Colorimetric sensing for transdermal phospho-Tau 181 detection mediated by wearable microneedle functionalized with gold nanoparticle (MN-AuNP)

Microneedles (MNs) have rapidly emerged as powerful tools in wearable biosensing, providing minimally invasive access to interstitial fluid (ISF). Among the neurodegenerative based biomarkers detectable in ISF, phosphorylated Tau at threonine 181 (p-Tau181) is reaching a clinically evaluable significance for Alzheimer's disease (AD) and other tauopathies. Elevated p-Tau181 levels are strongly correlated with abnormal Tau aggregation in the brain and with cognitive decline. Current diagnostic methods rely on invasive cerebrospinal fluid (CSF) sampling or costly laboratory immunoassays and radio-imaging which are unsuitable for routine or point-of-care screening. Here, we present a highly sensitive colorimetric microneedle-based immunosensor designed for non-invasive, transdermal detection of p-Tau181. For the first time, gold nanoparticles (AuNPs) are integrated into a three-dimensional (3D) microneedle geometry, where antibody antigen recognition occurs directly on MN in contact with ISF. The aggregation-induced optical shifts of AuNPs provide an immediate and instrument-free colorimetric signal, while two optimized coating techniques enable uniform immobilization and reproducible performance. The MN-AuNP platform achieves a limit of detection (LoD) of 16 pg/mL, a nearly 30-fold improvement compared to the reported 2D surface (460 pg/mL). This enhancement shoots from the 3D architecture, which offers greater surface area, enhanced probe loading, and improved analyte diffusion. Compared with existing diagnostic approaches, the proposed system offers multiple advantages: non-invasive operation, real-time readout without complex instrumentation, low fabrication cost, and potential integration into wearable or point-of-care formats. Collectively, these results lay the groundwork for advanced MN-based colorimetric biosensors for early Alzheimer's disease detection through accessible and patient-friendly neurodiagnostic technologies.