Magnetic Susceptibility-Based Protein Detection Using Magnetic Levitation.

Magnetic levitation, which is a magnetic phenomenon of levitating particles suspended in a paramagnetic liquid under a nonuniform magnetic field, is a powerful tool for determining densities and magnetic properties of micro- and nanoparticles. The levitation height of particles in the magnetic field depends on the magnetic susceptibility and density difference between the object and the surrounding liquid. Here, we developed a magnetic susceptibility-based protein detection scheme in a low-cost and miniaturized magnetic levitation setup consisting of two opposing magnets to create a gradient of a magnetic field, a glass capillary channel to retain the sample, and two side mirrors to monitor inside the channel. The method includes the use of polymeric microspheres as mobile assay surfaces and magnetic nanoparticles as labels. The assay was realized by capturing the target protein to the polymer microspheres. Then, magnetic nanoparticles were attached onto the resulting microsphere-protein complex, creating a significant difference in the magnetic properties of polymer microspheres compared to those without protein. The change in the magnetic properties caused a change in the levitation height of the microspheres. The levitation heights and their distribution were then correlated to the amount of target proteins. The method enabled a detection limit of ∼110 fg/mL biotinylated bovine serum albumin in serum. With the sandwich immunoassay developed for mouse immunoglobulin G, detection limits of 1.5 ng/mL and >10 ng/mL were achieved in buffer and serum, respectively. This approach sensed the minute changes in the volume magnetic susceptibility of the microspheres with a resolution of 4.2 × 10-8 per 1 μm levitation height change.