Elucidation of interfacial pH behaviour at the cell/substrate nanogap forin situmonitoring of cellular respiration

Abstract

In situ monitoring of cellular metabolism is useful for elucidating dynamic functions of living cells. In our previous studies, cellular respiration was continuously monitored as a change in pH at the cell/electrode nanoscale interface (i.e., interfacial pH) using an ion-sensitive field-effect transistor (ISFET). However, such interfacial pH behaviour on the nanoscale has not been confirmed using other methods such as fluorescence imaging. In this study, we have clarified the interfacial pH behaviour at a cell/substrate nanogap using a laser scanning confocal fluorescence microscope. The phospholipid fluorescein used as a pH indicator was fixed to the plasma membrane on the external side of a cell by inserting its lipophilic alkyl chain into the membrane, and used to observe the change in interfacial pH. As a result, hydrogen ions generated by cellular respiration were gradually accumulated at the cell/substrate nanogap, resulting in a decrease in pH. Moreover, the interfacial pH between the plasma membrane and the substrate became lower than the pH near the surface of cells not in contact with the substrate. The data obtained in this study support the idea that potentiometric ion sensors such as ISFETs can detect a cellular-metabolism-induced change in pH at a cell/electrode nanogap in real time.

Biocompatible and flexible paper-based metal electrode for potentiometric wearable wireless biosensing

“…As a result, a change in the interfacial potential between the solution and the oxide membrane surface is output as the Nernstian response for the change in pH, which is ideally calculated as 59.2 mV/pH at 25°C. Cellular respiration activities were monitored at a cell/ gate nanogap interface using a pH-sensitive FET in a real-time and noninvasive manner [21][22][23][24][25][26][27]. When polyvinyl chloride (PVC)-based ISMs with ionophores are coated on a gate insulator, the Nernstian response is similarly obtained for the change in the potential at the solution/ISM interface with ions of various concentrations (e.g., Na + and K + ) [1,16,18,19].…”

Biocompatible and Na+-sensitive thin-film transistor for biological fluid sensing

“…The operation of FET-based biosensors is based on the potentiometric measurement of biomolecular recognition events, which are specifically detected by modifying the gate sensing surface with various functional membranes such as ion-sensitive oxides and polymers [14,15]. That is, the changes in ionic or molecular charges caused by biological phenomena at/in such membranes contribute to the changes in the electrical properties of FET-based biosensors; therefore, FET biosensors have a significant advantage in that they enable the direct monitoring of biomolecular recognition events on the gate without using materials for labeling DNA [16][17][18][19], proteins [20,21], cells [22][23][24], and small biomarkers [25][26][27]. Moreover, a thin-film transistor (TFT) device contributes to the flexibility and stretchability [28,29] of the biosensors for the in situ monitoring of biological fluids such as sweat on the skin.…”

“…For a-InGaZnO-channel ISTFT sensors, in particular, a SiO 2 thin film, which was coated by sputtering, was utilized as the gate insulator and showed a pH responsivity (about 56 mV/pH) near the ideal Nernstian response at 25°C. This is why cellular respiration is monitored as the change in pH at the cell/gate interface using ISTFT sensors [24]. The oxide membranes used as a gate insulator, such as SiO 2 and Ta 2 O 5 , are the basic configuration of ISTFT sensors similarly to ISFET sensors [36].…”

Sperm-Cultured Gate Ion-Sensitive Field-Effect Transistor for Non-Optical and Live Monitoring of Sperm Capacitation

“…As one of the detection principles to analyze cellular behaviors, a cell-cultured gate ion-sensitive field-effect transistor (ISFET) is used to detect them quantitatively as a change in pH during cell culture in a non-optical and noninvasive manner [15,16,17,18]. Because the gate insulating membrane usually consists of an oxide with hydroxy groups at the surface in a solution, ISFET sensors are responsive to the change in the concentration of positively charged hydrogen ions based on the equilibrium reaction.…”

“…In fact, cellular respiration was monitored as the change in pH based on the amount of respiration products (CO 2 or lactic acid) released from cells. Furthermore, the change in pH at the cell/gate nanogap interface was found to be closely related to the quantitative electrical signal for adhesive cells using the gate electrode of an ISFET sensor [17]. Therefore, the cell-coupled gate ISFET sensor may allow for the quantitative monitoring of sperm activities based on sperm metabolism in a non-optical and noninvasive manner.…”