Magnetic MFe2O4 (M = Fe, Co, or Mn) nanoparticles with uniform diameters in the 4-20 nm range and with excellent material properties, reported previously, can be rendered soluble in water or aqueous buffers using a combination of alkylphosphonate surfactants and other surfactants such as ethoxylated fatty alcohols or phospholipids. Surfactant-modified oligonucleotides can be incorporated into the particles' organic shell. The particles can withstand salt concentrations up to 0.3 M, temperatures up to 90 degrees C, and various operations such as concentration to dryness, column or membrane separations, and electrophoresis. The particles can be selectively hybridized to DNA-functionalized gold surfaces with high coverages using a two-story monolayer structure. These particles may find valuable applications involving the magnetic detection of small numbers of biomolecules using spin valves, magnetic tunnel junctions, or other sensors.