I. Icp-ms Measurements of the Amount of Gold on Beads Due to the Binding of Gold-labeled Streptavidin

In the metal-amplified density assay (MADA), all the beads levitated at the same height in the developing buffer initially regardless of the concentration of gold-labeled protein that the beads were exposed to, suggesting that MagLev, at least in the form we implemented, was not able to detect the binding of gold nanoparticles onto the surface of the beads. The beads however, changed levitation height in the buffer differently depending on the concentration of gold-labeled protein that the beads were exposed to. The dependence on the rate of change of the levitation height on the amount of gold-labeled protein, allowed, when we used gold-labeled antibody in the detection step, to perform immunoassay with antibodies and antigens (DeLISA). We obtain a minimal model for the dependence of the change in levitation height to the concentration of gold-labeled protein in solution by adapting models developed in the early 1980s for the electrochemical deposition of metals onto electrodes [1–3]. The model captures the scaling of the time dependence of the change in levitation height of the beads and demonstrates that the change of levitation height depends on the number of nanoparticles adsorbed on the surface of the bead and the surface to volume ratio of the bead. The Supplementary Information is organized as follows. In Section I we report measurements using ICP-MS of the amount of gold present on the surface of the beads due to the binding of gold-labeled streptavidin to biotin-labeled beads. We obtained the binding isotherm by converting the mass data into the number of bound nanoparticles on the surface of the beads. In Section II we develop the model for the growth of the nanoparticles on the surface of the bead. In Section III we rederive the equations for the levitation height of small beads (of arbitrary shape) in the MagLev device from Subramaniam et. al [4], and in IV we modify the equation to take into account the change in the density of the bead due to the binding and growth of the gold-nanoparticle labeled proteins.