We demonstrate wash-free detection of C-reactive proteins (CRPs) based on third-harmonic signal measurement of magnetic markers. In the method presented here, the CRP concentration can be detected from the decrease in the third-harmonic signal from the sample solution. The relationship between the detected signal and the CRP concentration can be modeled quantitatively using a logistic function. The quantities of CRP that were detected using the proposed method showed good correlation with those obtained using the conventional optical method with a washing process. We also demonstrate CRP detection in a hemolysis sample solution that is not optically transparent.
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