Huei-Chun Liu, Hsin-Hsien Chen, Ming-Hung Hsu, Kun-Hung Lee and Shieh-Yueh Yang*
Immunomagnetic Reduction (IMR) is an ultrasensitive assay for low-concentrated circulating proteins associated with diseases, such as cancers, inflammation, preterm birth, and neurodegenerative diseases. In addition to exploring clinical utilities, investigations of the chemical kinetics of the associations between biomarker molecules and nanoparticles in IMR have attracted much interest. To this end, the real-time alternating current (AC) magnetic signal, χ ac, of the reagent after being mixed with a sample was recorded, i.e., χac -t curve. Total tau protein (T-Tau), which is associated with neurodegenerative diseases, was used as a representative biomarker. Detailed characterizations, such as characteristic times in the χac -t curve, instant reaction rates, and orders of reactions, were investigated in this work. In particular, the effect of the concentration of the biomarker (T-Tau) on these parameters was explored. The results show that the nanoparticle-biomolecule association consists of two steps: Diffusion and binding. The time to initiate the binding step decreases as the T-Tau concentration increases, but the time to finish the association increases. During the binding step, the instant reaction rate reaches a maximum before the first half of the binding step is completed. The period of the second half is longer than the first half by 65% to 112%. Furthermore, the nanoparticle-biomolecule association is found to be a first-order reaction. The contributions of various factors, such as the collision frequency, surface energy barrier, substrate mobility, numbers of nanoparticles/molecules, etc., are discussed to clarify the chemical kinetics of the reactions in the IMR assay.
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