On-chip manipulation of protein-coated magnetic beads via domain-wall conduits.

2010 WILEY-VCH Verlag Gmb Magnetic beads with functionalized surfaces are widely used as molecule carriers or labels for single molecule studies, cell manipulation, and biomagnetic sensing. For this reason manipulation at the nanoscale of surface functionalized magnetic beads in suspension is of paramount importance in biotechnology, nanochemistry, and nanomedicine as it leads to a precise control of the tagged biological entity. In the past few years many approaches have been developed both for the manipulation and transport of a massive particle population or of a single particle, e.g., microfabricated currentcarrying wires, micromagnets, and magnetic tweezers. More recently, thin films patterned into arrays of magnetic elements have also been proposed for the transport of single magnetic particles by exploiting their capability of focusing external magnetic fields. In addition, it has been shown that a fixed periodic landscape of magnetic domain walls (DWs) in a ferromagnetic thin film can be used to manipulate magnetic micro-particles at a solid-fluid interface. However none of these techniques combines 2D translation, rotation, and trapping of single magnetic particles along multiple trajectories with a control at the microand nanoscale and compatibility with lab-on-chip applications, which are the most prominent features of our approach based on magnetic strips presented in this paper. We demonstrate here the manipulation of individual microand nanobeads carrying proteins through the control of the motion of geometrically constrained DWs in magnetic nanoconduits patterned on the chip surface by means of a remote and low strength magnetic field. Magnetic beads are conveyed on the chip surface through a microfluidic channel and then captured by the stray field of a DW; their capture, transport and release is obtained via precise control over DW nucleation, displacement, and annihilation processes in a DW conduit structure. As a specific demonstration of biological application of our approach, we show that different proteins previously immobilized on the surface of microbeads can be singularly transported along a DW conduit, or complexes of two beads carrying proteins with high chemical affinity can be created and then manipulated on the same structure. By simply designing the shape of the DW conduit it is possible to manipulate a particle along the desired path on the chip surface, with the precision of 100 nm, as we demonstrated for circular conduits. These results demonstrate the great potential of our approach for realizing sophisticated biological experiments involving small volumes of samples in a lab-on-a-chip system, essentially consisting in controlled biochemical analysis and synthesis. In nanoscale ferromagnetic strips, shape anisotropy restricts the magnetization to lie parallel to the strip axis. Each magnetic domain has a head (positive or north pole) and a tail (negative or south pole). The resulting DWs are therefore either head-to-head (HH) or tail-to-tail (TT), and successive DWs along the nanostrip alternate between HH and TT configurations. These geometrically confined DWs exhibit intriguing properties and have become recently the focus of wide-spread theoretical and experimental research. Due to the geometrical confinement, the spin structure of a DW can be controlled via the lateral dimensions and film thickness of the nanostrip, while the DW size is on the order of tens of nm. Such DWs are ‘‘spin blocks’’ that behave as quasi-particles, which can be precisely manipulated by playing with external fields, spin-polarized currents and geometry. In particular, variations of the geometry such as constrictions, protrusions or corners, can be used to shape the potential landscape for the DW in order to obtain a finely controlled manipulation of the DW displacement. In addition it has been shown that DWs can be propagated through complex 2D and 3D networks of nanostrips that contain bifurcation and intersection points. All these peculiar properties have led to