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 mum 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 (5x15 mum(2)) using tapered Al current lines (10-20 mA current) and moved to and from adjacent spin valve sensors [2x6 mum,(2), magnetoresistance (MR) similar to5%]. Average MR signals of 1.2 and 0.6 mV were obtained for the detection of bulk numbers of 400 nm and 2 mum labels respectively using an on-chip field of 15 Oe and a sense current of 5 mA. The moment per label was calculated at 5x10(-13) emu for the 400 nm labels and 5x10(-12) emu for the 2 mum labels, illustrating the higher density of the 400 nm particles. MR signals of similar to100 muV were obtained for single 2 mum 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 similar to1 mV, with a noise level of similar to10 muV. The estimated maximum MR signal for one 2 mum label directly on top of the sensor was similar to400 muV. Biotechnological applications include high sensitivity biosensors and biochips for protein and DNA screening. (C) 2002 American Institute of Physics.