S. Anandakumar

Translocation of bio-functionalized magnetic beads using smart magnetophoresis

We demonstrate real time on-chip translocation of bio-functionalized superparamagnetic beads on a silicon surface in a solution using a magnetophoresis technique. The superparamagnetic beads act as biomolecule carriers. Fluorescent-labeled Atto-520 biotin was loaded to streptavidin-coated magnetic beads (Dynabead(®) M-280) by means of ligand-receptor interactions. The magnetic pathways were patterned lithographically such that semi-elliptical Ni(80)Fe(20) elements were arranged sequentially for a few hundred micrometers in length. An external rotating magnetic field was used to drive translational forces on the magnetic beads that were proportional to the product of the field strength and its gradient. The translational force at the curving edge of the pathway element of 6 μm diameter was calculated to be ∼1.2 pN for an applied field of 7.9 kA m(-1). However, the force at the flat edge was calculated to be ∼0.16 pN. The translational force was larger than the drag force and thus allowed the magnetic beads to move in a directional way along the curving edge of the pathway. However, the force was not sufficient to move the beads along the flat edge. The top and bottom curving edge semi-elliptical NiFe pathways were obliquely-arranged on the left and right sides of the converging site, respectively. This caused a central translational force that allowed the converging and diverging of the Atto-520 biotin loaded streptavidin magnetic beads at a particular site.

Optimization of the Multilayer Structures for a High Field-Sensitivity Biochip Sensor Based on the Planar Hall Effect

We have investigated the planar Hall effect (PHE) in three multilayer structures such as a bilayer, a spin-valve and a weak exchange bias coupling bilayer structure introduced a very thin Cu spacer layer between the antiferromagnetic and ferromagnetic layers. These thin films are Ta(3)/NiFe(10)/IrMn(10)/Ta(3) (nm), Ta(3)/NiFe(10)/Cu(1.2)/NiFe(2)/IrMn(10)/Ta(3) (nm), and Ta(3)/NiFe(10)/Cu(0.2)/IrMn(10)/Ta(3) (nm), respectively. The active layers in all three structures were kept constant. The field-sensitivity of the fabricated PHE sensors obtained for the respected structures are about 1.6 muV middot Oe-1, 5 muV middot Oe-1, and 12 muV middot Oe-1, respectively. The results suggest that the sensor based on a weak exchange bias coupling structure has the highest field-sensitivity compared with the others. The proposed weak exchange bias coupling structure emphasizes for the development of the PHE sensor materials.

Translocation of magnetic beads using patterned magnetic pathways for biosensing applications

We have designed, fabricated, and demonstrated a novel system for translocation of magnetic beads at specific sites of the sensor surface on a single chip for biosensor applications. The soft NiFe elliptical (9×4×0.1 μm3) elements are arranged as magnetic pathways connected to the model sensor surface. The patterned NiFe elements can generate different stray magnetic fields when they are subjected to the external rotating magnetic field. The inhomogeneity in stray magnetic fields can govern the magnetic bead motion on the pathways. We demonstrated the motion of Dynabead® M-280 magnetic bead on patterned pathways by controlling the external rotating magnetic field in clockwise and counterclockwise directions. The magnetic beads that were placed on the magnetic elliptical pathways are shown to be transported to the sensor surface, as well as be pulled out away from the surface. This technique enables microtranslocation of the magnetic beads coated with biomolecules to the specific binding sites of the senso...

Sensitivity Dependence of the Planar Hall Effect Sensor on the Free Layer of the Spin-Valve Structure

Planar Hall effect (PHE) sensors with the junction size of 50 mum times 50 mum were fabricated successfully by using spin-valve thin films Ta(5)/NiFe(x) /Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) with x = 4, 8, 10, 12, 16. The magnetic field sensitivity of the PHE sensors increases with increasing thickness of ferromagnetic (FM) free layer. The sensitivity of about 95.5 m Omega/(kA/m) can be obtained when the thickness of the FM-free layer increases up to 16 nm. The enhancement of sensitivity is explained by the shunt current from other layers. The PHE profiles are well described in terms of the Stoner-Wohlfarth energy model. The detection of magnetic micro-beads label Dynabeadsreg M-280 is demonstrated and the results revealed that the sensor is feasible for high-resolution biosensor applications.

