Polydopamine-based electrochemical immunosensor for sensitive and selective detection of cotinine

Tobacco use is a leading cause of preventable disease and death worldwide. Cotinine, the primary metabolite of nicotine, is widely recognized as a reliable biomarker for tobacco exposure. In this study, we developed a sensitive and specific electrochemical biosensor for cotinine detection using polydopamine nanoparticles (PDA NPs) conjugated with anti-cotinine antibodies. Dynamic light scattering and Fourier transform infrared spectroscopy confirmed successful antibody conjugation and nanoparticle functionalization. The optimized biosensor exhibited diffusion-controlled redox behaviour and a linear response across a wide cotinine concentration range (0.1-10 000 ng mL^-1), with a detection limit of 0.07 ng mL^-1. Differential pulse voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy validated sensor performance. Recovery analysis in diluted human urine demonstrated good reproducibility. The biosensor also showed high specificity against common interferents and retained over 80% of its signal after six weeks at room temperature. These findings demonstrate the potential of the PDA NPs-based electrochemical immunosensor for rapid, cost-effective, and point-of-care detection of cotinine in biological fluids.