Figure 4.
Accurate, calibration-free kanamycin measurements. Dual-frequency calibration-free operation can be achieved even for sensors that lack a nonresponsive frequency. (A) The kanamycin-detecting EAB sensor, for example, does not exhibit a frequency at which its output is independent of target concentration (Figure S15), likely because the binding of its aptamer is not two-state (e.g., the sensor output trends downward at high kanamycin concentrations, presumably reflecting a second binding event). The sensor’s output at a low-response frequency (250 Hz; open circles) nevertheless parallels its output at a more responsive frequency (750 Hz; solid circles). (B) That is, despite the modest target-concentration-dependence of the output current, iLR, observed at the low-response frequency the ratio i/iLR is both indicative of target concentration and largely independent of sensor-to-sensor variation. (C) Exploiting this (eq 5) we obtain calibration-free estimates of kanamycin concentration in buffer that are accurate to within ±10% over the range 40 μM to 1 mM (dark blue) and within ±20% over the range 30 μM to 3 mM (light blue). Additional data for individual sensors (illustrating, for example, the good precision of these sensors) are presented in Figure S19.