Detection of cyanobacterial toxins using electrochemical biosensors for sustainable and safe agriculture

Increasing environmental pollution and the impacts of climate change have triggered cyanobacterial blooms in water sources, leading to the accumulation of hazardous cyanotoxins such as cylindrospermopsin (CYN) and saxitoxin (STX). These compounds exert severe toxic effects on the liver and nervous system, posing significant risks to human health and the environment through their accumulation in surface waters. This thesis focuses on the development of electrochemical biosensors based on pencil graphite electrodes (PGE) for the highly sensitive detection of cyanotoxins in water, thereby contributing to environmentally conscious and sustainable agricultural practices. In the first phase, the interactions between double-stranded DNA immobilized on the PGE surface and CYN and STX were investigated via electrochemical impedance spectroscopy (EIS). The optimized PGE/DNA system achieved detection limits of 0.12 ng mL⁻¹ and 0.043 ng mL⁻¹ for CYN and STX, respectively. The genotoxic effects of cyanotoxins on DNA were further characterized in detail using FTIR, UV–Vis, and X-ray photoelectron spectroscopy (XPS). For the selective detection of CYN, four different biosensor platforms were developed. The first system involved immobilizing a CYN-specific aptamer (cynApt) onto the PGE surface, reaching detection limits of 0.78 ng mL⁻¹ in deionized water and 0.97 ng mL⁻¹ in lake water by EIS. This sensor exhibited high selectivity even in the presence of structurally similar toxins such as okadaic acid and saxitoxin. In the second system, the PGE surface was modified with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) to enhance conductivity, followed by cynApt immobilization. Electrochemical characterization of PGE and PGE/PEDOT:PSS electrodes was conducted via cyclic voltammetry (CV), while surface morphology was examined using scanning electron microscopy (SEM). Differential pulse voltammetry (DPV) analysis of this sensor yielded detection limits of 0.16 ng mL⁻¹ (deionized water) and 0.23 ng mL⁻¹ (lake water). The PEDOT:PSS modification effectively increased the surface area and electron transfer capability, resulting in the lowest detection limits among the developed sensors. The third system was constructed by electropolymerizing poly(3,4-ethylenedioxythiophene) (PEDOT) onto the PGE surface and immobilizing cynApt thereon. Electrochemical (CV and EIS) and microscopic (SEM) characterizations confirmed the sensor's performance, achieving detection limits of 0.23 ng mL⁻¹ (deionized water) and 0.27 ng mL⁻¹ (lake water) via CV. The PEDOT-modified electrode provided stable and reproducible results due to its homogeneous surface and enhanced conductivity. Finally, the fourth biosensor system involved electropolymerizing polypyrrole (PPy) onto the PGE surface, followed by cynApt immobilization. Characterizations by CV, EIS, and SEM demonstrated that this sensor exhibited competitive detection limits of 0.18 ng mL⁻¹ (deionized water) and 0.24 ng mL⁻¹ (lake water) via EIS. Overall, this thesis elucidates the electrochemical interactions of CYN and STX with DNA and develops highly sensitive, rapid, and reliable electrochemical biosensor platforms for CYN detection. The developed biosensors, characterized by low cost and disposability, enable ppb-level detection in environmental samples such as lake water. The use of conductive polymer/aptamer combinations on PGE surfaces provides innovative, reliable, and practical alternatives for water quality monitoring.