Background: It is difficult to discriminate healthy subjects and patients with Parkinson disease (PD) or Parkinson disease dementia (PDD) by assaying plasma α-synuclein because the concentrations of circulating α-synuclein in the blood are almost the same as the low-detection limit using current immunoassays, such as enzyme-linked immunosorbent assay. In this work, an ultra-sensitive immunoassay utilizing immunomagnetic reduction (IMR) is developed. The reagent for IMR consists of magnetic nanoparticles functionalized with antibodies against α-synuclein and dispersed in pH-7.2 phosphate-buffered saline. A high-Tc superconducting-quantum-interference-device (SQUID) alternative-current magnetosusceptometer is used to measure the IMR signal of the reagent due to the association between magnetic nanoparticles and α-synuclein molecules. Results: According to the experimental α-synuclein concentration dependent IMR signal, the low-detection limit is 0.3fg/ml and the dynamic range is 310pg/ml. The preliminary results show the plasma α-synuclein for PD patients distributes from 6 to 30fg/ml. For PDD patients, the concentration of plasma α-synuclein varies from 0.1 to 100pg/ml. Whereas the concentration of plasma α-synuclein for healthy subjects is significantly lower than that of PD patients. Conclusions: The ultra-sensitive IMR by utilizing antibody-functionalized magnetic nanoparticles and high-Tc SQUID magnetometer is promising as a method to assay plasma α-synuclein, which is a potential biomarker for discriminating patients with PD or PDD.
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