From Counting Single Biological Cells to Recovering Photons: The Versatility of Contactless Impedance Sensing

Two modern applications of contactless impedance sensing at the microscale are reviewed. In particular, the cross-disciplinary evolution of contactless conductivity sensing between planar micro-electrodes from cell biology to a novel application in the field of solid-state silicon photonics is presented. The versatility of the same equivalent model, identified by means of impedance spectroscopy, is here highlighted. The presence of single cells in ionic solutions can be detected either in static (for monitoring the growth of a colony of adherent cells) or dynamic conditions (in micro-fluidic impedance flow cytometry), thanks to the contrast in conductivity between the insulating cell volume and the physiological solution, probed bypassing the electrochemical double-layer interfacial capacitance. Analogously, it is possible to leverage impedance detection in order to monitor the power of light propagating in silicon waveguides and weakly interacting with the wall interfaces. Here, as well, it is necessary to bypass the insulating thin silica cladding by means of a couple of planar micro-electrodes in order to probe the conductivity of the silicon core (in the nS range), increased by free carriers (from 1 to 103 per µm of waveguide) generated through photon absorption mediated by intra-gap energy states, which are created by the defects at the semiconductor surface. Such a non-invasive power monitor, featuring a detection limit of -35 dBm, 80 dB of dynamic range, sub-ms response speed and requiring no process modification, can be easily parallelized and allows for closed-loop control of optical devices. In spite of the apparent distance between these two examples, common design criteria for both the micro-electrodes geometry and the sensing electronics are here briefly discussed.