Label-Free Electrochemiluminescence Immunosensor Based on Ru(bpy) ₃ ²⁺ and PEI-Carbon Dots for Ultrasensitive Tau Detection in Alzheimer’s Disease

Alzheimer’s disease (AD) is characterized by progressive memory loss and cognitive decline, significantly impacting the daily functioning of elderly individuals. The low abundance of AD-associated biomarkers in blood necessitates highly sensitive detection techniques. Tau protein, a critical biomarker of AD, plays a vital role in the early diagnosis and monitoring of disease progression. In this study, we developed an electrochemiluminescence (ECL) immunosensor based on the Ru(bpy)32+ luminophore and polyethyleneimine-carbon dots (PEI-NCD) as co-reactants for the detection of tau protein. The system employs a label-free format, where monoclonal Anti-tau antibodies are conjugated to PEI-NCDs, leveraging their abundant amino groups for efficient antibody attachment and co-reactant functionality. A two-electrode setup was established; upon the addition of tau antigen, the luminescence intensity decreased, producing a sensitive turn-off response. The immunosensor achieved a low detection limit of 6.103 pg/mL with high specificity and selectivity even in the presence of other biomarkers. Additionally, they demonstrated excellent recovery (93–104%) in spiked serum samples. Electrochemical characterization confirmed the stability and reliability of the sensor for tau detection. Overall, this Ru(II)-based immunosensor holds strong promise for the ultrasensitive and specific detection of tau protein, supporting early diagnosis and effective management of Alzheimer’s disease.