  The characterization of individual nanoparticles in a liquid constitutes a
critical challenge for environmental, material, and biological sciences. To
detect nanoparticles, electronic approaches are especially desirable owing to
their compactness and lower costs. Indeed, for single-molecule and
single-nanoparticle detection, resistive pulse sensing has advanced
significantly during the last two decades. While resistive pulse sensing was
widely used to obtain the geometric size information, impedimetric measurements
to obtain dielectric signatures of nanoparticles have scarcely been reported.
To explore this orthogonal sensing modality, we developed an impedimetric
sensor based on a microwave resonator with a nanoscale sensing gap surrounding
a nanopore. The approach of single nanoparticles near the sensing region and
their translocation through the nanopore induced sudden changes in the
impedance of the structure. The impedance changes in turn were picked up by the
phase response of the microwave resonator. We worked with 100 nm and 50 nm
polystyrene nanoparticles to observe single-particle events. Our current
implementation was limited by the non-uniform electric field at the sensing
region. The work provides a complementary sensing modality for nanoparticle
characterization where the dielectric response, rather than the ionic current,
determines the signal.
