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.

No abstract

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.

Gold, nickel, and two-segment nickel−gold nanowires have been synthesized by electrodeposition into alumina templates. The nanowires have ∼350-nm diameters and were typically 12−22 μm in length. The nanowires were removed from the templates and were functionalized with organic molecules. Adsorption isotherms were constructed for the binding of 8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21H,23H-porphine-2,18-dipropionic acid to nickel nanowires in ethanol solution at 298 K. Adduct formation constants of 9 ± 5 × 106 M-1 and limiting surface coverages of 8 × 10-10 mol/cm2 were abstracted from the isotherms. Surface functionalization conditions were identified where thiols bind selectively to gold and carboxylic acids bind to nickel. Nanowires with free amino or thiol functional groups were reacted with activated dyes to yield amide, thiourea, and thioether covalent linkages that were quantified by fluorescence microscopy. These reactions with two-segment nickel−gold nanowires produced materials that emitted light only on one segment of the nanowire or emitted light of different colors on each segment.