WS ₂ Nanosheet Quenched Erbium Doped NaYF ₄ Upconversion Immunoprobe for the Detection of Glial Fibrillary Acidic Protein: Prognostic Biomarker of Alzheimer's Disease in Serum

Glial fibrillary acidic protein (GFAP) is a clinically significant prognostic biomarker in blood associated with astrocytic injury, gliosis, and various neurodegenerative diseases specifically Alzheimer's disease (AD). The need for reliable, rapid, and cost‐effective methods adaptable for primary healthcare centers is highly demanding for enabling timely diagnosis and effective treatment protocols to reduce the mortality rate associated with AD. In this article, an immunosensor based on Ab‐GFAP conjugated polyethylene glycol functionalized @NaYF4: Yb, Er upconversion nanoparticle with emission at 655 nm was developed. It is then quenched with exfoliated WS ₂ NS as quenchers forming a fluorescence resonance energy transfer (FRET) pair. Upon GFAP addition, the antigen–antibody complex disrupts the FRET interaction, resulting in fluorescence recovery. The developed probe demonstrates a linear detection range from 32.38 to 287.53 pg/mL, with a detection limit of 0.82 pg/mL. The selectivity and cross reactivity against interfering proteins and serum recovery percentage were evaluated, and the results show potential diagnostic promise for GFAP sensing in clinical diagnosis.