Electrochemical Impedimetric Aptasensors

In Radi s work, a gold electrode is functionalized with thiolated a-thrombin aptamer (TEA) and blocked with neutral 2-mercapoethanol (MCE) to act as a sensing interface for a-thrombin (Radi et al., 2005). This negatively charged interface repels the anionic redox couple of [Fe(CN)6] and therefore repels the electron-transfer process. The electron-transfer resistance (Ret) produced is greater than that of bare electrodes. For a-thrombin the pH is nearly neutral, about 

with a molecular mass of about 35,000 Da. When it binds its aptamer on the electrode here, the Ret increases due to an integrated function of both its bulk size and the resistive hydrophobic layer insulating the conductive support. The protein is detected simply by this further increase in electron transfer impedance (Ret), with a sensitive detection limit of about 5 nM. Similarly, in Xu s work. 

STRATEGY FOR USE OF SMART ROUTES TO PREPARE LABEL-FREE APTASENSORS 

Undoubtedly, bulk proteins are very suitable targets for impedimetric aptasen-sors. However, the conditions are not the same for small molecules because most small molecules have a low molecular mass or low net charge that is not strong enough to affect conditions on the electrode surface. 

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An amplified electrochemical impedimetric aptasensor for thrombin has been also described [70]. A nice improvement in the detection sensitivity was achieved by constructing a sandwich platform where the thiolated aptamers were immobilized on a gold substrate to capture the thrombin molecules. Then, aptamer functionalized Au-NPs were used to amplify the impedimetric signals (Fig. 4.11). A detection limit of 0.02 nM, with a linear range of 0.05 to 18 nM was achieved. 

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