A novel bio-sensing technique based on the measurement of nanobeads' Brownian motion using a micro-particle tracking velocimetry (micro-PTV) has been successfully developed to detect antigen-antibody interactions. The rapid interaction between antigens (C-reactive proteins, CRPs) and nanobeads with conjugated antibodies (anti-CRPs) has enabled real-time detection of CRPs to be easily carried out. During the binding process of CRPs to nanobeads, the mean value of the beads' diameters increases so that the Brownian velocity decreases with the increase of time. Moreover, higher CRP concentration leads to a lower Brownian velocity of the nanobeads in the equilibrium state. From the results, the limit of detection 0.1 μg/ml was observed and could be further improved by using smaller nanobeads. Moreover, based on kinetic analysis, the average dissociation constant KD = (6.48 ± 1.43) × 10-7 found by this technique for anti-CRP was shown to be in close agreement with the literature values obtained by dual-polarization interferometry (DPI) and indirect competition enzyme-linked immunosorbent assay (ELISA). This simple sensing method can further be used to detect other bio-molecules and viruses.
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