Single magnetic microsphere placement and detection on-chip using current line designs with integrated spin valve sensors: Biotechnological applications

Superparamagnetic labels, 400 nm dextran iron oxide particles and 2 μm polymer encapsulated iron oxide microspheres, with biomolecules immobilized on the surface, e.g., the enzyme horseradish peroxidase (20–40 molecules per label) were controllably placed on chip sites (5×15 μm2) using tapered Al current lines (10–20 mA current) and moved to and from adjacent spin valve sensors [2×6 μm,2, magnetoresistance (MR) ∼5%]. Average MR signals of 1.2 and 0.6 mV were obtained for the detection of bulk numbers of 400 nm and 2 μm labels respectively using an on-chip field of 15 Oe and a sense current of 5 mA. The moment per label was calculated at 5×10−13 emu for the 400 nm labels and 5×10−12 emu for the 2 μm labels, illustrating the higher density of the 400 nm particles. MR signals of ∼100 μV were obtained for single 2 μm labels positioned over the spin valve sensor using an on-chip field of 15 Oe and 8 mA sense current. The corresponding sensor saturation occurred at ∼1 mV, with a noise level of ∼10 μV. The estimated maximum MR signal for one 2 μm label directly on top of the sensor was ∼400 μV. Biotechnological applications include high sensitivity biosensors and biochips for protein and DNA screening.