Step‐by‐step monitoring of a magnetic and SERS‐active immunosensor assembly for purification and detection of tau protein

We report a bottom‐up synthesis of iron oxide and gold nanoparticles, which are functionalized and combined to form a nanohybrid serving as an immune sensor, which selectively binds to tau protein, a biomarker for diagnosis of Alzheimer's disease. Detection of the target analyte is achieved by surface‐enhanced Raman scattering originating from the diagnostic part of the nanohybrid that was prepared from Au nanoparticles functionalized with 5,5′‐dithiobis‐(2‐nitrobenzoic acid) as a Raman reporter and monoclonal anti‐tau antibody. The magnetic part consists of Fe_xO_y nanoparticles functionalized with polyclonal anti‐tau antibody and is capable to separate tau protein from a complex matrix such as cerebrospinal fluid. We further identified and validated a set of analytical tools that allow monitoring the success of both nanoparticle preparation and each functionalization step performed during the assembly of the two binding sites by an immune reaction. By applying UV/Vis spectroscopy, dynamic light scattering, zeta potential measurements, X‐ray diffraction, small‐angle X‐ray scattering, and transmission electron microscopy, we demonstrate a proof‐of‐concept for a controlled and step‐by‐step traceable synthesis of a tau protein‐specific immune sensor.