A high sensitivity, 4.5 µm×4.5 µm InSb thin film micro-Hall sensor (micro-HS) with a minimum field detection (B min) of 77 nT/(Hz)1/2 was developed for bioscreening applications and used for the detection of a single 2.8 µm diameter superparamagnetic microbead by monitoring its ac magnetic susceptibility. The scalability of the InSb micro-HS was demonstrated by fabricating 500 nm×500 nm InSb nano-Hall sensors with B min of 0.72 µT/(Hz)1/2 that could potentially be used to detect 100 nm sized superparamagnetic particles.

Nanosized ferrite spherical particles, ∼30 nm in diameter as revealed by scanning electron microscopy, were synthesized from an aqueous Fe(OH)2 suspension (pH=7.6–8.0) at 25 °C by oxidizing it with H2O2. The nanoparticles were of a spinel structure of an intermediate between Fe3O4 and γ-Fe2O3, as revealed by x-ray diffraction. Compared to the nanoparticles synthesized by our previous method in which an aqueous solution of Fe2++Fe3+ was oxidized by air (oxygen), the nanoparticles increased in size, from ∼10 nm (previous method) to 30 nm. Also saturation magnetization increased, though slightly, from 76 emu/g (previous method) to 80 emu/g (present method). Therefore, the ferrite nanoparticles synthesized by this method will improve the efficiency of magnetic separation. Because synthesis is performed at room temperature and neutral conditions (pH=7.1–7.8), which are compatible with most bioactive molecules (e.g., antibodies and proteins), these molecules will be immobilized onto the surface of the nanoparticles during their syntheses.

No abstract

Hall-effect biosensors with a magnetic-field sensitivity of 23nT∕(Hz)1∕2 (30×30μm2, at a drive current of 400μA) were fabricated using pseudomorphic AlGaAs∕InGaAs∕GaAs heterostructures for detection of DNA labeled with magnetic beads. Magnetic beads with diameters of 120 nm were selectively immobilized onto the surfaces of the Hall biosensors using highly specific hybridization of complementary oligonucleotides and were subsequently detected by measurement of the ac magnetic susceptibility of the beads.

A compact, high sensitivity and low cost biosensing platform based on InSb thin film micro-Hall effect biosensors [30 ×30 µm2; magnetic field sensitivity of 1.8 nT/(Hz)1/2] was developed and used for the detection of DNA molecules labeled with magnetic beads. Magnetic beads (∼200 nm diameter) were functionalized and selectively immobilized onto Hall biosensor surfaces via highly specific hybridization of complementary oligonucleotides and subsequently detected by magnetic susceptibility measurements. The potential for parallel biomolecular analysis with 5 ×5 µm2 linear sensor arrays was demonstrated.