Simplified Fabrication of Magnetic Nanoparticles With Directly Adsorbed Antibodies for Bacteria Detection

Antigen–antibody reactions with conjugated magnetic nanoparticles (MNPs) are widely used for detecting viruses, bacteria, and proteins. In this study, we proposed a novel method for adsorbing antibodies directly on the surface of MNPs without coating any proteins. MNPs (Fe₃O₄; average size 4 nm) were synthesized by a thermal decomposition method using oleyl amine as a surfactant, the fabricated MNPs exhibit superparamagnetic characteristics, and the antibodies (anti- Bifidobacterium longum) were adsorbed. We found that 15 $\mu \text{g}$ /mg of antibody adsorbed to MNPs and were able to detect the bacteria bound to the produced antibody-MNPs based on the antigen–antibody reaction, offering potential for highly sensitive bacteria detection.

I. Introduction

Antigen–antibody reactions with conjugated magnetic nanoparticles (MNPs) are widely used for detecting viruses, bacteria, and proteins and evaluating and treating cancers [1]–[6]. The MNPs labeled by antibodies open various novel biomedical diagnoses and treatments, such as magnetic resonance imaging, magnetic particle imaging, magnetic immunostaining, magnetic drug delivery, and magnetic hyperthermia, with cutting-edge technologies [7]. The MNPs with antibodies effectively localize target areas by antigen–antibody reactions. Clinical trials and animal experiments under the application of the MNPs, whose size is approximately a few tens of nanometers, labeled by antibodies demonstrated the improvement of diagnoses and treatments. For example, the MNPs labeled by antibodies significantly promote the antigen–antibody reaction by applying magnetic forces to evaluate metastasis intraoperatively [8]–[10]. The efficiency of heat generation on magnetic hyperthermia for cancer therapy strongly depends on the size of the MNPs, and the optimum size of the MNPs effectively involves the annihilation of cancer [11]–[14]. For magnetic particle imaging, the optimum size of the MNP enables us to identify the objectives with high spatial resolution and high signal-to-noise ratio [15]–[17]. Furthermore, the magnetic characteristics attributed to the size play important roles in pharmacokinetics and identification of the MNP in biomedical tissues [18]–[21].