Fig. 4. Real time high sensitivity operation.
a Real time high-sensitivity ion detection in the concentration range 54 × 10−5 M – 1.27 × 10−3 M. The electrolyte concentration is increased every 30 s. The measurements are performed at VM = 0.415 V. b Measured steady-state output voltage VO (symbols) as a function of ion concentration c. Full red line is the linear least square fit to the measurements and yields a sensitivity SA = 640 mV dec−1. c Measured VO as a function of c in various sub-range of concentrations, covering the whole physiological range. The ion concentration where VO = VDD / 2 is c0 = [7.8 × 10−4, 7.8 × 10−3, 7.8 × 10−2, 7.8 × 10−1] M. Full lines are the linear least square fit to the measurements, yielding SA. d Measured (symbols) output voltage variation ΔVO = VO(c) - VO(cmin) as a function of c / cmin, where cmin = [5.4 × 10−4, 5.4 × 10−3, 5.4 × 10−2, 5.4 × 10−1] M. Dotted lines are guides for the eye.