annealed films exhibited a positive temperature coefficient due to the limiting carrier mean free path. The magnetoresistance (MR) exhibited a quasilinear field dependence with an MR effect as large as 5600 (560 000%) at 5 K and 2.2 (220%) at room temperature.

Magnetotransport properties of bent ferromagnetic nanowires

Magnetotransport measurements were performed on individual multisegmented Pt–Ni–Pt nanowires fabricated by electrochemical deposition in nanoporous alumina templates. The nanowires were removed from the template, and precipitated onto substrates from liquid suspension. The Pt end segments provide an oxide-free interface to the magnetic central segment of interest. Centrifugation prior to precipitation induces sharp bends in the nanowires. The angular dependence of the magnetoresistance of both straight and bent nanowires was used to observe domain switching. The magnetic response of straight nanowires is well described by the curling model of domain reversal. In the case of the bent nanowires, the general behavior of each individual straight segment is also consistent with this model, but evidence for interactions between the segments is also observed.