Bjarke Thomas Dalslet

Microstripes for transport and separation of magnetic particles

We present a simple technique for creating an on-chip magnetic particle conveyor based on exchange-biased permalloy microstripes. The particle transportation relies on an array of stripes with a spacing smaller than their width in conjunction with a periodic sequence of four different externally applied magnetic fields. We demonstrate the controlled transportation of a large population of particles over several millimeters of distance as well as the spatial separation of two populations of magnetic particles with different magnetophoretic mobilities. The technique can be used for the controlled selective manipulation and separation of magnetically labelled species.

Chip‐Based Measurements of Brownian Relaxation of Magnetic Beads Using a Planar Hall Effect Magnetic Field Sensor

We present a simple ‘click‐on’ fluidic system with integrated electrical contacts, which is suited for electrical measurements on chips in microfluidic systems. We show that microscopic magnetic field sensors based on the planar Hall effect can be used for detecting the complex magnetic response using only the self‐field arising from the bias current applied to the sensors as excitation field. We present measurements on a suspension of magnetic beads with a nominal diameter of 250 nm vs. temperature and find that the observations are consistent with the Cole‐Cole model for Brownian relaxation with a constant hydrodynamic bead diameter when the temperature dependence of the viscosity of water is taken into account. These measurements demonstrate the feasibility of performing measurements of the Brownian relaxation response in a lab‐on‐a‐chip system and constitute the first step towards an integrated biosensor based on the detection of the dynamic response of magnetic beads.

On-chip measurement of the Brownian relaxation frequency of magnetic beads using magnetic tunneling junctions

We demonstrate the detection of the Brownian relaxation frequency of 250 nm diameter magnetic beads using a lab-on-chip platform based on current lines for exciting the beads with alternating magnetic fields and highly sensitive magnetic tunnel junction (MTJ) sensors with a superparamagnetic free layer. The first harmonic out-of-phase component of the MTJ response gives the imaginary part of the magnetic bead susceptibility, which peaks at the Brownian relaxation frequency. This work paves the way to on-chip implementation of Brownian magnetorelaxometry in innovative “lab-on-a-bead” assays for biomolecular recognition.

Determination of Oxygen Transport Properties from Flux and Driving Force Measurements

We demonstrate that an electrolyte probe can be used to measure the difference in oxygen chemical potential across the surface, when an oxygen flux is forced through an oxygen permeable membrane disk. The oxygen flux as well as the total oxygen chemical potential difference is carefully controlled by an oxygen pump. The developed method is tested on a (La 0.6 Sr 0.4 ) 0.99 CO 0.2 Fe 0.8 O 3-δ membrane. An La 0.75 Sr 0.25 MnO 3 /Y 0.16 Zr 0.84 O 1.92 /La 0.75 Sr 0.25 MnO 3 oxygen pump was attached to one side of the membrane. A conical Ce 0.9 Gds 0.1 O 1.95 (CG10) electrolyte probe was pressed against the other side of the membrane. The voltage difference between the base and the tip of the CG10 probe was recorded with an applied oxygen flux through the membrane. This voltage was used to extract precise values of the surface exchange rate constant, ko. Using these values of k o , the vacancy diffusion factor, D°, could be extracted from data of the flux and the oxygen chemical potential difference across the membrane measured with the oxygen pump. Furthermore, upon a gas change, the transient voltage signals of the oxygen pump and the probe could be fitted to give values of D 0 v k o .