Direct electrochemical immunosensing of tau protein using shape-controlled gold nanoparticles for Alzheimer's diagnostics

Early diagnosis of Alzheimer's disease (AD) remains challenging, creating an urgent need for sensitive, rapid and non-invasive technologies for the detection of blood-based biomarkers. Here, we report an electrochemical immunosensor for the direct detection of phosphorylated tau protein (tau 181), a key biomarker associated with early-stage AD. The sensing interface combines a redox-active poly(toluidine blue) (p(TB)) layer with gold nanostructures, including spherical nanoparticles (AuNPs) and branched nanostars (AuNSs), enabling external redox probe-free detection. The electropolymerized p(TB) acts as an intrinsic redox mediator, while the nanostructures enhance surface area, antibody immobilization and electron transfer. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) confirmed that interfacial charge transfer processes are strongly dependent on nanostructure morphology, with optimal performance obtained using 0.06 mM AuNPs. Direct quantification of tau 181 was achieved by square wave voltammetry (SWV), providing a wide linear detection range (1 pg/mL-100 ng/mL) and an ultra-low limit of detection of 1.09 pg/mL, which is below the clinically relevant threshold for AD diagnosis (2-4 pg/mL). The immunosensor also demonstrated excellent reproducibility and selectivity. By coupling a redox-active polymer with colloidal gold nanostructures on a disposable electrode platform, this work provides mechanistic insight into nanostructure-mediated electron transfer and establishes a promising biointerface for fast and sensitive AD biomarker detection for clinical screening applications.