A "turn-on" electrochemical aptasensor for ultrasensitive detection of Cd2+ using duplexed aptamer switch on electrochemically reduced graphene oxide electrode

We report a versatile "turn-on" sensing strategy for the ultratrace level detection of Cd2+ ion based on structure switching aptamer duplex which is immobilized on electrochemically reduced graphene (ERGO) electrode. This approach utilizes a methylene blue (MB) tagged aptamer probe (Apt) and its partially complimentary DNA (cDNA), designed to form aptamer duplex (cDNA-Apt) which is immobilized on the ERGO electrode. The double helix structure of cDNA-Apt on the ERGO electrode impedes the efficient electron transfer of MB to the electrode, due to the far proximity of MB and electrode, resulting in the decreased oxidation peak current of MB. The presence of analytes causes cDNA-Apt duplex to unwind and release the cDNA which can quantitatively facilitate the efficiency of electron transfer of MB, leading to the enhancement of electrochemical signal. It is noted that we can successfully achieve ultrasensitivity of the sensor owing to the large conformational change of Apt from the proposed aptamer architecture and "turn-on" mode sensing upon target binding. This sensing platform can be used to monitor Cd2+ from 1 fM to 1 nM and very low detection limit of 0.65 fM (S/N = 3), which is among the best in all the reported electrochemical aptasensors.