Electrodeposited CoNiP Hard Magnetic Nanowires in Polycarbonate Membrane

An array of CoNiP magnetic nanowires were grown in polycarbonate membrane using potentiostatic electrodeposition technique under three electrodes configuration. The commercially available track etched polycarbonate membranes of thickness 6 mum with pore size of 50 nm diameter were used in these experiments. The electrolyte bath consists of NiCl2-6.81 g/l, CoCl2-2.76 g/l, NaH2 PO2-2.59 g/l, H 3BO3-2.49 g/l, NaCl-2.20 g/l, Saccharin-0.8 g/l was used for deposition of CoNiP magnetic nanowires. The main aim of this work focuses on growth conditions, structural and magnetic properties of the CoNiP nanowires. In this context first we observed three different growths of nanowire lengths 1.21 mu m, 4.31 mu m and 6 mu m at three different deposition times 30 min, 60 min, and 90 min, respectively. The X-ray diffraction patterns of CoNiP nanowires have shown the intermixture of fcc and hcp phases. The structural properties of the CoNiP nanowires were observed using scanning electron microscope (SEM). The magnetic properties of the CoNiP nanowires were observed using vibrating sample magnetometer (VSM), which show hard magnetic properties with no preferential magnetization direction of the nanowires having high coercivity values around 500 Oe.

Template Synthesis of Cobalt Nanowires Using PS-b-PMMA Block Copolymer

Cobalt nanowires with 18 plusmn 5 nm diameter were grown in the self-assembled diblock copolymer templates using electrodeposition technique. The diblock copolymer templates with hexagonally ordered nanoporous structure were successfully synthesized by varying the copolymer thickness from 50 nm to 430 nm and post annealing at temperature 180degC for 24 h. The minimum optimized pore sizes of the template were 18 plusmn 5 nm with well order hexagonal nanoporous structure at copolymer thickness from 350 nm to 430 nm. Cobalt nanowires were then successfully deposited using three electrode configuration potentiostatic electrodeposition methods. To remove the cross linked polystyrene of the template and to display the cobalt nanowires, heat treatment was carried out. The morphology of the templates and cobalt nanowires was observed using Scanning Electron Microscope (SEM). The magnetic properties of the cobalt nanowires are analyzed using Vibrating Sample Magnetometer (VSM).

Programmable Magnetic Actuation of Biomolecule Carriers using NiFe Stepping Stones

We have designed, fabricated and demonstrated a novel micro-system for programmable magnetic actuation using magnetic elliptical pathways on Si substrates. Lithographically patterned soft NiFe ellipses are arranged sequentially perpendicular to each other as stepping stones for the transport of magnetic beads. We have measured the magnetization curve of the ellipsoid (9 μm × 4 μm × 0.1 μm) elements with respect to the long and short axes of the ellipse. We found that the magnetization in the long axis direction is larger than that in the short axis direction for an applied field of ≤ 1,000 Oe, causing a force on carriers that causes them to move from one element to another. We have successfully demonstrated a micro-system for the magnetic actuation of biomolecule carriers of superparamagnetic beads (Dynabead ® 2.8 μm) by rotating the external magnetic field. This novel concept of magnetic actuation is useful for future integrated lab-on-a-chip systems for biomolecule manipulation, separation and analysis.

Numerical Analysis of Magnetic Field Distribution of Magnetic Micro-barcodes for Suspension Assay Technology

In this study, we have investigated real-time decoding feasibility of magnetic micro-barcodes in a microfluidic channel by using numerical analysis of magnetic field distribution of the micro-barcodes. The vector potential model based on a molecular current has been used to obtain magnetic stray field distribution of ferromagnetic bars which consisting of the micro-barcodes. It reveals that the stray field distribution of the micro-barcodes strongly depends on the geometries of the ferromagnetic bar. Interestingly enough, we have found that one can avoide the miniaturization process of a magnetic sensor device needed to increase the sensitivity by optimizing the geometries of micro-barcodes. We also estimate a magnetic sensor response depending on flying height and lateral misalignment of the micro-barcodes over the sensor position and found that control of the flying height is crucial factor to enhance the detection sensitivity and reproducibility of a magnetic sensor signal in the suspension assay technology. # 2011 The Japan Society of Applied Physics