Immunoassays Using Optical-Fiber Sensor with All-Directional Chemiluminescent Collection.

While chemiluminescent optical-fiber sensors present essential advantages for immunoassays of biomarkers, an inherent limitation hampering their detection sensitivity is the low collection efficiency of chemiluminescent emission induced by samples, owing to the microspatial scale of an optical fiber for transmission of light via total internal reflection. Here we present a robust approach to overcome this limitation, based on a uniquely designed all-optical chemiluminescent collection vial (CC vial) by using a concave mirror and a coaxial tubular mirror as its bottom and wall, respectively. Using cardiac troponin I (cTnI), a highly specific but low abundance cardiac biomarker, as the test sample, we show that accurate assays can be achieved in a wide linear range of 1-80000 pg/mL. The limit of detection is as low as 0.31 pg/mL, which is about 2 orders of magnitude lower than that obtained by a normal chemiluminescent optical fiber sensor. The method is successfully applied for determination of cTnI in real human serum samples with good accuracy and repeatability. Our study shows that the method can perform an all-directional collection of the chemiluminescent emission, thus, greatly enhancing the collection efficiency and improving the sensitivity of the immunoassays. It is also worth mentioning that the proposed strategy requires neither complex equipment nor additional chemicals, making it a cost-effective and universal approach for ultrasensitive detection of trace biomarkers.