Electrical tracking-assisted cascaded microring biosensor for alpha-fetoprotein detection

Label-free optical sensors have played a crucial role in advancing immediate and rapid health monitoring technologies. An electrical tracing-assisted, intensity-detection-based cascaded microring biosensor for real-time detection of alpha-fetoprotein (AFP) antigen-antibody binding is proposed in this paper. The cascaded microring consists of a sensing ring and a reference ring. The upper cladding layer of the sensing ring was removed and the waveguide surface was functionalized with antigen-specific capture probes. These probes can detect changes in the refractive index upon binding to AFP antigen. The reference ring was heated to track spectral changes. The intensity-based detection method utilizes a low-cost setup consisting of a broadband light source, a fixed-bandwidth filter, an optical power meter, and a current source meter. The electrical tracking scheme solves the problem of narrow linear detection range that usually exists in intensity-based detection, while maintaining low cost. Experiments were conducted to evaluate the sensor's performance in the refractive index sensing by using saline solutions and bio-sensing of AFP. The electrical tracking-assisted cascaded microring exhibited a sensitivity of 9475.5 mW/RIU, with a detection range of AFP of 1 ng/mL to 1 μg/mL. The sensor demonstrated a linear range of 8 to 64 ng/mL and a limit of detection (LOD) of 2.86 ng/mL, falling within the concentration levels typically assessed in AFP assays. This optical biosensor shows significant potential for detecting a variety of biomarkers, including nucleic acids and antigens.