  Microwave sensors can probe intrinsic material properties of analytes in a
microfluidic channel at physiologically relevant ion concentrations. While
microwave sensors have been used to detect single cells and microparticles in
earlier studies, the synergistic use and comparative analysis of microwave
sensors with optical microscopy for material classification and size tracking
applications have been scarcely investigated so far. Here we combined microwave
and optical sensing to differentiate microscale objects based on their
dielectric properties. We designed and fabricated two types of planar sensor: a
Coplanar Waveguide Resonator (CPW) and a Split-Ring Resonator (SRR). Both
sensors possessed sensing electrodes with a narrow gap to detect single cells
passing through a microfluidic channel integrated on the same chip. We also
show that standalone microwave sensors can track the relative changes in
cellular size in real-time. In sensing single 20-micron diameter polystyrene
particles, Signal-to-Noise ratio values of approximately 100 for CPW and 70 for
SRR sensors were obtained. These findings demonstrate that microwave sensing
technology can serve as a complementary technique for single-cell biophysical
experiments and microscale pollutant screening.
