Sensitive and reproducible detection of cardiac troponin I in human plasma using a surface acoustic wave immunosensor

We present a sensitive and reproducible surface acoustic wave (SAW) immunosensor that uses an internal reference sensor. A sandwich immunoassay was performed on the working sensing area of the SAW sensor that is able to specifically capture and detect cardiac troponin I (cTnI), while an immunoassay comprising rabbit IgG and anti-rabbit IgG was carried out on the reference sensing area. The analyte in human plasma was captured on gold nanoparticles (AuNPs) that were conjugated in advance with detection antibody. Introduction of these complexes as well as AuNPs conjugated with anti-rabbit IgG into the capture antibody-immobilized working sensor surface and its adjacent rabbit IgG-immobilized reference sensor surface resulted in a classic AuNP-based immunoassay format on both sensing areas. In order to achieve signal enhancement, a gold staining method was performed. Normalized sensor response (a ratio of working sensor response to reference sensor response) due to gold staining varied as a function of applied cTnI concentration. Furthermore, introduction of the internal reference sensor with the normalization technique reduced the coefficient of variation (CV) of the assay by a factor of 4–12, improving reproducibility of the SAW immunosensor. To validate the accuracy of the SAW system, correlation with a commercial system (Centaur®XP, Siemens) was investigated using the plasma of 21 patients and exhibited good agreement with a correlation coefficient (r) of 0.986 and a slope of 0.8 for SAW/Centaur®XP.