Figure 2.
Calibration-free means of estimating imin using a nonresponsive square-wave frequency. (A) E-AB signaling arises due to a binding-induced change in electron transfer kinetics, and thus the relative change in peak current observed upon the addition of saturating target is strongly dependent on the square-wave frequency used to interrogate the sensor. Enough so that there is a specific frequency (40 Hz for the cocaine-detecting sensor shown here) at which sensors of a given type do not respond to target. (B) The origins of this nonresponsive frequency are as follows. When subject to a potential pulse, E-AB sensors produce an exponentially decaying current, the lifetime of which depends on whether or not the aptamer is bound to its target. At the instant the “bound” and “unbound” current decay curves cross (t = 1/f NR) the currents produced by the two states are the same irrespective of which state (or mixture of states) the aptamer is populating. (C) Square wave voltammetry can be used to sample the current at this specific time by setting its frequency to f NR (for the cocaine-detecting sensors shown here, 40 Hz; black curve), at which the sensor’s output, iNR, is constant irrespective of target concentration. In contrast, at a responsive frequency, f R (here 500 Hz; red trace), the output is strongly dependent on the binding state of the aptamer. (D) The proportionality constant, α, which relates imin to iNR, is quite reproducible from sensor to sensor for a given type of sensor, as shown here for the same three cocaine-detecting sensors employed in Figure 1.