2. Materials and methods 2.1. Chemicals Chitosan, HAuCl4, N,N-Dimethylformamide (DMF), KCL, PBS, sodium acetate, 50% glutaraldehyde, atto565 NHS ester, HEPES, were obtained from Sigma Aldrich, Ireland. All solutions were prepared by diluting with ultra-pure Milli-Q water (18.2 MΩ.cm, Milli-Q). All synthetic oligonucleotides were obtained from Sigma Aldrich Ireland in a dried form. Upon arrival they were diluted with sterile DI water to 100 µM and stored at -20°C. Before use, they were diluted in 0.1 M PBS buffer to the desired concentration. The probe sequence for stx1 detection was selected based on ISO/TS 13136:2012 standard. The probe was modified with an amine group at the 5' end, enabling covalent attachment to the electrode. For fluorescent confirmation, the target sequence was modified on the 5' end with Atto565 fluorescent dye. The sequences used in this work are detailed in Table . Apparatus All voltametric measurements were undertaken using a CHI920 potentiostat while electrochemical impedance measurements (EIS) were undertaken using a Multi AutoLab M101. A three-electrode electrochemical setup was used for modification of one comb of a gold IDE (used as the working electrode), an on-chip gold counter electrode and an external Ag/AgCl reference electrode. A modified four-electrode setup was employed for DNA detection: a modified gold IDE was used as a sensor, an unmodified gold IDE was used as an accumulator and on-chip gold and platinum as counter and pseudo reference electrodes, respectively. White light optical microscopy images and fluorescent microscopy images of the non-modified and modified IDEs were acquired using an Axioskop II (Carl Zeiss Ltd.) microscope equipped with a halogen lamp and a charge-coupled detector camera (CCD; Coolsnap CF, Photometrics). The surface morphology and compositional analysis of AuNPs and Cht-Au modified IDEs were performed using a field emission scanning electron microscope (FEI QUANTA 650 HRSEM) with energy-dispersive X-ray spectroscopy (EDX Oxford Instruments INCA energy system).