Abstract: A simple immunosensor for rapid and high sensitivity measurements of protein biomarkers, such as SARS-CoV-2 nucleocapsid protein, in serum. The assay is based on a unique sensing scheme utilizing dually labeled magnetic nanobeads for immunomagnetic enrichment and signal amplification. This immunosensor can be integrated onto a microfluidic chip, which offers the advantages of minimal sample and reagent consumption, simplified sample handling, and enhanced detection sensitivity. An ultrafast magneto-immunosensor, which employs AC electrothermally driven flow (ACEF) for accelerated mass transport and enhanced immunocomplex formation, is developed for high sensitivity protein measurement in whole blood samples.