Improved Liquid-Phase Detection of Biological Targets Based on Magnetic Markers and High-Critical-Temperature Superconducting Quantum Interference Device

Abstract

SUMMARYIn this paper, we propose improved methods of liquidphase detection of biological targets utilizing magnetic markers and a high-critical-temperature superconducting quantum interference device (SQUID). For liquid-phase detection, the bound and unbound (free) markers are magnetically distinguished by using Brownian relaxation of free markers. Although a signal from the free markers is zero in an ideal case, it exists in a real sample on account of the aggregation and precipitation of free markers. This signal is called a blank signal, and it degrades the sensitivity of target detection. To solve this problem, we propose improved detection methods. First, we introduce a reaction field, B re , during the binding reaction between the markers and targets. We additionally introduce a dispersion process after magnetization of the bound markers. Using these methods, we can obtain a strong signal from the bound markers without increasing the aggregation of the free markers. Next, we introduce a field-reversal method in the measurement procedure to differentiate the signal from the markers in suspension from that of the precipitated markers. Using this procedure, we can eliminate the signal from the precipitated markers. Then, we detect biotin molecules by using these methods. In an experiment, the biotins were immobilized on the surfaces of large polymer beads with diameters of 3.3 μm. They were detected with streptavidinconjugated magnetic markers. The minimum detectable molecular number concentration was 1.8 × 10 −19 mol/ml, which indicates the high sensitivity of the proposed method. key words: high-critical-temperature superconducting quantum interference device (SQUID), magnetic marker, immunoassays, liquid-phase detection

Biosensing utilizing magnetic markers and superconducting quantum interference devices

“…As a result, the time-consuming washing process that was previously required for B/F separation can be eliminated in the proposed magnetic methods, thus enabling rapid and simple detection of the required targets. To date, high-sensitivity detection of various targets has been demonstrated using the ACS [21][22][23][24][25][26], MRX [27][28][29], and remanence-based [30][31][32] measurement methods, thus confirming the value of these magnetic immunoassay techniques. In the following, examples of the magnetic immunoassay will be given.…”

“…The detection of more realistic targets, such as IL8 and IgE proteins and the fungus Candida albicans, has been also demonstrated [30]. The detection method used was improved to reduce the so-called blank signal that occurs in the absence of any bound markers [32]. These blank signals were generated by both the agglomeration of free markers and the free markers that were absorbed at the bottom of the reaction well, and they reduce the target detection accuracy.…”

“…Magnetic biosensing systems based on SQUID sensors have been developed for in vitro medical diagnosis applications using the ACS [19][20][21][22][23][24][25][26], MRX [27][28][29], and remanencebased [30][31][32] methods. The markers that are bound to their targets are called bound markers.…”

The progress of magnetic sensor applied in biomedicine: A review of non‐invasive techniques and sensors

“…The minimum detectable biotins concentration of this system was 9.5 × 10 −19 mol/ml. In 2016, Ura et al 57 proposed a method to improve the performance of this detection system. It mainly aimed to the signal interference caused by free magnetic bead aggregation and precipitation.…”

(Invited) Biological Applications of Magnetic Nanoparticles for Magnetic Immunoassay and Magnetic Particle Imaging

“…Next, a measurement field B ex,mea = 1 mT is applied to measure the remanence signals from the bound markers. By changing the strength of the external field B ex , undesired aggregation of the free markers during the measurement can be significantly reduced (11).…”