Detecting the presence of nanoparticles in suspension in droplets with a coplanar differential capacitive sensor

We report a measurement methodology to sense electrically the presence of nanoparticles in suspensions in droplets and assess their type and density. The nanoparticle suspensions used in this work were solvent-based. A small amount of the suspensions was placed on the sensitive surface of an interdigitated capacitance sensor and left to evaporate at ambient temperature while the capacitance signal was registered. Our sensor has a driving and a sensing interdigitated spiral electrode array and the output signal is proportional to the impedance difference between similar sensors, reference and sensing. They were designed and fabricated over a glass-substrate. A low noise measurement system based on a Lock-in Amplifier monitored continuously in time the evaporation process of nanoparticle suspension droplet by registering variations of the imaginary component of the differential current. The results show that it is possible to infer the presence of nanoparticles in suspension and distinguish, at least between two types of nanoparticles, at very low volume concentrations.