This work demonstrates the quantitative assay of cardiac Troponin I (cTnI), one of the key biomarkers for acute cardiovascular diseases (the leading cause of death worldwide) using the fluorescence-based sandwich immune reaction. Surface plasmon coupled emission (SPCE) produced by non-radiative coupling of dye molecules with surface plasmons being excitable via the reverse Kretschmann format is exploited for fluorescence-based sandwich immunoassay for quantitative detection of cTnI. The SPCE fluorescence chip utilizes the gold (2 nm)-silver (50 nm) bimetallic thin film, with which molecules of the dye Alexa 488 (conjugated with detection antibodies) make a near field coupling with the plasmonic film for SPCE. The experimental results find that the SPCE greatly improves the sensitivity via enhancing the fluorescence signal (up to 50-fold) while suppressing the photo-bleaching, permitting markedly enhanced signal-to-noise ratio. The limit of detection of 21.2 ag mL-1 (atto-gram mL-1) is obtained, the lowest ever reported to date amid those achieved by optical technologies such as luminescence and label-free optical sensing techniques. The features discovered such as ultrahigh sensitivity may prompt the presented technologies to be applied for early diagnosis of cTnI in blood, particularly for emergency medical centers overloaded with patients with acute myocardial infarction who would suffer from time-delayed diagnosis due to insufficient assay device sensitivity.
Keywords: Troponin I; cardiac biomarker; fluorescence; lab-on-chip optical biosensor; surface plasmon coupled emission.