Effective Capacitance from Equivalent Electrical Circuit as a Tool for Monitoring Non-Adherent Cell Suspensions at Low Frequencies

Analyzing the electrical double layer (EDL) in electrical impedance spectroscopy (EIS) measurement at low frequencies remains a challenging task for sensing purposes. In this work, we propose two approaches to deal with the EDL in measuring impedance for particles and non-adherent cells in an electrolytic suspension. The first approach is a simple procedure to compute a normalized electrical impedance spectrum named dispersed medium index (${\mathrm{DM}}_{\mathrm{i}}$). The second is the EIS modeling through an equivalent electric circuit based on the so-called effective capacitance (${\mathrm{C}}_{\mathrm{ef}}$), which unifies the EDL phenomena. Firstly, as an experiment under controlled conditions, we examine polymer particles of 6, 15, and 48 μm in diameter suspended in a 0.9% sodium chloride solution. Subsequently, we used K-562 cells and leukocytes suspended in a culture medium (RPMI-1640 supplemented) for a biological assay. As the main result, the ${\mathrm{DM}}_{\mathrm{i}}$ is a function of the particle concentration. In addition, it shows a tendency with the particle size; regardless, it is limited to a volume fraction of 0.03 × 10⁻⁴ to 58 × 10⁻⁴. The ${\mathrm{DM}}_{\mathrm{i}}$ is not significantly different between K-562 cells and leukocytes for most concentrations. On the other hand, the ${\mathrm{C}}_{\mathrm{ef}}$ exhibits high applicability to retrieve a function that describes the concentration for each particle size, the K-562 cells, and leukocytes. The ${\mathrm{C}}_{\mathrm{ef}}$ also shows a tendency with the particle size without limitation within the range tested, and it allows distinction between the K-562 and leukocytes in the 25 cells/µL to 400 cells/µL range. We achieved a simple method for determining an ${\mathrm{C}}_{\mathrm{ef}}$ by unifying the parameters of an equivalent electrical circuit from data obtained with a conventional potentiostat. This simple approach is affordable for characterizing the population of non-adherent cells suspended in a cell culture medium.