Exonuclease I-Assisted General Strategy to Convert Aptamer-Based Electrochemical Biosensors from Signal-Off to Signal-On

In terms of how the signal varies in response to increased concentration of an analyte, sensors can be classified as either signal-on or signal-off format. While both types hold potentials to be sensitive, selective, and reusable, in many situations signal-on sensors are preferred for their low background signal and better selectivity. In this study, with the detection of lysozyme using its DNA aptamer as a trial system, for the first time we demonstrated that such an aptamer-based electrochemical biosensor can be converted from intrinsically signal-off to signal-on with the aid of a DNA exonuclease. The fact that the stepwise cleavage of antilysozyme aptamer catalyzed by Exonuclease I (Exo I) is entirely inhibited upon binding lysozyme leads to the selective removal of unbound DNA probes (thiolate anti-lysozyme DNA aptamer strands immobilized on gold electrode) upon the introduction of Exo Ito the sensor. With the aid of electrostatically bound redox cations ([Ru(NH3)(6)](3+)), we were able to quantitate the number of aptamer strands that are bound with lysozymes via conventional cyclic voltammetry (CV) measurements. We demonstrated that Exo I-assisted signal-on conversion protocol not only improves the sensing performance (10 times better limit of detection) but also promises a versatile strategy for DNA-based biosensor design, i.e., it can be readily adapted to other aptamer-protein binding systems (thrombin, as another example).