Ultrasensitive, label-free detection of cardiac biomarkers with optical SIS sensor

Acute myocardial infarction (MI) is the leading cause of high mortality and morbidity rate worldwide, early and accurate diagnosis can increase the chances of survival. In this work, we report a simple, ultrasensitive, label-free, and high-throughput solution immersed silicon (SIS) immunosensor based on non-reflection condition (NRC) for p-polarized wave for early diagnosis of MI. SIS sensor chips are just a thin dielectric polymer layer on the silicon surface, which can be functionalized for specific application. At NRC, SIS sensors are extremely sensitive to the growing thickness of a bio-layer on the sensor surface while independent of refractive index change of the surrounding medium. Therefore, SIS signal is free from thermal noise, unlike surface plasmon resonance based sensor. Also, there is no need of reference signal which facilitates fast and accurate interaction measurement. Here, SIS technology is applied to tackle two issues in MI diagnosis: high sensitivity with the direct assay and the ability to measure in human serum. Myoglobin, creatine kinase-MB, and cardiac troponin I (cTnI) proteins were used as the MI biomarkers. We were able to measure over a broad concentration range with the detection limit of 5 and 10 pg/ml for cTnI in PBS and blood serum, respectively. The response time is about 5 min. This novel technique is a suitable candidate for cost effective point-of-care application.