A microfluidic sensor in coplanar waveguide configuration for localized micrometric liquid spectroscopy in microwaves regime

In this work, we present a sensor suitable for performing the spectroscopy on a localized micrometric volume of a generic liquid, in the spectral range comprised between 1 and 20 GHz. The sensor is based on two ungrounded, open-ended coplanar waveguides, acting as Input/Output ports, with a microfluidic channel passing through them. It works by detecting the change of the reflection and transmission of microwaves due to the variation of the dielectric properties characteristic of the liquid. The devices have been designed and numerically simulated to obtain the electromagnetic response in different conditions: an empty channel for having a zero-reference response and for observing variations as a function of liquids filling the channel. We considered standard (i.e., common in literature) liquids (deionized water, ethanol, and glycerol); by means of that set of simulations we were able to start the creation of the calibration curves and defining the sensitivity of the sensor with respect to dielectric properties of the liquid under test. Then, we realized a batch of prototypes utilizing a standard photolithographic process. Later we proceeded to check the operation of the sensors. We recorded experimental data by a Vector Network Analyser connected to the devices by means of microwave probes. Sensors are designed to perform both reflection and transmission measurements on the liquid sample, and local microscopy on a micrometric volume of the solution under test. Actually, the device works avoiding any contact between the metallic parts and the liquid that can flow uncontaminated through the microfluidic channel.