Figure 2. A simulated situation showing the effect of counter-ion crowding on the sensor sensitivity.

At fixed counter-ion size = 8.6 Å, when pH_B = 5, the counter-ion concentration saturates near the sensing oxide surface (a), the surface H⁺ concentration bends and does not follow the Boltzmann model (b) and therefore larger electric potential occurs at sensor surface (c). At fixed pH_B = 5, when the ion size is larger, there is larger decrease on counter-ion surface concentration (d) and therefore greater increase on hydrogen ion surface concentration (e) which leads to higher electric potential at surface (f). The FG potentials are shown for various SG/FG area ratios at constant counter-ion size = 10 Å (g), there is larger coupling between the sensor surface potential and FG when the SG/FG ratio is larger. The electrostatic potential across the sensor when SG/FG = 6 (h) and SG/FG = 2,500 (i) are compared.