Live Monitoring of Cellular Respiration at Cell/Gate Nanogap Electrical Interface

Ionic or biomolecular charges induce a change in potential at the electrolyte solution/electrode interface. As a type of potentiometric biosensor, biologically coupled gate field-effect transistors (Bio-FETs), which are originally based on solution-gated FETs, are attracting attention worldwide. This is probably because various types of biomolecules with charges can be directly detected as electrical signals with the Bio-FETs in a label-free and real-time manner, and various semiconductive materials can also be applied to biosensing [1]. <br/> A general cell culture medium includes various ions and chemicals such as serum and glucose. In such a medium, the shielding effect caused by counter ions is a problem because Bio-FETs are very insensitive to the changes in the density of molecular charges based on biomolecular recognition events on the gate electrode in the cell culture medium, although the diluted measurement solution can be used for reducing the shielding effect by counter ions. In other words, nonspecific electrical signals can be prevented from interfering with species in the cell culture medium because some proteins contained in it have been nonspecifically adsorbed on the gate electrode during preculture. Then, what specific targets are detected by the Bio-FETs under this condition? Hydrogen ions, in particular, which have the smallest size, induce changes in pH. Actually, in our group, cellular respiration activities were easily and continuously monitored for any living cells (e.g., cancerous, autophagic, and normal cells, extracellular matrix production of chondrocyte, and embryo and sperm activities) using Bio-FETs with an oxide gate electrode in the cell culture medium [2–8]. Some proteins in the cell culture medium are adsorbed at the oxide gate surface during preculture, resulting in the adhesion of cells at the substrate. These macromolecules prevent targeted ionic charges from coming into contact with the gate, but hydrogen ions can easily attach to the oxide gate surface, where the equilibrium reaction between hydroxyl groups and hydrogen ions contributes to the change in the charge density at the oxide gate electrode. Moreover, hydrogen ions are concentrated in the closed nanogap space between the cell membrane and the oxide gate electrode [4,6]. This detection mechanism is very simple, that is, living cells are simply cultured on the oxide gate electrode of the original solution-gated FET (i.e., pH-responsive ion-sensitive FET) for monitoring cellular respiration. In addition, the cell culture medium with high ionic strength contributes to the reduction in the effect of other ionic and biomolecular charges on the output signal by minimizing the Debye length. This is a straightforward mechanism in the pH-responsive FET. Thus, it is also important to reconsider the intrinsic features of Bio-FETs, which allow the stable monitoring without additional modifications of the gate electrode. In this talk, we would like to introduce the above fundamental characteristics of the cell-coupled gate FETs and their applications. <br/> <br/><b>References</b><br/>[1] Sakata, T. [Review Article] <i>Commun. Chem.</i> 2024, 7, 134239. [2] Sakata, T.; Saito, A.; Mizuno, J.; Sugimoto, H.; Noguchi, K.; Kikuchi, E.; Inui, H. <i>Anal. Chem.</i> 2013, 85, 6633–6638. [3] Yang, H.; Honda, M.; Akiko, A.; Kajisa, T.; Yanase, Y.; Sakata, T. <i>Anal. Chem.</i> 2017, 89, 12918–12923. [4] Satake, H.; Saito, A.; Sakata, T. <i>Nanoscale</i> 2018, 10, 10130–10136. [5] Sakata, T.; Saito, A.; Sugimoto, H. <i>Sci. Rep.</i> 2018, 8, 8282. [6] Sakata, T.; Saito, A.; Sugimoto, H. <i>Anal. Chem.</i> 2018, 90, 12731–12736. [7] Saito, A.; Sakata, T. <i>Sensors</i> 2019, 19, 1784. [8] Satake, H.; Sakata, T. <i>Anal. Chem.</i> 2019, 91, 16017–16022.