The early diagnosis of diseases has become a trend in clinical medicine. One of the diagnostic methods for diseases is the analysis of disease-associated proteins, such as tau protein and amyloid b peptide for Alzheimer’s disease, in human blood [1-3]. In the early stages of diseases, the concentrations of marker proteins are very low. This has motivated researches to develop ultrasensitive assay technologies. Most of the technologies for protein assays, such as chemiluminescent enzyme immunoassays and electrochemiluminescence, are based on optical detection [4-7]. Optics-based immunoassays typically result in high background signals and significant interference from colorful biomaterials such as hemoglobin and bilirubin in the blood [8]. Hence, uncontrollable variations in the measured concentrations of proteins are a serious issue. Owing to the merits of low interference, magnetically labeled immunoassays have been proposed as promising candidates for ultrasensitive assays [9,10]. One magnetically labeled immunoassay technology is called immunomagnetic reduction (IMR) [11,12].

In IMR, the reagent consists of antibody-functionalized magnetic nanoparticles dispersed in a pH7.2 phosphate buffered saline (PBS) solution. Under an external alternative-current (AC) magnetic field, the reagent generates signals, referred to as AC magnetic susceptibility. After mixing the reagent with human blood, some of the magnetic nanoparticles bind to protein molecules in the blood. Significant decreases in the AC magnetic susceptibility of the reagent have been observed experimentally [13,14]. With the aid of a high-temperature superconducting quantum interference device (SQUID) magnetometer to sense decreases in the AC magnetic susceptibility of the reagent, an ultrasensitive immunoassay was developed [15]. The low limit of detection of proteins via SQUID-based IMR is as low as the fg/ml range [16]. Many groups have applied IMR for analyzing nucleic acids, chemicals, proteins, and viruses for research purposes [17-20]. Furthermore, the IMR assay has been used to detect proteins related to Alzheimer’s disease, Parkinson’s disease, etc., in human blood [21-23]. Given the substantial confirmation of its clinical value, the IMR assay has been approved for clinical use in Taiwan and Europe.

Although many experimental studies have revealed decreases in the AC magnetic susceptibility of the reagent in the IMR assay, detailed investigations on the mechanism underlying the observed decrease in the AC magnetic susceptibility of the reagent used in the IMR assay are rare. In this work, the AC magnetic susceptibility of the reagent used in the IMR assay was explored theoretically.

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