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In this work an electrochemical immunosensor for the toxic microalgae Alexandrium minutum (A. minutum AL9T) detection is described. A glassy carbon electrode (GCE) was modified by depositing gold nanoparticles followed by L-cysteine for obtaining a self-assembled monolayer. The SpyTagged nanobody C1, specific for the A. minutum toxic strain AL9T, was then covalently immobilized via SpyCatcher on the surface of the modified electrode and used for the selective capture of such microalgae strain. Electrochemical impedance spectroscopy (EIS) was used for the quantification of A. minutum cells present in water samples by measuring the charge-transfer resistance changes of the electrode with a hexacyanoferrate probe. Each electrode modification step was accompanied by cyclic voltammetry (CV) and scanning electron microscopy (SEM). The immunosensor provided highly reproducible data, was simple to fabricate at low cost, exhibited higher sensitivity than previously described alternative diagnostic methods and showed a broad linear range between 10 3 and 10 9  cells L -1 with detection limit of 3 × 10 3  cells L -1 of A. minutum AL9T. The immunosensor was successfully applied to quantify A. minutum AL9T in seawater and brackish water samples proving that it can be used for early detection of harmful microalgae without the necessity of pre-concentration or dialysis steps., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)