We report a facile one-step hydrothermal approach to the synthesis of iron oxide (Fe 3 O 4 ) nanoparticles (NPs) with controllable diameters, narrow size distribution, and tunable magnetic properties. In this approach, the iron oxide NPs were fabricated by oxidation of FeCl 2 ·4H 2 O in basic aqueous solution under an elevated temperature and pressure. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies reveal that the particles are highly crystalline and that the diameters of the particles can be tuned from 15 nm to 31 nm through the variation of the reaction conditions. The NPs exhibit high saturation magnetization in the range of 53.3 ~ 97.4 emu/g and their magnetic behavior can be either ferromagnetic or superparamagnetic depending on the particle size. A superconducting quantum interference device magnetorelaxometry (SQUID-MRX) study shows that the size of the NPs significantly affects the detection sensitivity. The investigated iron oxide NPs may find many potential biological applications in cancer diagnosis and treatment.

Sub-100 nm nanomagnets not only are technologically important, but also exhibit complex magnetization reversal behaviors as their dimensions are comparable to typical magnetic domain wall widths. Here we capture magnetic "fingerprints" of 10 9 Fe nanodots as they undergo a single domain to vortex state transition, using a first-order reversal curve ͑FORC͒ method. As the nanodot size increases from 52 nm to 67 nm, the FORC diagrams reveal striking differences, despite only subtle changes in their major hysteresis loops. The 52 nm nanodots exhibit single domain behavior and the coercivity distribution extracted from the FORC distribution agrees well with a calculation based on the measured nanodot size distribution. The 58 and 67 nm nanodots exhibit vortex states, where the nucleation and annihilation of the vortices are manifested as butterflylike features in the FORC distribution and confirmed by micromagnetic simulations. Furthermore, the FORC method gives quantitative measures of the magnetic phase fractions, and vortex nucleation and annihilation fields.

Anodization of Al is used to produce optically smooth porous alumina (Al(2)O(3)) films with pores approximately 60 nm in diameter and approximately 6 mum deep. The capture protein, protein A, is adsorbed to the pore walls by noncovalent, electrostatic interactions, and thin film interference spectroscopy is used to detect binding of immunoglobulin (IgG). The porous alumina films are stable against corrosion and dissolution in aqueous media at pH 7, allowing quantitative monitoring of steady-state and time-resolved biomolecular binding. The bare porous Al(2)O(3) surface displays a significantly greater affinity for protein A than for IgG. The known species specificity of protein A binding to IgG is confirmed; the protein-A-modified sensor responds to IgG derived from rabbit, but not chicken (IgG/IgY). A "cascaded", or multiprobe sensing approach, is demonstrated, in which a specific target, sheep IgG, is administered to a sample modified with a protein A/rabbit anti-sheep IgG assembly. Binding measurements are confirmed by fluorescence microscopy using fluorescein-labeled IgG.

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