This paper reports the development of
Metal-amplified Density Assays, or MADAs - a method of conducting quantitative or multiplexed assays, including immunoassays, by using Magnetic Levitation (MagLev) to measure
metal-amplified changes in the density of beads labeled with biomolecules. The binding of target analytes (i.e.
proteins,
antibodies,
antigens) to complementary
ligands immobilized on the surface of the beads, followed by a chemical amplification of the binding in a form that results in a change in the density of the beads (achieved by using
gold nanoparticle-labeled biomolecules, and electroless deposition of
gold or
silver), translates analyte binding events into changes in density measureable using MagLev. A minimal model based on diffusion-limited growth of hemispherical nuclei on a surface reproduces the dynamics of the assay. A MADA - when performed with
antigens and
antibodies - is called a Density-Linked
Immunosorbent Assay, or DeLISA. Two immunoassays provided a proof of principle: a competitive quantification of the concentration of
neomycin in whole milk, and a multiplexed detection of
antibodies against
Hepatitis C virus NS3 protein and
syphilis T. pallidum p47
protein in serum. MADAs, including DeLISAs, require, besides the requisite biomolecules and amplification
reagents, minimal specialized equipment (two permanent magnets, a ruler or a capillary with calibrated length markings) and no electrical power to obtain a quantitative readout of analyte concentration. With further development, the method may be useful in resource-limited or point-of-care settings.