Finding the Lost Dissociation Constant of Electrochemical Aptamer-Based Biosensors

Electrochemical aptamer-based (E-AB) biosensors afford real-time measurements of the concentrations of molecules directly in complex matrices and in the body, offering alternative strategies to develop innovative personalized medicine tools. While different electroanalytical techniques have been used to interrogate E-AB sensors (i.e., cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry) to resolve the change in electron transfer of the aptamer's covalently attached redox reporter, square-wave voltammetry remains a widely used technique due to its ability to maximize the redox reporter's faradic contribution to the measured current. Several E-AB sensors interrogated with this technique, however, show lower aptamer affinity (i.e., μM–mM) even in the face of employing aptamers that have high affinities (i.e., nM−μM) when characterized using solution techniques such as isothermal titration calorimetry (ITC) or fluorescence spectroscopy. Given past reports showing that E-AB sensor's response is dependent on square-wave interrogation parameters (i.e., frequency and amplitude), we hypothesized that the difference in dissociation constants measured with solution techniques stemmed from the electrochemical interrogation technique itself. In response, we decided to compare six dissociation constants of aptamers when characterized in solution with ITC and when interrogated on electrodes with electrochemical impedance spectroscopy, a technique able to, in contrast to square-wave voltammetry, deconvolute and quantify E-AB sensors' contributions to the measured current. In doing so, we found that we were able to measure dissociation constants that were either separated by 2–3-fold or within experimental errors. These results are in contrast with square-wave voltammetry-measured dissociation constants that are at the most separated by 2–3 orders of magnitude from ones measured by ITC. We thus envision that the versatility and time scales covered by electrochemical impedance spectroscopy offer the highest sensitivity to measure target binding in electrochemical biosensors relying on changes in electron-transfer rates.