| Summary: | Cadmium (Cd<sup>2+</sup>) is one of the most toxic heavy metals causing serious health problems; thus, designing accurate analytical methods for monitoring such pollutants is highly urgent. Herein, we report a label-free electrochemical aptasensor for cadmium detection in water. For this, a nanocomposite combining the advantages of gold nanoparticles (AuNPs), carbon nanotubes (CNTs) and chitosan (Cs) was constructed and used as immobilization support for the cadmium aptamer. First, the surface of a glassy carbon electrode (GCE) was modified with CNTs-CS. Then, AuNPs were deposited on CNTs-CS/GCE using chrono-amperometry. Finally, the immobilization of the amino-modified Cd-aptamer was achieved via glutaraldehyde cross-linking. The different synthesis steps of the AuNPs/CNTs/CS nano assembly were characterized by cyclic voltammetry (CV). Electrochemical impedance spectroscopy (EIS) was employed for cadmium determination. The proposed biosensor exhibited excellent performances for cadmium detection at a low applied potential (−0.5 V) with a high sensitivity (1.2 KΩ·M<sup>−1</sup>), a detection limit of 0.02 pM and a wide linear range (10<sup>−13</sup>–10<sup>−4</sup> M). Moreover, the aptasensor showed a good selectivity against the interfering ions: Pb<sup>2+</sup>; Hg<sup>2+</sup> and Zn<sup>2+</sup>. Our electrochemical biosensor provides a simple and sensitive approach for Cd<sup>2+</sup> detection in aqueous solutions, with promising applications in the monitoring of trace amounts of heavy metals in real samples.
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