Ultrahigh-Sensitivity Fiber-Optic Immunosensor Based on the High-Order-Harmonic Vernier Effect for Cytokeratin 19 Fragment Detection

Herein, we propose and demonstrate a fiber-optic immunosensor based on the high-order-harmonic Vernier effect (HVE) and cascaded dual Fabry–Pérot interferometers (FPIs) for detecting the cytokeratin 19 fragment 21–1 (CYFRA 21-1). The proposed sensor was fabricated by splicing a single-mode fiber (SMF), a hollow-core photonic crystal fiber (HCPCF), and an SMF pigtail. The high-order HVE occurs between the HCPCF (reference FPI) and SMF segment (sensing FPI). To enhance the interaction between the sensing FPI and external medium, graphene oxide (GO) with a large specific surface area and abundant oxygen-containing functional groups is deposited on the SMF pigtail end face. Subsequently, the SMF pigtail end surface is immobilized with the anti-CYFRA 21–1 antibody to detect CYFRA 21-1. Antigen–antibody binding changes the sensing layer thickness and roughness, resulting in changes in the reflectance spectrum. HVE can substantially improve the refractive-index sensitivity of a sensor. In our experiments, we obtained a high sensitivity of 162000 nm/RIU, which is two orders of magnitude higher than that obtained without HVE (1270 nm/RIU). Furthermore, the special structure of the proposed immunosensor results in a low noise level (±0.07 nm). On this basis, the proposed immunosensor was used for CYFRA 21–1 detection, and a limit of detection (LOD) of 1.6 fg/mL was achieved. Thus, the proposed immunosensor has the advantages of ultrahigh sensitivity, high fabrication tolerance, simple structure, strong stability, and good robustness. To the best of our knowledge, this study is the first to apply HVE to fiber-optic immunosensors.