An RF/microwave microfluidic sensor based on a 3D capacitive structure with a floating electrode for miniaturized dielectric spectroscopy

This paper reports on the design, fabrication and testing of a microfluidic sensor for a miniaturized measurement platform dedicated to dielectric spectroscopy at RF/microwave frequencies. The sensor employs a novel, three-dimensional, parallel-plate, capacitive sensing structure with a floating electrode integrated onto a PMMA microfluidic cap used for delivering the material-under-test (MUT) with a 9μL-sample volume. Requiring only an S₁₁ measurement and after a 6-point calibration, complex relative permittivity readings by the sensor in the frequency range of 14MHz to 6.5GHz agree very well with bulk-solution reference measurements conducted with an Agilent 85070E dielectric probe kit. Using ethanol and ethylene glycol as two different MUTs with the sensor, the rms errors in real and imaginary parts of the complex relative permittivity of ethanol over the full frequency range are 3.5% and 5.6%, respectively. The corresponding numbers for ethylene glycol are 5.4% and 4.5%, respectively.