We present here an
inexpensive method for generating a sensitive
direct electronic readout in bead-based immunoassays without the use
of any intermediate optical instrumentation (e.g., lasers, photomultipliers,
etc.). Analyte binding to capture antigen-coated beads or microparticles
is converted to probe-directed enzymatically amplified silver metallization
on microparticle surfaces. Individual microparticles are then rapidly
characterized in a high-throughput manner via single-bead multifrequency
electrical impedance spectra captured using a simple and inexpensive
microfluidic impedance spectrometry system we develop here, where
they flow through a three-dimensional (3D)-printed plastic microaperture
sandwiched between plated through-hole electrodes on a printed circuit
board. Metallized microparticles are found to have unique impedance
signatures distinguishing them from unmetallized ones. Coupled with
a machine learning algorithm, this enables a simple electronic readout
of the silver metallization density on microparticle surfaces and
hence the underlying analyte binding. Here, we also demonstrate the
use of this scheme to measure the antibody response to the viral nucleocapsid
protein in convalescent COVID-19 patient serum.