Sensitive electrochemical sensor for Cd2+ with engineered short high-affinity aptamer undergoing large conformation change

Cadmium ion (Cd2+) is a highly toxic heavy metal ion that threatens the environment and human health. To achieve rapid and sensitive detection of Cd2+, here we developed a reagent-less aptamer electrochemical sensor by immobilizing an engineered high-affinity DNA aptamer with a redox tag of methylene blue (MB) on the gold electrode. After testing a series of engineered aptamer sequences, we employed an optimal and new 15-mer aptamer with a short 3-bp stem for sensor fabrication, which underwent large conformation change upon Cd2+ binding. This aptamer retained high affinity with a Kd about 360 nM, verified by isothermal titration calorimetry (ITC) analysis. In the presence of Cd2+, this aptamer folded into a stem-loop structure, drawing the MB into a close proximity to the electrode surface and generating enhanced current in square wave voltammetry (SWV). Under the optimized conditions, this aptamer sensor enabled us to sensitively detect Cd2+ in a wide concentration range from 0.5 nM to 4 μM, and the detection limit was 90 pM. The developed electrochemical aptasensor has the advantages in easy preparation, rapid response, high stability, high selectivity and easy regeneration and reuse, showing the potential for Cd2+ detection in broad applications.