Flexible Gold-Based Electrochemical Biosensor for Highly Sensitive Detection of Saxitoxin in Water Samples

Saxitoxin (STX) is a highly potent cyanobacterial toxin, posing serious risks to aquatic ecosystems and human health. Conventional methods for STX detection, such as high-performance liquid chromatography (HPLC), mass spectrometry (MS), and enzyme-linked immunosorbent assays (ELISA), are effective but costly, labor-intensive, and nonportable, motivating alternative strategies. Here, we report a flexible electrochemical biosensor fabricated via photolithography of gold electrodes on affordable polymer-based substrates for STX detection in water samples. The electrodes showed high mechanical stability, low charge transfer resistance, and an electroactive surface area 55% larger than the geometric area. Anti-STX antibodies were immobilized using a simple sodium citrate-assisted physisorption method. The biosensor response, monitored by cyclic voltammetry in phosphate-buffered solution with [Fe-(CN)₆]^3-/4- as the redox probe, displayed a concentration-dependent current decrease upon STX exposure. The biosensor operated within the range of 0.5-6.0 μg/L and showed a detection limit of 0.2 μg/L, which is 15 times lower than the World Health Organization (WHO) guideline value. The device exhibited high specificity, with negligible responses to microcystin-LR and cylindrospermopsin, and remained effective in mineral water samples across different pH values (6.2-10.2). This study highlights the potential of flexible, low-cost photolithographic sensors for practical monitoring of cyanotoxins in water samples, paving the way for broader applications in the detection of other environmentally relevant toxins.