Highly Sensitive Refractive Index Sensor Based on Polymer Bragg Grating: A Case Study on Extracellular Vesicles Detection

The measurement of small changes in the refractive index (RI) leads to a comprehensive analysis of different biochemical substances, paving the way to non-invasive and cost-effective medical diagnosis. In recent times, the liquid biopsy for cancer detection via extracellular vesicles (EV) in the bodily fluid is becoming very popular thanks to less invasiveness and stability. In this context, here we propose a highly sensitive RI sensor based on a compact high-index-coated polymer waveguide Bragg grating with a metal under cladding. Owing to the combined effect of a metal under cladding and a high-index coating, a significant enhancement in the RI sensitivity as well as the dynamic range has been observed. The proposed sensor has been analyzed by combining finite element method (FEM) and coupled-mode theory (CMT) approaches, demonstrating a sensitivity of 408–861 nm/RIU over a broad dynamic range of 1.32–1.44, and a strong evanescent field within a 150 nm proximity to the waveguide surface compliant with EV size. The aforementioned performance makes the proposed device suitable for performing real-time and on-chip diagnoses of cancer in the early stage.