Recent increases in human activity and contamination of the marine environment have led to the advent of biosensors as sensitive, easy-to-operate, and rapid devices for water pollution screening. In the present study, a whole-cell electrochemical biosensor based on the native green microalgae Scenedesmus sp. MN738556 was developed, for the first time, to assess the biotoxicity of Cd2+ ions in freshwater. This electrochemical biosensor was constructed by immobilization of microalgae-BSA film on a glassy carbon electrode surface. The biosensor responses were based on chronoamperometric currents generated by alkaline phosphatase (ALP) activity. Since Cd2+ ions have an inhibition effect on ALP, the reduction in the general current can be used to determine the toxicity degree of these ions. The feasibility was evaluated and the application of the biosensor was optimized for parameters such as pH and cell density. The sensitivity of the biosensor was verified through the IC50 value of 0.26 ppb for Cd2+cation with the LOD of 0.1 ppb at pH 8.5 and 107 cells/mL. These values are practical for natural water environment monitoring based on the World Health Organization limit (3 ppb for Cd2+ ions in drinking water). The selectivity of the fabricated biosensor was investigated in binary mixtures of other divalent cations such as Zn2+, Ni2+, Cu2±, and Hg2+ at 1:1 and 1:10 ratios. The results indicated that the fabricated Scenedesmus biosensor is highly sensitive with a good selectivity at a 1:1 ratio for measuring the concentration of Cd2+ cations except in the presence of Hg2+. Moreover, this biosensor could respond with only one drop of an analyte (50 μL of 1000 ppb Cd2+ ions), resulting in suitability for simple and on-site water toxicity testing. Finally, two real freshwater samples were selected to prove the reliability, reproducibility, and performance of the Scenedesmus biosensor for the detection of Cd2+ cations discharges in the aquatic environment.

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