Highly sensitive surface-enhanced Raman scattering-based immunosensor incorporating half antibody-fragment for quantitative detection of Alzheimer's disease biomarker in blood.

Blood-based detection of Alzheimer's disease (AD) biomarker has become a prominent method for diagnosis of AD which can replace the complex and invasive cerebrospinal fluid (CSF)-based diagnostic method. However, the application of blood AD biomarker in actual AD diagnosis is hampered by the extremely low concentration of biomarkers in blood, as well as the existence of interfering proteins. Therefore, it is essential to develop a sensitive and specific detection platform to achieve blood-based diagnosis of AD. Here, a surface-enhanced Raman scattering (SERS)-based sensor is developed for the quantitative determination of tau protein in the plasma of AD patients. To acquire femtomolar-level detection limit, this platform involves the use of half antibody fragment immobilized onto head-flocked gold nanopillar SERS substrates and SERS-nanotags. The small size of the half antibody fragment maximizes the effect of plasmon coupling, by reducing the distance between SERS substrates and SERS-nanotags. Also, the use of half antibody fragment improves the antigen recognition ability by immobilizing the antibody with high density and efficient orientation of the antibody. The sensor using these characteristics showed a low detection limit of 3.21 fM and a wide detection range (10 fM - 1 µM). The platform was also able to accurately quantify the tau protein in the clinical plasma sample and correctly distinguish the AD patient from the healthy control. The ultrasensitive and specific SERS immunoassay platform facilitates accurate and early detection of AD biomarkers and can serve as a valuable tool for simple point-of-care testing in clinical diagnosis.