Label-Free, Sensitive, and Direct Detection of Cardiac Troponin Biomarkers Using Frequency-Locked Microring Resonators

Abstract

Silicon photonic microring resonators have emerged as promising sensors for Point-of-Care applications, where the readout of one or many biomarkers at once is required. In the context of rapid heart attack detection, a limit of detection reaching an ultra-low concentration of biomarkers is needed, however, such sensors are prone to fundamental noise influence in optical systems which can potentially jeopardize sensor readings. While noise reduction has previously been explored with the Pound–Drever–Hall (PDH) technique, its full implementation in microring biosensors has not been realized due to the complexity of the setup. Recent innovations in photonic integration and compatibility with MEMS structures have sparked new interest in validating PDH's potential to be used with chip-scale sensors. By enabling the signal readout of microrings through their phase response, instead of power transmission, the impact of optical noise can be greatly reduced. This study explores a proof-of-concept for this system against the cardiac troponin biomarker, demonstrating the sensor's capacity for selective measurement down to a limit of 10 ng mL⁻¹ while using frequency locking. An improved limit of detection for this system is achieved, down to 5.03 × 10⁻⁷ RIU, which is two orders of magnitude improved compared to the equivalent sensing based on intensity alone.